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timgunn

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Posts posted by timgunn

  1. The idea that there is a sharp transition from Oxidizing to Reducing is one of those oversimplifications that are very useful when trying to explain broad general principles, but which can get you into deep trouble if you take them as gospel.

     

    If you try to think in terms of there being a range over which the atmosphere varies between strongly Oxidizing and strongly Reducing, then factor in the position of the workpiece within the flame itself (within the flame, the Oxygen and fuel gas have not yet finished combining, so there will be more free Oxygen available to react with the steel in the early part of the flame), you may find it easier to work out what is going on.

     

    If you richen up the flame a little more (more gas for the same air or less air for the same gas), there's a good chance you'll see an improvement. You may see the forge temperature drop when you do this, so may then have to increase both gas and air in the same proportion to get the forge temperature back up to where you want it.

     

    There are 2 factors at play here: the temperature of the flame and the amount of flame.

     

    The amount of flame is quite easily understood.

     

    The temperature of the flame is dependent on the air:fuel ratio and will be highest at the stoichiometric ratio. This is the point at which all the Oxygen and all the fuel combine, leaving neither excess fuel nor excess Oxygen behind. It corresponds to the sharp transition point in the oversimplified model.

     

    We do not want excess Oxygen so we are always going to run on the fuel-rich side of stoichiometric. The heat produced will therefore be limited by the amount of Oxygen present. If we richen the mixture by adding more gas, the amount of heat energy released cannot increase because there is no Oxygen for the extra gas to react with. The extra gas will absorb some of the heat energy released by the original fuel burning and this will lower the overall flame temperature.

     

    We can turn up the gas-and-air flow to increase the amount of flame to compensate, but we can never get the forge temperature above the flame temperature. This means that the forge cannot be run as highly-reducing at high temperatures as it can at low temperatures.

     

    Part of the skill involved in using a gas forge with variable mixture is achieving an effective compromise between gas usage and oxidation of the workpiece. In my very limited experience, gas is a whole lot easier than using solid fuel, but it still involves a bit of a learning curve.

    • Like 1
  2. Just a heads-up on the TM902C pyrometers: I'd had about 30 or so that were boringly reliable and as accurate as big-name branded instruments costing more than ten times as much over the full range.

     

    I bought another ten last year and put them on the shelf. They were bought on ebay and came from China.

     

    Before I use one for the first time, I usually put it on the calibrator to check it is reading sensibly. I did this with several of the new batch and found they were all horribly inaccurate once the temperature got much above 800 degC (1472 degF).

     

    Below that temperature, accuracy seemed pretty good; the variation was less than that I'd expect to see between different thermocouples, even those made to special limits of accuracy, but above 800 DegC, accuracy got progressively poorer and I was getting readings that were off by 80 DegC with the calibrator set at 1200 degC.

     

    I still had a couple of the older ones and checked they were still reading correctly, mainly to check the calibrator. The old ones were fine.

     

    There are some visible differences between the old ones and the new ones, but you've really got to have them side-by-side to see them.

  3. There have been a couple of comments on the jet size that seem to suggest confusion over units. The link to Grant Thompson's video in the OP shows a .6mm tip, which is about .024". Probably about a .030" hole, as the hole is bigger than the nominal wire size it is intended to pass.

     

    If anything, it looks like it's too small to me.

     

    It's hard to tell what's going on with the choke in the last video, but I get the impression it is going too lean and going out when the choke is opened?

     

    If so, and it's a .6mm mig tip, I might try a .8mm mig tip and see if it improves things, but only if it is going to be used in open air.

     

    I would not try tapering the present tip. It looks like there's too much air at the moment and anything that increases the amount of air drawn in, relative to the gas flow, seems likely to make things worse.

     

    If you stick it in a forge with some back-pressure, that will reduce the amount of air. Grant's setup looks like it probably has quite a lot of back-pressure, so putting it in the furnace, with a crucible, seems like the thing to do if that's how you are intending to use it.

  4. The continuous edge for tile cutting does not seem to clear very well on anything but thin tile, so avoid them.

     

    Either the segmented ones or the ones with a continuous edge but alternating thick and thin bits when viewed edge-on should work well. The "turbowave" ones from tool station at 3 quid would be my choice.

  5. I'd try chain drilling out the corners with a masonry drill bit used without hammer, or perhaps a tile drill if you can find one long enough, then go at the straight runs with an angle grinder.

