O/T How do you inspect flatness?

[kunfuzed]

GD & T defines flatness as "The condition of a surface having all of its elements in one plane." So I was wondering what would be a good procedure for checking this on a granite plate(or what the best way is). I can't come up with anything becuase I don't understand how to get a proper reading with a test indicator if both surfaces aren't exactly parralell. Thanks.

[mcpgmr]

One way is put the part on Guage blocks on the Granite table and check the surface from underneath with a dial indicator.HTH!

[jeff]

just because the 2 sides are parallel doesn't mean they are flat.

think of the top surface that you are indicating and have that be "parallel" to the granite. Then that will tell you how flat that surface is.

 

HTH

[Guest]

Up on 3 points, from underneath, level the indicator at the 3 points. That is your plane, inspect the rest of the surface relative to that plane.

[Thee Dragracer1951]

Set it on three points as three points define a plane. then sweep it from underneith

[Müřlıń®]

Clean off the surface plate and degreese with alchol or orange stuff.

 

Hone part to remove all the burrs.

 

Get a good interapid indicator and affix in surface guage.

 

Set part on surface plate and move in a circular motion to remove dust from between the part and table.

 

Zero off surface guage on any area of the part.

 

take your fingure and press around on the top of the part in several places and check for indicator movement.

 

If no movement, flip part over and repeat process.

 

If you get indicator movement, the concave side is now facing you. If not your part is perfectly flat.

 

Observe total indicator movement and you will have your flatness.

 

Sliding an indicator around on you part wil tell you a hell of alot more than just sweeping in your part along a couple points.

 

You can detect defects, paralism, flatness, warpage, surface consistancy, and measurement.

[Kevangel]

I would raise the surface that needs to be flat up, and place it on 3 tooling points(3 1-2-3blocks or or 3 gage blocks same height or make up 3 riser blocks of 1"dia. etc.) Part will not "rock" on a 3 point stance. Height masters are 3 point stance, my jig grinder is 3 point stance etc. Sometimes on prints they will call out where tooing points are, so that everyone checks the same way. Also I try to place points to reduce any sag in the part. Then indicate bottom side that is resting on risers.

[Kevangel]

I am gonna have to type faster...

[kunfuzed]

Yeah, that's great! I was kinda thinking the 3 point was the way to go, since that's the minimum amount of points for defining a surface. So now I just have to see if all of my points lie within my tolerance zone? Thanks a bunch everyone!

[Müřlıń®]

LOL Kevangel....me too

[Zoober]

+1 to the three points, but I don't like 123 blocks or jo blocks for smaller parts. the surface contact is too much. We use conical blocks that have about a .05 dia of contact.

Also, don't forget to sweep the points you are resting on to make sure that they are all zero to each other.

[Ocean Lacky™]

of course, there's always a Faro arm.

[Zoober]

isn't that cheatin?

[Ocean Lacky™]

Cheatin? Nahhhh!

[Rick Henrickson The Boeing Co.]

+1 to john and dragracer, better to use jacks on 3 points. 1, 2, 3, blocks have to much surface contact.

you could use cmmm take hits to set datumn, then take multi hits to look at flatness

[ToolMan184]

3 gage balls is what I use.

[Zoober]

Just got a vision of Toolman chasing the plate around the rock as it rolled on balls! ROFLMAO!!

 

I don't know why I saw it as so funny, as I've used tooling balls for three points for ever.

[Jim at Gentex]

Just finished my GD&T training yesterday!

 

Remember, flatness is a form like straightness, circularity, and cylindricity, which are very difficult to measure since they have no datum reference. If you really want flatness, you can also check for parallelism from a known datum plane which gives you flatness and straightness 'for free'.

(Source: "The GD&T Hierarchy Guide" by Don Day)

 

The problem with the 3 point method is that in theory the 3 points (ideally) must be the highest points on the surface which we have no real way of knowing. If you must do it that way, then do so by all means, but without a CMM and checking MANY points, we can't REALLY be sure of the results.

[Thad]

quote:

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in theory the 3 points (ideally) must be the highest points on the surface which we have no real way of knowing.

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I see no need for the 3 points to be the highest. As long as they are all the same, the plane is defined. There is also no need to indicate the bottom of the part. If you establish the points on the top, the top can be checked for flatness.

 

Thad

[Guest]

quote:

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just because the 2 sides are parallel doesn't mean they are flat.

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parallelism tolerance applied to a plane surface also controls the flatness of that surface.thus,if no flatness is called out,the flatness will be at least as close as the parallelism.

[Zoober]

From Brown and Sharpe:

quote:

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"The condition of a surface or axis equidistant at all points from a datum plane or axis". Parallelism tolerance specifies one of the following: a zone defined by two planes or lines parallel to a datum plane or axis, or a cylindrical tolerance zone whose axis is parallel to a datum axis.

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A few others, including Zeiss, use the first part of the quote (in "") word for word.

[Jim at Gentex]

quote:

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the flatness will be at least as close as the parallelism.

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That's what they mean when they say you get flatness 'for free' when you check for parallelism.

 

quote:

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I see no need for the 3 points to be the highest.

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Thad, this is all theoretical, of course. If they aren't the highest points of the surface, you are not getting full indicator movement and thus you have NOT determined whether ALL points which define that surface lie within the tolerance zone.

That's why 'forms' like flatness are MUCH more difficult to inspect than 'orientations' like parallelism.

 

quote:

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There is also no need to indicate the bottom of the part.

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If you indicate the top of the part sitting on the surface plate, you are checking parallelism of the top surface relative to the bottom surface. If you use the 3 point method with the part sitting on gage pins or balls and indicating the bottom, you are checking flatness of one surface relative to the surface plate and independent of the opposite surface. There IS a difference!

 

See a more detailed explanation here.

 

[ 12-21-2006, 01:04 PM: Message edited by: Jim at Gentex ]

[Thee Rickster ™]

I just give it to my Q.A.Manager

 

He always knows what to do.....mostly.

 

If he finds it out of spec I give to

my apprentice, then it become inspec...lol

 

RMS

[Thad]

Jim,

 

Let me clarify...

 

Put your part on 3 jacks and jack the part up or down until the area above the jacks (on top of the part) all read 0 with an indicator. Now move your indicator around the top surface of the part. Some areas may be + and some may be -. This variation is how you measure flatness. Since the top surface of the part is what establishes your plane, there is no need to indicate the bottom of the part.

[Chao]

just for discussion:

 

In order to obtain the form deviations (straightness, flatness, roundness and circilarity),two steps have to be gone through:

 

step#1 - collect data

Generally the data collected by indicator or CMM or whatever are not the deviation value.

 

step#2 - process data

setting up two "parallel" geometries on data themself by NIMMAX principle, then the distant between the two"parallel" geometries is the deviation value.

 

Parallelism and Flatness have nothing to do each other no matter what how perfect they are.

 

regards!