Measuring Height with flatness

[HemiBrian]

I have a 5.000 +.060 -.000 high forging with a oof of.060
my problem is that the top surface has some waviness and there are low areas on both faces not across from each other
so I placed part on a surface plate measured the top face with a height gage found the lowest spot, then subtracted the worse oof on the bottom face (example: lowest height gage reading was 5.050 and oof was .040= 5.010)
My question is do I subtract the gap from the surface plate to the part (oof) from the height??
If I could use a 6" dial caliper and if the lowest areas on both face where across from each other.Then I should get 5.010 also

 

[fsincox]

No, if no other conditions are called out then multiple (2) point directly opposed measurments are fine. Remember also if ASME Y14.5 is used, then there is a requirement for perfect form at MMC or 5.060 unless stated.
Frank

 

[HemiBrian]

What if I use a caliper to measure height and get a reading of 4.990 which is .010 low
Then I use a height gage and get a reading of 5.020 which is higher because of the oof
which is correct?

 

[dgallup]

If any 2 point measurement is below 5.000 then it does not meet your specification.

 

[HemiBrian]

Frank,
Thank you for your responses. If any two point measurement is below 5.000 is non-conforming then if I use a height gage on that same part I would have to subtract any gap on the bottom to obtain the same result as I did with the caliper.. right?
Don't you inspect flatness by it's self
and the height dimension by it's self
not together?
thanks Brian.

 

[SeasonLee]

Whenever GD&T Y14.5 applies to a FOS ( for your case 5.000+.060-.000 ) that consists of two parallel planes, an automatic indirect flatness control exists on both surfaces. For your case, the MMC part ( 5.060 ) would have to be perfectly flat on both sides, the LMC part ( 5.000 ) could have .060 flatness error on both sides.

For the part inspection, we will check :
1. Size tolerance---The actual local size at any cross section must be within the maximum and minimum limits of the size dimension.
2. Rule #1 boundary---The part must be within the MMC envelope.

You need to check the flatness if there is a flatness callout on your part, all surface elements of the toleranced feature must be between two parallel planes that create by the flatness control.

SeasonLee

 

[fsincox]

HemiBrain,
Your answer is given by the others and conforms, in my opinion, with what I stated. A (2) point measurment of 4.990 is out of print.
Frank

 

[HemiBrian]

ok, what I'm not understanding is if my only option is to check the height with a height gage I will not get the lowest point unless I subtract the gap between the part and table.So do I subtract?

 

[fsincox]

HemiBrain,
If I understand what you are asking, yes, you would subtract if you believe you are at exactly the same point as another height gage point measurment on the directly opposing side and assuming it is a concave area.
Frank

 

[PeterStock]

You take point to point measurments, the part would not be on a surface plate for those meaurements. These dimensions should be within the 5.000-5.060 limits. The part would then be put on a surface plate and measured again, no point should be above 5.060. I would suggest doing that with one with one side, then with the other on the plate.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net

 

[HemiBrian]

the part does have concave areas on both faces but may not be across from each other

 

[fsincox]

Really, you have to do the best you can to meet the spec with the tools you have at hand.
Frank