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timgunn last won the day on May 19 2016

timgunn had the most liked content!

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About timgunn

  • Birthday 03/15/1962

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    Lancashire, UK
  • Interests
    Tools, science, food, wine. Making things that "just work".
  1. timgunn

    More heat

    “Venturi” is the term used to describe the shape of the classical Naturally-Aspirated mixer. It tends to get used (incorrectly) for any Naturally Aspirated burner: one which does not use an air blower to provide the air. Get the back-pressure down by reducing the restriction on the openings and see if it improves. If you still can’t get it hot enough, have Dragons Breath and the gas jets are changeable, fitting the next size down jets will probably get the temperature significantly higher.
  2. timgunn

    Info on this drill press please

    Looks like it's a high-speed drill press from a quick google: possibly 1500-10000 RPM, though the one in the link below looks to be 3-phase and is in Europe. If the pulleys and motor pole count are the same, it'll be around 1800-12000 RPM on 60 Hz mains. A bargain if that's what you need, but not really a general-purpose tool. http://unimachines.at/tischbohrmaschine-super-valmer-model-6-1970-14105.html
  3. timgunn

    rigidizer question

    Note that it makes it more rigid, rather than hard: When you press a thumb into blanket, it doesn't take much force to press it down by, say, 1/8" and it springs back. With rigidizer, it'll take noticeably more force and it won't spring back. The rigidizer seems to get fairly rigid as soon as it is fully dry, then get more rigid once it has been fired to high temperature. If your plan is to rigidize, then coat with a castable refractory, I'd not worry too much about it. The general consensus seems to be that the refractory bonds better to wetted blanket and the usual practice is to spritz the blanket with water before applying the refractory. If you wait until the refractory has crisped up at the surface, spritz it and apply the refractory, you should be fine. If you rigidize and apply the refractory while it's still wet, you should also be fine, but your drying time is likely to be much longer: moisture can escape from the exposed blanket face much more easily than it can escape through the layer of refractory. Drying time is VERY location-dependent. Some places, you'll need to slow initial drying down by covering the forge with plastic to give the (castable) refractory time to set properly. Other places, you'll struggle to get it dry at all. I'm in the wetter bit of England, just North of Manchester, and fall into the latter category. Dry is IMPORTANT: if the low-permeability layer is not dry, it'll flash off steam inside and blow it apart. It may not be impressive or even immediately apparent, but it'll leave cracks where the steam forced its way out. I'm pretty sure this has caused the early demise of many forges built by impatient would-be smiths. Using a fireclay-based refractory cement/mortar as a coating, the steam flash-off causes paper-thin bubbles to rise, harden and break up. If I really need to make a forge with refractory cement, it gets dried over 8+ hours in the oven (the actual time taken depending on how long I can be sure the wife will be out).
  4. timgunn

    HT oven from ceramic kiln

    1/4 DIN is definitely better for my middle-aged eyes, but 1/16 DIN is cheaper. My first homebuilt HT oven used a relay-output controller and contactor. It was noisy and when I did some testing with a borrowed high-end controller, several thermocouples and a datalogger, it was clear that shorter cycle times gave better stability. It was also clear that radiative (over)heating was a potential issue, particularly at tempering temperatures, and that ramp/soak capability would be worth having. The controllers I have been using for my more recent homebuilt ovens have been either Omega CN7823 or Automation Direct SL4848VR. They seem to be the same controller with different badges and I buy whichever is cheapest at the time. They have ramp/soak capability. The graphs were from a series of different-diameter thermocouples, intended to (loosely) approximate the heating effect at different thicknesses on the bevels. The thermocouples were out of my box-of-bits and I knew nothing about them except that they were Mineral Insulated typeK and of different diameters. Exp had an exposed junction. I do not know whether the others had grounded or ungrounded junctions, though ungrounded seems most likely. The setpoint in both cases was 250 degC, 482 degF. The control thermocouple was a 6mm Mineral Insulated typeN with an insulated (ungrounded) junction and Nicrobel sheath. The 30-min (-ish) slow ramp-to-temperature gave a peak radiative overshoot of 19 degC, 34 degF. Without the ramp, the peak radiative overshoot was 119 degC, 214 degF. Further testing showed that slower ramping helped and there were small errors due to thermocouple tolerances which may account for as much as 10 degC, 18 degF. There was also a saw-tooth variation about the setpoint once it was reached, with the pitch and amplitude directly proportional to the output cycle time. There were clear improvements from reducing the cycle time down to 5 seconds. I "think" there was an improvement between 5 seconds and 2 seconds and that I could tell which was which from the charts. Unfortunately I lost the data to a hard-drive failure and these 2 charts were all that I had backed up. I use a 2-second output cycle and fit an LED indicator to the control panel that is lit when the element is powered. By watching it for 2 seconds, you get a surprisingly good idea of how much of the output cycle is powered, even from 20-30 ft away.
  5. timgunn