     

    The diamond disks for angle grinders are cheap and extremely effective. Screwfix or Toolstation are probably the best source over here. Buy the cheap ones. The expensive ones are better for specific tasks, but only really worth it if you need to cut 300 slabs for an awkward-shaped patio.

     

    Don't use your best grinder. Work outside. Wear a mask. Expect a huge amount of extremely fine dust.

  6. KBAC 27D looks like a good drive. Lots of guys use it Stateside.

     

    As far as I can tell, it has 2 big advantages there.

     

    First is that it will take either a 110V or a 220V supply, though the motor is limited to 1 1/2 HP on 110V. This probably does not matter to you in Oz.

     

    Second is that it is a sealed drive. This is essential for a grinder. However, there are lots of other manufacturers of sealed drives. Invertec are probably the European go-to sealed VFD manufacturer with their IP65 and IP66-rated drives and there are sealed drives available from most of the big manufacturers. I'm half a world away and don't know what's available to you. Look for something sensibly priced and supported locally.

     

    Other points that I suspect are in the KBACs favour in the US knifemaker market: it is US manufactured, and setup is by jumpers and trimpots, rather than a menu-driven keypad/display.

     

    The KBAC 27d seems to be a V/Hz drive and does not have Sensorless Vector capability as far as I can tell.

     

    You are on 50 Hz mains in Oz. This means that your motor rating plate will show the rated speed at 50 Hz. With the VFD, you don't actually need to worry about the rated speed very much. The important thing is the number of pole pairs on the motor.

     

    50 Hz is 50 cycles/second and there are 60 seconds in a minute, so 50 Hz is 3000 cycles/ min (60 Hz is 3600 cycles/min). Each pair of poles takes one cycle, so a 2-pole motor will only need one cycle per revolution and will turn at "about" 3000 RPM on 50 Hz. A 4-pole motor has 2 pairs of poles, so needs 2 cycles/rev and turns at "about" 1500 PRM on 50 Hz. 6 pole, "about" 1000 RPM and so on. On 60 Hz mains, as used in North America and some other places, the speeds would be 3600 RPM, 1800 RPM, 1200 RPM and so on.

     

    An unloaded motor will run pretty close to the speeds given in the last paragraph, but as the motor is loaded, it slows down and produces "slip". This slowing down is not a bad thing: it is just part of the physics of electric motors, with the slip causing the motor to produce torque. The 2800 RPM motor you mentioned would have been slowed down from a bit under 3000 RPM at no load to 2800 RPM at its rated output: about a 7% speed reduction. The 1400 RPM motor sees a similar speed reduction in percentage terms.

     

    Some of the NEMA 56-frame motors may be slightly different because the 56-frame was originally intended for Fractional HP motors and there has been a lot of work put into getting 2HP or more from that small a frame size, but generally it is the case that manufacturing economics have driven manufacturers to standardize their products as much as possible. You can be pretty sure that all the IEC 90-frame motors in a particular range from a particular manufacturer will use the same bearings, casing, rotor, etc. and that the rotating assembly will have been designed for 3600 RPM (a 2-pole motor running on a 60 Hz supply). The 90-frame has a 24mm shaft, so I assume that is the frame size you are looking at.

     

    The only major change for the 4-pole, 6-pole, etc variants will be the windings, which are static and are fixed to the inside of the casing.

     

    The design speed of 3600 RPM sets the upper end of the speed range (you can run faster, but you'll probably be outside the manufacturers design range unless you can find a spec for the motor you are intending to use that gives a higher maximum speed.

     

    Most basic (V/Hz or V/F) drives will run OK down to about 10 Hz happily. Much below that, my experience is that they start to feel "coggy" by about 7 Hz and become noticeably less smooth. Sensorless Vector drives have some additional electronics that allows them to run a motor smoothly at lower frequencies: I've used them at 3 Hz with no apparent loss of smoothness and they may go lower still. I've never need to try.

     

    You want a 10:1 speed range or better (500-5000+ SFM).

     

    Running a 4-pole motor (rated for "about" 1500 RPM on 50 Hz) from 10 Hz (about 300 RPM) to 120 Hz (about 3600 RPM) will give you a 12:1 speed turndown and ticks that box.

     

    If we do the belt speed calculation at the bottom end, where you are likely to be using very light pressure and not slowing the motor to generate much torque, we can get a drive wheel diameter.

