Monday, 19 September 2011

Imperial vs. Metric

Was just told:

I was rereading one of my books the other night and came across something that applys to you. Measure in metric, multipy by 40, call that 0.001"s. If you want the last cut to be 0.1mm move it 0.004" instead, you'll be out by 0.000063" or 0.016mm, not often that that will matter.

Might help me get my metric mind around my imperial Myford!

Normally most wouldn't aim to take that much off to get to size without measuring along the way.
But the math would be

5x40=200. wind 200 on your crossslide. 200/1000 is 5.08mm so in a perfect world you would be out by 0.08mm.

Normally I would take somethign like 4mm off and measure again.
4x40=160 (4.064mm). That would leave you(in a perfect world) with 0.936mm x 4=37.44. Call it 37.5 on the dial. That would take you to a total of 197.5/1000. total of 5.0165mm. See how the error is much reduced and in most cases not worth talking about(depending of course what you are making, but it you are working to that close a fit you would likely use more steps)

p.s. this ignores that metric lathe crossslides are normally moving half the distance. imp crosssildes aren't like that.

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