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Measuring a bowl?

mhh

Plastic
Joined
Oct 11, 2009
Location
Denmark, Them
Hi! I have bit of a problem! At work we are about to make a few of the parts similar to the one below. My question is: How do you go about measuring the diameter of the sphere cut? Part is to be made on a lathe so it is circular.

Any help is more than welcome!

EDIT!
Yes it is in metric, sorry about that, forgot to say at first.
At the moment it is only a few but in time it could be a much larger order, I have acces to CMM but it is important that I can measure it while it is in the machine. :)

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Would it be practical to make the part thicker to start with so that more of the sphere was there to measure ,then use a bore guage to check the diameter ,finally facing off to the 18mm thickness. an extra 2mm should be enough assuming you can trust the tool path of the machine otherwise go up to 36 mm thick to take more measurements.
 
If you "assume" your curve is consitant, find or machine a ball surface that is easy to measure
blue up the bowl and see what the fit is
make a few of the ball "gauges" in smaller steped sizes up to your max Diam or Radius
You will still need to have you diameter at surface equal the spec
As mentioned above, start with thicker materila and face off to adjust
Mike
 
In addition to bryan's suggestions, you can place the part on a flat, put three balls of appropriate and known size in the recess and use a plug gage. If you do this more than once, with different sized balls, you can also get a (only moderately precise) reading for where the center of the sphere is relative to the face on the flat.
 
I'd also use gage balls. But, I'd probably use 2 balls and Jo blocks.

If you have two 20mm balls, make a stack of blocks to get 8.723mm. Put the balls on a surface plate with the gage blocks in between. Flip your part over and put it over the balls. It should sit flat on the plate if it is at a min dia of 55mm.

As sfriedberg said, you need to measure more than one point, to check it correctly. You'd need different size balls, stacks of blocks, and some trig to measure the radius, and it's center.

Unless you have a CMM. If you do, forget everything that's been said, and use the CMM.
 
How do you go about measuring the diameter of the sphere cut? Part is to be made on a lathe so it is circular.

Any chord of a circle, when bisected by perpendicular line, that bisecting perpendicular line will pass through center of the circle - see pic.
So one might measure the radius (and so then the diameter) via the intersection of two "T-squares" laid appropriately across the 'cords' - I.e across the curves cut in the work.
 

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A gage ball that is slightly bigger then the diameter and a comparator is all that is needed. Where the ball intesects the plane will give the points to measure the diameter. The other way is with a gage ball and an indicator. The distance from the top of the gage ball to the plane is used to calculate the diameter. I just don't remember the math part of it.
 
Any chord of a circle, when bisected by perpendicular line, that bisecting perpendicular line will pass through center of the circle - see pic.
So one might measure diameter via the intersection of two "T-squares" laid appropriately across the 'cords' - I.e across the curves cut in the work.

You might notice that there are some similar triangles show in that picture.

The 1/2 chord and the Saggita similar to the half chord and the radius, minus the sag.

I would need to open a book to recall clearly, it's late....
 
You might notice that there are some similar triangles show in that picture.

The 1/2 chord and the Saggita similar to the half chord and the radius, minus the sag.

I would need to open a book to recall clearly, it's late....

Yes you reminded me CalG - there is another (perhaps easier) way to solve the radius by using the chord length, and the perpendicular distance from mid chord to the (midpoint of) arc - call that the 'height' (or the saggita I think its called?)...then:
Radius = (height squared + 1/4 Chord length squared) / 2x height
Or same thing less text:
Radius = (S² + 1/4C²)/2S ...where 'S' is the sagitta (the perpendicular height from (mid) cord to (mid) arc), and 'C' = chord length
The cord length and the sagitta height should be fairly easy to measure from object.
 
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A 55 mm ball should blue up in one spot or blue up the entire surface. A 55.025 mm ball should only blue where it contacts the ID and OD. When these two procedures occur at the same time you can assume the radius is within tolerance. You can then measure from the face of your part to the top of the ball to find the proper depth.

EDIT: The 55.025 mm ball should only contact at the OD (mouth diameter)

It would be difficult to do this if the dimensions were in inches.

Gene
 
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The easy way to go about this is to get a shallow bore gage with ball contacts (Mueller Gages Company - 77 Series ) set the depth with gage blocks and set the distance between the contacts with gage blocks with ears and your ready to rock.
Cheers Don

I agree. A bore gauge is how I would do it. Ball contact with a depth stop for in process measurement. Double check everything offline with a CMM.
 
I agree with easymike299. I think it would be difficult to be precise measuring with a single ball. All you can tell that way is whether it's too big or too small. But how much? If it were me I would make 3 NOGOLO-GO-NOGOHI balls and use the bluing method described. Then you can determine whether or not the radius is in tolerance.

Best Regards,
Bob
 








 
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