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Forges 101


Mikey98118

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Exhaust size and shape

One thing backyard casters and blacksmiths both worry over is how large to make the exhaust openings on their equipment. Too small and you have high back pressure killing burner performance; too large and you can't get enough heat to stay in the equipment to do your work. Of course, the closer to the "right" opening size your equipment is the stronger the forge or furnace can be built. Just don't get suckered into confusing the right size for a “perfect” size. As long as burner output can by varied (turn-down range), there can't be any such thing as a perfect opening size. The right size is what is needed to accommodate the burner's highest output (the highest you are willing to take it to).

    Variable is the optimal opening size; all other dimensions can be outright wrong, but seldom just right, with a burner flame that can be varied. This is one of the many reasons for controlling exhaust with an external baffle wall beyond a larger than needed ringed opening; thus, allowing the least heat loss through radiation, while maintaining optimal back pressure in the forge. Why include a ring around the exhaust opening? To divert hot exhaust gasses away from the metal shell, where it could otherwise overheat the metal.

Doors

Maximum clearance can be provided with a hinged and latched forge door that contains built-in changeable baffle plates. A door makes building the refractory structures of equipment much easier, and permits larger pieces to be heated then would pass through a narrowed opening. Best of all it allows movable internal baffles to be used, which would not pass a narrowed exhaust opening; this promotes the use of single burners for short pieces, saving money in a five-gallon propane cylinder furnace run by two ½” burners.

The door is a big step up from an exterior brick baffle wall; it should include a parts entrance that can be varied in size; for instance, with several round (or hexagonal) kiln shelves with different openings to for passing stock through, which can be exchanged, and held in a pocket on the door. All of these improvements don’t need to be seen to at once, so long as a hinged and latched door is included in the forge shell.

    If you choose a simple brick baffle wall in front of the forge, keep the bricks at a small distance from the exhaust opening, to allow hot gases to move up and out, between forge shell and brick wall, while bouncing radiation off of a re-emissive (heat reflective) coating, back into your forge. Keep the stock entrance only as large as is needed to move parts through.

    This arrangement helps to slow the flow of expended gas in the forge interior, as it heads toward the exhaust opening; and then speed the gas up through the opening; another highly desirable trade off. So, you are gaining hang time for the heated gas in the forge, and recuperative savings from bounce back of radiant energy; another win-win situation. A baffle wall also minimizes infrared and visible light from impacting your eyes and skin, improving your health and comfort.

    While hinged and latched doors can do just as much on box shaped forges, all the examples I have seen slide up and down.

    High alumina kiln shelves are seven times more insulating than hard fire brick; they are also tougher at forge temperatures, which is an important consideration for something you will end up shoving parts back and forth through. Using alternate kiln shelves, with different part openings is fine, but building an elaborate system of moving kiln shelf parts to ape the ability of bricks to change their openings comes under the heading of "gilding the Lilly." The additional energy savings it provides probably isn't worth the effort. Make up new openings in shelve baffle walls sparingly. Diamond coated and carbide coated rotary burrs (and coated hole saws) are the preferred way to drill holes in kiln shelves. Friction cutoff blades and diamond coated blades are the best way to cut lines between those holes.

    You want to coat the hot-face of the door with one of the re-emissive coatings, use a formula of 95% zirconia silicate (zircon) and 5% Veegum (or bentonite as an alternate); this mixture makes a tough re-emissive coating for wear surfaces. Zirconium silicate can also be mixed with fumed silica to make a tuff and effective coating on refractories (but not on ceramic fiber products). There are other choices, Like Plistix 900F, but none of them are as economical or easily purchased in other countries.

    All these advantages can be applied in casting furnace mode, if a round kiln shelf is placed in a hoop, which can be swung into position above the furnace and swung out of the way during crucible removal. A mall center hole in the shelf allows observation and metal to be added to the melt; it also provides a rest for preheating metal to make sure it is thoroughly dry before placement in the crucible.

