# GDT Flatness measurement question

#### hogheavenfarm

Quick question here -
I have a GDT callout for flatness on a rectangular welded product of 0.060" . This rectangular piece is 48" long and 12" wide. It is made up of several welded ribs lengthwise down the panel and is about 1" thick. Due to welding (stainless steel) there is a bow to the center which is pretty normal for SS. The customer is measuring this flatness by placing the unit on a granite surface and using gage pins to look for gaps greater than 0.060". (I know, this would not detect any deviation in the panel itself, only the edges).
I am using our CMM to take surface points across the entire face and look for the "form" deviation, basically a "best fit" of all taken points within a rectangular "envelope" around the part. The reasoning I had was that placing the part on a flat surface might actually be measuring the "rock" ( off the bow towards the middle) and artificially doubling the deviation from the plane of the part itself, as the contact point on the table would be considered "flat" and thus basing the datum on the table, and not the part itself.
IE - the part, if bowed will contact the table near the center, pressing one side down will raise the other over 0.060", while it may be 0.035" on each side if left alone. I am thinking of a rectangular envelope 0.060" thick by using the CMM, but I think measuring this way (on the table) would exaggerate the gap on the other side. Also note, we were not supplied with the customers method prior to this project, only after the fact.
Is my thinking incorrect? I think the callout is improper anyway since the purpose is to align the panel with existing panels, so the existing panels would actually be the datum, and I have no knowledge of the dimensions and fit of those panels. All I can work with is our panel.

#### Ron Rompen

Trusted Information Resource
I agree with your thoughts about measuring with the CMM - flatness is the deviation of a surface to itself, and any rocking of the part would artificially skew the form deviation.
Best idea is to review this with the customer, and get their agreement as to the method of inspection. Is it possible for you to duplicate their method (do you have a surface plate of sufficient size?)?

#### Miner

##### Forum Moderator
Flatness deals with one surface only and does consider deviations from the best fit plane. Both your customer and you are attempting to do this, but as you correctly noted, your customer cannot check interior points. However, as soon as you use a surface plate as a datum on one surface and start measuring points on the other surface, you are no longer measuring flatness, but parallelism. In addition, if you rock the plate, you are no longer close to a best fit plane.

You initial method of best fit plane on the top surface is correct.

#### hogheavenfarm

I can duplicate the method on our table, but I have the rocking issue I spoke of. I can get measurements with pin gages over 0.060 if I hold down the opposite end. If the situation were reversed I would get measurements over 0.060" in the middle somewhere, but that still would be relative to an external datum (the table) and not to itself, no?

#### Emmyd

##### Involved In Discussions
I'm probably way out of line, but when I have this type of situation when measuring a flatness, I end up having to elevate the part (as close to defined points as possible) and measure the form of the surface. This takes out the rocking issue and parallelism concern by only checking the profile to itself. I'm sure this wrong and there will be plenty of people who chime in to tell me. But, if by some miracle, I'm not, I hope this helps.

#### hogheavenfarm

Well, the lowest point would be in contact with the table, the highest point would be the gap between any other edge and the table. This is what the customer is doing. I contend that this is not giving the plane of the part (flatness), but is giving parallelism using the table as the datum. It also is subject to twist, so the part can be rocked meaning is it not giving a measurement off the plane of the part, but the table. This can (in theory) double the deviation, so I can measure 0.119" but the two points can be within 0.060" of the plane of the part. So I contend using the part "form" gives me the "envelope" that confines the part within 0.060".

#### Johnnymo62

##### Haste Makes Waste
The flatness is that all points of the surface are between two imaginary planes 0.060 apart from each other, no matter how it's measured.

#### Matt's Quality Handle

##### Involved In Discussions
I've run into a similar issue. It was a machined oil pan sealing surface that was being warped in the fixturing process. The part was rectangular, with one set of opposite corners high, and the other set of opposite corners low.

The overall flatness tolerance was .004". We didn't have the CMM capacity to 100% them, but I got my (internal) customer to agree to place them on a surface plate. They would naturally rock across the high diagonal. We rocked one of the low corners to the plate, forming a 3 point plane, then checked the 4th corner by attempting to slide a .007" feeler gage. We then rocked the other direction to check the 2nd low corner.

Both the customer and I agreed that mathematically, we COULD use an .008" feeler, but he wanted to be conservative.

EDIT: Grammar because it's the end of the day, and coffee has worn off.

#### Miner

##### Forum Moderator
@Matt's Quality Handle

You were actually robbing yourself of tolerance. In this case the measured flatness would be when you had the two corners resting on the surface plate and the other two corners both equidistant from the surface plate.

Think of it in two dimensions. You have a cloud of X/Y measurements and you draw a best fit line through them. Now expand to 3 dimensions. You have a cloud of X/Y/Z dimensions and you draw a best fit plane through them.

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