DaSalo
Mechanical
- Apr 27, 2010
- 213
Hello,
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.
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.
