True Position Question

[John Kugler]

Just a quick question. When you folks measure the true position of a hole, do you measure at the top of the hole and at the bottom? Or just once somewhere in the middle?

Just asking...

[dfiola66]

For me, it's sheet metal almost all the time. I try to hit the cut vs. the break of the material.

I'd imagine if it's more of a cylinder, you'd want to at least measure both and decide if reporting both is necessary.

[kbotta]

Depends on how "thick" the hole is. Most of my parts are 3mm, with a shear break at around half way, so I only have about 1.5 mm of effective measureing surface - so I only check in 1 Place (mid).
For larger or "thicker" holes, I would check in a few places.

KB

[Paul Sarrach]

Sheetmetal also. I make sure I get into the sheer. I take 3 prehits and go only as deep as nessicary, and its projected to that surface.

[John Kugler]

Thanks for the responses. I'm just trying to get a feel for the rest of the world as to how they do it.

I know that the right way is to do both top and bottom to ensure the hole is perpendicular to the surface. But is it safe to say that when a hole is machined into a part, it IS perpendicular within a reasonable amount?

[Paul Sarrach]

usally +/- 1 degree is accectable

[Mike N.]

Just a quick question. When you folks measure the true position of a hole, do you measure at the top of the hole and at the bottom? Or just once somewhere in the middle?

Just asking...

I usually determine that (top,bottom) by the process that created the hole

If there is potential for it to "walk" or deviate then two places is a must

To "split hairs" on a TP of a hole that has a sizeable thickness it could be made a cyl and TP of it instead of a circle.

[STANLEY]

We machine casting here. Most of my prints have t.p. and perpendicularity callouts.If it doesn't have both callouts, i still call out perpendicularity as a referance for inhouse inspection.

[Underspec]

I would check the hole to see if it's perpendicular to the plane (datum). If it is, you shouldn't have to call out different depths of the hole.

[Cumofo]

Try it for yourself, the requirements for a cylinder meeting T/P are tighter than a circle. The tolerance zone for t/p is cylindrical, and therefore perpendicularity is implied.
Ok, so that's the theory, but in real life toss the question to the Quality Assurance Engineer, or the Q.C. manager, and have him put his %$# on the line. Because, one way the customer will complain, the other production will, so: "Run Forest, run!".

Gabriel

[Schrocknroll]

It depends on the tolerance of the hole. If the true position is .005 " or greater and the part is less than 1/2 inch thick I'll usually go .100 from the top. If the hole goes alot deeper or if the tolerance is alot tighter than either 2 circles or a cylinder with top and bottom reported is the way I'd go.

[george frick]

i work with precision machined parts not sheet metal. i usually measure between .075 to .100 deep in the part when using a circle for tolerances of .010 to .014 true-position. this duplicates an open set-up check on the surface plate using a height gage & indicator or a microhite. HOWEVER- cmm software calculates true-position not necessarily correctly. if you measure a cylinder the output is the "pierce point" of the axis which IS NOT correct according to ansi y14.5. pc dmis does have the option to output axis at worst , best, and axis average when using a cylinder. true-position is a cylindrical tolerance zone encompassing the entire length of the cylinder in which the axis must fall. SO- depending on the TOLERANCE & application i use different methods to evaluate true-position location.

[Shelley Liebman]

I agree with Mike N, depends on what created the hole, but also on tolerance.
If its pretty wide open, and your process is sound, tried and true, I would just measure the hole once around the middle.
We do alot of bolt hole patterns here, typically with t.p.s of around .02" at mmc.
Knowing our processes, unless the print calls for a perpindicularity, I would only check in one place.

[KillGorack]

Stampings here, steel usualy around 3-4mm thick. We never ever worry about how perp the hole is to the sirface. just TP and Profile. Just like Dave Fiola up there, we try to measure the shear and not the break away.

[dwade]

My 2 cents.

I think the process that the hole is created with is greater in machining than the tolerance.

There are multiple process scenarios that need to reviewed before you decide which method to use.

What operation are the part datums machined? What operation is the hole machined on? What type of tool and process (lathe, mill, drill, boring head, interpolated, etc.) was used to put the hole in the part?

For machining with a boring head on the first op whether using a lathe or mill you can get away with checking the hole in one place and feel confident. If you have tight tolerances or the bore is put in on 2nd or 3rd op load, then it should be checked top and bottom. If it's interpolated with an end mill, the same applies to checking the hole as if it were made with a boring head.

If it's drilled or drilled then reamed, it needs to be checked top and bottom or insert a "tight" gage pin, measure the pin as a cylinder, then tolerance.

Holes put by wire or sinker EDM should be checked both top and bottom.

Sheet metal is as stated earlier, check it on the shear and not the break.