Friction thread

[Mr Ron]

I want a tapped hole to have a friction fit with a screw or set screw. For example, if I want an 8-32 set screw to be a friction fit, I use a #30 tap drill in place of the recommended #29. I haven't done this in metal, but just in wood for a saw plate insert in a table saw. I used set screws to level the insert in the saw and didn't want the screws vibrating loose. Is this a legitimate process for metal? a set screw for a motor pulley for example.

[ChipMaker4130]

Not at all reliable, one size out of twenty might give you the result you want and you have a much better chance of breaking a tap. Loctite is much more dependable.

[earlgo]

Or a nylock type of screw. The type with the little nylon plug set in the side of the threads.
--earlgo

[Mr Ron]

Thanks for the quick replies. Can I achieve the same results by using a tap with a different class tolerance?

[Bill Shields]

Theoretically yes ..but in reality...no.

Use threadlocker or something that used to be called vibratite....reusable thread locker.

[Harold_V]

Is this a legitimate process for metal?

No. The H28 Handbooks specify that the pitch diameter is where interference is to be generated, not at the minor/major diameter. If memory serves, what you're talking about is a class 5 fit. I could be wrong, as it has been many years since I last read on the subject.

You might enjoy success in wood because it gives (compresses). In metal, you'd have greater difficulty in tapping, and most likely would just break taps instead of forming interference, as the minor diameter of the tap would be forced to removed any metal that might have been used to create the desired interference.

H

[GlennW]

Thanks for the quick replies. Can I achieve the same results by using a tap with a different class tolerance?

Taps are sized by "H" limits, not class of fit. You can achieve the desired class of fit of a tapped hole by choosing the proper "H" limit of the tap to accompany the male thread pitch diameter being used. (or come pretty close!)

http://www.tapmatic.com/tapping_questio ... imits.ydev

[Richard_W]

I want a tapped hole to have a friction fit with a screw or set screw. For example, if I want an 8-32 set screw to be a friction fit, I use a #30 tap drill in place of the recommended #29. I haven't done this in metal, but just in wood for a saw plate insert in a table saw. I used set screws to level the insert in the saw and didn't want the screws vibrating loose. Is this a legitimate process for metal? a set screw for a motor pulley for example.

I have used studs that have an interference fit. I don't know if they still make studs you can buy off the shelf with an interference fit on the short threaded end. We would use a stud driver to screw the studs in. What we did to make studs with an interference fit was to use a geometric die head. The geometric die head has a lever for a roughing in the threads, then you would run it again with the lever in the finish cut position. We would only thread with in the rough cut position. This would give us an interference fit on one end of the stud. Then cut the thread on the other end with both rough and finish cut. So once you drive the stud in with the stud driver then you could use an off the shelf nut on the other end. I have run into studs with an interference fit on the exhaust manifold on 350 Chevrolet engines. As I see it you would have to make your own over size pitch screws to do what you want.
I think you might try using 2 shorter set screws and locking the first set screw in and using the second one to lock the first one in place. Just a thought, since I don't have a clear picture of your application.

Richard W.

[Mr Ron]

I want a tapped hole to have a friction fit with a screw or set screw. For example, if I want an 8-32 set screw to be a friction fit, I use a #30 tap drill in place of the recommended #29. I haven't done this in metal, but just in wood for a saw plate insert in a table saw. I used set screws to level the insert in the saw and didn't want the screws vibrating loose. Is this a legitimate process for metal? a set screw for a motor pulley for example.

I have used studs that have an interference fit. I don't know if they still make studs you can buy off the shelf with an interference fit on the short threaded end. We would use a stud driver to screw the studs in. What we did to make studs with an interference fit was to use a geometric die head. The geometric die head has a lever for a roughing in the threads, then you would run it again with the lever in the finish cut position. We would only thread with in the rough cut position. This would give us an interference fit on one end of the stud. Then cut the thread on the other end with both rough and finish cut. So once you drive the stud in with the stud driver then you could use an off the shelf nut on the other end. I have run into studs with an interference fit on the exhaust manifold on 350 Chevrolet engines. As I see it you would have to make your own over size pitch screws to do what you want.
I think you might try using 2 shorter set screws and locking the first set screw in and using the second one to lock the first one in place. Just a thought, since I don't have a clear picture of your application.

Richard W.

I don't have a particular application in mind. Just thinking out loud.

[SteveR]

Couple of questions:
1) Why the special name of "friction thread" or is it that you just want a tighter fit of the bolt to the hole?
2) How deep is the hole you are threading? IIRC, Machinery's HB gives drill sizes for just that depending on the class of fit and hole depth. It's been a while since I've checked. TLDR - short hole, drill smaller, long hole drill larger.
2A) You said

if I want an 8-32 set screw to be a friction fit, I use a #30 tap drill in place of the recommended #29

. I had a similar problem with a shallow (5/16 deep) hole for 10-32 threads. The drill for 10-32 is #21, but I thought the threads looked shallow on a test part that I cross sectioned so I redid with a #23. Much improved. Very careful tapping.

Not the best cross sections, but enough to make the point:

10-32 w/21 drill

10-32 w/23 drill

3) Have you considered using back to back setscrews like they use on Bridgeport mills?
4) And there's always 1-800-Loctite.

Thanks,
SteveR

[Harold_V]

I had a similar problem with a shallow (5/16 deep) hole for 10-32 threads. The drill for 10-32 is #21, but I thought the threads looked shallow on a test part that I cross sectioned so I redid with a #23. Much improved.

I'd suggest to you that the drill you used cut oversized. Recommended tap drills are usually in the vicinity of a 70% thread. A used drill, or one that is not properly sharpened tends to yield oversized holes, often bell-mouthed as well.

If you wish to achieve properly sized holes when drilling for tapping, I suggest you double drill the holes. Start with a twist drill that is a couple sizes smaller than the target diameter, then open the hole with the desired drill. It's highly unusual for that procedure to yield an oversized hole.

Use caution when drilling some of the copper alloys, as the drill tends to self-feed when opening the hole with the second drill.

When working at Sperry Utah, we were required to double drill all holes, to assure proper size.

H

[Mr Ron]

Couple of questions:
1) Why the special name of "friction thread" or is it that you just want a tighter fit of the bolt to the hole?
2) How deep is the hole you are threading? IIRC, Machinery's HB gives drill sizes for just that depending on the class of fit and hole depth. It's been a while since I've checked. TLDR - short hole, drill smaller, long hole drill larger.
2A) You said

if I want an 8-32 set screw to be a friction fit, I use a #30 tap drill in place of the recommended #29

. I had a similar problem with a shallow (5/16 deep) hole for 10-32 threads. The drill for 10-32 is #21, but I thought the threads looked shallow on a test part that I cross sectioned so I redid with a #23. Much improved. Very careful tapping.

Not the best cross sections, but enough to make the point:

10-32 w 21 n1.jpg
10-32 w/21 drill

10-32 w 23 n1.jpg
10-32 w/23 drill

3) Have you considered using back to back setscrews like they use on Bridgeport mills?
4) And there's always 1-800-Loctite.

Thanks,
SteveR

I've tapped holes in wood using a one size smaller drill in order to get a friction (tight) fit so a set screw won't vibrate loose. It is used on insert plates for circular saws. Works fine; just wondering if the same procedure works in metal.