Lead screw / DRO accuracy

After reading the article in the latest MEW I thought I would check the Y-axis leadscrew of my chinese mill against the cheap chinese DRO
I have fitted.
I noted the DRO reading for every full revolution of the leadscrew for about 400 mm ie., 100 points. According to the DRO the the leadscrew is short by 1 part in 2000 with a random error of plus or minus one thou.
Given the low workshop temperature (about 15C) at present and the high temperature in chinese factories I have visited in the past perhaps the error is due to thermal expansion/contraction of the leadscrew? Any ideas as to how significant this effect would be?
Russell.
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message

DRO
for
leadscrew
high
None whatsover unless you are trying to make parts to a greater accuracy of +/- half a thou and even then you can compensate by relying on external measuring equipment that is accurate to your tolerance needs. (ie 1/10 thou micrometer)
AWEM
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On Tue, 28 Feb 2006 22:57:31 +0100, Russell Eberhardt

Given the curvature of the earth from the UK to China this is well within tolerance.
No seriously any screw with this small amount of error is well within manufacturing tolerance, in fact I would go as far as to say it's bloody marvelous and I'd like to see reading taken off similar machines.
-- Regards,
John Stevenson Nottingham, England.
Visit the new Model Engineering adverts page at:- http://www.homeworkshop.org.uk /
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On Tue, 28 Feb 2006 22:37:11 GMT, John Stevenson

Yes, it's better than I expected - maybe both the leadscrew and the DRO are out by miles in the same direction! Not bad though for a ฃ500 mill with digital scales thrown in for free.
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Well, you always get +/- one digit. Thats the way it is with digital electronics. But 1/2000 is 0.05% and that is not bad. And, who is wrong? The leadscrew or the scale? You need gauge blocks to check that.

The scales relay on a thin (0.6mm) PCB (printed circuit board) to be precise. The PCB is glued in a recess of the scales body. One scale I took completely appart had the PCB glued in with some ... ummm ... gum. Worse than contact cement. So the PCB will shrink and grow with temperature. And I _guess_ faster than the steel body. Anybody got the thermal expansion coefficient of a PCB?
Nick
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wrote:

FR4 epoxy which is a fairly common circuit board material is 5E-5/oC
Steel is around 1.2E-5/oC
So its most likely that the scale will expand faster than the screw
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Thanks for that number! Yes, it looks like FR4.

But not too bad. I would have expected more.
Nick
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wrote:

There's a lot of glass in it - which lowers the expansion quite a lot
I got my figure from a student's thesis - I've seen lower figures quoted for FR4 - down close to steel - my guess is there's not a huge amount in it
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----- Original Message -----
Newsgroups: uk.rec.models.engineering Sent: Wednesday, March 01, 2006 6:35 PM Subject: Re: Lead screw / DRO accuracy

for
Of cource if you are doing work so accurate this maters... the work pice will need a specification which includes the tempreture it's to be measured at, and you will need corection tables for both your DRO and your lath... or a tempreture controled workshop.
Having just had to warm my DRO to get it to work *at all* in a workshop just above freesing the tempreture contrled workshop seems very appealing :-)
Regards Jonathan
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