Thread: A question for the metrologicly (??) minded...

Originally Posted by Michael G

I wish to bore out a part section (target diameter is around 5"/ 127mm) but can't think how to best measure what the diameter/ radius is. This trough will not be uniform but will have some diameter changes so full form gauging would be problematic - although not impossible
This is not it, but an example of the type of profile I'm looking to measure. Measurement will need to be in situ so that I can adjust the boring head to be close to target (less than a mm)
Attachment 360901

The best method I've thought of is a couple of knife edges on a known length of beam with a dial gauge in the middle measuring the amount the surface deviates from flat. A bit of maths and I could calculate the radius
Attachment 360902
The downside is that to get reasonable accuracy the indication must be good to micron levels (I have one to 2 micron), which with a machined surface may be asking a bit much. (0.002mm steps in d where 2l= 50mm gives steps in radius of 0.045mm. An error then of 0.01mm means the radius could be around 1mm off. Reducing 2l makes things better but the gain on any error is still large)
Any other ideas out there?

Michael

Michael, can you clamp a slave piece of metal diametrically opposite so you can measure diameter with normal instruments?

G'day Michael, the process I described is how I set a boring head to size, but it could just as easily work backwards to determine the radius that you'd just bored. I've never had to do that myself, as I'm always working to a radius and not from, but there's no reason it wouldn't work both ways. So in other words, you may set the radius to 127 mm, bore the arc on the casting and find it didn't clean up. Let's just say you then decide to increase the radius and just keep increasing without measuring or noting any difference in radius until it cleaned up.

What you could then do to find the radius you've bored (say to make it's mate) is put an angle plate or similar opposite the casting where you have room on the table and set the angle plate to be tangent to the boring bar on the X axis. Once you've found that point, clamp the angle plate down, remove the boring head and find the edge of the plate. The distance between the point at which you bored the hole and the edge of the angle plate is the radius you bored. You could do this on the fly, just remember to move the angle plate out of the way before increasing the radius

Measuring the chord and height will definitely work, and with a long enough chord should be quite accurate, but I honestly can't see the point unless you're wanting to measure an existing arc you didn't create in situ.

G'day Michael,
You mentioned your tolerance is less than a mm, assuming this is in the range +/-0.5mm might a simple method be to machine a piece of say 3mm sheet to a circle of the required diameter then cut out a section large enough to cover your required arc which could then be used like a simple radius gauge?

A more accurate method would be to bore a full circle of the required diameter into some plate, position your dial gauge tool as you have described and zero the gauge. This could then be used to check your workpiece as you go with the correct diameter being achieved when the gauge reads zero in your machined arc.
Cheers,
Greg.

Hi Michael,
I think this idea should work. As your good at what you do, draw your circle at 127mm, draw a line through the centre and offset them left and right a length that will fit your cord. Draw in a block of appropriate length and measure the distance required. Make a block of the length you chose, with a hole in the centre and mount a micrometer in it, so it looks similar to a depth gauge, once you get to the depth marked on your drawing, you should be spot on.
Kryn

Originally Posted by OldRustyToolie

Can you clamp a slave piece of metal diametrically opposite so you can measure diameter with normal instruments?

I could but supporting it properly then adds extra to the set up. Another option though

Originally Posted by kwijibo99

Might a simple method be to machine a piece of say 3mm sheet to a circle of the required diameter then cut out a section large enough to cover your required arc which could then be used like a simple radius gauge?

Another possible but a radius gauge will only tell me when I'm on size, not how much to go to get there and after I've machined this cylindrical feature I then want to machine a mating part that locates closely in it. If the sizes are too different they will spring/ distort. Turning an OD is easy, so I was was hoping to measure and avoid hit and miss "I'll just take a little more off" actions. With larger diameter circles a small amount in radius can make a big difference. I'd prefer to know I had to take say 5 thou off diameter rather than just do 10 thou jumps and discover I have a sloppy fit after the cut.

Originally Posted by KBs PensNmore

...as you're good at what you do...

Not sure I can live up to the claim of "good at what I do". That illusion is partly maintained because I don't starting threads titled "the things I stuffed up this weekend" I think there would be lots of people on this forum who could do the things I do better if they had the gear I have.

Originally Posted by KBs PensNmore

...draw a line through the centre and offset them left and right a length that will fit your cord. Draw in a block of appropriate length and measure the distance required. Make a block of the length you chose, with a hole in the centre and mount a micrometer in it, so it looks similar to a depth gauge, once you get to the depth marked on your drawing, you should be spot on.

Neither of my depth Mics (metric and imperial) are graduated down past thous or 0.01mm. I am thinking of making a base for the 2 micron indicator I have and using the roller idea, use that for measurement, so similar to what you suggest. I like the roller idea because it means not having to know exactly what the length of the beam is. More tooling to make...

Michael

Michael,

A tad late but

You could place 3 or more very accurate cylinders touching each other tangentially on your radius and then take a measurement across the outside of those. The smaller the radius the smaller this dimension will be, the larger the radius the closer the dimension the will be to the sum of the diameters of the cylinders.

Eric