     

    500 SFM / 300 RPM = 1.67 feet per rev = 20" per rev.

     

    Drive wheel diameter = 20" / Pi = 6.36"

     

    Call it 6", which is 0.5'

     

    At the top end of the speed range, you'll probably be leaning on the belt and hogging, so you want maximum torque.

     

    At 50 Hz, the 4-pole (1500 RPM unloaded) motor is doing 1400 RPM.

     

    1400/1500 = 0.93333

     

    3600 x 0.93333 = 3360 RPM

     

    3360 x 0.5' x Pi = 5277 SFM

     

    The 500-5277 speed range on a 6" wheel is a safe bet. You could maybe go higher at the top end if you are happy to exceed 3600 RPM, and maybe go a tad lower at the bottom end even with a V/Hz drive and certainly with an SV drive. This is something you'll only really be able to determine once it is running.

  7. Looking at the photo of the motor rating plate, it's rated as a 2.2 kW motor at 50 Hz and as a 2.5 kW motor at 60Hz, so you'll want the 3 kW VFD to allow the possibility of running to 60 Hz. You'll need to connect in Delta, but I assume you are already running in Delta on the capacitors.

     

    The speed you can actually run to will probably depend on the current draw and you won't know the speed/current relationship until you actually run it.

     

    I had some brief hands-on experience of a Chinese hammer on a HuanYang VFD when I helped set the VFD up, It worked, but I don't honestly know how well it has held up since.

     

    If you can get your hands on one, an optical (laser) tachometer is a very useful thing to have when setting things up: Stick a bit of reflective tape on the top die and you can read out the BPM directly. They cost around 10 Euros/bucks from China on ebay, but the delivery time might be a problem. If you know anyone into RC aircraft or drones, it's worth asking them if they have one you can borrow.

     

    Note that the VFD will need protecting from metal dust, especially if there is grinding done in the vicinity.

  8. IME the best way of running a 3-phase motor on single-phase mains is with a Variable Frequency Drive.

     

    Search for "2.2kw VFD" on ebay and you'll find plenty of HuanYang drives. Filter for buy now and sort by price+postage. Best to go advanced and limit location to somewhere that is not going to mean import charges. They are cheap drives but seem to work pretty well.

    • Like 1
  9. The apology should be mine: It didn't read the way I intended at all.

     

    I tend to be somewhat anal and pedantic about stuff like this, partly because it tends to get picked up and taken out of context when someone has a problem and Googles it months or years down the line. It was with such a future Google-jockey in mind that I posted.

  10. Can you post pics?

     

    It's quite difficult to suggest what can most usefully be shown by photos with no prior information. Best approach is to give it a run, take photos of everything you can and post the long shots first. If the guys that know what they are doing ask for specific details after that, you have the photos ready.

     

    The most useful shot is usually taken in the dark from a distance off to the side and shows the Dragons Breath from both the front and rear ports. Long shots in bright daylight don't normally show the Dragons Breath at all and are of limited help when it comes to diagnosing the overall problem.

     

    Close shots into the chamber can sometimes be useful, but digital cameras usually mess about with the white balance so badly that the color in the photo is nothing like the color in the forge. They are most useful for showing temperature distribution: is the color even, or are some parts hotter than others?

     

    Sometimes video is better than a photo, particularly when trying to capture images of flames.

     

    If pics are not an option right now, what are the burners? Atmospheric or Naturally Aspirated? Whose plans did you follow (a link helps)? What changes/improvements did you make? What jets did you use? What is the actual ID of the forge and what size are the openings?

     

    The burner to forge volume thing seems to be a rule-of-thumb, rather than an absolute thing: A forge that is well-designed and well-constructed to "normal" proportions from the appropriate materials will work with X burners of Y size per Z cubic inches. The surface area and particularly the opening area seem to be the most important factors affecting the heat retention and these are accounted for in the rule-of-thumb by the "normal proportions" bit.

     

    If you just stick a brick in the forge to occupy some of the volume, you still have the same size forge, but with a brick in it. It will not help much. If you add another layer of insulation to reduce the inside dimensions, it may help quite a lot.

     

    Reducing the size of the openings usually helps and that's where the brick might do most good.

     

    I'm not a smith and you'll get better advice from those who are. I do mess about with gas burners for a living though.

