More tapping problems

I brought this up once before and got a lot of good advice. The original problem was tapping 6-32 holes in side of 1/4" aluminum
round. Each piece has one hole. I use to do this by drilling the holes on the press, then tapping with a power drill. Sounds primitive, but it worked... On hundreds of them, no problem. Then it stopped working and I never did figure out why. I can't get through 15 without breaking a tap now and I've tried everything. The first time the consensus seemed to be that it would be best to use a combo drill/tap bit on a tapping head. I agree, except, I never got around to it. The need for these parts hadn't happen since the last time... but I decided I would do it that way when it comes up again. Well, it came up again... Except this time the rounds already had the #36 holes drilled in them... Hundreds of them. So I have no choice but to tap them as is. Long story longer, I went through about $50 worth of taps today just trying to get through 40 of them. And even just that took a ridiculous effort.
So now what? I haven't tried it yet, but I'm thinking maybe opening the holes to 7/64" would help? Or would that weaken the thread too much? I don't know, I'm pretty much desperate for a solution at this point. I've tried every type of tap, tap fluid, technique... I'm out of ideas.
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Dave D wrote:

Bypassing obvious questions but one:
I understand these came with the hole already in them, so maybe the supplier used a different grade of AL than you used the first time.
and not so obvious, are you holding them differently, maybe squeezing them tighter, or out of round than the first time?
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Thanks... To my knowledge it is 6061. I don't really have a way of knowing more than that. That's what they were supposed to be before, and that's what they're supposed to be now. The way I do them now is exactly as I did them before, but I have changed everything just to see what the problem is, and nothing seems to work. I clamp them in the same way, don't put pressure where the hole is going... Hold the drill the same way. Tried going fast, tried going slow... I tried by hand and the tap seems tight to me, but maybe that's me. I did check the holes and they are for sure #36. I could use one of those hand turn jigs where it lines it up, but that still wouldn't guarantee the hole is clamped straight, plus it would be brutally slow. I have no idea how I use to go through hundreds of these no problem. But however I did, I'm stuck with these. The only thing I can think to do is drill them out with a 7/64, but I'm afraid that would weaken them.
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Is this stock more or less "gummy" than previously?---
Could it be that the heat treatment has changed from earlier lots where things worked well? You never know what the purchasing dept might have done.
Have you tried tapping some holes in some totally different pieces of 6061?
See: http://www.galcit.caltech.edu/~tongc/html/data/elastic/Extruded_Alloy_6061.pdf to see all the heat treatment options.
Pete Stanaitis -------------------
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Did the problem start in the middle of a successful run, after changing a tap?
This happened to me, rigid tapping in a VMC. Everything was fine, except after someone broke the tap, and the new tap tore up the threads, no matter what we did.
The consensus was that the first tap had a coating or finish conducive to clean threads in 6061 Al. With the new tap, the coolant in the vmc was not adequate, and the solution was manually coating the tap between cycles with tap magic. There were other tapping fluids people really liked, forgot what they were -- Moly D might have been one.
You need to develop a sleuthing strategy -- mebbe try tapping other sized holes -- 5-40, 8-32, see if you have similar problems, which would indicate a material problem. You should get a feel for manually tapping a hole, to try to assess if the hole is "right", by how hard the hand process feels. Mebbe the material changed unbeknownst to you, got gummy/shitty.
What's nice about tapping heads is that you can peck, with the safety of a clutch. Speaking of which, mebbe a cordless drill with a decent clutch should help your breakage problem, in the meantime.
But a tapping head would hedge a few bets at once.
--
EA



