GH3 class thread and 2B class thread-mutually exclusive??

[Neil]

Dear Forum,
It's been a long time. I am a design engineer and I was asked to use GH3 threads for an automotive bracket. I am only used to class 1,2 and 3 threads and 99.99% of the time simply designate them 2A and 2B and leave it at that.
I went to google school and learned that GH3 are ground threads with a larger pitch diameter. I read Machinery's handbook section on classes of threads but only classes 1,2 and were there.
I searched this forum and General forum and found nothing about GH3.
My question is this: are GH3 class thread and 2B class thread-mutually exclusive?? In my drawing can I specify a 2b class thread that is further refined by being a GH3 thread? The quoted text below from a CNC forum says I can and that GH is merely a type of tap (I thought GH3= ground threads!). What say you roomful of wise men and women?

"2b simply indicates an internal thread class, or size range. 3b is a tighter tolerance.

A gh3 is a size specification of the tap's pitch diameter...where a 1 is the smallest and a 5 is the highest, in increments of .0001", depending on the size of the tap.

If you need sloppy threads, you might specify a 2b, and you'd use a gh5 tap. You might also use a gh5 if you're going to plate the part and the effective pitch dia will get reduced."

 

[Gordon B. Clarke]

Dear Forum,
It's been a long time. I am a design engineer and I was asked to use GH3 threads for an automotive bracket. I am only used to class 1,2 and 3 threads and 99.99% of the time simply designate them 2A and 2B and leave it at that.
I went to google school and learned that GH3 are ground threads with a larger pitch diameter. I read Machinery's handbook section on classes of threads but only classes 1,2 and were there.
I searched this forum and General forum and found nothing about GH3.
My question is this: are GH3 class thread and 2B class thread-mutually exclusive?? In my drawing can I specify a 2b class thread that is further refined by being a GH3 thread? The quoted text below from a CNC forum says I can and that GH is merely a type of tap (I thought GH3= ground threads!). What say you roomful of wise men and women?

"2b simply indicates an internal thread class, or size range. 3b is a tighter tolerance.

A gh3 is a size specification of the tap's pitch diameter...where a 1 is the smallest and a 5 is the highest, in increments of .0001", depending on the size of the tap.

If you need sloppy threads, you might specify a 2b, and you'd use a gh5 tap. You might also use a gh5 if you're going to plate the part and the effective pitch dia will get reduced."

A 2B isn't a "sloppy thread". It is a standard internal tolerance UN thread. If no tolerance is specified on an internal UN thread it's ALWAYS 2B and 2A for external.

If you Google you can find just about anything you want or need to know about GH3 etc.



It's in mm but use 25.4 to convert to inches.

 

[Neil]

The quoted text is from another forum, sorry!
Here it is:
""2b simply indicates an internal thread class, or size range. 3b is a tighter tolerance.

A gh3 is a size specification of the tap's pitch diameter...where a 1 is the smallest and a 5 is the highest, in increments of .0001", depending on the size of the tap.

If you need sloppy threads, you might specify a 2b, and you'd use a gh5 tap. You might also use a gh5 if you're going to plate the part and the effective pitch dia will get reduced."

I always specify 2a and 2b since they are the most used normal tolerances, I myself do not consider them sloppy at all.

 

[Neil]

This reply above did not address "can I use GH3 in conjunction with class 2B?". I generally understand the difference between 1B, 2B and 3B threads which you addressed, but I really need to know how to apply GH3 alongside 2B since GH3 only appears to talk about the pitch diameter modification and nothing else. Thank you!

 

[Gordon B. Clarke]

"gh3 indicates a ground (g) thread tap that is . 0015" high/above/over (h) nominal pitch diameter. The class of fit obtainable from any given tap depends upon a great number of variables and can't be guaranteed by the tap specification alone."

What is wanted is an internal thread where the pitch diameter is slightly above nominal pitch diameter. As the pitch diameter tolerance on an internal thread is always larger than the external pitch diameter then that is a way to make sure there is an allowance. This would be most relevant for a 3A to 3B fit or to compensate thin plating.

Normally, with standard UN and metric threads, the nominal pitch diameter is the same as the lowest pitch diameter tolerance on an internal thread.

Metric threads, although more complicated than UN threads re tolerance make an allowance for this. Standard internal metric is H but G for example will give an allowance.

 

[Neil]

My limited understanding of this is that you want the RESULT to be a GH3 spec thread. My original question is can you call out 7/16-20-2B-GH3? Would that even make sense? Do those callouts work together and make sense? I understand it may take some experimentation with taps and deal with springback and other issues to get to GH3 or even 2B for that matter. Am I over simplifying this?

 

[Gordon B. Clarke]

Sigh. What are you designing? I'm pretty sure you weren't asked to make a GH3 thread. What you were probably asked was that it was to be made using a GH3 tap.

Re "Am I over simplifying this?" No, what you are doing is complicating the issue.

Talk to your customer.

 

[Gordon B. Clarke]

So what you're looking at, is confusion...
The GH3 is a tap that has been made (by grinding) a little "oversize" (effective/pitch diameter).
This would also be known as a pre-plate tap.
And depending upon what plating type, and thickness, in production you can easily tap/produce the thread "slightly large" by using this tap at no extra production cost (ie a tapped hole is a tapped hole).
Then when you get your parts plated, the oversize thread will be plated and reduce to correct ("normal class") size.
In theory...

So my guess is...you're being asked to produce a part with a larger thread, so when it gets plated, the thread then becomes standard?
But as always, If in Doubt - Ask!

I didn't have © on my posts so no problem