     

    One of the things I see when I visit smiths is lots of Dragons Breath and not enough temperature. This is often easily fixed by going to a smaller gas jet.

     

    It's easy to lose tight of the need for enough air to burn the gas in the forge and think in oversimplified terms of "more gas = more heat". As soon as there is more gas than the amount of Oxygen present can burn, adding more gas just reduces the temperature in the forge and increases the DB outside it. This may or may not be part of your problem. Pics would help, particularly the one from the side showing the DB.

  11. Here's a quote from another forum regarding forge insulation that I got from Ed Caffrey MS and the source of my info:

     

    "Something that caught my attention in your post... you mentioned 2" of kaowool. I've been seeing that a lot lately, and have to point out that with kaowool, more is not necessarily better. In fact, most folks don't realize that when it comes to 1" versus 2" of kaowool, 2" does not insulate as well as 1". Kaowool is designed to absorb a given amount of heat before it starts reflecting, and the % of heat it reflects is dictated by the thickness...... 2" absorbs more heat before it starts reflecting, and due to it's thickness, reflects a lower percentage of heat. So what does that mean? It means that in the same application, 2" kaowool requires more energy (whether that be electricity, propane, natural gas, ect), and reflects a lower percentage of heat then 1" of the same density, which means it requires more energy/fuel to achieve/maintain a given temp with 2" kaowool, then it does with 1" (again, assuming they are the same density)

     

    I've asked a number of people why they use 2" of kawool instead of 1"... and the answers I get have lead me to believe that its the old mindset of "More is better"....but that's simply not true when it comes to ceramic fiber blanket such as Kawool."

     

    I hope that this helps make sense of it all.

     

    Gary

     

     

    Gary, is there any chance of a link to the original source?

     

    I'm struggling to find it with Google and I'd like to try to get full the context so that I can understand the bigger picture. In particular, I'm interested in what effect any surface coating might have on things.

  12. "Pressure gauge" in this case is likely to mean a U-tube.

     

    Take a transparent tube, fix it in a U shape and half fill it with water. The water in both legs will be at the same level until you apply pressure or suction to one leg. When you do, the height difference between the legs is the pressure. Measuring in inches gives "Water Column. Measuring in cm gives millibar.

  13. First thing to say is that I've built a few forges and they seem to work ok. I've spent most of my working life dealing with gas burners and temperature control and I think I understand it fairly well. However, I'm no smith and there are plenty of guys out there with more experience than me.

     

    That said, my take on it is this:

     

    There are a number of factors that determine how much gas you really need and those rules of thumb are just rules of thumb.

     

    Most of the stuff that is out there on BTUs and forges is aimed more at blacksmithing than at bladesmithing. Welding temperatures for mild steel, and particularly for wrought iron, are quite a lot higher than those for typical bladesmithing steels.

     

    2200 degF is around 1200 degC and it is certainly possible to weld at that temperature. However, when I've measured the temperature of forges being used for making Damascus, by smiths who are good at making Damascus, they have been around 1300 degC (2372 degF). Even then, it is high-Carbon steel that is being welded.

     

    Blades also tend to be able to fit through small openings, so it's quite easy to build a bladesmithing forge with small openings and a large volume. A typical blacksmithing forge will have a much larger front opening and often a large rear opening.

     

    In my experience, the area of the openings has much more effect on the heat input needed to maintain temperature than the forge volume does.

     

    For the (blacksmithing) rules of thumb, there is probably quite a strong correlation between forge volume and open area when building "normal" forges.

     

    The BTU rating of the burner is based on the heat output assuming all the gas is fully burnt. In reality, it seems quite usual to run with a rich (reducing) forge atmosphere to control scaling and for there to be a significant amount of Dragons Breath. The DB represents wasted heat: BTUs that are not actually released in the forge itself.

     

    When using MIG tips for gas jets, it should be noted that the tips are sized for a nominal wire size and that I have found the bore of the tip is typically .006-.008" bigger than the nominal tip size (Hybridburners give .007-.009" near the bottom of the page at http://www.hybridburners.com/BTU-charts.html#oneerchart) A .045" nominal mig tip would therefore be .051-.054" in the BTU calculator. The nicely-tapered lead-in probably also raises the discharge coefficient of the jet to at least 0.75 and possibly close to 0.8. This will also affect the BTU calculator results.

