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Thread: Jig to measure flatness ?
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9th Aug 2018, 08:12 PM #1Golden Member
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Jig to measure flatness ?
Cobbled together this thing today. Very simple.
The more I think about what it does and the results produced the more I think that this gadget is useless.
Any advice - hints - recommended websites ? ............. gratefully received.
See photos - Dial Gauge Indicator mounted on long piece of Pactene with a weight at one end to standardize the pressure on the base of the pactene.
As you move it around the granite plate you can detect variations in the surface height. Holding it at the end furthest from the gauge and the weight and just pushing horizontally minimises effect of hand pressure on the measurements - which were significant prior to putting the weight in place. Taking your hand away completely removes hand pressure and ensures all measurements taken under identical conditions.
According to the data I gathered the centre of the granite plate is all within 0.03mm of itself and there is a 0.05mm dropoff all around the edge of the plate. See attached spreadsheet.
But what is it actually measuring ? The three highest points under the plastic will define a plane and assuming the base of the plastic has been cut perfectly flat, the base of the plastic will sit on that plane and the dial gauge will measure the difference between the local spot and the base of the plastic.
I wanted to see if I could estimate how flat the granite plate is - but after thinking about it I don't think what I've done is valid.
Any comments from brains trust gratefully received.
I suppose I could mount the plate on the mill table and hang a DTI off the end of the quill and move the table around under the DTI - will have a bash at that tomorrow - but that will then include any errors associated with the mill table not being perfect,I know it slopes slightly.
Bill
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9th Aug 2018, 08:53 PM #2Philomath in training
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What you have done is not too bad in principle but won't be accurate enough and is probably a bit flexible.
Have a look here //metalworkforums.com/f65/t1781...-surface-plate
where we toss around a few thoughts on checking surface plates.
Michael
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9th Aug 2018, 09:38 PM #3Most Valued Member
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From my view I estimate it's very flat.
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9th Aug 2018, 11:16 PM #4Member: Blue and white apron brigade
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What you are more likely to be measuring is the deviation of any one measuring point from the average local flatness as determined by the plastic ie not very reliable.
If the plastic itself has variations and/or the surface plate has a systematic variation (ie slopes down or up) along the measuring axes these wont be picked.
I suppose I could mount the plate on the mill table and hang a DTI off the end of the quill and move the table around under the DTI - will have a bash at that tomorrow - but that will then include any errors associated with the mill table not being perfect,I know it slopes slightly.
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9th Aug 2018, 11:35 PM #5Golden Member
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Thanks for the comment Pipeclay, I've estimated that it is quite flat as well. In another thread somebody once said if you can see decent reflections in the proposed piece of granite then its worth checking out further and that's what I was hoping to do but brain started getting into a muddle when I was looking at the results I'd gathered and tried thinking about what they really mean.
Michael I've started reading that thread you recommended, I remember reading it a few years ago and not really understanding a lot of it, interesting that it means a lot more to me now.
I still think that any proposed measuring gadget will sit up on the three highest points under its base no matter if its my plastic one or a precision ground stiffer cast iron one and then you are only comparing the height of the local measured point against the plane of the base plate which might not be representative of the rest of the plate. ie any single point can be measured by approaching from different directions and could measure a different height depending upon the direction you approach it from. ..... Cant quite express this idea clearly enough .... am off to continue reading that other thread.
(Edit) G'day BobL - you were typing same time as I was
Bill
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9th Aug 2018, 11:43 PM #6Member: Blue and white apron brigade
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