Flatness strange question
Flatness strange question
(OP)
EDIT: the below drawing is representing the final machined part, a perfect part would be a perfectly square 1-2-3 block, datum a just represents the surface plate method.
I'm learning the CMM now and the GDT is really hitting home now, thanks for all your help in the past.. I get tripped up from time to time and the below issue is really bothering me.
Flatness....I was taught manual inspection on a surface plate. If we inspect a 1-2-3 block, you place it on the surface plate(datum A touches the plate) and run a dial indicator over the top surface. The dial min vs max is your flatness amount.
Now my mind wonders why the above method is wrong...if someone machines a part with a top surface so bad that it maximizes the plus/minus height tolerance, but the top has a nearly perfectly ground surface, you have a part that is bad, but within the plus/minus height tolerance...and is well within a flatness tolerance due to it being almost a ground like surface quality. BUT if I check flatness on the top feature by placing the bottom on the surface plate, I am going to get a massive 0.25 variance on the min max dial indicator. What is the appropriate way to measure flatness outside the CMM and on a surface plate? Am I confused?
I'm learning the CMM now and the GDT is really hitting home now, thanks for all your help in the past.. I get tripped up from time to time and the below issue is really bothering me.
Flatness....I was taught manual inspection on a surface plate. If we inspect a 1-2-3 block, you place it on the surface plate(datum A touches the plate) and run a dial indicator over the top surface. The dial min vs max is your flatness amount.
Now my mind wonders why the above method is wrong...if someone machines a part with a top surface so bad that it maximizes the plus/minus height tolerance, but the top has a nearly perfectly ground surface, you have a part that is bad, but within the plus/minus height tolerance...and is well within a flatness tolerance due to it being almost a ground like surface quality. BUT if I check flatness on the top feature by placing the bottom on the surface plate, I am going to get a massive 0.25 variance on the min max dial indicator. What is the appropriate way to measure flatness outside the CMM and on a surface plate? Am I confused?

RE: Flatness strange question
The manual inspection method you described would be correct for Parallelism, not Flatness. For Flatness, you don't need to align the bottom surface of the part to the plate. You can level the part to get the best possible indicator reading on the top surface (adjustable jacking screws are often used).
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Flatness strange question
RE: Flatness strange question
3DDave, what you said made some much simple sense I feel as dumb as you can feel without caring that I feel dumb. Thanks, cleared me all up. You all are an awesome resource I can't say thanks enough.
RE: Flatness strange question
I've asked several manual inspection questions and you seen quite keen on the exact ways to approach the measurement. Do you mind sharing what your experience is in manual and how/if it relates to what you have seen in cmm? Not looking for an essay, just your thoughts however long they are I'll read em all.
RE: Flatness strange question
RE: Flatness strange question
If they really care they can take a dial indicator and put the indicator through a hole in a flat plate and move the part so the indicator can check those as well.
RE: Flatness strange question
RE: Flatness strange question
Your Mileage May Vary.
RE: Flatness strange question
But even better is the method mentioned above with a dial indicator mounted through a little hole in the inspection plate; then your part would be flipped upside-down so that the surface in question sits directly on the table. See the attached graphic for a crude image of this idea.
RE: Flatness strange question
But many seem to say that the method is actually commonly used. Is it true?
I would appreciate if by any chance someone could show a picture of actual full inspection setup.
RE: Flatness strange question
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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.
RE: Flatness strange question