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timgunn

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Posts posted by timgunn

  1. OK. A couple of things jump out.

     

    First is the Propane cylinder. It's a Patio Gas cylinder inended for patio heaters and, as far as I can tell, only takes a 37mbar clip-on regulator.

     

    Second is the amount of Carbon on and in the forge. This shows a very rich burn: you are not getting enough air in to burn the gas. You may not be getting enough gas in to make temperature either, but the lack of air is the first thing to address.

     

    You don't show the burners you have tried. I'm guessing they were atmospheric burners. If you are going to use the Patio gas, I'd expect you to have to go to a blown burner.

     

    If you are intending to use an unblown burner, you'll really want to start with a "normal" Propane cylinder with the female threaded fitting, and a high-pressure regulator giving 0-4 bar, 0-60 PSI.

     

    If you've been building burners to "known good" plans, the cylinder and regulator change may be enough to fix your problems.

     

    Otherwise, can you provide some more information about the burners, preferably with pictures?

  2. Borax is very viscous at Carbon Steel HT temperatures. I tried it once in a mini salt pot and it was just too thick to use for O1. I tried low-sodium salt next (Class 5 HT salt, but from my local supermarket) and it was much better.

     

    Borax might be usable at higher temperatures, but I'm not sure. In the forge, it seems to go properly runny at about welding temperature and I think that might be too hot.

     

    It does have a tendency to dissolve refractories too, which can be something of a nuisance.

     

    I'm sure there are other good reasons Borax is not used, but the viscosity was enough for me.

  3. The C706 and C715 actually look to be CuproNickels; Copper/Nickel alloys with no Zinc.

     

    As I understand it, the Nickel Silvers are Copper/Nickel/Zinc alloys. I gather the 18% Nickel C752 is the one to use for the "silver" colour. The 10% Nickel stuff tends to look a bit brassy, with a definite yellow tinge.

  4. I cannot be sure; do you have a link to the one you can get?

     

    The ones we use are a 3/8 brass valve with coil to suit the supply and an asymmetric timer that fits between the DIN connector and the coil. The timer is universal supply. Price here is 30-35 GBP, so I'm guessing it's basically the same thing.

     

    They look a lot like this: http://www.cccme.org.cn/products/detail-8063679.aspx

  5. We use them at work a lot; usually on 3 HP piston compressors with around 40-gallon tanks running 24/365 to provide control air on remote sites. The motor/pump duty cycles are usually in the 10-25% range.

     

    The drains are unbranded, made in the Far East and are pretty reliable as a rule.

     

    The most common failure mode we see is little tiny balls of steel from when the tank was welded, getting trapped in the valve and stopping it from closing properly. It tends to be more of a problem with new compressors.

     

    Timer failures and coil failures are much rarer. We see perhaps one every year or 2, with somewhere between 40 and 60 installed; maybe 40-120 years MTBF.

  6. Using a Propane forge indoors is a fairly high-risk activity. Carbon Monoxide is produced in quantity and is a very poisonous gas.

     

    It can be managed safely, but there are a few things to consider and they may not be obvious until you've got some experience and done some research.

     

    Stick with plan A for now.

     

    Whatever forge you build, you'll find that you reach some limit at some point and need to build another.

  7. Are you getting Dragons breath?

     

    There seem to be 2 possibilities:

     

    First is that you are not getting enough gas in. This condition is likely to give the same effect as the forge just being run at lower pressure than you want and whether or not there is any Dragons breath depends on the normal burner mixture.

     

    Second possibility is that you are not getting enough air in. If there is a restriction to the air supply, there will be a lot of Dragons breath.

     

    The dragons breath should narrow down where to start looking.