Floors

If you don’t plan to do much forge welding, laying down a Kast-O-lite 30 floor over ceramic fiber insulation (or K26 insulating brick) will be tough enough, and more insulating than kiln shelves. For occasional welding, some people use a steel baking can, filled with the kind of kitty litter that is made from pure bentonite clay bits to shield the floor from welding flux, but frequent welding is certain to slop flux onto a forge floor. A kiln shelf, that is trapped in slightly oversized slots in the forge shell, will pay for itself with the ability to be slid out and resurfaced or replaced, once its top is fouled with glue like flux. Once the flux cools into a glassy layer, a grinding disc can remove it with modest effort. Don’t forget to where a dust mask.

    If you’re going also going to use a tunnel forge as a casting furnace, the kiln shelf can simply lay on the round wall, when your forge/furnace is in the horizontal position.

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Flame positioning

Burner designs have improved in recent years, and are continuing to develop. Refractory product improvements are advancing apace with burners. Both changes are shifting the ground rules of forge design. Top-down facing burners were the practical choice in times past, for good reason; mainly, it integrated well with the limits of reasonably priced refractory wall materials, by permitting flames to impinge on forge floors, which needed to be made of tougher materials anyway.

    But while new materials (including Morgan’s K26 bricks), can be used to improve performance of standard forge designs, they are even better, when combined with greater distance between flame tips and heating parts, to ensure complete combustion before impingement. Therefore, a reduction in scale formation will be gained by pointing burners up and away from the parts in tunnel, "D," and oval forges; or high on a side wall of box forges.

    Ribbon burners (and other multiple flame nozzle designs) have little or no problem completing combustion before their flame paths impinge on work pieces, or forge walls. Single flame burners must be aimed to provide sufficient distance, before impingement occurs. Two or three smaller burners provide far more distance for combustion to complete, than a single larger burner.

    A neutral flame is not only hotter than a reducing flame, but it is much better for your health; employing lightly reducing flames in a gas forge has long been standard practice, in order to decrease scale on heating parts. Increasing the distance between flame tips and parts is the cleaner choice.

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Burner placement

The first question asked about burner positioning should be why; not where. Burner positions are always dependent, but not primarily on best circulation of hot gases; that is a secondary concern. Flame impingement should be your first concern. The point where a burner's flame is aimed must be physically tough, up to thermal stress, and as far from the flame tip as you can manage.

    If your insulation is only protected by rigidizer and a thin seal coat, the flame needs to impinge on a high alumina kiln shelf or an exceptionally tough high alumina cast refractory (like Kast-O-lite 30). On the other hand, if the equipment’s interiors are a 1/2" or thick layer of such a refractory, wall impingement is no longer a big issue; it's just one more factor, to be balanced against others of equal concern.

     Circulation is a secondary concern; fortunately, this takes nowhere near as much encouragement as is commonly supposed. There is a strong natural tendency for hot gases to circulate within most forge and furnace shapes; including box forges. In fact, the only burner position that would greatly interfere with sufficient circulation of hot gases would be with the flame aimed directly toward the exhaust opening!

Note: Positioning burners near the exhaust opening makes a close second to the previous example of exceptionally bad planning.

    You also want it to avoid flame impingement on work-pieces or crucibles. If top mounted in a tunnel or “D” shaped forge, you would want the flame to strike as far from the first surface it will impinge on as possible. In that case, I prefer to aim the flame at the near edge of the floor, with only enough forward angle to assure that the heated gasses bounce toward the far edge of the floor, and continue up the opposite wall. An equally good position is with the burner mounted at about three o’clock, with the flame facing diagonally, at about forty-five degrees down and impinging on the far side of the floor. In either position, the flame isn’t aimed directly at the work pieces.

    In most box forges, the burner (or burners) was previously placed at the top. And the flame was aimed at the floor; the point of this was for the flame to impinge on the toughest surface that could be provided, so that weaker wall surfaces might be spared the ravages of flame impingement. High (purity) alumina kiln shelves, or alumina-based refractories are one the most effective choices to employ as an equipment floor; this was the limitation for decades, but with better wall materials available at reasonable prices, this limit is fading; this allows burners place high on side walls, facing across the interior at the opposite wall, and passing over the work.

    The latest development in burner positioning is placing them low on the wall, or even in the floor, and aimed upward in tunnel and “D” forges, and aiming across the top of oval forges; this requires cast refractory surfaces in floor, walls, and ceiling.

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Mounting burner ports

Typically, a burner port (entrance) consists of a short tube or pipe about 1/4” to 3/8” larger than a burner’s mixing tube all the way around; about 1/2” to 3/4” larger inside diameter than the outside diameter of the burner’s mixing tube. This allows enough space to aim the burner somewhat within the portal. You also want the port large enough for the burner’s flame retention nozzle to slide through it.

    The burner is held in position and aimed with two rows of thumbscrews; each row has three equidistant screws. One of the advantages of these screws is that they can hold a pipe or tube in place within the portal, and resting exactly where the flame is intended to impinge, while the portal opening is being ground into an oblong shape (to allow the tube to be aimed at a desired angle). This method ends up with a very close fit in the shell opening, to promote easy silver brazing of the port’s tube to the equipment shell. You’re building a burner, so why not employ it to help you construct its forge?

    Alternatively, you can drill and mount a burner port in the shell with two bent flat bars and some pop rivets, or self-drilling screws. Bracketing parts together can end up looking tacky if you don’t manage to keep the shell opening tolerances close. Employing screwed brackets can be a be a minor pain, if the burner’s port tube is positioned at an angle.

    Welding equipment parts, such as burner ports unto a steel shell, takes a wire feed machine and a learning curve. Some people are reworded with distortion in the shell, because of welding contraction; it just takes a little time to learn to run a wire feed welder, and somewhat more to bridge gaps with one; but it takes a lot more time to learn where and how much to weld without creating distortion.

    Hard brazing requires an oxy/fuel torch, or an air/fuel torch, propylene fuel, and a lot of skill, Silver brazing can be done with an air/fuel torch, propane, and close attention to setup, but most silver brazing alloys won’t bridge gaps very easily.

    Silver brazing by hand torch benefits from a low temperature filler with broad melting  range such as Ufhauser silver braze filler A-54N (54% silver/ ) that has a broad elastic range (250 °F), and bridges irregular gaps; it can be considered a capping alloy, but if heated too slowly it can suffer from liquation (where the alloy separates into solid and liquid zones); it will melt between 1325 °F (dark red) and 1575 °F (bright red). The high temperature portion will melt only above the normal brazing temperature afterward. For this reason, alloy A-54N should be heated rapidly through its melting range. If you are joining a thin shell from a tin can to a thicker tube, keep the flame mostly on the tube.

    This filler alloy has a good color match to steel. Reasonable care with a sanding drum or grinding stone in a die grinder or electric rotary tool, will easily produce a sufficiently close-fit in the joint between a burner portal tube and the forge shell opening. Burn polypropylene fuel gas if you employ an air/fuel torch, and employ a high temperature black flux meant for brazing on stainless steel.

     Car mufflers are zinc coated, and silver brazing parts to this kind of forge shell will ensure lots of damage to the plating. Zinc-based solder sticks may be employed afterward. Most zinc-based soldering alloys are zinc-tin-lead (avoid these), zinc-tin-copper (excellent), or zinc-cadmium (use fume rated respirator with these and follow all safety guidelines to the letter).

Note: The main ingredient in zinc flux is zinc chloride (follow safety guidelines on container); it is the only ingredient in many of them; it tends to “tin” the surface of steel, rather than just cleaning it. If steel is freshly cleaned and power buffed with stainless steel wire wheels, it can be zinc soldered without flux, but why do things the hard way? Zinc’s melting point is 787 °F; comfortably below its boiling point (1665 °F). Zinc fumes are easily seen and smelled; avoid them. Unlike lead fumes, it takes a heavier dose of zinc vapors to cause fume fever. Unlike lead, the body can tolerate a little zinc, but keep your dose tiny; none is best. No metal fumes are good for your lungs.

