I'd like to make a tap for a one time threading operation. I need to be able to tap a 12mm x 1.0 mm through-hole. I can't do metric on my lathe (9" South Bend), but in this instance, close is good enough. It looks like 26 TPI is a pretty close match. Actually my problem is hardening the stock after I cut the threads. How hot does it have to be. The best I can do for a torch is MAPP gas. If this would work, what would you quench it in, oil or water (I'm using either CRS or HRS). Do I just mill slots in the stock to allow it to cut?
The tap needs to be above the curie point (ie no longer magnetic) which for most steels is dull to cherry red. Make your tap, coat it in hard soap to protect the threads, heat with your torch and try offering a small magnet to it to see if you are there yet. I don't know how much carbon you have in your steel, but ordinary mild steel won't harden this way. You would be far better using silver steel (drill rod US speak) and oil quench.
I'm not sure re. your "close enough" theory but as far as heat treating goes, this should suffice:
Cut the basic tap into a suitably sized piece of drill rod. Cut some flutes into the threads, 2 flutes should suffice. Put a small taper, say, over 2 or more threads at the end - this can be done on a grinder. Then relieve the tapered (nose) threads by backing them off - best done carefully with a file. Thread relief is done by removing a small portion of each thread from behind the thread at each flute. The relieved portion should extend to the next flute opening. Relieve only the nose threads.
Heat the new tap with MAPP gas or whatever to a good even "red". One way to gauge the red is to see if it will attract a magnet. When it will no longer attract a magnet, it is "red enough". Quench in oil. After quench, place it in a pre-heated kitchen over at 375 - 400 degrees for 45 minutes to 1 hour.
Bob Swinney
PS: Credit here to RCM's own Ed Huntress. (Ed is an editor and/or contributor to a major machine magazine) After *tapping 101* from Ed, I got a copy of the book, "Tool Steel Simplified". TSS is highly recommended for basic heat treating in the home shop.
Regarding my "close enough" need; I recently bought a cheap quill stop for my mill. It's the type that you squeeze the two knobs, it opens up and you place it where you want your quill to stop. The threads on it are 12mm x 1.25mm. My Grizzly mill's depth stop threads are 12mm x
1.0mm. So the threads don't actually revolve on anything. It only clamps onto existing threads with relatively light pressure. I'm figuring that if I clamp the two halves shut and drill and tap it, it should work fine. Thus "close enough" should be "good enough". It actually works now, but is a bit cocked to one side because of the thread differential.
I've found the tap available commercially. But to pay about $10 - $20 (prices I've found so far) for a one time use tap goes against my grain. The quill stop only cost me about $4.00.
========================== First I would check and see what a 12 mm X 1mm tap would cost.
Second consider milling/grinding a tapering flat on a good grade
12X1 cap [allen] screw like a tool makers' reamer, cut off the head and drive with your drill chuck to maintain alignment [no power]. Third, it is doubtful you can get normal h/r or c/r steel to harden as the carbon content is low. If this is a one time thing in soft material like aluminum you most likely can use it without hardening. If several holes or in harder material you can case harden the tap using caseit and a mapp or propane torch.
See
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reminded that some versions of this use cynaide, caution is required for use and extreme care must be taken with storage.
Much less distortion and the case will be harder than the the last alternative which is to use drill rod [silver steel for our Brit members]. O1 (oil quench)will distort less that W1 (water/brine quench) when you quench and temper. A1 (air hardening no quench)is best but most expensive. see
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hi-shock -- most likely overkill for your described use]
This is for 1/2 inch [12.7 mm] 12 mm available but at higher price. Unka' George (George McDuffee) .............................. Only in Britain could it be thought a defect to be "too clever by half." The probability is that too many people are too stupid by three-quarters.
John Major (b. 1943), British Conservative politician, prime minister. Quoted in: Observer (London, 7 July 1991).
Your torch will get the part hot enough. But CRS or HRS won't cut it. Those steels will not harden. You would be better off buying the right metric bolt, but if you can't find that buy a 1/2 inch bolt and make your tap from it.
You will probably need to use some sort of insulation to contain the heat if you use your torch for heating. Insulating fire brick is good.
Robert has described a method I've used with some success for short holes, but trying to cut a deep thread in steel with one of those simple home-made taps is an excercise in frustration--although you'll learn a few things about taps from the effort.
I don't recall if I've ever made one by cutting a thread first, but then, my memory isn't what it used to be. Usually I use a steel bolt and case harden it with Kasenit. If you're using CRS or HRS (use the CRS, if you must do it that way), you'll have to case harden that, too. It won't have enough carbon to just heat-and-quench.
You can easily relieve the crests of the threads with a file or a stone but relieving the flanks is a pain. If the tap just binds too much in your application, buy a commercial tap and store away the lessons you've learned.
Taps aren't trivial things, so it's satisfying to get a job done with a home-made one. If you have to buy a piece of oil-hardening steel to do the job, though (and oil-hardening will be much better than case-hardening a piece of CRS, as others have explained), it's just not worth it in terms of costs.
OTOH, if we really cared about costs first and foremost, not many of us would own lathes.
Thank you again for all the advice. I just HAVE to try to build the tap and do this myself. I'm going to try to use a 1/2" bolt (Dan's advice) and go from there. If it doesn't work out I'll buy a darned tap. I spent one day building little knurled screws for my lathe and mill, replacing various screws previous owners had substituted whatever they had that would fit. My stepson, who is quite the fabricator, told me he doesn't stoop to building fasteners. I thought it was a great exercise in not only building the screws, but building multiple screws that were more or less identical. Simple for experienced machinists, but for someone who is learning as he goes, it was great. This tap making exercise is just another part of the learning curve. Know what I mean?
Well, numerically 26TPI might not seem far off, but if the threaded portion is more than a turn or two then this is not the right route. The screw will start but very soon you'll start binding and/or ripping up threads.
Try putting a 10-32 screw into a M5x0.8 nut to feel the principle in action :-).
All the big suppliers (e.g.
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have M12x1.0 taps in stock. Not the most common size but in stock.
Why not just mold your half nuts out of JB-weld.... Just greese up your shaft and make a box to hold the JB-weld and make the split with wax paper... Let it get hard and open the box and split the nut... Been there... Done that...
Why not just mold your half nuts out of JB-weld.... Just greese up your shaft and make a box to hold the JB-weld and make the split with wax paper... Let it get hard and open the box and split the nut... Been there... Done that...
Lessee... You're telling some guy to grease his shaft, make a box, let it get hard, and split his nut. Are you sure this is appropriate for a family newsgroup? ERS
We make taps when they are not available. Unless this is a matter of pride or I don't know what, try to put a value on your time, eh? If you're willing to do this sort of work for $10/hour or less, you can come to work for me =)
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