    HT oven from ceramic kiln

    The kiln is almost certain to be able to achieve Austenitizing temperature for Carbon steels at its design Voltage. There's a good chance it'll be able to handle most of the less-exotic stainless steels too. I don't know which of the controllers from Banggood you are looking at. They list several and I don't think they are all suitable for a HT setup. Some, particularly the Rex C100, seem to have factory-configured input ranges which cannot be changed by the end user. A 400 degC maximum on a typeK thermocouple seems to be the most common input and this is not much use to us. There is an XMT612 controller listed and this "may" be the same controller as a tet612 that I used a few years ago. The tet612 had fully-user-configurable input ranges and worked pretty well for me. As Dan says, the controller needs to have some means of modulating the heat input to the oven. This is usually done with a Solid State Relay switched to provide time-proportioning control. I have found a 2-second output cycle time gives about the best results. The controller needs to have a DC pulse output to trigger the SSR. On US 220V supplies, I understand 2 SSRs are normally used, one for each "hot". Most controllers with pulse DC outputs can trigger 2 SSRs in parallel. Whatever controller you get, make sure you have downloaded, read and (mostly) understood the manual BEFORE you buy. The download needs to be from a site accessible without opening some sort of account or logging in with anything that could be interpreted as a "feel free to spam me" permission. I would expect the potential supplier to be able to provide a link to the manual As you say, this is not really in your comfort zone and you may need to ask for help. If you ask for help with a link to the manual, someone with a general understanding of PID controllers can probably help. If they cannot access the manual, you are on your own. One thing worth mentioning is that it is wise to use a 48mm x 48mm (1/16 DIN) controller and to leave a bit of extra length on the wiring: maybe 2". 1/16th DIN is the most common size for controllers and a later upgrade to a ramp/soak controller is easy later on. Any 1/16 DIN controller will fit the mounting hole, but some are longer than others and the terminal layout may be different, Having the wires long enough to reach can save a lot of cussing if you ever change the controller.
  6. Some conversion factors might prove helpful. 64 kg/m3 is 4 lb/cu ft 97 kg/m3 is 6 lb/cu ft 128 kg/m3 is 8 lb/cu ft 160 kg/m3 is 10 lb/cu ft The 128 kg/m3, 8 lb/cu ft, density is usually recommended. 160 might be marginally better, but it's not usually easy to come by. 2 layers of 25mm, 1", are usually best in a round forge, as single layer of 50mm, 2", is more difficult to wrap smoothly. In general, the material recommended for forges is high-temperature Ceramic Fibre Blanket. Usually rated to around 2600 degF or 1400 degC. Insulfrax products are made by Unifrax and I think the Insulfrax range is a series of Low BioPersistence products made from Alkaline Earth Silicate fibres. The LBP fibres are soluble (I assume pretty slowly) in body fluids. The presumption seems to be that they are likely to be safer than insoluble Ceramic fibres over the long term. A ceramic fibre inhaled today will still be in the lung in 50 years. An LBP fibre inhaled today will not. Different products have different temperature ratings, but the Low BioPersistence fibres generally tend to have significantly lower temperature ratings than the Ceramic fibre products. In the UK and much of Europe, it is becoming increasingly difficult to obtain non-LBP blanket. The lower temperature rating of the LBP products means that the material used to provide a hot-face coating should ideally also provide insulative properties. This will help to limit the temperature at the interface of the hard refractory and fibre to less than the rated temperature of the fibre product. If using a 1400 degC-rated Ceramic fibre blanket, insulation is less of a consideration for the coating layer. The big names in refractories are Insulfrax and Morgan Thermal Ceramics. Insulfrax are best known for fibre products. Morgan Thermal Ceramics cover the full range of castables, Insulating Fire Bricks and fibre products. In my (admittedly fairly limited) experience, the Ceramic fibre blankets from other manufacturers seem to perform just as well as the big-name products of similar density and temperature rating. The best materials to use for the hot-face layer tend to be castables. A lot of folk use refractory mortar because it is cheap and readily available, but it does not last nearly as long as a well-chosen castable. For burners, I would strongly suggest an Amal atmospheric injector from Burlen Fuel systems. You'll need gas fittings to connect to the injector, but the burner tube can be a straight section of pipe, threaded at one end to screw into the injector: I'd source a long stainless steel nipple from ebay or similar. This does away with any concerns over galv pipework, saves a lot of time and effort on your part and produces a burner that is extremely adjustable. Buy the burner factory-jetted for Butane, not for Propane, as it will give a higher maximum flame temperature on Propane than will the Propane-jetted injector. I'd only use a single burner, mainly to avoid the hassle of trying to balance two burners to the same mixture/temperature, and size it for your chamber. A 3/4" burner is good for about 350 cu in and a 1" burner is good for about 600 cu in, both to welding temperature. A rear pass-through port is a good idea if there's any chance you'll want to do long stuff. You can block it off if you are only doing short stuff. I pack in an offcut of blanket personally, but I've seen others use IFB cut to fit. With an Amal injector-based burner, the temperature control available with the screwed choke means that you are very unlikely to need a muffle tube for HT, though you MUST do your HT outdoors. Closing down the choke to get HT temperatures causes massive amounts of Carbon Monoxide and death is a very real possibility if you run the forge in an enclosed space.
  7. timgunn

