inspectining large diameter

[HemiBrian]

I have a large forging (36" ID). the tol. is 36.000 +/-.060
when measured with a vernier the id max diameter is .060 smaller than what our CMM max diamater. caused from the part being .090 O.O.R.
when inspected on CMM and graph the results the actual shape of the part is shaped like a three leaf clover. there is a high area across from a low area. so there is an error using a vernier
is there another method to inspect the min/max diamater manually without CMM.

 

[drawoh]

HemiBrian,

If you rotate your part on a turntable, you can use a dial indicator to test for roundness. This ought to be enough to make your vernier measurements usable.

A precision slewing ring would be ideal for you, but your tolerances are loose enough that you can improvise. I wonder how good the bearings are of a lazy susan from IDEA?

JHG

 

[HemiBrian]

drawoh,
we have tried a 3 jaw chuck on a table as a turntable, but when unchuck and rotate the part and re chuck it we get a different roundness. how do you know if the part is on center?

 

[MikeHalloran]

You could turn some discs from plate to make go/nogo gages.



Mike Halloran
Pembroke Pines, FL, USA

 

[ajack1]

Okay this is a bit of a bodge but might do what you want.

Turn a disk 1” undersize and pack it as centrally as you can using ½” dowels, drill blanks or the like, then simply use the vernier to measure how far out it is from .500” at various points around the circumference.

 

[GregLocock]

Crazy thought, isn't the bore designed to fit over a mating part? why not offer it up and measure the clearance?


Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ajack1]

A similar idea from Greg, however by making it a known amount undersize this allows you to use a go/no-go pin to check it is within limits, this is pretty much how they check trim lines on automotive stamped parts.

By simply scribing a series of lines through the centre of the disks at angular increments this would allow you to actually measure the “diameter” but in a very crude way.

 

[drawoh]

HemiBrian,

You would not centre the part on a simple turntable on the first try, but you would know where your error was. You could then tweak its position and try again. A three jaw chuck might be bending your part a bit. I would rely on gravity as a clamp.

The suggested go/no-go gauges are good too. With a tolerance of ±.06", you might be able to make them out of plywood.

JHG

 

[mfgenggear]

OK fellows

a vernier caliphers are accurate, must take at least three minmumum measurements. it will tell you how much out of round the part is. What it is measuring is an elipse.
major & minor diameter. what is your out of roundness value allowed.

CMM is accurate for this loose of tolerance.
problem with the Inspector/operator?

it appears your ring is out of round.
use a pie tape on the outside diameter.
it will give the exact diameter. measure the wall thickness.

 

[rb1957]

it sounds to me like the IML of your part is made up of several arcs. in that sense you don't have a centre, maybe you could develop a least squares fit. i guess it's slow (maybe expensive?) using the CMM, but it is giving you the results you need. if this is a bought-in part, how is the machine shop checking the dim'n ? how are they making the cut(s) ?? what is the NC tape telling the machine to do ???

 

[mfgenggear]

what is important here is that

A) the forging meets the grain flow & met lab requirements.

B) the dimensions should be a stock on condition.

C) Make there is enough stock for clean up. on all surfaces major, minor & on the faces. to the finished part.

D) rough turn one forging to verify it meets the above statement. if not reject them back to the forging house.