Caution: Metals give off toxic fumes upon reaching their boiling points. Using zinc coated sheet metal or parts (such as car mufflers) is okay if you're careful about doing it. The boiling temperature of zinc (the point at which it makes fumes) is

1665 °F (bright red heat). Your forge shell should not get higher than one-fourth that temperature. But you do need to be careful to keep the shell well away from the edge of the exhaust openings, by not making the openings in ceramic fiber, kiln shelf, or cast refractory even with, or even near the shell. But zinc coated flame retention nozzles or mixing tubes are out. There is no reason at all to avoid zinc coated reducer fittings on a burner’s air openings. In other words, keep zinc away from part surfaces that may become incandescent (above 1200 °F or 649 °C).

Note: Preheat temperatures should be kept down to 600 °F (315 °C on zinc coated surfaces, such as car mufflers, to avoid damage to the existing coating on their surfaces, and to keep scale formation down on the steel; “tinning” the bare steel with a zinc chloride-based flux will help with this. Remove all residual flux with hot water and a clean rag after soldering.

    Larry Zoeller (of Larry Zoeller Forge) is credited for first mounting schedule #40 pipe to a forge shell with conduit locking rings; he calls it a “burner holder assembly.” If you’re looking for fast and easy, he sells them for $25 and shipping from his website. Their main limitation is that they can only be positioned at right angles.

    The burner port should be completely external to the forge shell; it should not extend inside the forge.

    A washer should be provided to slide up and down the burner’s mixing tube above the portal, so that it can limit how much secondary air the burner flame can entrain through the gap between the burner’s mixing tube and the portal wall. A nut can be silver brazed onto the washer, so that a thumbscrew can keep it positioned at the right distance away from the portal edge; limiting secondary air into the forge to only what is needed for complete combustion, without lowering internal temperatures needlessly.

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Mike,

Just to make sure I understand the placement and angle of the burner you're suggesting.  I will be using a  small helium tank, 2" ceramic fiber, .5" satanite, Bubble wash, K26 on the bottom.  I was planning on using a top mounted firing to the far side of the K26 (Green) but your saying one of the other colors would be a better option?  I will be using a 1/2" Frosty if that matters.

 

Thanks

Forge layout.jpg

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No; to be perfectly honest, I had not even consider the position you contemplate (green line). So long as your flame face is sufficient, I would consider it an excellent plan. Normally, I would suggest a slight change in direction, so the the flame impinged on the floor, but normally" the floor would have a high alumina kiln shelf do withstand impingement, or Kast-O-lite 30 layer over that brick, which I suggest, but it's up to you.

 

Glenn sells small bags of Kast-O-lite 30 for moments like this. I would hate for all that expensive work you're doing fall apart.

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25 minutes ago, Mikey98118 said:

No; to be perfectly honest, I had not even consider the position you contemplate (green line). So long as your flame face is sufficient, I would consider it an excellent plan. Normally, I would suggest a slight change in direction, so the the flame impinged on the floor, but normally" the floor would have a high alumina kiln shelf do withstand impingement, or Kast-O-lite 30 layer over that brick, which I suggest, but it's up to you.

 

Glenn sells small bags of Kast-O-lite 30 for moments like this. I would hate for all that expensive work you're doing fall apart.

I thought the k26 coated in satanite and bubble wash would be OK for direct impingement.

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I don't know whether it will or not, and am not inclined to suggest something without that knowledge.

I haven't even heard of a Satanite and bubble alumina wash before, and had assumed it was something you were mixing up yourself. It sounds like a fine idea--generally--but when it come to the impact point of flame impingement...I get all conservative, having spent twenty years seeing just how hot I can make that spot :unsure:

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I feel that YOU will probably do just fine with your arrangement; that doesn't mean the next guy will do the job exactly the way you did, or place his burner as for from the impingement point, or use Morgan K 26 bricks under it. If your methods become the next "obvious path," and lots of guys tread it...good. But it takes lots of successful builds before enough voices speak up, and keep people on that path. Welcome to the way things are :P

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No, it's a component maybe but it's main use is to increase the insulation value of cast refractory. A few of the guys here have made a kiln wash with it but it's not very common.

