Pilot hole size tolerance for 10-32 threads

[CanosSSCS]

Hello,

I'm getting an extruded part in Aluminum, and I've set a pilot hole for a 10-32 tap at 0.159" +/- .010. The extruder is having issues meeting this spec, and requests to increase the tolerance by .006" bilaterally. Should I be concern that at the high end the extruded hole would be to large to tap for a 10-32 male thread? the new dimension would be 0.159" +/- .016". I'm inclined to say that this shouldn't be a problem but do appreciate any advise.

Thanks.

 

[powerhound]

To clarify, you're actually having the hole extruded and the supplier wants .006 more tolerance on the hole size? When you say "bilaterally", that implies location to me, not size.

If this is the case of size tolerance, I certainly wouldn't recommend -.010 much less -.016. Good luck tapping 10-32 in a hole that is .143 in diameter.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[mfgenggear]

CanosSSCS

In according to ASME B1-1 Unified Inch Screw Threads minor diameter for 10-32 unf 2B thread should be .156-.164 diameter for full thread strength.
the larger the the thread minor diameter hole because of force required to torque there is possibility of stripping the threads. especially aluminum.
edit: and if the hole to small it will be harder or not possible to tap. jamming the tap causing breakage.

 

[3DDave]

Is this a cutting tap or a forming tap? Nominal cutting tap pilot - 0.159; Nominal forming tap 0.177

The spec minor diameter, usually set by the pilot hole diameter, is 0.156 to 0.164 for a 2B thread, so the small end will likely break a cutting or a forming tap and the high end will not conform to the hole thread standard for the cutting tap.

That is a lot of variation in such a small feature - 10%?

 

[mfgenggear]

Good point 3DDave if it is a forming tap the starting minor diameter is specified by the forming tap manufacture. because it has to be larger to extrudes to
the proper minor diameter after forming, but must hold minor diameter from the specification.

 

[drawoh]

CanosSSCS,

I have Machinery's Handbook 26th Edition here.

For tapped holes with depths/thickness of up to 1/3D, they recommend [⌀].160/.156. For tapped holes of 1-1/2D to 3D, they recommend [⌀].1661/.1621. That sort of gives you a range of [⌀].1661/.156. Using as[‑]extruded holes for tapping does not sound like a high[‑]strength strategy. Can you change to a larger screw?

A coarse thread would be more tolerant of an oversized tap drill/hole.

--
JHG