    belt grinder motors and vfd uk advice required

    A VFD takes mains power in, rectifies it to DC internally, then synthesizes something that looks to a 3-phase motor sufficiently like a 3-phase sinusoidal waveform for the motor to behave as if it is powered by one. The clever bit is that the VFD can vary the apparent Voltage and Frequency and make the motor run at variable speed. VFDs are available for single-phase 230V input or for 400V 3-phase input. If you get a 230V single-phase one, it can run on UK domestic mains. The biggest you'll be able to run from a 13A socket will be a 3 HP/2.2 kW. It's not really worth getting any other size IMHO. They can run motors smaller than their maximum rating, but not bigger. If you use a 230V VFD, the output will be 3-phase 230V. The vast majority of 3-phase motors up to 3 HP/2.2 kW are wound for 400V connected in Star (Wye) or 230V connected in Delta and can run fine on a single-phase-input VFD. You'll need to check when ordering though. Over 3 HP/2.2 kW, motors tend to be wound for about 700V in star and 400V in Delta to enable star-delta starting (an old-school way of reducing motor starting current. It has largely been supplanted by VFDs). These cannot run on 230V 3-phase. Here in Europe (for the present), we have 50 Hz mains: 50 cycles/sec, 3000 cycles/min. Other parts of the world have 60 Hz mains: 60 cycles/sec, 3600 cycles/min. It's the reason you'll see the different motor speeds quoted on opposite sides of the pond. A 2-pole motor will run at an RPM equal to or just below the frequency of its power supply (3000 or 3600 RPM). A 4-pole motor will run at, or just below, an RPM equal to half the power supply frequency (1500 or 1800 RPM). 6-pole: one third (1000 or 1200 RPM), 8-pole; one quarter (750 or 900 RPM), and so on. We tend to use either 2-pole or 4-pole motors and in Europe, motors generally conform to IEC standards. Across the pond, they tend to use motors to NEMA standards. NEMA motors are pricy over here and offer no inherent benefit. They tend to be used where expensive machine modifications would be necessary to change to an IEC motor. For a serious belt grinder, you'll probably want a 90-frame motor in a long casing (90L). This will most likely be 1.5 kW/2 HP if it's a 4-pole or 2.2 kW/3 HP if it's a 2-pole. The shaft size of 90-frame motors is 24mm. Because half the world uses 60 Hz mains and the other half uses 50 Hz, meaning that maximum mains speed is 3600 RPM, motors are designed to run to 3600 RPM. It is not cost-effective for motor manufacturers to design a completely different motor for each speed, so the only difference between the 2-pole, 4-pole and 6-pole motors in a particular frame size is the winding. The winding is attached to the inside of the outer casing and is static. This means that all the moving parts are good to 3600 RPM. We can run a 4-pole motor to 120 Hz to get 3600 RPM using a VFD, or we can run a 2-pole to 60 Hz to get the same 3600 RPM. At the bottom end of the speed range, most