0.5mm (~0.020in) Flatness Measurement

dksoba · Jan 29, 2013

I have a drawing that calls out a 0.5mm (~0.020in) flatness. The part is 1.25x4x0.125 aluminum. I am going to cut it out of sheet stock. How can I inspect this flatness? I have a surface plate and a nice height gage. At 0.02" tolerance, I'm thinking I might be able to put it on the surface plate and use a feeler gage. The only way I've seen flatness measured was using a CMM (I know there's other ways). That just locates multiple points on surface in 3D space and computes the distance between two planes that all the points fit between.

Thoughts?

Thanks,
Matt

dsergison · Jan 29, 2013

make 2 appropriate height 0.145" ? spacers and set a 1-2-3 block or similar over them on your surface plate. if you parts pass through the "garage" they pass the test.

Chobyn · Jan 29, 2013

The garage idea works, IF, if you can fit the entire surface under the garage at once.

Place your part on 3 pointed standoffs on your surface plate. These need to have the same height from the surface plate. These three points establish a plane.
Using an indicator mounted in a surface gage, check the bottom of the part as thoroughly as you can. If you get .020 or LESS total indicator movement from
high reading to low reading, you win.

The real Leigh · Jan 29, 2013

Chobyn said:
Place your part on 3 pointed standoffs on your surface plate. These need to have the same height from the surface plate. These three points establish a plane.
Using an indicator mounted in a surface gage, check the bottom of the part as thoroughly as you can. If you get .020 or LESS total indicator movement from
high reading to low reading, you win.

Sorry. That won't work.

Your three support points do not establish the low point of the workpiece.
You would have to survey the entire bottom surface to establish a low point, then
survey the entire upper surface to establish a high point, and compare those two values.

That procedure adds one entire sweep of the workpiece that's not required if using the garage gage.

- Leigh

dksoba · Jan 29, 2013

Doesn't the garage check the thickness of the part? And, in the 3-point method, I would only have to survey the lower surface. I'm only concerned about if one surface is flat, not if the surfaces are parallel and flat. Maybe, given the tolerance, a precision straight edge and feeler gage? The quote is only for 60 pieces.

The real Leigh · Jan 29, 2013

Thickness and flatness are two unrelated parameters.

Take a sheet of aluminum and roll it into a tube. It still has uniform thickness, but the flatness is out the window.

- Leigh

dandrummerman21 · Jan 29, 2013

I use the three point method. I don't see how you have to survey both sides with an indicator when finding out how flat a single side is. If they call flatness on both sides, then I'd agree.

Like when I make a part from 7075 that is 26x18" that starts off at 1.08" thick and ends up at .984", and require .008" flatness on one side, I just set it up on jo blocks and sweep it all over with an indicator. We don't care how flat the other side is (really; it could be off .02", wavy, tapered, etc, doesn't really matter). I do understand that if I do not inspect to find three "high/low" spots on this plate, and instead set it up on three random places, the measure of flatness may say that the part has more deviation from a plane than it really has, but never less.

Imagine throwing a wide banana-shaped part on jo blocks, and having the part supported on one half of the banana. Sweeping this will give the impression that the surface is WAY OUT on one end. Distributing the blocks evenly (both ends of the part) will result in your banana having much less deviation from the same plane.

But anyway, enough of that rambling. I believe that the indicator method would work well. And given the tolerance, I don't see why the feeler method on a surface plate wouldn't work well too, assuming the parts don't come close to being off .02"

Chobyn · Jan 29, 2013

The real Leigh said:
Sorry. That won't work.

Your three support points do not establish the low point of the workpiece.
You would have to survey the entire bottom surface to establish a low point, then
survey the entire upper surface to establish a high point, and compare those two values.

That procedure adds one entire sweep of the workpiece that's not required if using the garage gage.

- Leigh

The three points establish a plane. The flatness callout specifies the TIR allowed relative to this plane.
If, when surveying the surface on the 3 points (established plane), the TIR is within the tolerance this indicates
that that surface in question is within spec. If not, then it is out.