  8. Search for "TM902C" on the well-known auction site. If you think in Farenheit, find an conversion table and print it out as the TM902C reads in degC and only works with type K thermocouples. It's around 5-6 bucks, delivered. It's cheap, accurate and battery-powered

     

    You'll still need to get a suitable type K thermocouple with cable and a miniature plug, but you'd need that for the PID controller as well (though not the plug). The one that comes with the TM902C is only good to 400 degC/752 degF

     

    Type K tops out at 1370 degC/2500 degF (as do all of the TM902Cs in my experience, though I have some that are only marked 750 degC). If you want a thermocouple that goes hotter, you are probably talking Platinum-based and fairly serious money. I've not tried Tungsten-Rhenium, so don't know if thet would be a realistic option.

     

    The Auber link shows a range of 0-2300 degF for a type K input. That misses off the top 200 degF of the type K range. I think this might reduce its usefulness significantly.

  9. I have done a few remote boxes like this. It works for most drives I've come across, but one or two need separate start and forward/reverse inputs. It's just a wire to move in the remote box to make it work on these.

     

    The cable needs to be at least 7-core. I make mine long so that I can use the same drive for different machines by moving the box to the appropriate machine and plugging its motor into the drive.

     

    For the same reason, the forward/reverse switch is keyed; If the machine is not reversible, I can take the key out.

     

    I lucked into a bunch of multiturn pots. The one on this box is one of them and takes 10 turns from min to max. After using a single-turn it feels horrible to me, but the other guys using my remote boxes don't seem to notice.

     

    The pot works as a voltage divider, so the actual resistance value is not critical. It does need to be a linear trim though (not logarithmic, as is used in a lot of audio applications).

     

     

     

    DSCF0010.jpg

     

     

    DSCF0004.jpg

     

    DSCF0014.jpg

  10. That doesn't look like a sealed VFD in the pic, though it's not one I recognize. I guess you were controlling the speed from the front panel in the vid. If it's not sealed, I'd strongly advise a remote control box at the grinder and a sealed enclosure round the VFD.

     

    Conductive steel dust in the power electronics tends to get exciting. Briefly. And irreversibly.

  11. Not using PID control means you have to do all the control manually.

     

    As one of the advantages of salts is the huge thermal mass involved and it's ability to damp out temperature fluctuations, things tend to happen slowly. The intention is to control over a very narrow temperature band. The PID controller, if correctly set-up, can see the small temperature changes occuring over relatively long timescales and respond appropriately.

     

    If you do the control manually, you need to see the temperature readout change, usually by whole degrees, before you can respond.

     

    Even if you are good at it and alert, the PID controller (again I stress it needs to be correctly set-up) will typically keep the temperature within a significantly narrower band than you will be able to.

     

    Personally, I'd use a Venturi burner instead of a blown burner. The reasons being; Firstly that I can build a really good Venturi burner, with a commercially-made gas mixer, for significanly less than I'd have to pay for a blower or a big enough solenoid valve to cut off the air supply. Secondly, a Venturi with pilot/full settings only needs one (small and therefore cheap) solenoid valve and is its own pilot. Thirdly, they are what I know and I like them. Obviously YMMV.

  12. Just a thought on the parts list. You've not mentioned any parts for a choke. It could just be that you have the bits already so didn't need to order them, but if you've not allowed for chokes, I'd recommend you do. Getting both welding and HT temperatures from the same forge is much easier with good air control.

  13. http://amalcarb.co.uk/downloadfiles/amal/amal_gas_injectors.pdf

     

    Available from http://burlen.co.uk/?___store=default

     

    I think it'll almost certainly be a phone call job, as they are not really standard enough to sell online: jet sizing is fairly critical.

     

    Amal jet sizing is a little esoteric but they should be able to work things out.

     

    I did some playing with jet sizes on high-pressure Propane, ending up with a 0.6mm Mig tip, with about a 0.74mm diameter orifice, in a 1" LV Amal injector. It seems about right. It should be ok to scale this, so about 0.55mm for a 3/4" and about 0.37mm for a 1/2", but Owen may have better information.

