RE: [CAD_CAM_EDM_DRO] Positioning

The point made was they were inaccurate because they were cheap - that's not the case.

They're cheap because they're made with 1970's era PCB manufacturing. Hell, some hobbyists could make the PCB for those callipers themselves.

Cheap doesn't mean bad - a $2 digital watch will outperform a Rolex (measuring time at least).

+/- 0.02mm sounds about right given the display. Imperial would be 0.001" as they show 0.0005"

Tony

> You are confusing resolution with accuracy. Mitutoyo who specify each
> separately specify a resolution of 0.01mm but an accuracy of +/- 0.02mm for
> their 150mm digital calipers.
>
> Keith
>
> > -----Original Message-----
> > From: CAD_CAM_EDM_DRO@yahoogroups.com
> > [mailto:CAD_CAM_EDM_DRO@yahoogroups.com]
> > Sent: 03 June 2014 10:02
> > To: CAD_CAM_EDM_DRO@yahoogroups.com
> > Subject: 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)
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > [Non-text portions of this message have been removed]
> >
> >
> >
> > ------------------------------------
> > Posted by: "Tony Smith" <ajsmith1968@...>
> > ------------------------------------
> >
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> ------------------------------------
> Posted by: "Keith Burton" <kb@...>
> ------------------------------------
>
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> davemucha@... [Moderators] URL to this group:
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>
> OFF Topic POSTS: General Machining
> If you wish to post on unlimited OT subjects goto:
> aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach it if
> you have trouble.
> http://www.metalworking.com/news_servers.html
>
> http://groups.yahoo.com/group/jobshophomeshop I consider this to be a sister
> site to the CCED group, as many of the same members are there, for OT
> subjects, that are not allowed on the CCED list.
>
> NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING
> THEM. DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
> bill
> List Mom
> List Owner
>
> Yahoo Groups Links
>
>
>