I think you're running into one of the serious problems trying to use online video how tos to build  most anything. Most Youtube videos aren't worth spit, the only qualification the poster needs are a camera and connection. Some videos are down right dangerous.

There is a craft jargon that is almost never gotten right in online videos. That's where "bubble wash" comes from, the guy posting the video didn't know what he was doing but wanted to get out there with something. <sigh>

We spend a lot of time trying to straighten these things out. Don't sweat it, it's just part of the learning curve we all climb. Nobody is born  knowing this stuff.

Stick around, we'll get you up and going.

Frosty The Lucky.

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The Pass-through

Even in sword making, only about 6” at a time can be worked. For wrought iron twisting, only short sections are twisted in a given direction. For long twists, cold forming at low RPM produces much more uniform twists. So, even long parts can be heated in relatively short forges, if they have a pass-through; that is a small opening in the rear wall of a box forge, or the far end of a tunnel, “D”, or oval forge. Anything from an insulated hinged flap, to a mere brick can block this opening, when not in use.

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  • 4 weeks later...

 

 

12V 300W Brushless 75mm (3”) Cordless Angle Grinders

Found on eBay and Amazon.com; they can be used as a saw by mounting a cutoff disc; this is the safe and sane alternative to high power die grinders, for grinding and surface cutting, but can’t be used as a rotary tool. Other ads for the same tool, and for the Parkside version can be found on eBay. The side mounted power switch slides forward and locks in position, with some effort; but the same spring that you must fight to turn the tool on makes stops easy, and an electronic motor brake makes stopping fast; both are important safety devices for surface cutting . This grinder has a maximum speed of 19,500 RPM; it has all the power you can safely use in surface cutting. The guard can be changed into three different positions, but only after you buy a set of Allen wrenches; no wrench is included in the tool for this purpose. Since the first tool I ordered through a drop shipper on Amazon did not arrive in the time frame they allowed for, I reordered from a second source through eBay. The first toll finally arrived, too late to cancel an order for the second one; this was just as well, as the first tool came without all the promised accessories or second battery, and with a frozen spindle brake, which makes it good for nothing but spare parts. The second tool works fine and has every accessory promised, including the second battery.

    Even the best angle grinder will be limited by the accessories you mount on it. Cheap resin bonded cutoff discs are generally a bad bargain. Buy a Weiler, Shark, Miller, or 3M cutoff disc.

    3” and smaller flap wheels and grinding discs don’t come in junk brands—yet.

    Using 3” diamond coated cutoff discs is not a safe choice. Its smaller torque, and built-in handle makes a 12V grinder a lessor risk than a 120V electric die grinder, but it is still not a smart move for most people. If you can’t resist the advantages, be sure to surface cut. Don’t plunge cut, and deliberately employ the guard as a rest to keep the disc from sinking deeply past the work’s surface.

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  • 2 weeks later...

Of course, the better a tool is the more we desire to use it for every possible task. Once you acquire one of these wonders, you will start scheming to somehow add the ability to use it as a die grinder or rotary tool, which brings us to conversion chucks; these are quite successfully employed  one full size grinders for this very purpose, and much less successfully employed to use them for drilling holes in steel (their RPM is too high for that).

The why and how a conversion chuck works is all in the adapter's thread, because the problem of run-out is mainly about axial miss-alignment between the chuck and a motor, etc., that it mounts on. Every angle grinder's spindle ends in a threaded hole in the center of a  small disc; this forces accessories to spin at true right angles to the spindle. Keeping accessories centered on the spindle is pretty well taken care of by the threads on the bolt that screws into it. The final factor is the minor amount of slop that threaded parts provide; this allows the bolt and the accessories it traps in position to be forced into true right angles with the spindles face,

Thus, the adapter thread on a conversion chuck can be made to allow the chuck to run centered and exactly parallel to the grinder's spindle.