drive/motor combinations are able to run smoothly down to about 10 Hz. Below this, running from the simpler V/Hz drives, things tend to feel "coggy". The V/Hz drives use a fixed (usually linear) relationship between Voltage and Frequency to determine what will be supplied to the motor and this linear relationship tends to break down once it gets that far from the design frequency. There are also drives which have "Sensorless Vector" capability. These measure the time difference between peak current and peak Voltage internally, calculate to determine the phase angle between them, then fine-tune the Voltage in real time to maintain the design angle (the motor Power Factor defines this angle, being its Cosine). These can keep the motor running smoothly well below 10 Hz and usually down to 1 or 2 Hz. A 2-pole motor on a V/Hz drive has about a 6:1 speed range (600-3600 RPM, 10-60 Hz). A 4-pole motor on a V/Hz drive has about a 12:1 speed range (300-3600 RPM, 10-120 Hz). Either motor has a much greater speed range when run from a SV drive with smooth running down to 1 Hz achievable if needed. VFDs switch large currents very fast and produce some heat, which must be dissipated. Most VFDs have ventilation fans and allow airflow over the power components to cool them. They are intended for use in clean conditions (usually sealed electrical enclosures). If there is airborne steel dust (which is both conductive and magnetic) it will flow across the power electronics, where the magnetic fields caused by the switching will capture it and attract the metallic dust right onto the power components. The short-circuit that results is usually quite spectacular and is invariably expensive, killing the VFD completely. If they are going to be in the same room as a grinder, we need to use VFDs that are protected against such dust to IP66 or NEMA4 standards. We can either mount an unsealed drive in a sealed enclosure ourselves, with sealed control switches and speed control knob, or we can buy a drive that is designed to be sealed to IP66 from the factory and which has the sealed control knobs, etc on the front. The latter is by far the better option for the non-electrician. To buy and enclose an unsealed (IP20) drive properly, with sealed controls, to IP66 is about as expensive as buying an IP66 drive to begin with. In the US, the KBAC series of sealed drives from KB Electronics are the go-to. In Europe, the Invertec IP66 drives tend to be the ones people use for grinders. The current ones are SV drives so you get the low speed capability. I'd recommend a 90L-frame motor and an Invertek ODE-3-220105-1F4Y VFD. The drive is expensive, but it's a cry once thing. I'd try for a 2 HP, 4-pole motor for personal preference, but would be pretty happy with a 3HP, 2-pole. If you are anywhere near Lancashire, I can sort you a foot-mount 90L 2-pole 3 HP from a compressor, gratis.
  8. Hi Tim,

    Knowing of your expertise, especially on AMAL gas injectors, I wonder if you could please advise on my comments below? 

