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4 point surface plate? And recalibration

Glorifire

Aluminum
Joined
Jan 25, 2016
Location
Miami
I just picked up this collins microflat 18x24 surface plate, advertised as grade A. I plan on getting it resurfaced/calibrated to AA so i was checking the mounting points to see if pads were already attached. I found that there were 3 pads attached but there were 4 points marked in yellow ink shown in the picture below( with the single point that should be centered being instead two points positioned off to either side). This wasnt original was it? Should i just reattach the pads at the normal 20% positions before having the plate resurfaced and calibrated?

Screenshot_2017-06-17-15-17-21.jpg
Also the plate seems to be in relatively good condition, for the most part very smooth. There is a small chip(maybe .070") here and there, that shouldnt hinder it from being recalibrated to AA should it, since they are small low spots and dont really affect the bearing surface?
 
Per the national spec, surface plates should be supported at three points.

Geometrically, that's because three points define a plane, unambiguously.

Four points do not define a plane.

- Leigh
 
My 8x12 Starrett plate that I bought new came from the factory like this.
That would accomplish the same thing as long as the two feet were the same thickness.

Most likely they do that so they can use the same feet at all locations.
If they used a single foot at the left, it would have to be twice as strong as those on the right.

- Leigh
 
Hmm so it may just be original, looks alot like the layout on that starrett, i guess ill just put new pads at the marked locations. Still wondering if the small chips will cause calibration issues though
 
You may want to curb your hopes of AA grade unless you plan to keep it in a climate controlled environment. I just had my plate (much larger, but same idea) calibrated and the technician told me they won't even try to lap better than A in a non temperature controlled environment. It makes sense. If the plate has a temperature difference from top to bottom causing a bow, lapping the plate could make it much worse if the temperature differential is reversed.
 
Thank ewlsey, i plan on dropping it off at their facility so theyll equalize the plate's temperature before lapping. Im not sure what the temperature tolerances are, though my shop doesnt fluctuate very much, i figure if i have it lapped to a higher accuracy in a controlled environment then i would have a better end result when it warps slightly in my shop that does vary by a couple degress throughout the day
 
That would accomplish the same thing as long as the two feet were the same thickness.

Most likely they do that so they can use the same feet at all locations.
If they used a single foot at the left, it would have to be twice as strong as those on the right.

- Leigh

We recently bought a 24x36 Starrett and it had only 3 pads. They were square, and appeared to be all the same size/thickness. I would guess (not at shop) they are about 1.5" square x .25-.38 thick?
 
Thank ewlsey, i plan on dropping it off at their facility so theyll equalize the plate's temperature before lapping. Im not sure what the temperature tolerances are, though my shop doesnt fluctuate very much, i figure if i have it lapped to a higher accuracy in a controlled environment then i would have a better end result when it warps slightly in my shop that does vary by a couple degress throughout the day

Do you need a AA for some reason? I wouldn't recommend paying for that if you aren't going into a temp controlled environment, minimum. Can you measure something that would need a grade AA plate? I ask because if you are using a tenths indicator, for example, checking a 1x2x3 sized block...

http://www.tru-stone.com/pages/faq.asp#question14 (no affiliation, first Google response)
 
Im going for AA because the company seem to be charging a flat rate for any grade surfacing so i figured go for the best possible haha. But ill be using it for spotting for scraping and for measuring slidesways
 
Good point Leigh im going to inquire further, maybe i havent gotten the whole picture. Is anyone familiar with Precision Metrology out of Milwaukee,WI? They have a branch here in Dade City,FL that i plan on getting the work done at
 








 
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