  14. I don't know if I can explain this well. When I've tried face to face, folk either get it straight away or their eyes glaze over.

     

    It is important to understand the difference between heat and temperature.

     

    We can adjust the temperature at which gas burns by adjusting the air-to-fuel ratio. Maximum temperature is achieved when all of the fuel gas burns with all of the oxygen in the air, leaving no unburnt gas and no unburnt oxygen. This is a neutral flame.

     

    If we add more air, it can't contribute to the burn, because all the gas has already been used up. All that happens is that the extra air absorbs some of the heat energy and reduces the temperature of the flame. Because there is unburned oxygen remaining, this gives an oxidizing flame.

     

    If we go the other way and increase the amount of gas, it can't contribute to the burn, because all the oxygen has already been used up. All that happens is the extra gas absorbs some of the heat energy and reduces the temperature of the flame. Because there is unburnt gas remaining, this gives a reducing flame. Once the hot unburnt gas leaves the forge, it mixes with air, gets the oxygen it needs and continues to burn as the dragons breath.

     

    What I think you need to do is adjust your burner to give a flame of the correct temperature. I think you'll be choked down quite hard on the air port. You should then be able to increase your gas pressure to make the flame bigger (but not hotter). The area by the burner will always be hotter than the last few inches of the forge, but increasing the size of the flame should reduce the differential. At some point, you'll get an even enough temperature to be usable.

     

    Running with the flame much hotter than the temperature you want will just cause uneven heating.

     

    If at all possible, get hold of a thermocouple and readout and measure the flame temperature whilst adjusting the air.

     

    Sputtering is often a result of too low a gas pressure. When the flame travels through the mixture faster than the mixture is moving in the opposite direction, it will run down the burner tube until it runs out of mixture to burn. The flame will then go out. The gas continues to be fed, draws air, mixes with it and, shortly after, reaches the forge, where it ignites and the process repeats. Often, increasing gas pressure wil cure the sputtering because it increases the mixture speed until it is faster than the flame speed.

  15. As Podmajersky says, AC motors on VFDs can be run above their rated speed and this is very likely to be what you have seen.

     

    With a DC motor, the only thing that the controller can vary is the Voltage. Speed follows Voltage and once the maximum voltage is reached, that's it.

     

    AC drives control both frequency and Voltage. Below rated frequency and Voltage, both increase together, giving a constant torque characteristic. Once the rated Voltage is reached, the drive stops increasing the Voltage, but can continue to increase the Frequency. The motor speed will continue to increase with the frequency, but the torque will reduce as the speed increases, giving a constant power characteristic.

  16. I didn't try a star-wheel dresser, though I have one and use it on bench grinders. When I've used them, I've always had the impression they worked by pulling out the high grains and they seem to give a more agressive wheel than a diamond does to me. It might just be my imagination though.

     

    I was after fine, so didn't even think about using it on the wet wheel. With hindsight, it may work better for truing up the wheel. I found it quite hard to true it up freehand, as it's so slow; I'd hit the high spot and then it took a second and a half to come round again. Any slight pressure on the dresser just kept it in contact with the stone and resulted in a smoothly-surfaced but eccentric stone.

  17. Dan, check out the motor and drive carefully (pics would be interesting). I think you'll find it's much slower. I have a 4" x 24" natural stone wheel on the original motor and drive. It turns at 42 RPM, which gives a 3 MPH surface speed.

     

    For dressing, one of the diamond jobbies that looks like a 2" long piece of 1/2" box section with a diamond-coated face and a handle, worked pretty well, if very very slowly, on mine. I tried a single-point diamond, but the slow speed made it awkward to avoid cutting threads.

     

    Watch out for the big nut, visible in the pic in post #2, backing off and letting the stone move; you;ll never get it back in exactly the same place and you'll have to true the wheel with the dresser again. It gets tedious remarkably quickly.

     

    Nice score though.