Which chuck to pick? A snall keyed chuck used for micro-drills. Unfortunately the last part--the adapter bolt--is the sticking point, unless you have a lathe, or a buddy who is a machinist. How badly do you want what you want?

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    Which chuck to pick? A small keyed chuck used for micro-motors, is your best bet, since they have reasonable quality, but aren’t too heavy. Unfortunately, the last part--the adapter bolt--is the sticking point, unless you have a lathe, or a buddy who is a machinist. How badly do you want your own way? It is inevitable that conversion chucks will become available for this tool in the next few years, but then, everyone will have them…

    An alternative choice would be the Dremel 4486 Keyless Chuck (1/32” to 1/8”)—do not settle for any of the look alike junk. A spindle can be stripped from a cheap or broken rotary tool, and partially re-machined to match the thread on a 12V grinder’s bolt.

Any survey of conversion chucks on eBay 0r Amazon.com will show numerous other choices, which at first glance appear easier and cheaper to use; a look into their customer reviews should quickly change your mind.

 

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  • 3 weeks later...

Surface cutting has different rules

en you run a chop saw, or plunge cut with a hand-held circular saw, the last thing you want to do is stop the saw during the cut; more often than not, doing so will cause kickback. The opposite is true when surface cutting through sheet metal products, like pipe and tubing.

    Those OEMs (like Dremel Tools) who bother with thorough safety tips in their rotary tool instruction manuals, all advise the operator to run the cutoff disc back and forth on the part surface, gradually deepening a groove at the cut line, until the disc begins to break through the groove, which is then called the “kerf.” Unlike chop sawing or plunge cutting through lumber, the operator is supposed to bring the disc to a halt before exiting the kerf; it is dissimilar to other processes, because your disc isn’t deeply buried in the part. There is very little material for the disc to “walk up,” creating an opportunity for kickback, as the disc stops. So, surface cutting creates a unique situation, where stopping the disc before removing it from the kerf is safer than removing the disc while it is still in motion.

    Die grinders are treated the same as rotary tools for surface cutting (my own description for the technique).

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Mounting a collet chuck on the 3” 12v angle grinder

(from my book notes)

 

Once you acquire one of the 3” 12V angle grinders, you will start scheming to add the ability to use it with drum mandrels, stones, rotary files, small wheels, and small cutoff disks; extending the number of jobs it can do well. Why cutoff discs? To make intricate internal cuts for rectangular air openings in small burners.

    Which brings us to conversion chucks; these are quite successfully employed on 4” and 4-1/2” angle grinders for this very purpose, and much less successfully employed for drilling holes in steel (their rotational speed is way too high for that).

    The “why and how” a conversion chuck works, is all in its adapter's thread (the threaded mandrel used to connect a chuck to a motor, etc. is called an adapter). The worst run-out problems (wobble) are mainly caused by axial miss-alignment between the chuck adapter and a motor (or tool) that it mounts on, although poorly constructed chuck can also create run-out, along with other problems.

    Every angle grinder's spindle ends in a threaded hole in the center of a small disc shape, which forces grinding wheels and cutoff discs to spin at true right angles to the spindle. Keeping accessories centered on the spindle is pretty well taken care of by the bolt that screws into it.

    The final factor is the minor amount of slop in axial alignment that threaded parts can provide; this allows the bolt and the accessories it traps in position to be forced into true right angles with the spindle’s face. Thus, the adapter thread on a homemade conversion chuck can allow the chuck to run centered and exactly parallel to the grinder's spindle.

     It is best to screw a keyless chuck into your grinder before you try to mount an accessory in it, and use the grinder’s spindle stop, when tightening or loosening the accessory. But which chuck to pick? 

    Any survey of conversion chucks on eBay or Amazon.com will show numerous choices. But dependable 3 jaw keyed conversion chucks are too massive to work well on one of these grinders. Every start and stop would load down (stress) the motor, and with every stop, its electric breaking would jar the chuck’s jaws, loosening their hold.

