Re: [CAD_CAM_EDM_DRO] Reality bites was Re: Step count / inch problem

I'm not sure about the accuracy of them, but it seems like what's needed here,
is to set a std using one of the tape reader type position readers WRT the
actual position vs. what the software/machine says it is. This would give us an
actual map file that would be good for around NXX hours of operation, depending
on the machine. The file could be referenced by driver software in an absolute
way regardless of what mode the rest of the machine might be running in. It's
basically a 3 dimensional array map with one row per dimension, unless you might
want to totally map a machine that's the next thing from flying apart. Seems to
me that this would give us accuracy that would have the aerospace johnnies
drooling, and one guy with a tape/reader could service a big area, and all kinds
of machines.

What thinkest thou?

Bill

On Wed Feb 20 02:11:52 2002, ballendo, <ballendo@...> wrote:

>Oops,
>
>Part of that last reply left off...
>
>Ballscrew "errors". Poor choice of words on my part. What you have
>said, and the Thomson engineer confirmed is correct. I mis-spoke.
>
>This is what I "meant" to convey:
>
>Since the lead error is "constant"(even in the chreapies), in that it
>either is too much or too little as a result of the rolling process
>used to make the screws, it can be cancelled out easily. This means
>it won't "stack up", as we have specified the "correct"(real) number
>of steps per inch/unit. What we cannot cancel out by setting the
>steps per inch/unit is the cyclical errors...
>
>In other words, we can "un-slope" the line by adjusting our steps per
>unit to match the rolled screw error(which eliminates the "stacking
>error"), but the waviness(cyclical variations) will still be there...
>
>Hope this helps.
>
>Ballendo
>
>P.S. The graph you looked at answers your question about the
>relationship of cyclic to cumulative errors... If you define the
>slope part of the graph as .004 (or whatever), you can interpolate to
>get the value of the cyclical "bumps" by their relation to this
>distance on the graph axes.
>
>So what you need are more of these graphs; and the values used along
>the axes when they were created.
>
>
>--- In CAD_CAM_EDM_DRO@y..., "Carlos Guillermo" <carlos@v...> wrote:
>> Hi Ballendo -
>>
>> The Thomson rolled ballscrew literature shows a graph of the type
>> of variation you are likely to see with their ballscrews. They
>> clearly show that the errors DO stack up for longer lengths (they
>> refer to the "Cumulative Representative Lead"), and they also show
>> the superimposed error component that is cyclical, like what you
>> describe ("Lead Fluctuation"). The graph is of a sloped line,
>> with waviness added. Just imagine taking a ballscrew with
>> cyclical errors, and changing its temperature. The overall length
>> will change, and every point along the way will shift
>> proportionally (with even temperature changes), but the cyclical
>> errors will still be there. The cyclical errors are in fact small
>> compared to the cumulative errors. It's just part of the
>> manufacturing process. I spoke to a Thomson engineer who said if
>> you could use software to factor out this cumulative error, you'd
>> compensate for the majority of the error. When they give
>> accuracies of .004"/ft, they're mostly talking about the
>> cumulative errors. I tried to get some numbers on the cyclical
>> errors with no luck. I believe they will map out their more
>> pricey ballscrews for a fee, however. Anybody have this kind of
>> data? It would be interesting to compare the magnitude of the
>> cyclical variations against the cumulative errors, and to see the
>> error characteristics of an actual ballscrew (slope, waviness,
>> arcing, etc)
>>
>> HTH,
>>
>> Carlos Guillermo
>> VERVE Engineering & Design
>>
>> Ballendo wrote:
>> <snip>
>> Ballscrew errors, even with the cheapies, do not usually stack
>> up; but instead are cyclical variations within shorter distances.
>>
>> <snip>
>
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