  18. One of the reasons for going wider than seems necessary, is to reduce the radiative heating effect of the elements on the workpiece.

     

    The elements radiate heat and this radiated heat effectively reduces with distance from the hot elements. If you read the manuals for the big-name ovens, I'm pretty sure they'll mention this.

     

    I don't think it is likely to be much of an issue at Austenitizing temperatures, but it can seriously louse up tempering.

     

    What happens is that the elements radiate heat in all directions along line-of-sight. Most of the radiated heat will hit the walls, ceiling,etc, where it will be absorbed by the inside of the oven, heating the structure of the oven. This has a lot of thermal mass and relatively little surface area exposed to the radiated heat.

     

    Some of the radiated heat will reach the blade directly. The blade has lots of surface area and not much thermal mass, causing the temperature to rise rapidly. If we are talking about a blade being tempered, it may well have a black surface from the hardening process and it will certainly have a cross-section that gets thinner as it approaches the cutting edge. This means that the effect of radiative heating will be to raise the temperature of the edge more than that of the spine, unless steps are taken to mitigate the radiative heating effect. Placing a single blade in a rack which supports it so that both sides are exposed to the elements is pretty much guaranteed to maximize the unwanted effect.

     

    There are several things that can be done.

     

    First is to put a barrier between the elements and the workpiece to put the blade in shadow.

     

    The second is to position the thermocouple so that it is the fastest-heating thing in the oven. The thermocouple itself will need to be specified for the fastest possible response and the PID tuning will need to be done on the second firing; the first will establish a nice black oxide layer on the thermocouple, which is then a permanent feature. The black layer will mean the thermocouple responds differently to the first heat (when it was shiny) and probably heats faster when subjected to radiative heating.

     

    The third is to minimize the duration of radiative heating by bringing the oven up to temperature before loading the workpiece, open it, insert the workpiece and close it as quickly as possible to avoid the temperature dropping far.

     

    The fourth is to program a slow ramp up to temperature. This obviously needs a ramp/soak controller, but will eliminate the problem. It does require the user to actually use the ramp/soak facility though, and the ramp needs to be fairly long; perhaps an hour from room temperature to tempering temperature.

     

    I suspect most people use method 3, probably without even being aware of the issue, and rely on the oven manufacturer to have dealt with method 2, so far as is reasonably practicable.

     

    The oven manufacturer will usually also have dealt with at least part of method 4, by providing for ramp/soak control. The user decides whether or not to avail himself/herself of this facility.

     

    Method 1 is easy, as long as the oven is wide enough. I know at least one good knifemaker who gets impressive results tempering his knives in a tray of dry sand. I did some testing with stainless-steel sheet "screens" and that seemed to work, as did ceramic tiles.

  19. Read up on the subject as much as you can. Look at what others have done and try to picture how some of the "improvements" that people add would affect the way that you work. Be sceptical.

     

    The Mypin controller looks like it can do what you need it to do BUT....

     

    In the hands of a half-way decent control engineer, I am sure it will be no problem.

     

    Because you needed to ask the question, it seems fair to assume that you are not a control engineer.

     

    There are guys on this forum, and doubtless many others, who either are control engineers or have enough experience and knowledge to pass for one in a poor light (I count myself among the latter). If they know what you are struggling with, they can probably help.

     

    To help, they will need access to the manual.

     

    I would not buy a controller unless I had already downloaded the manual from a publicly accessible website and added the location to bookmarks/favorites. If help is needed, it's just a case of asking for help with a link to the manual. I'd also want to check the manual is usable; there's a world of difference between a manual written by someone who writes technical manuals in a language they, and you, are fluent in, and a babelfish translation to English of something written in Chinese or similar.

     

    As a first step, I'd ask the ebay vendor if they can provide a link to the manual before you make any buying decision. If so, post the link here.

