Re: [CAD_CAM_EDM_DRO] Positioning

on 2014-06-18 15:46:04 UTC

Gentlemen, I've had enough! Very few hobbyists need to machine to any of the tolerances that have become the core of this thread. The arguments have become too technical for most without an engineering degree to understand it's relationship to their own situation. Even professional shops use a sensible approach. OK, so the aerospace industry needs this precision, but the majority do not. Under most circumstances, a machine is deemed adequate for it's purpose if it produces work within certain tolerances.

For most hobbyists and small professional shops with CNC equipment, there are other sources of error which will add to the cumulative error in the dimensions of the final work piece. Knowing that rigidity is an issue I bought a small lathe built like a tank. I have a basic Acme threaded screw and a modified Acme nut. I have done what I can to reduce sources of mechanical errors in this lathe and my small milling machine. I have educated myself in tools used for roughing, final finishing cuts and, where necessary, hand polishing and lapping methods. I have no plans to build a titanium-skinned hypersonic aircraft, anything going into orbit or for automobile racing over 300 Kph. I have 2 sets of measuring tools, one for regular use (Chinese, but OK) and another Japanese/US set that are more accurate. I accept what Mitutoyo, Fowler and Brown & Sharp tell me. I do not need to subject these tools to measurements to the nearest nanometer.

To set up my machinery, I used web text and video resources and troubleshooted any problems. I also referred to an extensive library of engineering 'how to' books available at home, but available from any public library. I had a couple of machinist friends check my set up after I had completed the task (beer being the standard local currency). I am happy that my machines will do what I want them to do. If I need better, then I will work out how to achieve it.

Now I will send this message and unsubscribe from future feuding posts. Enjoy yourselves! Dr Dave

On Thursday, June 12, 2014 4:54:56 PM, "'David G. LeVine' dlevine@... [CAD_CAM_EDM_DRO]" <CAD_CAM_EDM_DRO@yahoogroups.com> wrote:

On 06/11/2014 02:32 AM, Steve Blackmore steve@...
[CAD_CAM_EDM_DRO] wrote:
> On Wed, 11 Jun 2014 01:11:23 -0400, you wrote:
>
>
>> Let's look at your points in order:
>>
>> Thermal expansion should not affect backlash, it should affect
>> positional accuracy. Since backlash is clearance or lost motion in a
>> mechanism caused by gaps between the parts, and such lost motion is
>> possible to eliminate, thermal expansion may or may not matter. The
>> joint may not be rigid, but that is another matter.
> What other matter?

Rigidity and positional accuracy are different from backlash.

Assuming reality causes some other things to come into play, like
measuring below the atomic level.

Let's use a ball screw with two nuts and one of those nuts spring loaded
against the other. Free space is NOT necessary as long as the spring
compliance can handle the loads. Lubricant films will only matter at
the point where positional inaccuracy is affected by the lubricant film
differential thickness, and since the backlash is well below this
number, can be compensated by servoed adjustment with a measuring system.

> There has to be some free space to allow for initial
> thermal expansion in the joint as it warms up during use, and still some
> left to allow the surfaces to move. No space, no move.

Wanna bet? The above example has NO free space (but has compliance),
and can be horribly inaccurate while having no backlash.

>
>> Rolling and sliding friction can cause backlash, it is true, but that
>> assumes material contact. Magnetic suspension in a vacuum does not have
>> rolling or sliding friction. It may not be practical for your
>> operation, but it exists.
> What Vacuum - we are talking about machine ballscrews, not some fantasy
> sci fi set in space?

How about magnetic positioners used in typewriters (yes, Brother had one
of these many years ago), and old disk drives? Zero backlash!

>
>> Lube films do not need to be matter. Magnetic suspension in a vacuum
>> does not have any lube film, nor does it have a wear issue.
>> Electrostatic suspension in a vacuum is similar.
> Forget your fantasy and get back to the task in hand - machine
> ballscrews..

Okay, with air bearings, the lube film (dry, clean gas) does not cause
backlash, but has a compliance of a million (or so) pounds per inch,
Newway Air Bearings describes sub micron machine tools and the
leadscrews can run similar bearings. There is a compliance error, but
no backlash error. I think you are confusing the terms.

>
>> As I said, the caveats are pretty severe, and the positioning system may
>> be too compliant, but the backlash can be zero.
> Try working in the aerospace or precision engineering industry for a
> while then come back and tell us how it works in the real world.

When I worked in metrology, measuring linear encoders, we were looking
at errors which could not be seen (the wavelength of light was just too
big!) Knowing the difference between backlash, compliance, accuracy,
resolution, etc. really did matter.

>
>> For a common, backlash free system used in real life, look at a speaker
>> voice coil mechanism. It has no backlash, but it is far from rigid.
> Again - BALLSCREWS - not speakers...
>
> Steve Blackmore

Your initial point was that no backlash free system is possible, Several
backlash free systems have been shown here, INCLUDING some which can be
applied to leadscrews. What you seem to be missing is that there are
OTHER errors which need to be examined, for example, quantizing errors
due to the atomic structures. No matter can be fine enough when looking
in the 10^-15 meter range, or lower. For that the measuring mechanism
based on atoms is WAY too coarse. And backlash based on averaging many
particle positions can be less than that. Electrostatic measurement can
be in that range, and backlash can be much less than that.

You are correct that the hobbyist will seldom (if ever) be looking at
that level, so FOR THE HOBBYIST, zero backlash is pretty simple since
the measurement error can easily swamp any small backlash. For the
metrologist (who sees the thermal effects of opening and closing an
airlock door as huge), backlash at the below the 1 hydrogen atom range
can be further reduced.

In the aerospace and precision engineering arenas, the question of
errors below 10^-15 meters don't matter, most machining is not able to
cut a hydrogen atom in half. When the backlash ceases to matter, other
effects do matter and backlash is swamped by quantizing errors due to
the size and distance between atoms and molecules.

Dave 8{)
--

"A word to the wise ain't necessary - it's the stupid ones that need the
advice."

Bill Cosby

Discussion Thread