     

    When I was playing with jet sizes for burners, I built a forge using cheap IFB and with a 6" x 6" x 13.3" chamber. With a single 3" x 3" opening, I got a temperature of 1545 degC (2813 degF) on a 0.6mm (.023") MIG tip measuring between .030" and .031" diameter and 4 bar (59 PSI) gas pressure. BTU calculator gives just under 104,000 BTU/hr (.031", 59 PSI, 0.8 discharge coefficient) which works out to 214 BTU/cu.in (less than half the rule of thumb figure) even at over 2800 degF.

     

    DSCF0005_zpsdaf77aa2.jpg

     

    DSCF0006_zps51cf6b93.jpg

     

    And the effect on the Thermal Ceramics JM23 IFB I'd used to restrict the opening:

     

    DSCF0011_zpsa1906ee3.jpg

     

    The cheap IFBs held up to the temperature pretty well, despite only being rated to 2300 degF, but are pretty poor insulators compared to the JM23s.

  14. I'm guessing the burner throat is (maybe) too big, or you are not putting enough fuel/air through it.

     

    Does the following make sense and/or sound like it could explain things?

     

    In the burner tube, the mixture needs to be moving towards the forge faster than the flame-front travels through the mixture in the opposite direction.

     

    The speed of the flame-front is not a constant. It depends on the air:fuel ratio, the pressure and the temperature.

     

    As the forge heats up, the temperature of the burner tube tends to rise a little and the speed of the flame-front increases.

     

    At some point, the flame-front moves slightly faster through the mixture than the mixture is moving along the tube.

     

    Often, there is a pop (at least with a NA burner: I don't get to play with blown burners) as the flame accelerates down the tube to the gas jet, where the flame goes out, followed by a brief pause as the burnt gases clear the tube, then another burnback when fresh mixture hits the hot chamber and everything repeats. If the operator is quick, turning up the pressure often sorts things. If the operator is not quick, each successive flashback heats the burner tube a little and increases the flame-front speed until the flame stabilizes at the back end of the burner tube close to the gas jet.

     

    If that sounds like your problem, your best approach is to increase both gas- and air-pressure to increase the mixture speed. If you have found your sweet-spot and would be wasting gas by doing this, you'll need to reduce the burner throat diameter so that when you increase the pressure to increase the mixture speed, you get back to the gas- and air-flow that you have now.

  15. Kaowool looks fine.

     

    Burner looks like it should be OK.

     

    Meter looks fine. Personally, I search for TM902C on ebay, filter by shipped price and buy the cheapest. The TM902C reads in degC only but that's not a problem when you are aiming for a target temperature you've decided on beforehand. I've had 20 or so and they've all worked to 1368 degC (just under 2500 degF), regardless of the range marked on the front. I've had them on the calibrator at work and accuracy has been just as good as brand-name instruments at 20 times the price. At around 5 bucks delivered, they are a bargain.

     

    The probe is really the awkward bit. It will not usually be cheap.

     

    The one in your link has a 6" probe and 3' of cable. If you fit it through the casing, you'll be able to measure the temperature about 4" from the wall wherever you fit it. Being a type K, the junction will measure to 2500 degF. The rest of the thermocouple assembly will probably not take that temperature though.

     

    Over here, I'd use something along the lines of the 405-041 with a miniature plug 724-102

     

    http://www.tcdirect.co.uk/Default.aspx?level=2&department_id=190/2and http://www.tcdirect.co.uk/Default.aspx?level=2&department_id=280/1

     

    I appreciate this doesn't help you guys in the states much, but it should give the general idea. I've been struggling to find a US equivalent of TCDirect with online pricing, though I suspect my UK-centric search settings don't help

     

    It's well worth reading up on thermocouples, but it seems pretty difficult to find a good basic thermocouple primer anywhere. Most seem to get quite technical, quite quickly. Probably the best I've found is the Labfacility Handbook:

     

    http://www.labfacility.co.uk/temperature-handbook.html

     

    About the biggest name in temperature control worldwide is Omega: http://www.omega.com/temperature/

     

    They can supply pretty much everything you need. They are not the cheapest (IME) but have genuinely outstanding technical support that more than justifies the premium (again, IME). If you need good advice, they are well worth a phone call. They have an online thermocouple configurator that will give prices, but there's not an online pricelist that I've found.