    I have built a forge from Vitcas Grade 28 Insulating Fire Bricks; the aperture (as originally constructed) measures 7.5” w x 4.75”h x 18” l (195 x 120 x 460) and have been using it for a couple of years without problems.

    Forge Performance Test.jpg

    I use a ¾” AMAL injector (on a burner made to the well-proven design) by simply locating it into the front of the forge to heat the forge where required. For forging mono steel and pre-made san mai this works fine and I have successfully forged blades in a wide range of carbon and stainless steels.

    I recently hand-forged some san mai (140mm x 35mm x 3mm pieces) from spring steel/mild steel as a test piece and found that there was incomplete fusion in some small areas; I put this down to insufficient heating but it could of course be poor hammering technique. I don’t have a power hammer as I can’t stand the noise and our neighbours certainly couldn’t! But, the attraction of forging my own san mai and Damascus is growing so I’ve given thought to my forge’s capability hence the performance test. Additionally, I am contemplating building a hydraulic press.

    In addition to the AMAL burner, a year ago I bought a ¾” T-Rex burner from Hybrid Burners to evaluate against it, I concluded that the T-Rex has no performance advantage over the AMAL.

    So, last weekend I modified my AMAL by adding the support tags, bored a hole into the forge top, reduced the forge volume (only by blocking which is not substantive but guess the reduced length helped) to 6.25” wide x 4.75” high x 12” deep (155 x 120 x 305) and then ran some tests the results of which are graphed  below. I monitored the temperatures with a thermocouple the tip of which just protruded into the top of the chamber. 


    As you see, the maximum temperature I eventually reached was 1220 at a gas pressure of 1.75barg. The latent heat in the blocks kept the temperature quite high even after I had reduced the gas pressure; I ended the test at 80 minutes.

     So, based on your knowledge of these things: 

    • Is this the performance and temperature I should expect from the AMAL with its standard 0.036" jet in this size forge
    • Should 1220 degrees be sufficient to allow forge welding by hand or do I need to higher temperatures and if so how to get there - fit 2 AMAL’s ....
    • What’s your view of modifications to the forge for an optimised forge design  

    This is such a wide area that I hope these few questions will set my development path and would appreciate any comments or suggestions.

     Clive Witton 


  9. timgunn

    Spring rate

    Given that a failing hammer can also take parts of the operator off, I'd feel happier with: yes, mild would have worked in that application just fine as long as the fatigue limit didn't get exceeded.
  10. timgunn

    Forklift tine anvil

    What with? With 2 cuts and probably some tidying up with a grinder, it should be less than a days work to make a better post anvil than many smiths are ever likely to own. That seems like an excellent use of your time to me.
  11. timgunn

    thermocouple recommendations?