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If the holes that you're tapping are off the centerline of the 1/4" round, that could explain why you're having problems. (One side of the hole could be slightly longer than the other side and might force the tap to bend or bind.) Could you sand or machine a slight flat before tapping the hole?
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The off center thing might be an issue I hadn't thought of. But they're as accurate as before, so that doesn't explain the huge breaking problem now and not before. It may be a difference in the aluminum like mentioned.
What if a drilled them out to 1/8" and used a form tap? Those are much stronger, aren't they? Would I have to counter sink again after the threading doing that?
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At least you won't have the aluminum tearing with the form taps. I'm not sure if countersinking will help a lot (unless you go deep). It doesn't seem that you have much meat on the sides of the holes. You still might have to do some deburring depending on what is expected for this part.
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Do you think there would then be too little material on the side, so deforming could be an issue?
I'm still wondering if using a 7/64 hole with the cutting tap might loosen it up just enough. I would think that would still give a decent thread %.
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I was taking to someone about this, and they said the way they did small round, was to lay them in a half circle bed that is slightly wider than the stock, and that way it allows it to adjust itself to the tap as it goes in. I'm looking at a tapping head, but the machine I want to dedicate to it doesn't have the separate arbor... so I'm not sure if the built in arbor would fit the head. I'm wondering if it would work to use a straight shank arbor on the head and then put that in the press chuck?
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I always like to try the easiest solutions first. Opening them to 7/64" would work, but first check to see if the shank of a #36 drill fits the existing hole. Maybe the hole got distorted somehow (metal relaxed, bumping around, cut-off operation?). Maybe redrilling them with the #36 drill bit will solve the problem.
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wrote:

Wouldn't that work harden the surface too maybe making it harder to tap?
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I think that the OP was working with aluminum (6061), so I don't think that he should have this problem. It's more common to see work hardening problems with stainless steels.
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wrote:

A lot of aluminum alloys work harden too. 5052 sure does. That is one of the two reasons its very important to blow or wash chips clear when machining. The other is that the oxidized coating is harder than the base metal.
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wrote:

Which brings up another question. How long are your aluminum pieces sitting between processes? 5 minutes? 5 hours? I don't know about from a machining perspective, but from a welding perspective that is a huge difference.
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Thanks for the PDF Dan, I'll check it out. Since the hole is twice the screw width, I think it should be OK. I could probably even do #35. I did a stress test and I couldn't get it to strip by hand.
If the work hardening thing is true, that could be something. I don't know how they drilled these... But on the last batch that I drilled myself and started having problems, I believe I did run the drill faster than other times. But not even up to recommended speed though. But maybe it still made a difference. There was probably more heat. There's considerable time between drilling and tapping, but there always has been.
I checked all the bits today and they all seem to spec. The #36 shaft does fit back in the finished hole fine. I drilled out some of them to 7/64 then tapped by hand to see what it felt like. It was easier, but I still felt like it would be likely that they would break fairly easily. It gets that clogging feeling where it stops and you need to back up a bit. That was on a spiral gun three flute. I'm using that because it seems stronger. Helical taps seems to go through easier over all without the binding, but they are weaker overall.
The only thing left is just the fact that it's 6-32, which I know is not a favorite. But it is what it is. I can't believe changing the thread size would be the difference between all the breaks and no breaks. I still think there's something inherently wrong with how we're doing it.
So has anybody ever tried using a tapping head with a straight shank arbor chucked directly in the drill chuck?
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I really think you've got a problem with the alloy. I've never had a problem with 6061-T6 work hardening to the point that it made tapping difficult. 6-32 is definitely the nastiest thread size for tap breakage. I recently made a simple bracket out of some scrap 1/8" aluminum panel material. It was HORRIBLE stuff to work with. The aluminum was chewy, and stuck instantly to the drill bits. The chips were stringy, and clogged things up terribly. I sincerely doubt I would have been able to tap a 6-32 thread in it without fits of one sort or another.
Doug White
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Yes it works fine, just run the drill press at a slow speed. I seem to recall the setup requiring an extension to the Tap-Matic to keep it from spinning.
You might try getting a custom tap made that has a longer than normal taper so the tap does not have to remove as much material in each pass, yet you still end up with a 75% thread.
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Roger Shoaf

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Thanks... I think I'm going to try that. Someone I talked to says he thinks it is due to side forces, which will be eliminated with a taping head. I hope so. But it still feels like it's catching when I do it by hand though, so we'll see. It may very well be the aluminum.
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    Just out of curiosity -- is it possible that someone handed you a batch of titanium rods instead of aluminum? I would expect serious tapping difficulties with that.
    And, BTW, I would prefer a two-flute gun tap to a three-flute one, especially in 6-32 size.
    Good Luck,         DoN.
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