  16. I've not tried 3/4" kaowool (or even seen it). If it's the high-density stuff (128 kg/m3, 8 lb/cu ft) I think it "should" work, but will give you a bit less headroom for getting the rest right than the thicker stuff. I'd rather expect the lower densities to give marginal insulation at 3/4", though if you get everything else pretty much perfect, it should still work OK.

     

    I'd recommend a long type K mineral insulated thermocouple with a 1/4" sheath in type 310 stainless, Inconel or Super Omegaclad XL. That way you can put the tip anywhere you want it and can move it about to profile the temperature distribution. Once you are happy with the distribution, you can put the probe in a representative position and use it for the adjustments. I find 6mm diameter (1/4") is stiff enough to use horizontally at 1000mm (40") long at HT temperatures. Vertically, there should be no problem at all except for keeping the handle and the hand holding it cool. getting a 6' transition junction thermocouple and bending it is probably the best solution.

  17. Would stainless wire work to hold the blanket up?

    Depends on the stainless and the the temperature you are working, but tentatively yes.

     

    What kills stainless in heat cycling is loss of the Oxide layer. When it gets hot, an oxide layer forms. When it cools, the mismatch between the expansion coefficients of the Oxide layer and the underlying Stainless Steel causes the Oxide to spall off. After a number of cycles, depending on thickness, there's nothing left. Different stainless steels will hang onto the Oxide layer through different temperatures. 316 keeps the Oxide layer intact through around 850 degC/1560 degF, so is good enough for most Carbon steel HT.

     

    310 stainless has higher (25%) Chromium for a stronger Oxide layer and higher (20%) Nickel, which brings the thermal expansion coefficient closer to that of the Oxide, and will keep the Oxide layer intact through cycles up to about 1100 degC/2000 degF.

     

    Finding 310 is usually difficult, so Nichrome or Kanthal are often used because they can be found in small quantities on ebay.

     

    316 in conjunction with rigidizer works quite well for blanket retention: by the time the stainless wire fails, the shell is rigid.

  18. There are many more things going on in a gas forge than there are in a gas cooker and it is very dangerous to assume that a forge will be as safe as a cooker simply because both burn gas.

     

    The "well-tuned" part is where most of the complexity comes in when referring to a gas forge.

     

    For heating things with flames, the flame needs to be hotter than the thing that it is heating because heat transfers from areas of high temperature to areas of low temperature.

     

    For cooking, the thing being heated (the food) tends not to need to reach a temperature much above about a couple of hundred degrees Centigrade; around 400 degrees Fahrenheit. None of the common fuel gases will sustain a flame at temperatures this low, so it's fair to assume that if there is a flame present, it is hot enough to cook with.

     

    The steeper the temperature gradient, the faster the heat transfer, so higher flame temperatures are better for efficiency. The highest flame temperature for each of the common fuel gases tends to be at around the stoichiometric air:fuel ratio. This is the air:fuel ratio at which all the fuel and all the Oxygen are consumed with neither fuel nor Oxygen remaining.

     

    Moving either side of the stoichiometric ratio, the flame temperature reduces because there is either excess air or excess gas to be heated to flame temperature and this takes some of the energy released during combustion.

     

    If we have excess gas, we don't get complete combustion and produce some Carbon Monoxide, a toxic gas. If we have a small amount of excess air, the flame temperature is still very hot and there is enough energy to cause some of the excess Oxygen to combine with Nitrogen and form Oxides of Nitrogen, often abbreviated to NOX, which are toxic gases.

     

    The OP lists auto racing and modifying among his interests, so he is probably somewhat familiar with some of this. An automotive exhaust Lambda sensor usually supplies a signal to the engine management system, which it uses to adjust the fuelling and maintain a Lambda value of 1. Lambda is just the scientific notation for the stoichiometric ratio. An exhaust reading below 1 is a rich mixture, above 1 is a lean mixture.

     

    If the mixture is very lean, the flame temperature is much lower. There is not enough energy to react the excess Oxygen with Nitrogen and little or no NOX is formed. Carbon Monoxide will tend not be formed either.

     

    The gas cooker will therefore be set up to burn very lean. That way there is minimal risk of causing large numbers of American mothers to keel over while cooking the thanksgiving turkey. The reduced heat transfer efficiency is generally felt to be an acceptable price to pay for this.

     

    Wikipedia lists the CO concentration near properly-adjusted gas cookers as 5-15 PPM whilst OSHA place a long-term exposure limit of 50 PPM of Carbon Monoxide on workplaces.

