RE: [CAD_CAM_EDM_DRO] Capabilities of SPC (digital output) from indicators?

"John P. Flanigan [Software Technology Initiative, Inc.]" <JFlanigan@...> wrote:

Thank you Ron. You have given substance (and confidence) to my vague
notions about how to proceed. I was thinking of using an angle plate
clamped to the table.

///Any good vertical surface.

On further thought, isn't the test I described actually a backlash test and
not a repeatability test?

////Not really, If I understood the question correctly. Moving in lets say X+ then take a reading then moving in X- then reversing back to X+ and reading the dial indicator once again is actually irrelevant. A machine that failed this test would have debris on the leadscrew or something is loose (the very reason 3-jaw scroll chucks should be disassembled and cleaned frequently as they are trash collectors and a chip on the scroll can result in one jaw progressing sooner than the others). One is returning to the same position with all the same factors that resulted in the first reading. Most machines will perform this test quite well under a no-load situation. Whats more important is the amount of backlash present which will display errors in motion related to arcs or anytime a + and - direction is performed in one operation. A test equally important is the third test referred to and that is accuracy over distance. Which would effect overall dimensions i.e. dowell to dowell alignment
in work pieces. A few thous difference could result in parts that will not assemble. /////

I hadn't thought of the jog test you mentioned as
a test for positioning accuracy. I guess this test would require an
indicator with a decent range.

////.001" resolution and .100" range is sufficient.

Regards,
John

-----Original Message-----
From: R Rogers [mailto:rogersmach@...]
Sent: Monday, December 20, 2004 12:00 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] Capabilities of SPC (digital output) from
indicators?


Its measuring repeatability. It is commonly measured and can be achieved by
a dial indicatior mounted in the spindle touching off on a vise or any
vertical surface at right angle to the table. Touch off and zero indicator
and machine control, move away then return to zero then indicator and
machine control should be at zero once again. Any discrepancy is the value
of "repeatability". Another test is one for backlash. Touch off again with
dial indicator and jog control in .001 increments, the number of inputs that
fail to read on dial indicator is the amount of backlash present. Many
softwares allow the user to compensate for this. Another test requires a
precision ground standard that involves touching with the indicator to zero
then traveling a known distance to another portion of the standard, and zero
on the indicator to test the accuracy of the pitchlead of the leadscrew,
handwheels and control over a distance. A 1-2-3 block mounted to the table
with a gage-block clamp!
ed to one
end would work also. A good test in looking for a used mill is simply
grabbing the end of the table and see how much it will shake in and out at
right angle to the "Y" dovetail. Quickly tells how much use its had. If
adjusting this out of the table with the wedge in the dovetail results in
the table locking up towards the end of its travel, the machine is shot.

Ron





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