6-32 UNC-2B Threads are Tight after Cleaning before Painting?

[CarolX]

Do they form the thread or cut them ? I am going through a tight thread issue on some extruded parts since switching to a form tap. I have not figured it out yet but we are seeing .010 variation in minor diameter after threading even though we make a consistence drill hole. As you can see this is causing tight threads on some parts. Seems limited to blind tap holes for me.

Just off the top of my head - you may want to check with your tooling supplier - a form tap requires a larger drill hole to allow for material flow.

 

[Jim Wynne]

I've seen a lot of instances where it was possible to check a threaded hole with the fastener it was designed to mate with, thus bypassing all of the trouble with exact fit to thread gages. Sometimes a screw will fit just fine, and function perfectly, when a thread gage won't go or is excessively tight. Of course, care must be taken to insure that the method is valid, but oftentimes it is. If you can't make a reasonable performance prediction though, all bets are off and you're stuck with the gage.

 

[qcman]

Yes we drill a larger hole than you would when cutting threads. We actually found it necessary to go .002 under recommended size to prevent going oversize on minor diameter. We do not see the variation when cutting that we do forming however tap breakage is now small and no chips to deal with. I agree with Jim about the fastener working fine even when the gage is tight or even fails.

 

[QCAce]

Re: Threads are tight after cleaning?

My guess is that some debris is disturbed or otherwise mixes into the threads themselves during the wash process.

The other thing to consider is galling with the clean aluminum. Google the term "thread galling" or "cold welding" to get some insight.

Update.....
I did some research and found this about thread galling:
http://www.accuratescrew.com/TechTips/?TipNO=4

After reading this, I tried a test where we put a little lube on our thread gage and sure enough, it slipped in and out of the threads just fine. The No/Go side of the thread gage did not go, so I would guess if the threads were not correct, the Go side would not have fit either.

It makes a little more sense to me. The supplier is checking their parts with their thread gage at the machining station, where of course there would still be some nice, lubricating cutting fluid already in the hole.

Short-term I'm having a small bottle of lube available at Assembly.

Wes -- were you a mfg engineer for a screw manufacturer in a former life? Not sure how you know about all the different things you do.

 

[Wes Bucey]

Re: Threads are tight after cleaning?

Update.....
I did some research and found this about thread galling:
http://www.accuratescrew.com/TechTips/?TipNO=4

After reading this, I tried a test where we put a little lube on our thread gage and sure enough, it slipped in and out of the threads just fine. The No/Go side of the thread gage did not go, so I would guess if the threads were not correct, the Go side would not have fit either.

It makes a little more sense to me. The supplier is checking their parts with their thread gage at the machining station, where of course there would still be some nice, lubricating cutting fluid already in the hole.

Short-term I'm having a small bottle of lube available at Assembly.

Wes -- were you a mfg engineer for a screw manufacturer in a former life? Not sure how you know about all the different things you do.

"Not sure how you know about all the different things you do."
It's MAGIC!

 

[andygr]

Do not overlook the propert of AL to form a oxide on the surface soon after exposure. When welding accetone is a recomendation to remove the oxide.This oxidization process would be accelerated if exposed to temperature and moisture ( plus possably the additives in cleaning stations). Some form of protection based on the alloy to the threads soon after tapping plus some extra care in masking can help to minimze this condition.

 

[AndyN]

I've seen a lot of instances where it was possible to check a threaded hole with the fastener it was designed to mate with, thus bypassing all of the trouble with exact fit to thread gages. Sometimes a screw will fit just fine, and function perfectly, when a thread gage won't go or is excessively tight. Of course, care must be taken to insure that the method is valid, but oftentimes it is. If you can't make a reasonable performance prediction though, all bets are off and you're stuck with the gage.

Jim Wynne! How could you stand there, riding that bicycle and say such a thing! I'm amazed that one so learned would propose such heresy!

Which mating fastener are you going to use to 'accept' the threads?

Depending on the thread fit and the variation of the fasteners would have some influence on whether all the parts were good at assembly. I've personally had painful experience where someone proposed such acceptance of threads.

I'd only suggest qualified gauging be used..............

 

[Wes Bucey]

I've seen a lot of instances where it was possible to check a threaded hole with the fastener it was designed to mate with, thus bypassing all of the trouble with exact fit to thread gages. Sometimes a screw will fit just fine, and function perfectly, when a thread gage won't go or is excessively tight. Of course, care must be taken to insure that the method is valid, but oftentimes it is. If you can't make a reasonable performance prediction though, all bets are off and you're stuck with the gage.

Should this be placed in the Coffee Break Forum? This statement by Mr. Wynne is a statement which probably needs some smileys to indicate it is not to be taken seriously by a Quality professional, but is merely a weak attempt at injecting some humor into the thread.

 

[Stijloor]

Should this be placed in the Coffee Break Forum? This statement by Mr. Wynne is a statement which probably needs some smileys to indicate it is not to be taken seriously by a Quality professional, but is merely a weak attempt at injecting some humor into the thread.

Stijloor.

 

[56flh]

I have some 6-32 UNC-2B threads machined into an aluminum die cast part that are failing (too tight) from our supplier. The supplier is claiming that this is because of the wash cycle prior to paint. Has anyone heard of this condition? Any way to fix the problem other than re-tapping the holes after paint?

Haven't heard of wash cycle disrupting thread form unless its possible that corrosion may be taking place.

The part could possibly be run with a different pitch diameter tap.

Pitch diameters limits for 6-32 taps:
H1 .1177/.1182
H2 .1182/.1187
H3 .1187/.1192