Restraining Plastic Part 1

[Rubicon500]

I have a question in-regards to restraining a plastic part using datum targets.

I've specified a .5 kg force to restrain a plastic part on 4 datum targets. I believe when stating that a part needs to be restrained you need to specify a magnitude of the restraint section 7.20, 7.20.1 Y14.5-2018 (Torque, Force etc..)

I'm getting feedback from our quality and vendor saying that they cannot measure the force, as they just use standard metrology clamps to retain the part on the CMM table.

Does anyone have knowledge how a part clamping force is determined in standard practice?

On another note, does anyone know a consultant in Ontario, Canada that can be used to use as an expert opinion on Y14.5-2018?

 

[chez311]

I'm getting feedback from our quality and vendor saying that they cannot measure the force, as they just use standard metrology clamps to retain the part on the CMM table.

I would think a simple hand calculation to verify that the clamps utilized provide the restraint force specified within a reasonable approximation would be sufficient. I would be surprised if very many people had a force measurement/feedback in their metrology setups unless extremely precise verification of force was required but I could be wrong. Of course with fasteners this is much easier with the help of a torque wrench and/or torque/angle measurement however even that is an approximation relating torque (and possibly angle) to linear force.

If you are going by Y14.5-2018 I would note some of the changes/additions to the "restrained condition" section 7.20 - this is probably the default setup in most cases anyway with datum targets, but with the "entire surface" specification (b) that might require some additional thought - ie: not just say it needs to be restrained with X force on Y surface and leave it up to the metrology department to place clamps wherever they see fit because the standard requires in lieu of a specification the force be applied over the entire datum feature.

(a) When datum targets are specified, the restraint load
shall be applied over each datum target, normal to the
surface at that location and the same size and shape as
the datum target UOS. In cases in which restraint
applies to a datum target line or area specified on a
surface with more than one normal vector, the direction
of the restraint load should be specified.

(b) When the entire surface is specified as the datum
feature, the restraint load shall be applied over the entire
datum feature, normal to the true geometric counterpart
and the same size and shape as the datum feature UOS. In
cases in which restraint applies to a datum feature with
more than one normal vector, the direction of the restraint
load should be specified.

 

[Rubicon500]

Thanks, that is what I would assume would be done.

It would be interesting to see what currently people do to meet this.

 

[tim_member]

If part geometry allows, some people would simply put four 0.5 kg weights over each datum target.

 

[pylfrm]

Rubicon500,

The kilogram (kg) is a unit of mass, not force.

It's often more appropriate to specify an upper limit on the restraining force instead of an exact value. This doesn't work in all situations though, so it would be interesting to know more about your application. How exactly have you specified the restraining force? What is your rationale for that specification?

Perhaps you've specified an exact value for the restraining force to more closely match the conditions of the final assembly. If so, can the force application method from the assembly be used for inspection as well?

It should be possible to apply a specific force with reasonable accuracy through controlled compression of springs with known stiffness. This might be somewhat more complex than "standard metrology clamps", but it could still be practical.

Alternately, if the part can be shown to meet the requirements when restrained with approximately 0.1X and approximately 10X the specified force, then it's likely to also meet the requirements when restrained with the exact specified force.


pylfrm

 

[3DDave]

In a force application I found that calibrating a compression spring to an applied load is very useful. It doesn't matter so much what the spring rate is as long as there is noticeable deflection - set the spring under the load and measure the length; tie a tag to record that length to the spring. Use a clamp to compress the spring to the recorded length against the part. It should be repeatable within a few percent with little effort.