Close tolerance thread fits?

[Terry Keeley]

In basic terms I want to cut an internal thread to closely fit an existing externally threaded part, probably similar to a Class 3B.

In the past I've single pointed the thread and just "cut and tried" until I got the desired tight fit. I've got a few more parts to do and was hoping to use a tap instead.

Without knowing the class of the external thread would I use a thread mic to measure, then (hopefully) find a (H rating) tap to match? Any other way short of having a tap ground?

This is for 1/4-32 in 2024 aluminum.

 

[Conrad Hoffman]

IMO, you really need to know the pitch diameter of the external thread. How many is "a few"? You might want to just keep on keeping on.

 

[Terry Keeley]

IMO, you really need to know the pitch diameter of the external thread. How many is "a few"? You might want to just keep on keeping on.

Ya, starting to see that now, have to know the external thread size to be able to match it (hopefully) with a tap.

Have a dozen to do now, couple at a time down the road.

 

[pavt]

Without knowing the class of the external thread would I use a thread mic to measure, then (hopefully) find a (H rating) tap to match? Any other way short of having a tap ground?

This is for 1/4-32 in 2024 aluminum.

Sounds like a ME thread (model engineering)
I think measuring the external thread and getting a tap with the proper H value is the way to go. You can probably do it with "off-the-shelf" components.

I have heard of "packing the flutes" with material on an undersized tap to gain a few more "tenths" of clearance if need by, but I am not up to speed on that practice.

 

[Milland]

I have heard of "packing the flutes" with material on an undersized tap to gain a few more "tenths" of clearance if need by, but I am not up to speed on that practice.

Could that be packing one of the flute gullets, to push the tap over a bit? I can't see how packing all the gullets would do any good (just seems like asking for trouble).

 

[Conrad Hoffman]

I'm an expert on asking for trouble and invariably my requests are granted! Ideally, the root and crest of the thread shouldn't touch anything, only the flanks. Thus, the PD is all that matters. 1/4"-32 is similar to stuff I've worked on all my career. Nice for adjustment screws and such. The thread depth is 0.0169". The PD is 0.2297" (MMC, basic thread). Because the thread is standard NEF, I'd just buy a few taps of various fits and try them. Cost shouldn't be that much. I like Besly- see page 17 of their catalog. https://www.besly.com/images/BeslySpecCat.pdf If the screw is a bit off, but consistent, they'll grind you any fit you like.

 

[pavt]

Could that be packing one of the flute gullets, to push the tap over a bit? I can't see how packing all the gullets would do any good (just seems like asking for trouble).

You're probably right, but like I say I'm not up to speed on that method.

 

[Terry Keeley]

I'm an expert on asking for trouble and invariably my requests are granted! Ideally, the root and crest of the thread shouldn't touch anything, only the flanks. Thus, the PD is all that matters. 1/4"-32 is similar to stuff I've worked on all my career. Nice for adjustment screws and such. The thread depth is 0.0169". The PD is 0.2297" (MMC, basic thread). Because the thread is standard NEF, I'd just buy a few taps of various fits and try them. Cost shouldn't be that much. I like Besly- see page 17 of their catalog. https://www.besly.com/images/BeslySpecCat.pdf If the screw is a bit off, but consistent, they'll grind you any fit you like.

Thanks for the resource, haven't heard of them before.

It might be a moot point since after searching around all I can find is standard H3 taps in 1/4-32 and I know from experience that fit is looser than what I want.

Not enough to do to have a tap ground unfortunately.

 

[magneticanomaly]

Aluminum expands a lot. Tap them in boiling watrer, see how the fit is when cool..

 

[D Nelson]

Use a form tap and use the drill size to get the fit you want
Don


Sent from my iPhone using Tapatalk Pro

 

[Terry Keeley]

Just discovered thread measuring wires, gonna p/u a set tomorrow: FISHER MACHINE,FISHER MACHINE THREAD MEASURING WIRE SET,1-811-131,KBC Tools & Machinery

A little cheaper than the Mitutoyo thread mic I was looking at...

 

[Conrad Hoffman]

Those are probably good. Don't buy the really cheap ones, especially for fine threads, as the surface finish may not be up to snuff. Try to grow a third or fourth hand while waiting for them to arrive.

 

[rons]

A fine thread in aluminum. I use Optimoly paste. The color is gray and it does tighten up a loose thread.

In the tap size you are dealing with it wouldn't hurt to buy your best guess for that tight thread and it would probably be good.
If you are not happy with the fit then a next size up or down. You can't lose.

 

[rogertoolmaker]

In basic terms I want to cut an internal thread to closely fit an existing externally threaded part, probably similar to a Class 3B.

In the past I've single pointed the thread and just "cut and tried" until I got the desired tight fit. I've got a few more parts to do and was hoping to use a tap instead.

Without knowing the class of the external thread would I use a thread mic to measure, then (hopefully) find a (H rating) tap to match? Any other way short of having a tap ground? This is for 1/4-32 in 2024 aluminum.

