RE: [CAD_CAM_EDM_DRO] Positioning

Capacitive type scales are cheap because of the way they are manufactured, that doesn’t not affect their accuracy.



It’s the same reason why bolts are cheaper than nuts – you can forge and roll the thread on a bolt, while you need to cut the thread on a nut. The operations to make a bolt are cheaper than making the nut for it.



Most capacitive scales have a resolution of 0.0005” of an inch, and can be made to measure less than that. Yes, even under a micron (and mostly better electronics, btw).



If you have a source for their inaccuracy (apart from your rather fertile imagination) let’s see it. 0.04mm? Hah.



Capacitive scales have two drawbacks – they’re affected by oil & crud and have slow-ish data rates (50 readings per second?). Doesn’t stop people using them though. The ‘jitter’ people find with those scales is power supply problems, usually caused when powered from a DRO rather than battery. Or RF, but that happens to everything.



If you bothered to calculate the thermal coefficient of your 600mm screw, you’d find for every degree (Celcius) change in temperature it will expand by that micron you claim to machine all of your parts to (including those “high-precision” spanners you were going to make – your funniest post ever).



Unless you have a temperature controlled room (to a degree!) AND are keeping every part of your machine at that temperature, your parts are not the size you imagine they are. And don’t forget your scales are change shape at a different rate. Oh, and of course your screws are liquid cooled as well.



Sure, add scales. In theory they should tell you where everything thing is (you believe the brochure?), in practice close enough. I realise that’s unsuitable for the fantasy-land you do your machining in, but anyway.



Machines with backlash shake themselves apart? Lol.



You really should do some machining one day, you might learn something.



Tony









From: CAD_CAM_EDM_DRO@yahoogroups.com [mailto:CAD_CAM_EDM_DRO@yahoogroups.com]
Sent: Tuesday, 3 June 2014 6:10 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] Positioning



The specs on the screw accuracy are irrelevant.
What IS relevant is that there be no slop, as a machine with dual-feedback, ie glass scales, will shake itself to pieces is it has backlash.

Thats why you cannot use the crap chinese scales, because they are off, ie inaccurate, by upto 0.00" or 0.04 mm, in short distance, and the error varies.
So using such, would make the machine jitter (sometimes, in some places).

This causing back-and forth motion, upto hundreds of times a second, depending on how fast the secondary positoning error-correction mechanism is.

It is impossible to make a (working) secondary-feedback mechanism if you have backlash in the system.

Tony..
Unlike some people here, I try to be factual and polite.
Grin.

Re: screw accuracy.
Everything is inaccurate. Its just a question of how much.
One good part about glass scales, is that they are usually accurate (to 1 micron, or down to 0.01 micron with some Mori Seiki machines).






Sure, when you read the specs on the average screw, and that "Dummies Guide To Fantasy Machining" you've got.

Do you know what a drunken thread is?

Do you know what a temperature coefficient is? (Go work out the expansion on a 600mm steel screw for a 1 degree delta.)

Tony

(You and your microns, lol)





--
-hanermo (cnc designs)










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