Threaded features as Datum's

[Madhu454]

Hi All,

I am very new to GD&T and started learning it by reading books and refereing to GD&T drawings. I have a doubt on using the threaded feature as datum.

When threded features used as a datum feature, the default is the pitch diameter as the datum. when the datum referes to either major dia or minor dia we need to mention this on the drawing.

Question is
1) Under What functional requirements we need to go for PD, Major Dia, Minor Dia as datum feature?
2) How to simulate the datum, when the Minor dia is used as the datum feature in external thread.
3) How to simualte the datum when it is applied to Pitch dia?
4) How to simulate the datum when the major dia of external thread is used as datum feature.

Please see the attached drawing where minor dia(LD) is used as datum feature A, what if we use major dia as the datum. which one is dificult to measure? what is the differnce it makes to the final product.

Please can anyone guide me on this?

Madhusudhan Veerappa
Mechanical Engineer

 

[ptruitt]

I don't think you necessarily need any datums at all to comply with ASME Y14.5-1994 or -2009.

Peter Truitt
Minnesota

 

[drawoh]

Madhu454,

A good, quick and dirty rule about datums is that you should study them, and ask yourself how you will fixture to them.

The major diameter of a male thread can be picked up by a hole. I believe that if you screw something onto your thread, you are picking up the pitch diameter. Getting at your minor diameter will be a challenge. On a female thread, the minor diameter is easily picked up by a pin, and the major diameter is inaccessible, or at least, very hard to get at.

Check your handbooks on screws, and look at the tolerances on threads. They are surprisingly loose. Threads are not ideal datums if you require accurate tolerances.

On your example, I would use one face as datum A, and make the second face parallel to it. Use the thread minor diameter as datum_B, and use this to position the outside diameter. You can specify the perpendicularity of your thread to datum_A.

An important trick when designing any lathe fabricated part is to ensure that all the critical dimensions can be done on one setup. Then, they should be very accurate. Your biggest error will be your second setup.

JHG