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How to place controls on gage contact points

How to place controls on gage contact points

How to place controls on gage contact points

I am trying to understand the best practice for applying controls to gage points that function as datum simulators. For example:

I have a simple gage that uses three domed carbide contacts to simulate the primary datum plane of the part. Two of these contacts are at the same height and one is .010 lower. I need to produce a drawing for the manufacture of this fixture.

These three contacts are pressed into a steel baseplate. I will call the bottom surface of the baseplate "datum A" and control with a flatness callout of .0002. Now I want to apply a callout that will control the height of the two in-line contacts so that they will effectively create a tangent line that is parallel to datum A within .0002. The exact height above datum A doesn't matter at all and I don't want to control it here. All that matters is that a line tangent to the heighest points of the two contacts is parallel to datum A.

I then want to define this tangent line as datum B and call out the top of the third contact as .0100 +- .0001 below datum B. Actually, what I want is for the drop to be calculated from the average height of the two in-line contacts. With the CMM software that we use this means that inspector should pick up the heighest points of the two in-line contacts, construct a line through them, set that line as the Z origin and then pick up the heighest point of the third contact. If inspected on the surface plate I want the inspector to find the height value of each of the in-line contacts, average them and then find the drop from that average value to the third contact.

How do you properly apply controls to a tangent line like this and how to properly define a tangent line as a datum? One thought that I had is to show a phantom line tangent to the tops of the two in-line contacts and apply a parallelism control to this phantom line by pointing to it with a leader. I would then hang a datum indentifier from this feature control frame. Below the feature control frame I would place a note saying something like "Line Tangent to Gage Point Contact Surfaces". But then how to dimension down to the third contact to communicate the intent mentioned above?

I realize there might not be a way to exactly acheive my intent using symbols alone but I would like to get as close as possible with symbols and use notes only as a last resort.

Thanks in advance for any thoughts.

RE: How to place controls on gage contact points

This is a make only one ever fixture?

Why bother trying to describe this with drawing controls at all?  

Just take your post above and make it a "setup procedure" and you're done.


RE: How to place controls on gage contact points

We're only making this fixture once but this situation comes up constantly in all kinds of variations. I could add a note describing what I want but I'm interested in learning more about how to do this sort of thing properly and improve my drafting knowledge.  

RE: How to place controls on gage contact points

Checking fixtures should be produced with no more than 1/10th the part tolerance. If there is a requirement of flatness on the primary datum of, say, .005, all points on the checking fixture would have to be within .0005.

This concept is shown in ASME Y14.43

Hope this helps.

Dave D.

RE: How to place controls on gage contact points

Hi Dave,
Thanks for your reply. The tolerances that I mentioned were determined based on the general rule that you mentioned. The toolmaker will never see the print for the part that is manufactured with this fixture so I need to communicate all of the necessary information on the tooling print. My question isn't really about how to decide how much tolerance to allow but how to apply ASME Y14.5 conventions to the concept of a tangent line in a situation like the one I describe. We have ways of doing this that work, I am just trying to learn what the best practice would be for this type of situation based on either the 1994 or 2009 ASME Y14.5 standard.

RE: How to place controls on gage contact points

I have two proposals for achieving what you seem to be after, using GD&T.


From your OP, there area a couple of issues; Datum-B doesn't do anything ... it doesn't eliminate any dof after Datum-A, and it can't stand alone as a primary datum as it's not stable.  Next, what is the functionality of using the average height of the two pins?  If the tangent planes of the two pins have to be within two-ten-thousandths of an inch, then you're not going to resolve a difference between them with a CMM without having it in an environmental isolation chamber.

Option A relates the two same-height pins together using a composite profile control; the first level provides a general location for the tangent plane established by the two features wrt the datum, and the second level refines the orientation of the common tangent plane wrt the datum. Note that the leader for the composite fcf contacts the phantom line between the two pins; this establishes that a relationship exists.  The second surface profile control locates (and orients) the tangent planes wrt the datum.

Option B locates the height of one of the pins using a linear tolerance, then uses the profile control to locate the two tangent planes of the tall pins within a restricted zone (i.e. inter-feature location) and restricts the orientation wrt the datum at the same time.  The second profile control establishes the relationship between the tall and short pins.

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: How to place controls on gage contact points

Thank you so much Jim. That is incredibly helpful. Just to clarify a few points:

1) I assume I would need to add a basic dimension to example B defining the drop between the pin heights. The same as you did with the "5" dimension in the example A.

2) What is the purpose of the pointing to both the phantom line and the lower pin with the single-segment FCF in example A? The upper pins are completely controlled by the composite frame so wouldn't it be enough to have the basic dim. defining the drop and then the single seg. profile control on the lower pin?

Thanks again! I appreciate your time very much.


RE: How to place controls on gage contact points

1) yes
2) it controls the relationship between the upper pins and lower pin, as you needed.  I could have put the lower pin wrt datum-A only, but this ties the relationship together rather than separate.

No problem.

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

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