    I've had pretty good results with a few of the cheap Chinese pyrometers. I have access to a thermocouple calibrator and always check the accuracy of the pyrometers across the intended working range before using them in anger. Until a couple of years ago, I used to recommend the TM902C, available for around 5 bucks delivered. I'd had maybe 20, ordered in ones and twos over maybe 4 or 5 years. They were boringly accurate from 0 degC to 1370 degC (32 degF to 2500 degF) and came with a glassfiber-insulated bead probe that was good to 400 degC (750 degF) and was flexible enough to be closed in an domestic oven door to check tempering temperatures. A couple of years ago, I ordered ten of them. When they arrived, I checked them on the calibrator and found they were accurate up to 800 degC but then became progressively inaccurate. I can't remember whether they showed 1370 degC at an input of 1290 degC, or showed 1290 degC at an input of 1370 degC: either way I was not happy with them. They were very consistent and all read the same on any given input signal. Side-by-side, there were external differences between the good ones and the bad ones, but these would not be obvious on an ebay listing so I stopped using or recommending them. I have since used DM6801 digital thermometers and found them boringly accurate. 10-20 bucks delivered from China. The supplied bead-type probe is PVC insulated and only good to a little over the boiling point of water: not much use to most of us. Whatever pyrometer you get, the supplied thermocouple will not be any use in a forge and you'll need to go to a thermocouple specialist. The "best" probe I have used for "our" purposes is an Omega KHXL-14U-RSC-24 which is a handled type K Mineral Insulated probe 1/4" diameter and 24" long below the handle. The sheath material is Omegas proprietary "Super Omegaclad XL", which is not really necessary for Heat-Treating, but seems to be the best I've found for surviving being used to occasionally check forge temperatures without going to painfully expensive Type S, Platinum-based, thermocouples. The assembly includes a curly cable and miniature plug. https://www.omega.com/pptst/KHXL_NHXL.html Using the part number builder on the page does not allow anything longer than 18", but putting the desired part number into the search box, top-right, will get you a price. For sword-length stuff, a 36", 48" or even 60" might even be worthwhile. I am cheap and tend to use similar thermocouples from the company we use at work. However, these "only" have a type 310 Stainless steel sheath, which is rated to 1100 deg (2012 degF) and will last pretty much indefinitely for HT, but doesn't last many cycles to 1300 degC-plus when used for checking welding temperatures are being reached.
  12. timgunn

    Kiln - to buy or not to buy

    A Kiln/HT oven will certainly allow you to do things that no forge can realistically manage. Power input is typically around 3 kW. I have built HT ovens up to 27" long that will reach 1300 degC (in about an hour and 45 minutes) and will run from a standard UK domestic outlet: 13A at 230V nominal rating, so 3 kW. Bigger ones might use more power, smaller ones might use less. Lower temperatures are reached much faster. When I first tested the 27" oven from cold (maybe 10 degC, 50 degF), 800 degC, 1472 degF was reached in 22 ½ minutes, 1100 degC, 2012 degF took 54 ½ minutes, the temperature at an hour was 1125 degC, 2057 degF and 1177 degC (2150 degF) took 71 minutes. As Brian says, once the desired temperature is reached, the PID controller will cycle the power on and off as needed to maintain temperature, so the average power drawn during the "hold" will be less than the heat-up, which will usually be at full power unless you are "ramping". A fairly typical HT cycle for O1 might be to heat the oven to 800 degC and stabilize it (call it 30 minutes at 3 kW, for 1.5 kWhr), triple normalize; Work in, allow the temperature to recover, soak for 10 min, pull out and cool to black. Repeat. Repeat. (Call it an hour at 33% power, so 1 hr at 1 kW for 1 kWhr) Then Austenitize: work in, allow temperature to recover, soak for 20 min (call it 30 min at 33% power, 30 min at 1 kW for 0.5 kWhr). Quench workpiece(s) and switch off oven. Total power consumed 3 kWhr. You might expect to maybe double that for a Stainless HT, but to put things into perspective, unless you only do single blades each time, 6 kWhr is still unlikely to significantly exceed the cost of HT foil. The numbers I've given are probably high-side figures. I don't have hard data, but I'm pretty sure the same homebuilt 27" oven was actually showing a steady-state output cycle of only 18% during the 1100 degC, 2012 degF hold section of a Stainless Heat Treat: when I delivered it, the new owner gave it a test run while we drank tea. I use a 2-second output cycle and always have an LED that is switched with the elements. I was surprised the "on" part of the cycle seemed so short and pulled up the output cycle display on the controller. It read 18%. The heating load of a HT oven is entirely resistive: it does not have the starting surge of a motor load or the horrible Power Factor of some modern electronics. So long as you do not exceed the outlet rating, I would not expect any new and interesting problems with mains power supplies, even in the middle of nowhere. The short-cycling 3 kW output would be harsh on a small generator, were you to run it from one, but I'd not anticipate a problem once the generator gets above maybe 10 kW or so.
  13. timgunn

