Re: (Acme) 29deg stub thread 9TPI - Thread dimensions & tolerancing and root dimensio
Hello all I am back and I have a question. I have a customer print that is calling out "2/7 Pitch x 2/7 Lead left handed thread 29 degree acme thread" can anyone tell me what this means. Thanks in advance. Dean
Dean;
There are several parts of this, and it also appears that some parts of the thread designation are missing.
ACME threads are defined in ANSI/ASME
B1.5 with STUB ACME defined in ANSI/ASME
B1.8.

The specifications encourage ACME threads to be designated in inches and threads-per-inch. The specification defines specific classes-of-fit for best operation of the ACME thread.

The normal thread designation for an ACME thread includes:
[nominal major diameter]"-[threads-per-inch] ACME-[class-of-fit][other optional modifiers].
Where the customer's print states: "29 degree acme"; it is being redundant. In the world of thread designation ACME = 29? included angle. The preferred designation would be just ACME except many engineers make-up their own ACME thread parameters even changing the 29? included angle.

This adds to the confusion, which is why many people designate the 29? on the drawing with the word ACME. So, in this case, we know for sure that they intend to use the standard ACME form.
The customer print statement "2/7 Pitch x 2/7 Lead" is very odd

and seems at best poorly stated, but you need to get more data from your customer.
First and again; the print is being redundant because when pitch = lead only pitch is stated.
Second; the pitch is usually stated in threads-per-inch (TPI). Many times on European drawings TPI is stated as pitch in mathematical formula. Example 10-TPI would be stated 1/10. If you do the math on 1/10 (one divided by 10) you get 0.1" pitch which = 10-TPI. (A side note; some people may say it is: "10 pitch". While this is functional for communication, it is poor practice and should be avoided).
Third; taking the drawing at face value the 2/7 = 0.2857 pitch = 3.5-TPI. This is a common-ish TPI.
Fourth; Because the customer was so specific to include the 2/7 twice and the use of the '2' instead of a '1' leads me to believe that the customer may have intended this to be a 2-start (double lead) thread, but just does not understand the correct screw thread engineering nomenclature.
Where the customer's print states: "Left Handed Thread" I am sure he means what he says.

ACME screw threads are often Left Handed. Normally this is just designated as LH following the class-of-fit.

Things which are missing in this designation are the nominal major diameter (which I expect is dimensioned elsewhere on the drawing), and the class-of-fit which determines the location and tolerances for the Major Diameter; Pitch Diameter; and Minor Diameter.
You need to go back to your customer for clarification on his requirements

because there are too many ways to interpret this designation, and it is very incorrect per B1.5.
Here is a link to a tool which may help with the thread parameters once you have gained answers from your customer:
ThreadTech Free 30-Day Trial.
Other links which may be helpful:
The World of ACME Screw Threads
General Purpose ACME Screw Threads