Loose threads for 6-32 taps

Dave99 fired this volley in news:974c16a8-21b8-44a9- snipped-for-privacy@q39g2000hsf.googlegroups.com:
It's probably the tap, unless it's a very high-quality one. It could be dull. Even more likely, you're overdoing it, and not clearing the chips often enough. They'll make even a good tap cut oversize. And 6061 is just gummy enough to "gum up the works".
However, the grind on your drill bits is every bit as important. If the tip is the slightest amount off-center, or if one cutting edge is doing all the cutting, you'll end up with an oversized hole.
For critical threading (especially small sizes), I'll usually run a new (good) tap through a couple holes with a little medium polishing compound, to remove any burrs.
If you want really accurate threads, you might consider a thread forming tap, instead of a cutting tap. Because there are no chips to interfere with the working surfaces, the holes end up tapped to exactly the dimensions of the tap.
And of course, keep your tap perfectly aligned to the axis of the hole, or you'll cut with difficulty and cut oversized.
LLoyd

You might want to check the actual screws going into the holes. If they are coming from China, etc, they may be the problem. I have found socked head cap screws to fit much tighter than off the shelf machine screws.
Paul

Good advice already. Note that the tap drill tables you usually see are for a 75% thread anyway, so going one number drill smaller sure isn't going to hurt as long as you can get the tap through. There's only about 2 1/2 thou difference between #36 and #37. Are you using a "gun tap"? This kind "shoots" the swarf ahead of the tap, so there shouldn't be too much to clean out. I agree that a little troubleshooting is in order, ie: measure screws, check tap and drill bit for size and sharpness, centering of drill point, etc.. Are you using a lubricant? I use diesel fuel, since it's handy and similar to kerosene. You need to keep galling to a minimum. Is it possible that you are using a bottoming tap instead of a plug or taper? That might mush up the threads somewhat.
Pete Stanaitis -------------------
Dave99 wrote:

Try some smaller bits if you wish and see if things change. I often drill with smaller drills than for 75% threads, sometimes going right to 100% threads. I know it's unnecessary, and wouldn't be tolerated in a production environment, but if it makes me feel better, so what? It's been a long time since I've broken a tap. This is all hand tapping, though. You do have to be careful, and feel what the tap is telling you.
That said, 6-32 is a tough one. The threads are quite coarse for the diameter, giving a root diameter to nominal size ratio that is lower than for most other common screw threads. It's weak. Go carefully.
You may find that the drill size has little effect on the fit of your threads - it could be undersize fasteners or the tap itself. You might try an H2 instead of an H3 - you should be able to find both in 6-32 without any trouble.
John Martin

Thanks for the help... Upon further investigation... I examined a few different screws by holding them up to the light in front of the holes before tapping. I wasn't able to see light on any of them. If anything, it looked like the #36 holes might actually be a hair smaller than the shank of the screws. So it must be something in the tap process. As it turned out, I'm using screws with small nylon locking strips. I found them at a local surplus and they're pretty handy little things. The addition of the nylon actually makes it workout pretty well in the end.
But since I'm on the subject... What is the best process for dressing threaded holes that have been made in round stock? I'm putting these 6-32 holes through the sides of 1/4" and 3/8" round stock. Then those pieces go in holes with tight tolerances. If there's a little lip remaining on the edge of the holes from the tap, it sometimes prevents the round from fitting in the hole. What I've been using is just a countersink on each side of the hole. That cleans it up a bit and kind of makes an oval shaped opening. The down side is that it can make the start of the thread a little screwed up, so it's not always easy to start the screw. This isn't a problem for me, but other people assemble these parts.
Thanks,
Dave

Dave99 fired this volley in news:59b1900a-5a1f-4698- snipped-for-privacy@p69g2000hsa.googlegroups.com: Were it me, I'd counterbore the tapping hole before tapping. A good, sharp counterbore won't leave shucks for a burr.
The other rational approach would be to mill a small flat on opposing sides before drilling. This has the added advantage of allowing you to spot the drill dead on center, since you can drill with the same setup used to mill the flat.
LLoyd
LLoyd

Dave,
One thing you might consider is to use a finer thread than 6-32.
The 6-32 thread is a kind of force fit, not leaving much on the ID of the screw. It is by far the most often broken tap and the weakest screw for this reason.
Since it is aluminum, if you have to use 6-32 I like the idea of the thread form taps.

Thanks for the tips... For some reason I never even thought to countersink first. I suppose I could try 4-40. My thinking was just that, it being a softer metal, that a larger thread was the way to go.
DD

Dave99 fired this volley in news:c54a1902-a0fe-433f- snipped-for-privacy@i7g2000prf.googlegroups.com:
Not "countersink", Dave; Counter_bore_. Different tool.
LLoyd

You could use either 5-40 or 6-40 also. They are standard taps. the finer thread makes it easier to tap also. ...lew...

This is Al isn't it. Seems to me that a finner pitch tap would drill a hole. The force on the soft metal would tear out the threads as it went.
The idea of fine pitch was to have multiple - 3 or more - into the metal. e.g. sheet or small brackets.
What is the real facts - please.
Martin
Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net TSRA, Endowed; NRA LOH & Patron Member, Golden Eagle, Patriot's Medal. NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member.
Lew Hartswick wrote: