Seems I'm all screwed up.

[Susquatch]

The last time I had a major screw thread pitch size problem (that I remember) was with the height screw on a BXA tool holder.

I hit a new one today. It makes no sense. I can't make heads or tails of it.

Here is the hairy beast that is causing all my angst:



It is supposed to be an MT3 with a 3/8-24 screw post on it. You can even read that on the arbour if you look closely. The MT3 part is fine. The 3/8-24 part is all F'd up.

Here are the pertinent dimensions:

OD - 0.343" 8.71mm
Thread Pitch
26 excellent fit
1.0 poor fit (closest I have)

A 3/8-24 nut will go on it but not without mangling the threads near the MT3 taper as the nut starts cross threading.

So what the heck is it?

My guess is that it's the product of a CNC programmer somewhere in a developing nation who is metric and is trying to make imperial parts without a single clue what he is doing. I'd love to be wrong.

 

[Susquatch]

In case anyone is wondering, I'm making a universal arbour for my rotary table which has an MT3 center. I'd like a fine thread 3/8 stud at the center to use as a center pivot location for future arbour bushings of various sizes. The stud will also help remove the MT3 taper by acting as a Puller.

 

[Tom O]

Is it used it looks like someone used a slitting saw or something and stripped the thread with a nut.

 

[RobinHood]

An out of the box idea: maybe the 3/8-24 reference is the internal thread on the opposite side -to be used for a drawbar?

 

[Tom O]

Or only 3/8 of the thread is 24 tpi.

 

[RobinHood]

Or only 3/8 of the thread is 24 tpi.

Good one Tom!

 

[Susquatch]

An out of the box idea: maybe the 3/8-24 reference is the internal thread on the opposite side -to be used for a drawbar?

That's the very kind of thing I'd have gone "dooh" over! But no, the drawbar thread is a standard course thread 3/8-16.

Good thinking though!

 

[Susquatch]

Is it used it looks like someone used a slitting saw or something and stripped the thread with a nut.

I did that by cross threading the 3/8-24 nut that it said should be on it. My bad, but in my defense, it did start, the nut was as labelled, and the difficulty was expected as I was pulling the taper out of the female taper in the rotary table. It wasn't until I removed the nut and saw the mangled threads that I realized something was seriously wrong.

 

[RobinHood]

Well, now you get to fix the arbour. Cut off the stud, drill and tap for a 3/8-24 stud (or whatever thread you prefer). Hold it in the RT‘s female socket on the mill.

 

[Tom Kitta]

I actually have a decent number of chucks that have a thread a the back and I will need to get MT3 or similar and do a thread... I was planning to machine these on a lathe from stock.

 

[Arbutus]

It looks like the threaded part is not hardened. Could you bore out the arbor and install a new (hardened) stud with the correct thread?

 

[Tom Kitta]

It looks like the threaded part is not hardened. Could you bore out the arbor and install a new (hardened) stud with the correct thread?

Maybe - but this has to be done precisely - i.e. you do not want the chuck to have too much of a runout. I think biggest problem with a thread on is that a taper is more precise then even well done thread. I am sure Dabbler knows more about this than me.

 

[thestelster]

What?!....you've never heard of the #22-26 thread, also know as 11/32"-26? It's mainly used for the screws that secure the riding crop holder on rickshaws in Lower Slobobia.

 

[Susquatch]

Well, now you get to fix the arbour. Cut off the stud, drill and tap for a 3/8-24 stud (or whatever thread you prefer). Hold it in the RT‘s female socket on the mill.

It looks like the threaded part is not hardened. Could you bore out the arbor and install a new (hardened) stud with the correct thread?

I just bought it. I plan to return it unless they suggest I keep it. If I keep it, I may do exactly that.

I suppose I could also turn it and make it a 1/4-28.

Anyway, my big question is answered. Nobody knows what it is. I will tell the seller that in an effort to help them fix their problems. I'll also order something else to replace it.

 

[whydontu]

I love a challenge. 3/8”-26 60* Cycle Engineering Institution thread, DIN 79012

Thread Data Charts

www.ring-plug-thread-gages.com

 

[Susquatch]

I love a challenge. 3/8”-26 60* Cycle Engineering Institution thread, DIN 79012

Leave it to you to find something like that! Too funny!

Now if you can just find the right diameter to go with it....

 

[slow-poke]

I have a small bucket with a handful of yet to be determined bolts from Asia, not SAE, not metric, they are who knows what? They are close to a standard thread pitch, but then the error accumulates as you move down the thread.

 

[mbond]

It looks more like a mistake than a metric / SAE mismatch. When the thread pitch is misaligned, there should be a short region where the threads are fully compatible (how long depends on the tolerance) followed by a progressive increase in the stiffness of the nut (and the corresponding destruction of the threads). You didn't show the nut that you used, but there seems to be a sudden change in how the threads look. There isn't much length, but they don't seem to look worse further on

 

[Susquatch]

It looks more like a mistake than a metric / SAE mismatch. When the thread pitch is misaligned, there should be a short region where the threads are fully compatible (how long depends on the tolerance) followed by a progressive increase in the stiffness of the nut (and the corresponding destruction of the threads). You didn't show the nut that you used, but there seems to be a sudden change in how the threads look. There isn't much length, but they don't seem to look worse further on

I agree. That sudden transition bothers me too. I suspect that it is an artifact of trying to use a 3/8-24 threading die to fix the damage done to the threads by the nut I first used or that it's a consequence of the way the thread mismatch accumulates over the course of tightening the connection.

Years ago, I did an analysis of what happens when you deliberately mismatch a female 22x1.6mm 55deg thread face with a 0.858-16 60 degree male thread.

The results were very interesting. The two will fit for a certain length, but the difference stacks up as the overlap gets greater. Sooner or later they start to jamb and the threads starts to deform and become damaged. However, it happens at both ends not just one end.

I suspect that is the case here too but the damage at the top end is in the nut while it's on the stud at the bottom end. I don't know why that should be or if it is even true,

Unfortunately, I either threw the nut away or lost it so I can't check that. However, I do agree that it shouldn't look like this one does without an equal but opposite damage inside the nut or a corresponding length of damage at the start of the thread or some combination of the two.

 

[mbond]

I have a small bucket with a handful of yet to be determined bolts from Asia, not SAE, not metric, they are who knows what? They are close to a standard thread pitch, but then the error accumulates as you move down the thread.