     

    We tend to need higher flame temperatures for forging than a very lean flame can provide. For welding, we tend to need temperatures that can only be reached with mixtures quite close to stoichiometric.

     

    When we have Oxygen present and high temperatures at the surface of our steel, we get scaling and decarburization. We tend to consider this undesirable, so usually run our forges with excess gas instead.

     

    This means that we are intentionally operating our forges to produce Carbon Monoxide.

     

    Most of the Carbon Monoxide produced in the forge leaves the mouth of the forge and mixes with air, where it burns with Oxygen to produce Carbon Dioxide. This burning is often referred to as the dragons breath. Because the mixing is uncontrolled, parts of the mixture will cool before combustion is complete, so some of the Carbon Monoxide will not be burnt and will remain in the air.

     

    I am less familiar with solid fuel combustion than I am with gaseous fuel combustion, but production of some level of Carbon Monoxide is pretty much inevitable when using a coal, coke or charcoal forge, or even when burning wood. Wikipedia lists the CO content of the exhaust from a typical home wood fire as 7000 PPM.

     

    The received wisdom is to put forges outside.

     

    If US workplace safety legislation is anything at all like ours in the UK, there will be an obligation on your employer to assess the risks and control them to an acceptable level: killing the workers is generally frowned on. I assume the OSHA workplace exposure limits will need to be met for all substances likely to be present, including Carbon Dioxide and Oxides of Nitrogen.

  19. I have a 2' diameter factory-motorized natural wheel, running in a water bath. I think it probably dates from the 1950s and it turns at a heady 42RPM thanks to a reduction gear on the motor and a reduction v-belt drive. That's so close to a surface speed of 3 MPH that I'm sure it was the target speed when they specified the drive.

     

    It works a lot better wet than it does dry, but it is very, very slow going. It always seemed pretty good for recovering badly-abused cutting edges but was no good at all for removing stock.

     

    At that speed, the water doesn't get thrown off. However, it does get stripped off by contact with the workpiece and then runs everywhere. In a warm climate, it would probably be fine. Here in Lancashire, it's not pleasant.

     

    I don't have the 3-phase, 400V supply needed to run it any more, so it's not been used for 3 or 4 years. I can't honestly say I miss it.

  20. Unless you are pretty good at scrounging, electric is probably out of your budget.

     

    A Don Fogg style 55 gallon drum HT forge is cheap, about as simple as it gets and sounds very much like Jesus' suggestion.

     

    Basically, it is a 55-gallon drum, mounted horizontally and lined with a single layer of 1" Kaowool blanket, with a small burner mounted into the original 2" bung hole low down at one end. High up at the other end, a fairly small (maybe 2" wide and 3-4" high) opening is cut for workpiece access and exhaust gas egress. In the two I've seen, the workpiece is hung in wire loops which are just extra to the wire holding the blanket in place.

     

    The burner is not a tight fit in the bung hole: the temperature adjustment is basically done by putting a thermocouple in and adjusting the gas until the temperature is where you want it to be. It's the balance between the amount of burner flame and the amount of extra air coming in around the burner that gives the temperature control, so it does not need a fancy forge burner, a cheap torch will do the job.

     

    I'm pretty certain the large volume is a big factor in getting even heating (radiative heat transfer follows an inverse-square law). Temperature can be held within surprisingly tight limits once correctly adjusted.

     

    The design is very elegant: just about any attempt to improve it seems very likely to make it perform worse: more insulation, ITC100 and multiple burners are just three that I've heard suggested by folk with an "if it ain't broke, keep fixing it until it is" outlook.

     

    If a 55 gallon drum is too short, I'm pretty sure any other suitably-sized cylinder should work, even a couple of 55-gallon drums joined together. I think the diameter drives the airflow by providing the effective chimney height for the draft, so a long, skinny cylinder might not work as well. I think standing the drum on end would probably not improve things at all either, but I could be wrong.

     

    I've had a play with a PID-controlled version as well. It worked very well but needed a pilot burner plus the main burner and it took a fair amount of faffing about to get reliable ignition of the main burner. It did not seem to perform any better or worse than the manually-adjusted version. Given the potential consequences of filling such a large volume with gas/air mixture and then lighting it, I'm inclined to feel that PID control does not save enough adjustment time to be worth the risk and effort. Obviously YMMV.