    Grinder and VFD Programming

    The first question is probably "what does the motor rating plate tell you about the motor?" and the second is "what do you want it to do and how do you want to tell it to do it?" The motor rating plate should give you a bunch of values that need to go into group 2 of the programming. The values you are likely to have are 02-01, 02-03, 02-04, 02-05, 02-06. If the motor plate gives you any others in an obvious way, put them in. Otherwise leave the settings as the defaults. The default command source seems to be the keypad. If you have a speed control potentiometer (twiddly knob) on the front, set parameter 00-05 to "1" to enable speed control from it. It looks like it should run on the start and stop buttons on the keypad and should go to whatever speed the knob is set to. Once running, adjust the knob to adjust the speed. It looks like the default acceleration and deceleration times are 10 seconds, so there will be a lag if you turn the knob fast. The drive is environmentally protected to IP20: effectively fingerproof. It is not sealed against dust and metallic grinding dust WILL kill it very quickly unless you take steps to seal the drive. Bear this in mind when you first fire it up: do not get over-enthusiastic, start grinding and cause fireworks. The quickest anyone I know personally has killed a drive this way is around 2 minutes from first switching it on. You'll then need to decide how you are going to protect the drive and how you are going to control it in whatever protection you come up with (if it's in a box, you won't be able to get at its front panel). You can also decide on the speed range you want. Some of the remaining parameters will set the drive up for the speed range and command source you decide to use. Many of the others are there to interface with control processes in industrial automation and will not be needed for a grinder with a human operator. I mount IP20 drives in IP66 enclosures and fit a remote control box on a trailing lead with start and stop buttons, a keyed forward/reverse switch (the key is taken out when running a machine that should not be reversed) and a speed control potentiometer. The box has a 3-phase socket on the front so I can plug in different machines. It's usually as cheap to buy an IP66 drive to begin with, and is much less effort.
  14. timgunn

    I think I bought the wrong burner

    It looks like the burner is choked down to me: The air holes are covered so there is not much air flow for the gas to burn with. Try changing absolutely nothing except the choke sleeve position: move it towards the forge and uncover more holes to get more air in. See what happens and let us know. If you can, take photos in the dark. The most useful shot is often one across the mouth of the forge to show how much Dragons Breath there is, and what colour it is: In daylight, the DB flame does not show particularly well and digital cameras tend to do strange things to the white balance so that the flame colour in the photo is not the actual colour. Taking a tight shot directly into a forge can be completely useless because the camera will adjust the colour and a real dull red will look in a photo just like a real bright orange in another photo.
  15. timgunn

    Heat Treat Foil

    Are 17-4 and 18-8 foils actually that much more readily available at .001"-.002" thicknesses and in knifemaker-type quantities? 321 is not all that far off being an 18-8 stainless: it's effectively 18-10 with up to 0.8% added Titanium, which apparently improves high-temperature stability. I think 321 was developed to counter cracking in aircraft engine exhausts. Neither 304 nor 316 are particularly recommended for high temperatures. 309 is higher in both Chromium and Nickel for use at higher temperatures. They get used because they are known to work reliably in their specific applications. "Someone" did some work to identify suitable materials for the tasks and that is what they came up with. Most folk seem to accept the consensus view and pony up for the proven product. Using 321 when heat-treating steels that "need" 309 has been known to fail and I'm pretty sure an evening spent searching the knifemaking forums will provide some insight into the sort of things that can go wrong: I'm pretty certain I've seen posts about the foil welding to the workpiece and to failure of the envelope allowing contact of the workpiece with air. If you have easy access to other grades of SS foil and can do some testing, I'd certainly love to see your results.