    1/8” and ¼” keyless conversion chucks, at first glance, may appear easier to convert; a look into their customer reviews should quickly change your mind. If you decide to take this course, the Neiko 20753A ¼” Keyless Chuck Conversion Tool, is presently, your best bet. Most of the keyless conversion chucks are, presently, so poorly made as to be nearly unusable for this purpose. A ¼” chuck will mount the stronger die grinder accessories with ¼” shanks, and the keyless model is lighter than a keyed chuck. The Neiko chuck is the best of a poor lot; that said, it can be repurposed with little more than a 3/8” drill motor, an M6 x 1 thread die, cheap digital calipers, flat file, and the 3” 12V angle grinder you plan to use this chuck in.

 

Whatever chuck you repurpose for use on this angle grinder well not be an asset for drilling in steel, and the smaller and simpler 1/8” collet chucks found on rotary tools are less likely to make problems on this grinder, easier to mount correctly, and no worse for drilling than a repurposed conversion chuck.

    An alternative choice would be a 1/8” collet chuck and spindle, stripped from a cheap or broken rotary tool, and partially rethreaded to match the M6 x 1.0 thread on a 3” 12V grinder’s spindle bolt, provided that no mention of run-out or other problems with its chuck are listed among the other customer complaints.

 

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  • 2 weeks later...

Getting refractory edges to meet

I like working on drying castable refractory. Drilling, grinding, and shaping with a rasp goes very well after the first 24 hours; but, far less easily after firing. During the first week the refractory is curing to “green hardness; very like concrete.” In fact, castable refractory is a form of concrete; what sets it apart is that the chemically locked water that remains after curing can be—CAREFULLY—steamed out of the finished form by firing. Concrete can’t be fired; it explodes. Aside from firing, the more familiar you are with working concrete the more you already know about working with castable refractory.

    The first thirty minutes of set up time is the most important, as the mix is changing from thick liquid mixture into a solid. If you have cast horizonal mating surfaces for the upper and lower halves of a clamshell forge, or vertical mating surfaces for the forge shell and door in a horizontal forge, you want to use the edge of a steel square, etc., to flatten the mating surfaces by scraping. If you did a good job of cutting and grinding part edges for close tolerance, this is where that pays off; if not, aren’t you glad you read this section before you started pouring refractory?

    What if you didn’t? It’s not too late to fix you bobo. Low places, which don’t meet up with mating surfaces can be filled in with refractory cement. Be sure the cement is rated for temperatures as high as your refractory. Thoroughly clean, and then wet the surfaces where you lay the cement. Place wax paper over the top of the cement, and then close the mating surfaces against it during drying and curing. The wax paper should come away from the surface of the dry cement easily; if not, just let it burn away during firing. High spots must be ground away. If you grind to far, just use the refractory cement to correct your mistakes. Don’t try to do the whole job at one time. Correct you mistakes one at a time.

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How much firing?

Once the chemically locked water is driven out, is firing finished? No; most people consider this to be good enough, but without frequent firing during wet weather, the refractory can still slowly regain some water content from ambient air; necessitating the same careful fire drying routine you use during the initial firing, to keep the accumulated water content from cracking the refractory from built up steam pressure; unless you take firing to the next step, which is called calcining. Basically, you heat the fired refractory up to yellow incandescence all the way through the form. It will begin on the inside surfaces (flame faces) and slowly soak through the refractory until it reaches its exterior surfaces (cold faces).

    Calcining is the process of removing, by very high temperature (but below melting point), any volatile particulates, and finishing the oxidization of anything that can be oxidized, in a substance. Many of the constituents of a refractory mixture are separately calcined long before being included in the blend. But the cast refractory article may also be “calcined” at the melting point of silicon, to improve strength and durability. One example would be fine china, which is fired at higher temperatures for extended periods versus a ceramic coffee mug, which is minimally fired at much lower temperatures. One of the binding agents in most refractories is silicon. Some of the other constituents in many refractories are materials that contain silicon (like clay, which is likely to contain up to 40% silicon). When a fired refractory product is kept at high temperatures for an extended period, the silicon content begins to liquefy, gluing the other ingredients together more thoroughly, and filling in any micro gaps between particles; effectively toughening and waterproofing the refractory.