  21. Can anyone advise what the best fire proof wadding to use in my gas forge. I have some that came with the forge but it is melting away quickly where the heat hits it. If not wadding is there any other way of protecting the inside of the forge. Also where can I buy some

    Andy

     

    Whereabouts are you? As Alan says, some stuff is harder to find in the UK. Other stuff is easy and, on a little island, pretty close.

     

    What is the forge? Pics or a good description would help. Pics are usually better: since you are asking the question, there's a fair chance you are new enough to this that you don't yet know the right questions to ask.

     

    What do you use it for? The temperature makes a difference and if it involves flux, the answer will be very different to if it doesn't involve flux.

  22. I've not often seen it with torches, but it's a fairly common effect with Naturally Aspirated burners that draw primary air.

     

    Normally, the speed of the mixture moving away from the nozzle is higher than the speed the flame can move through the mixture all the way to the burner exit. After the exit, the mixture can spread out over a larger area, slowing down as it does so, to give a narrow band somewhere near the burner exit where the flame speed matches the mixture speed and you get a stable flame.

     

    Rather unhelpfully, the flame speed changes with the air:fuel ratio, temperature and pressure. Things can be going fine until the forge reaches a particular temperature, then it starts misbehaving.

     

    What happens with my Venturi burners and is probably happening with your torch is this:

     

    The Flame-front radiates heat, some of which goes forward and is absorbed by the unburned mixture. The flame-front speeds up slightly, still radiating heat and generating a bit of a pressure-wave in front as it speeds up. The hotter, higher-pressure mixture burns faster, the flame front accelerates and it keeps accelerating until it runs out of mixture when it gets so close to the gas jet that the gas and air are not mixed. The flame goes out for lack of anything to burn. The gas and air continue to flow, mixing as normal, until the mixture hits the hot chamber, ignites and repeats the process.

     

    The usual way to deal with this is to increase the mixture speed by turning up the gas pressure. What do you have by way of a regulator?

     

    With a choked Venturi burner, there is also adjustment of the air:fuel ratio available using the choke. The fastest flame speed is generally at the stoichiometric mixture (effectively a neutral flame) and adjusting to give a rich (reducing) flame (closing down the choke) will usually slow the flame speed to some degree.

     

    If your gas jet is restricted at all, your mixture is likely to be lean and this may be the cause of the problem.

     

    The first 2 things to check, in whichever order is easiest, are therefore crud in the gas jet and operating pressure.

  23. My thoughts on a dedicated tempering setup are as follows:

     

    A length of reasonably thick-walled box section, big enough to take the anticipated work (say 4" x 4" x 1/4"+ wall).

    A length of reasonably thick steel flat on the bottom of it (say 4" x 3/4").

    This needs to have a hole drilled in it to take the thermocouple, but it can be drilled from the side so it's not a big deal.

    A rod-type electrical heating element on the bottom of that. Diameter seems to be around 8mm or 5/16" for the ones I've looked at.

    Two strips of steel flat, whose thickness is the same as the diameter of the rod element, on either side of the element (say 5/16" x 1 3/4").

    Another, wide, strip of steel flat to go on the bottom (say 4" x 1/2"). Drill through and bolt everything together tightly, maybe with heat-sink compound on the joint surfaces.

    Fit Mineral-Insulated thermocouple and wire up rod element.

    Insulate with around 4" or so of Rockwool (mineral wool) slab.

    Control with a PID controller.

     

    The idea is that the insulation is good enough and the mass of steel is great enough to give even temperature

    distribution inside the box-section, albeit slow to get there.

    The thermocouple is between the element and the workpiece, so will prevent overheating of the chamber.

    The element does not have line-of-sight to the workpiece, eliminating any radiated heat issues.

    The heat input is even along the length of the oven.

    The elements are available pretty much any length and wattage from ebay, and are fairly reasonably priced.

    My best guess at required Wattage would be around 500W per foot of chamber length.

    A very basic PID controller will run this setup and the low power/large thermal mass/good insulation should

    make it very stable.

    A downside to this stability is likely to be a very slow autotune.

     

    The main reason I've not actually built one yet is that it'll be too heavy to move and I don't have the space for it as a permanent fixture.

    I wanted to build one to go with a 42" HT oven I built a while back, but the HT oven needed wheels to move it,

    and there was no way I was going to be able to manhandle a deliberately heavy tempering oven.

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