     

    Personally, I'd always use a ramp/soak controller. I built my first HT oven with a basic PID controller, which was free. I have bought ramp/soak controllers for each of the 4 HT ovens I've built since, despite having access to free basic controllers. Obviously YMMV.

     

    I would not reduce the width to 3"; The area is more important than the internal volume in determining heating rate and maximum temperature.

     

    With a 5" x 3" x 18" chamber, you'd have 318 sqIn of area on the inside. Going to 5" wide would give 400 sqin total. The smaller volume only saves 20% of the area, but I'm sure it would lose considerably more than 20% of its "usefulness" in doing so.

     

    Enjoy it. It's a fun thing to do and very rewarding.

  20. Do you know what the "spring steel" actually is?

     

    I know that EM45 is a spring steel available in the UK. It's a Silicon-Manganese steel and needs higher-than-normal-Carbon-steel temperatures for Austenitizing: well above non-magnetic. Could you have something like this?

     

    It will harden, it just needs a higher temperature, so all is not lost if that's what you have.


  21. Scott,

     

    There's some risk attached to everything we do and common sense goes a long way towards keeping us safe.

     

    My day job involves designing, modifying and maintaining machinery. I have no control over the end users of the machines, so I tend to assume they will find some new and interesting way of killing or maiming themselves if given the chance. Common sense doesn't enter into it. That thought process tends to get ingrained.

     

    When I've built ovens for other people, it's seemed pretty safe to assume they have no real interest in the technical details of how the oven works (if they did, they'd have built their own), so I try to build something that is as close to foolproof as I can make it. Also, once it has left my hands, I have no control at all over who gets to use it.

     

    The first HT oven I built was for an electrician. I could reasonably assume he'd understand the risks, so the control circuitry was broadly similar to the majority of the wiring diagrams on the web. The second was for a young knifemaker who was still at school, so I added a couple of extra safety features to bring it up to a standard I'd be happy to let my own kids loose on. The extras only added about $15 to the cost and perhaps 15 minutes to the build time, so I've stuck with that design on the others.

     

    In the wiring diagram in your original post, there is mains power to the SSRs at all times, unless the main power is switched off by the operator. The toggle and door switches only interrupt the switching signal to the SSRs, so if one of the SSRs fails in the way that yours seems to have failed ("closed" in relay terms), the elements will be live. I add an electromechanical contactor (basically a big relay) in the AC feed to the SSRs and pull in the contactor with the door and toggle switches.

     

    I gather there are some significant differences between Europe and North America in the way mains power is distributed and these may affect the danger posed by contact with conductors; here in Europe, our domestic supply is 230V on the hot leg and the Neutral is at Ground potential. Touch live and you get the full 230V to ground. It's not nice.

     

    I'm not entirely sure what you'll get if you do the same on a 220V circuit in the USA. I hava the impression you usually have a centre-tap to ground on the transformer, so would only effectively see half the voltage to ground if you touch live.

     

    Things are also different when building for ones own use; the user will have had the technical knowledge to build the oven, so will be able to make informed choices about its use based on that knowledge. The combination of the control circuit and competent operator normally provides adequate safety for most real-world situations.

     

    Apologies if my earlier post came across as doom-mongering. It wasn't my intention.

     

    Regards

     

    Tim

     

     

     

  22. Relays or new SSRs?

     

    I've tried both SSRs and contactors for the output stage and find SSRs preferable for the speed of operation; I find that a 2-second cycle time is definitely better than 4 seconds.

     

    I can't see any improvement going to 1 second, but I'm not sure whether that's because there's no improvement or because my testing setup is not good enough to detect any improvement there might be.

     

    I've found Fotek branded SSRs, bought from the far East via ebay, to be pretty good.

     

    All of the five HT ovens I've built so far have gone to people I like.

     

    I fit RCDs (GFCIs?) and use an "extra" electromechanical contactor in the mains supply to the SSR, simply because I'm not happy with the idea of an SSR as the only thing between a friend and electrocution.

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