    So, “calcining” is a word with a double meaning; its proper use is one thing, and the second use is closer to industrial slang. Despite all the good and honorable intentions of English teachers everywhere, industrial slang follows an extension of the ‘golden rule’ (them what has the gold makes the rules). This extension is that OEM sales departments choose what are proper industrial terms concerning their products. And as so many other lessons from the school of hard knocks, we can like it or lump it.     

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In recent years there has been growing interest in traditional metal trades, such as forging and casting. While the equipment, tools, and skills needed for each endeavor are similar, there is strong resistance among beginners to combining these pursuits. Tunnel vision is only to be expected in a novice; going forward, 3D printing should make the utility of casting more obvious for art and tooling.

    Hobbyists commonly look for ways to profit from their avocation, which is where their troubles start. Whether you dream up a better widget, or just “go into art,” opportunists will be overjoyed to undercut your product shortly after you market anything worthwhile. If what you produce requires little knowledge or effort, it also won’t take much for it to be copied to death. “Keeping things simple” is okay, until that includes keeping things easy; don’t “curse the locusts” in that case.

    The first problem that newbies get hung up on is taking the “easy way," which usually isn't, and then along comes the idea of " building big enough". Big enough for what? For some nebulous plans for someday when you are a “success”? Getting there someday requires making the right moves this day. All the rich and famous artists tell various versions of the same basic story; it goes something like "I just stumbled into this thing for fun, and things kind of got out of hand; and that's how I got here." It wasn't some secret knowledge or virtue that paved their yellow brick road. Being unhurried allowed them to avoid the maze that others got lost in.

    What these generalities boil down to is that only two classes of person isn’t foolish to buy their heating equipment; dabblers who are only playing with it, and journey level craftsman who have previously built properly operating equipment, and therefore, already have the insights that teaches. Any serious craftsman has no business being ignorant about such important tools.    

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Drill-tap-bits are a combination of drill bit and starting tap, in one tool. You can buy sets of them very economically through Amazon.com and other online sources. Why? Because they get very poor reviews. Drill tap bits are designed for use in CNC machines, but people attempt to employ them in electrical drills. When they use the drill and tap functions in power drills, their ends get snapped off. However, they work just fine in manual hand drills, for drilling through pipe and tubing walls, using the same care you normally take in hand tapping. Drill-tap-bits can also be used in a drill press, if you shut the power off after the hole is drilled, and then tap by hand. Release the bit from the chuck, once the thread starts forming, and continue tapping by hand (see Drilling and threading stainless steel; the next chapter). Not for use for internal threading of gas tubes for MIG contact tips, or 3D printer nozzles; use separate drill bits and taps for such work.

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Thinking Safety

 

There is a lot of safety advice given on this forum, which seems long on “legal,” and short on practical. When being quizzed by an insurance company adjuster, with your burnt shop smoking in the background, you may feel differently—I did.

    What this is leading up to is; I’m not Mr. Political Correctness from the safety committee. These concerns are real; don’t learn this the hard way. Working with tools is a lot like gambling. There’s no such thing as a sure bet. Safety is about making smart bets.

just hinted at what it feels like to deal with legal problems, after burning down a garage. How much worse do you think that would be if your garage was part of your house? If you don’t have a separate shed or garage to do your hot work in, put wheels on your equipment, and work out in the yard; that includes welding, and brazing jobs. You may expect it will be inconvenient, but you will instead find it is surprisingly fun—and a whole lot safer! These days, all kinds of work tables and benches are designed to be set up on job sites (and easily transported when you change addresses), so assuming that doing “shop work” out of doors will be inconvenient is very limited thinking.

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