There is "no class of fit" on external threads.
External threads are "Always" made to the "Basic" dimension for the thread. The fit of the thread is made by changing the size of the internal thread for the "fit". Starting at the "basic" dimension and going smaller or larger .0005 at a time in the Unified Thread System. An H1 (H for high) is .0005 over the Basic thread dimension. If H2 +.001 over Basic dimension. Etc. If L1 (L for low).0005 below the basic dimension. Etc. A tap 1/4-32GH2-UNS The "G" a ground tap. "H" above the basic dimension. H2 is .001 above the basic dimension. If you obtain the correct tap, you will obtain the fit your require. Your description "close"? Putting trash in the flutes is not recommended unless you enjoy removing broken taps. The flutes are for carrying away the chips. Just trying to out guess you for the tap you need from what you said, order a 1/4-32GH2-UNS. A 2B fit. This thread size is in the Unified Thread System as an Extra Fine Thread. Use Kerosene as tap lube. Known as Lamp Oil at the hardware store.
Contact "Drills and Cutters" on the Internet for your tap. A good chance they will have it in stock.
All the Best
Roger

 

[Terry Keeley]

There is "no class of fit" on external threads.
External threads are "Always" made to the "Basic" dimension for the thread. The fit of the thread is made by changing the size of the internal thread for the "fit". Starting at the "basic" dimension and going smaller or larger .0005 at a time in the Unified Thread System. An H1 (H for high) is .0005 over the Basic thread dimension. If H2 +.001 over Basic dimension. Etc. If L1 (L for low).0005 below the basic dimension. Etc. A tap 1/4-32GH2-UNS The "G" a ground tap. "H" above the basic dimension. H2 is .001 above the basic dimension. If you obtain the correct tap, you will obtain the fit your require. Your description "close"? Putting trash in the flutes is not recommended unless you enjoy removing broken taps. The flutes are for carrying away the chips. Just trying to out guess you for the tap you need from what you said, order a 1/4-32GH2-UNS. This thread size is in the Unified Thread System as an Extra Fine Thread. Use Kerosene as tap lube. Known as Lamp Oil at the hardware store.
Contact "Drills and Cutters" on the Internet for your tap. A good chance they will have it in stock.
All the Best
Roger

This document says there are classes 1A, 2A & 3A for external threads:

https://www.natool.com/wp-content/uploads/2019/06/classes_of_fit_web_catp101-102.pdf

No mention of tap class limit size: 1/4"-32 UNS HSS Taper Tap | DWTST1/4-32T | DrillsandCutters.com

 

[rogertoolmaker]

This document says there are classes 1A, 2A & 3A for external threads:

https://www.natool.com/wp-content/uploads/2019/06/classes_of_fit_web_catp101-102.pdf

No mention of tap class limit size: 1/4"-32 UNS HSS Taper Tap | DWTST1/4-32T | DrillsandCutters.com

The external thread is always noted as an "A" and the internal thread is always noted with a "B". It is sometime hard to get our head around threading. The "basic" dimension is half way between the root and the crest of the thread and is the one constant. If we have a system that changes the fit with both the external and internal thread we there no good place to start for our fit. And it will drive you nuts. We need a place to start that is always constant. The fit is "Always" with the Internal thread. Order a "plug" tap. 1/4-32GH2-UNS HSS. Aluminum 2024T4 is excellent for machining.
Roger

 

[pavt]

I checked drills and cutters yesterday. They only have H3 and H5 in that size and pitch combination.

 

[atex57]

The external thread is always noted as an "A" and the internal thread is always noted with a "B". It is sometime hard to get our head around threading. The "basic" dimension is half way between the root and the crest of the thread and is the one constant. If we have a system that changes the fit with both the external and internal thread we there no good place to start for our fit. And it will drive you nuts. We need a place to start that is always constant. The fit is "Always" with the Internal thread. Order a "plug" tap. 1/4-32GH2-UNS HSS. Aluminum 2024T4 is excellent for machining.
Roger

Rogertoolmaker, you have this backward. The "constant" is the internal thread created by the tap, the H numbers being minor compared to the male thread minimum and maximum sizes.

An example is 1/2-13 2A, pitch diameter min is .4485 and max is .4435 for difference of .005 which would be 10 H numbers. That is a simplistic explanation but shows where to look for your tolerances.

Ed.

 

[Bobw]

The "basic" dimension is half way between the root and the crest of the thread and is the one constant.

No its not. Otherwise the outer diameter of a 3/8-16 would be around .410.. And ITS NOT!!!

That doesn't even work for the pitch diameter, because if it was halfway between the
crest and root, we would have a sharp V, and that would suck.

I'm not a thread GURU.. But the "BASIC" dimension is the maximum size of an external
thread. I know there are formulas somewhere for how they figured all this out, but
they also aren't a sharp V. Every external thread has a flat (per the specs), so
your pitch diameter isn't even half way between your major and minor.

No.. Just NO!!! That is just wrong.

 

[rogertoolmaker]

If that is the case why is the fit obtained by changing the tap size for the required fit?