I made several tight toleranced parts yesterday. All of them about .2800" round with a number of different diameter steps that call for tolerances down to .0003. I made them from W-1 drill rod. The parts turned out nicely with all fits and tolerances just right. Then I hardened them...
Problem is that after hardening, the diameter of the parts increased anywhere from .0005 to .0007. Of course, they are now unusable. I hardened them by heating to bright red then quenching in water.
I'm wondering what I can do the next time to avoid this problem? Could it be that I heated the part too hot before quenching? Does W-1 always have this problem and/or is there a better steel to use? Thanks for all suggestions.
Generally, steel will always change it's size when hardened. This is due to the fact that the carbon-atom changes it's position in the crystal. But there are special steels that will not react that much when hardened. I think they are called "invar". But I can't give you any numbers because they are hard to obtain and also we (Krauts) have an other nomenclature.
In your case, I would just lapp/oilstone the parts down.
Nick PS: Thanks for another set of nice plans in MEB!
Hardening carbon steel converts the phase to martensite, which is less dense than the unhardened phases (ferrite, mostly). Thus, it expands when you harden it.
Usually such precision parts are turned, hardened, and then ground. In a hobby shop, they may be lapped after hardening, rather than ground.
All tool steels will change to some degree when hardening. W1 is about as bad as it gets. Following proper hardening and tempering procedure will minimize the problem. A2 is probably the most stable among the more common alloys, but requires more care and higher temps to harden. O1 is pretty forgiving and falls somewhere in the middle.
You may be able to pull this off with a little experimentation and careful control of your hardening process. If you're heating by eye with a torch it's going to be tough. Are you aware of the magnet test for the steel's transition temperature?
(clip) Problem is that after hardening, the diameter of the parts increased anywhere from .0005 to .0007. (clip) ^^^^^^^^^^^^^^^^^^^ This may sound like a dumb question to those in the know, but why can't you turn the parts about .0006 under, and then heat treat? Would it be a surface finish problem, or distortion or something I haven't thought of? Just curious.
The main problem is that the growth will be nearly impossible to predict. In some metals (like H-13) some areas may actually shrink. The growth usually is proportional to the size of the part, where small parts grow very little, and larger parts grow more. In most materials, you can count on around .0007" growth per inch. Usually, depending on the quality of the heat treatment process, you can expect different amounts of growth in different areas. Plan on the part coming back out of round. If you're trying to hold a .0003" tolerance, you better count on machining after heat treat.
I don't have photos to post, but my Dremel adapter is just a block of aluminum. I fitted the bottom of the block into the compound slot and drilled a vertical hole for a hold-down bolt, then turned the compound parallel to the ways and used the lathe to drill & bore a horizontal hole that accepts the neck of the Dremel. It's shaft is automatically at lathe center height and the compound screws feeds it straight in.
Nice picture of a dremel tool made into a tool post grinder. The author mentions that the bearings aren't up to real precision work, but I'm thinking they could be upgraded.
One of these days I gotta get me a tool post grinder...
Robert, Lacking proper grinding capabilities, you should be able to spin polish the hardened parts to achieve size. It's the common way to accomplish close tolerance work, heat treated or not, under such a circumstance. It requires a little skill to achieve without rounding edges, but it's certainly possible, assuming the turned lengths are not short.
OK...I'll ask a dumb question...Isn't drill rod already pretty hard? How much harder could you make it? Anyone know the Rc hardness of drill rod ? Thanks
I've always taken advantage of the highest spindle speed available to me, secure in the knowledge that grinding demands high surface speed. The faster the surface speed, the better (and faster) abrasives perform. Heating, naturally, is a problem.
Typical procedure for me has been to machine to roughly .0005" oversize, then polish for size and finish. Constant cooling and measuring is required when shooting for a tight tolerance. Seal and bearing fits are a good example of where I use this process.
Grits? Depends on how much is to be removed and the finish required/desired. I've used as coarse as 60 and as fine as 600. Regardless of where you start, though, it's smart to use them progressively, which eliminates polished surfaces with scratches.
What works very best is strip, which you hold by the ends. You can spot polish by using an edge, but care must be exercised, otherwise it's easy to create undercuts. You also must learn to avoid edges, which are easily rounded off. Short turns are very difficult for that reason. I must confess, the majority of the polishing I've done has been on soft materials (anything machinable, not requiring a grinder). With rare exception, I've had the use of precision grinders for hardened work. Still, the principles are the same, it just takes a little longer.
It's easy for strip to get wound around a spinning object, so care must be exercised to avoid wrapping well enough for the strip to do so. Keeping the ends apart helps avoid problems. If you find you're applying pressure at the part with fingers instead of holding the ends, the strip is inclined to wrap around, dragging your fingers in with it. Be careful. Never permit the strip to complete a circle.
I never use files, which make it easy to lose roundness, and are generally too aggressive for fine work when the part is spinning. I've always avoided using anything rigid (stones, for example), with one exception. I ground a set of stepped plug gages for a defense installation, but had to use a tool post grinder. There was no precision grinder in that particular facility. I ground the diameters on top tolerance, then followed up with polishing cloth, quite fine, and placed between a pair of parallels. My purpose was strictly to bring up the finish, which, typical of a tool post grinder, wasn't very good. It took only a tenth or so to enhance the finish. The parts had a lapped appearance. To be perfectly honest, I was surprised, and impressed.
Not much guidance, I realize. I think each of us develop what works for us------which is what I did. I don't recall anyone training me to achieve the end result in the way I do.
Incidentally, parts so created will pass extensive inspection procedures. It's not a hack way to go, not if applied well. .
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