Clamps in checking fixtures used to drive datum features.

O

OmarEn

Hi guys,

I have a question, hopefully, you could help me:

In our checking fixtures, when we have any part surface called out as datum feature or datum target, we used to install clamps at the opposite side to push the surface against its counterpart in the gage and have maximum possible contact. But one customer has been questioning about these clamps arguing that they force the part to be in-tolerance, and they should be removed.

I have seen this made this way from years in different companies, but I cannot find where in the Y.14.5 Standard states that the use of these clams. Could you guys please help me with this Issue?

Thanks in advance!
 

Stijloor

Leader
Super Moderator
Hi guys,

I have a question, hopefully, you could help me:

In our checking fixtures, when we have any part surface called out as datum feature or datum target, we used to install clamps at the opposite side to push the surface against its counterpart in the gage and have maximum possible contact. But one customer has been questioning about these clamps arguing that they force the part to be in-tolerance, and they should be removed.

I have seen this made this way from years in different companies, but I cannot find where in the Y.14.5 Standard states that the use of these clamps. Could you guys please help me with this Issue?

Thanks in advance!

There is a general dimensioning rule in ASME Y14.5-2009 [(Page 8 (m)] that states that all dimensions apply in a free-state condition. However, if it's necessary to inspect the part in a fixture (datum target simulators in your case), then a restraint note must be placed on the engineering drawing. The restraint requirements are on Page 79, Par 4.20 and an example in Fig 4-42 on page 81. There must be a note and a possible separate instruction on how and with what force (torque) the part must be restrained.

Hope this clarifies.


.
 
Last edited:
O

OmarEn

Thank you guys, excellent answers.

These drawings do not have such restraint notation. Therefore my part is considered rigid.
I think I can not make anything to solve this Issue and give an answer to my customer that can help me.
 
B

Bruser

A question is how is the part used? is it restrained in any way when in the "in use" position? is it a sub-component of another assembly that is clamped together in assembly?
an alternative to hard clamping is using spring loaded clamps that provide a fixed amount of pressure on the part, this eliminates putting "extra" pressure on the part to make it conform to the fixture, only applying enough pressure to keep the part from slipping.
 
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