Could you elaborate on why this will not work?

Flatness is a form, not a location, or orientation, so what does the upper surface have to do with it?
If I am comparing the TWO surfaces I would be checking parallelism, would I not?

Gordon B. Clarke · Jan 30, 2013

I wrote a suggestion that seems to have vanished. Was it deleted or did it get lost in cyber space?

Gordon

dksoba · Jan 30, 2013

Gordon B. Clarke said:
I wrote a suggestion that seems to have vanished. Was it deleted or did it get lost in cyber space?

Gordon

Not sure, but a minute ago the forum wasn't working for me.

Gordon B. Clarke · Jan 30, 2013

dksoba said:
I have a drawing that calls out a 0.5mm (~0.020in) flatness. The part is 1.25x4x0.125 aluminum. I am going to cut it out of sheet stock. How can I inspect this flatness? I have a surface plate and a nice height gage. At 0.02" tolerance, I'm thinking I might be able to put it on the surface plate and use a feeler gage. The only way I've seen flatness measured was using a CMM (I know there's other ways). That just locates multiple points on surface in 3D space and computes the distance between two planes that all the points fit between.

Thoughts?

Thanks,
Matt

Is 1.25x4x0.125 mm or inches? As you start of with a call out in mm (0.5mm) I'm not sure when you mean inches and when you mean mm.

dksoba · Jan 30, 2013

Gordon B. Clarke said:
Is 1.25x4x0.125 mm or inches? As you start of with a call out in mm (0.5mm) I'm not sure when you mean inches and when you mean mm.

1.25 inches x 4 inches x 0.125 inches

The real Leigh · Jan 30, 2013

Gordon B. Clarke said:
Is 1.25x4x0.125 mm or inches? As you start of with a call out in mm (0.5mm) I'm not sure when you mean inches and when you mean mm.

This is an American board. We customarily deal in inches.

Your earlier missing comment was never posted to the thread. I get an email with every new post, and none for that one.

- Leigh

Gordon B. Clarke · Jan 30, 2013

The real Leigh said:
This is an American board. We customarily deal in inches.

Your earlier missing comment was never posted to the thread. I get an email with every new post, and none for that one.

- Leigh

Lighten up. The OP started "I have a drawing that calls out a 0.5mm (~0.020in) flatness." Now why would mm even be mentioned???????????
I'd consider it a relevant question. It's far from the first time a post has been read and misunderstood.

Gordon B. Clarke · Jan 30, 2013

dksoba said:
1.25 inches x 4 inches x 0.125 inches

Thanks

Just out of curiosity why the 0.5mm call out? My suggestion isn't relevant now as you've now made it clear that all other dimensions were in inches.

Gordon

dksoba · Jan 30, 2013

Sorry, my customers drawing is in millimeters but I put inches for the convenience of us American machinists. I have a much better knowledge of metric system units (that's all we used in school), but my "intuition" is in inches.

ewlsey · Jan 30, 2013

Gordon B. Clarke said:
Thanks Just out of curiosity why the 0.5mm call out? My suggestion isn't relevant now as you've now made it clear that all other dimensions were in inches.

Gordon

You can sure beat a dead horse. How about contributing something?

Three point support. Indicator on a height gage.

I don't like to use a test indicator due to the potential tangential error.

dksoba · Jan 30, 2013

ewlsey said:
You can sure beat a dead horse. How about contributing something?

Three point support. Indicator on a height gage.

I don't like to use a test indicator due to the potential tangential error.

To be fair, Gordon responds to a lot of threads with very useful information. For every question I post, there's a multitude of threads that have been answered by Gordon which saves me from having to post the question.

ewlsey · Jan 30, 2013

For every solution he offers, there are dozens of worthless, non-machining related posts.

The real Leigh · Jan 30, 2013

Enough off-topic.

Back to the OP's question.

- Leigh

0.5mm (~0.020in) Flatness Measurement

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