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PM25 is here, DRO and table feed installed - now a question

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Dawn

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#1
Perhaps I'm just not understanding (it could happen...) but, I trammed the head per the manual and got it within.0005 from one end in relation to the other. So, then I got curious. By putting an indicator in the quill I wanted to see how 'true' the table runs from one end to the other in relation the quill position. If I start from right to left, stop to stop, it indicates within .0005 at each end. However, throughout the traveling distance the indicator picks up a .0015 'dish' toward the middle of the table.

Any reason I should be concerned about that...?

Thank you all!
 

markba633csi

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#2
Certainly reason to be concerned, however this amount of error may be within acceptable limits- Talk to PM and see what they say
mark
 

francist

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#3
Just throwing this out there, but my immediate thought was is it an actual dish in the table, or is that when the table is at each extreme of travel (the stops) the cantilever is enough to cause the end beneath the quill to elevate in the gibs ever so slightly?

Don't know if that would happen or not -- seems to me it might if the gibs were not snugged. A good straightedge should be able to pick out an actual dish in the flatness of the table surface.

Just my thought, possibly way out in left field.

-frank
 

RJSakowski

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#4
Frank has hit it.

This is probably being caused by the table tilting when at either at either extreme position. At those positions, the center of gravity of the table extends past the support. I have that problem with my Tormach 770 when I move my table more than 10" to the left.

Loose gibs in either or both the x and y ways will cause this and tightening will reduce the amount of lift. Since there must always be some clearance in the ways, the phenomenon won't be completely eliminated but it can be significantly reduced.

Verify that the table is indeed flat with a straight edge..
 

Tony Wells

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#7
When the table is extended to the travel limits, manually lift on the overhanging end and see what your indicator tells you. In all likelihood Frank and RJ are spot on on both counts; it's table tilt caused by gib clearance and can only be reduced but never totally eliminated. It's something that will only affect your long work, but is worth working out as the machine wears. Obviously clearance will increase in the most used area, and gradually you will see this problem grow more pronounced. All mill owners have this issue whether they realize it or not.

I scrapped a drive mandrel cutting some fairly long splines without considering the weight added to the table by the dividing head and part. I didn't realize it at the time but they were cut on a "curve". This was a sizable vertical mill. A horizontal would have scrapped it too, because the depth would not have been consistent, but I would have caught it in-shop instead of having assembly problems at the customer's facilities.
 

Dawn

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#8
Hi All,
After discussing this issue with Precision Mathews it turns out that Frank, and likewise everyone else, was correct. Thank you Frank and everyone.

The .0015 (which by way of gibb adjustment) I have reduced to .0001. According to PM, this is actually really good for this mill/drill.

So, I am happy :chunky:
 

wrmiller

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#9
That is really good for any mill really. In my experience anyway. :)
 
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