proper way to dimension......

[Mark Farren]

Have this part, see attached photos, -A- is the flat area between the inner and outer edges, -B- is the hole where the trihedron is located and -C- is a hole approx 38" from -B- and aligned to -B-. The outer surface is a contour surface, not necessarily a radius, it is also slightly angled in towards the -A- plane. The .098" diameter holes that go through this wall are called out TP .020m ABC. The wall thickness is .080". My alignment is set up per the datum structure, CAD=PART and all is good. My question is what would be the proper axis to tolerance for the true position of these holes through the contoured surface? There are no linear or radial dimensions to the holes, just the TP call out. Mind you that these are not located on a radius surface, there in lies my confusion of which axis to use for location. If this is unclear, I can attempt to explain it better or add more screen shots.

3.7 MR2 on a Global Image

[Matthew D. Hoedeman]

I would use all 3 axis, then toggle the TP to report PERP TO CENTER LINE.

[Mark Farren]

Thanks Matt, I'll give it a whirl . Did not you could use all 3 axis!

[Matthew D. Hoedeman]

Yep, sure can. I don't always use all 3, but I ALWAYS use PERP TO CL for TP dimensions (that way, if you use an incorrect axis, the TP will still be right)

[Wes Cisco]

Yep, sure can. I don't always use all 3, but I ALWAYS use PERP TO CL for TP dimensions (that way, if you use an incorrect axis, the TP will still be right)

Matt, Can you think of a case where you would need use true postiion without the deviation being PERP to CL?

[Matthew D. Hoedeman]

Not off the top of my head and to my recollection, I have not encountered one in my travels of the world of inspection.

[Matthew D. Hoedeman]

Got a PM on this subject, thought I would put this info here for EVERYONE to read:

Well, the perp to CL is mainly for use on surfaces that are NOT 100% square to your current alignment. Say you have a hole with a vector of -.109, .617, .778 (just pulled from a program), which axis do you report? Well, Y & Z are the main axis for TP, but does the X axis ALSO push the hole off location? Yeah, a little bit. I just toggled perp on and off (same one as the vector) and using just the YZ axis, the TP reads 0.598 (perp off) but when I turn it ON, the TP is 0.712. Big difference, caused by the minor vector of the hole. A large surface deviation will on a 3-D vectored hole will also cause some error to creep in, and it could make it worse or better, depending, but the PERP TO CL will always give you the correct distance of the actual center point of the hole to the center line of the vector of the hole, no matter if you report 1, 2 or 3 axis.

I also checked it by adding the 3rd axis, then the difference from PERP ON to PERP OFF was only 0.006mm, but when the guys on the floor have to move the hole, they will not be moving it in the X axis and the X axis will also show surface deviation, so I don't want that as part of the report. The PERP ON will give the correct value, simulating you checking it with a stab pin.

[Wes Cisco]

Thank you Matt,
I am guessing this is mostly a sheetmetal application? I thought it was important to always be properly aligned before using the trueposition dimension. I do not care to get into the whole check the "use datum" box can of worms here. But other than to avoid having to create a new aligment, I just don't get it. Maybe it is because of the nature of the work I do. I always create an alignment to match the datum structure. The program I am currently working on has 16 alignments now, and will probably have twice that many before it is finished. The engineer has 26 datums on the print. I know downhole oilfield is pretty far removed from automotive die and sheetmetal, but if this is something I should be using I want to understand. In your example I would either have leveled to the plane or to a cylinder created in the hole. Am I being dense? Please enlighten me. TIA

[Matthew D. Hoedeman]

Sheet metal, you bet! You get ABC datums, everything is dimensioned to those (99+% of the time). Part sits on a fixture. Fixture is aligned. Form, location, trim line, everything is dimensioned back to the same 3 primary datums. Here is a picture, part is aligned in car position and the XYZ 'thingie' is shown in the picture. How else would you dimension that round hole except perp tp CL for TP? It is the greatest thing to happen to sheet metal TP that Wilcox ever added to Pcdmis.

[kbotta]

i think (matt, correct me if I am wrong) that this function is taking the place of translating your alignment - (so you are normal to the hole- at the hole) - and addressing the tolerance as normal to the feature. Still in "2-d" though right?
How I would do this w/out that function - offset coordinates, and rotate theo perfect to the hole. Then position as 0,0,0 nominal.

matt, this takes the place of that right?
Kev

[Matthew D. Hoedeman]

Yeah, if I get what you are saying, that is correct. That would simulate looking 100% straight at the hole and gives you the '2-D' distance of the actual hole center point to the perfect center point vector line.

How would you do it WITHOUT that function? Well, construct points from the origin to the nominal center of the hole and the origin to that constructed point, level, rotate, etc to OTHER points constructed (NONE MEASURED!) to give you the plane and a line around the hole, then TP the hole in that alignment. You could even offset the alignment by the nominals of the hole so that the XYZ nominal numbers will be right, the actuals might show a little wierd, but will be close. Sure sounds a lot easier to me to use PERP TO CL, unless you are using a version that does not have that function?

[kbotta]

yep, yep and... yep.
thanks Matt.
Kev

[Ravenmaniac]

Gentlemen...that was some very interesting reading. Never took a look at perp to c/l in quite that way....thanx.

[Paul Sarrach]

You would think that Dimensioning would be one of the biggest section of PCDMIS training and its not.

[Jeffrey Lovely]

It will simulate a gage pin (locating pin)
years ago (3) I have verified this using cad - center lines of a hole
works great