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Starrett Granite Plate support point locations?

Luke Rickert

Hot Rolled
Joined
Oct 24, 2007
Location
OSLO
I can call them tomorrow but would like to finish up a design today. Does anyone know where Starrett puts their support pads on the bottom of their plates? It doesn't appear to be listed on the website. I really just need the distance in from the ends? (1/5th, 1/3 etc) this is a 36x48x8 (in) plate. From the pictures they appear to be rather close to the ends, I might suggest more ideally placed points further in.

thanks
Luke

G-80710 36" x 48" x 8" - Grade AA, No Ledge
 
I dont know the number but if you wanted the plate better than say, 50 millions flat, (4times better than it is?) the three points would need to be in the same place as they were during the lapping process. Once the plate is rotated 180 relative to the 3 point support it wouldnt be flat anymore.

This may or may not matter for that plate, but it would for a thinner one.
 
Doesn't help you today, but new Starrett plates have pads glued to the support points to make it obvious.

L7
 
Johansen is absolutely correct. The exact distance does not really matter, what really matters is that the pads were on the plate somewhere near the Bessel points when they lapped the plate and they are in the same position when you set it up. If you are planning to get the plate re-lapped, just glue them on somewhere near the Bessel points in both X and Y and have them lap and calibrate the plate, and make sure they never move after that. BTW The Bessel points will give you maximum flatness while the Airy points will give you vertical sides, doubt that matters for a surface plate.

A comparison of Airy and Bessel points :)

Airy points - Wikipedia

This what Starrett says about the subject of supporting a plate:

How should my surface plate be supported? Does it need to be level?
A surface plate should be supported at 3 points, ideally located 20% of the length in from the ends of the plate. Two supports should be located 20% of the width in from the long sides, and the remaining support should be centered. Only 3 points can rest solidly on anything but a precision surface.
The plate should be supported at these points during production, and it should be supported only at these three points while in use. Attempting to support the plate at more than three points will cause the plate to receive its support from various combinations of three points, which will not be the same 3 points on which it was supported during production. This will introduce errors as the plate deflects to conform to the new support arrangement. All Tru-Stone steel stands have support beams designed to line up with the proper support points.

If the plate is properly supported, precise leveling is only necessary if your application calls for it. Leveling is not necessary to maintain the accuracy of a properly supported plate.

Source: Tru-Stone frequently asked questions: The world's largest manufacturer of custom precision granite




dee
;-D
 
The supports for most plates are two in the front (along the long side) and one in the back at the 20% positions, as mentioned above.
So really there are only to ways of doing it; on one or other side. If done correctly (50% chance...) the plate will be flat. If done in reverse, the surface plate will show a twist and need to be turned around.
I would say to put the supports in the right pattern of the triangle, to mount the plate and if twisted to rotate it 180 degrees. The twist can be measured with a auto-collimator, precision clinomete or precision level (one arc second range) or the type of indicator known as Repeat-O-Meter. One of those tool is needed in any case when one needs to re-inspect the surface plate after a period of use.
 
Johansen is absolutely correct. The exact distance does not really matter, what really matters is that the pads were on the plate somewhere near the Bessel points when they lapped the plate and they are in the same position when you set it up. If you are planning to get the plate re-lapped, just glue them on somewhere near the Bessel points in both X and Y and have them lap and calibrate the plate, and make sure they never move after that. BTW The Bessel points will give you maximum flatness while the Airy points will give you vertical sides, doubt that matters for a surface plate.

A comparison of Airy and Bessel points :)

Airy points - Wikipedia

[snip]




dee
;-D

I actually suspect that surface plates are suspended from the minimal sag points, which are the points where the “beam” has equal sag in the center and edges. Minimum sag points are slightly (<2%] farther apart than Bessel points which are mostly used in length measurement between reference points.

I doubt it matters as long as the support points are repeatable since the preload is known ( 8” of granite in the OP’s case)and accounted for in lapping. But if there were a change in gravity( which does vary up to about 1%) it would make a difference.

If you buying a surface plate, you absolutely want to use the original points, or you paying for flatness you won’t get. ( if reusing an existing plate, you useage may dictate the that support barely matters.)


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The matter is academic regarding 20% in from the edges or 30% in.

But the location matters. Imagine holding a foam mattress up on three cardboard boxes. Now shave the top flat. Moving the boxes 10% will only affect the flatness a small amount. But rotate the foam 180 degrees and where you shaved the foam off, is now a valley.


How much this matters for a 36x48x8 plate i habe no idea. On the order of 50 millionths is my guess.
 
The matter is academic regarding 20% in from the edges or 30% in.

But the location matters. Imagine holding a foam mattress up on three cardboard boxes. Now shave the top flat. Moving the boxes 10% will only affect the flatness a small amount. But rotate the foam 180 degrees and where you shaved the foam off, is now a valley.


How much this matters for a 36x48x8 plate i habe no idea. On the order of 50 millionths is my guess.

We have a 4x8 surface plate outside that failed inspection, and it was decided not to be relapped, the actual support points look like 3x5 rubber pads. I think it’s 12” thick- weights about 5k ( the forklift barely could pick it up). I had never bothered to look until today. Also saw someone hit it with a car- that can’t have been pretty.

I have no feel for deflection in granite, but I know you pay more to go from a 50 psf to 100 psf load rating, so it’s obviously measurable, even when the load is a fraction of the plate weight (150 lb/cu ft)






Sent from my iPhone using Tapatalk
 
from Tru-Stone Technologies: The world's largest manufacturer of custom precision granite, standard metrology products, surface plates

Granite has a coefficient of thermal expansion of approximately .0000035 inches per inch per 1°F. As an example: A 36" x 48" x 8" surface plate has an accuracy of .000075" (1/2 of Grade AA) at a gradient of 0°F, the top and bottom are the same temperature. If the top of the plate warms up to the point where it is 1°F warmer than the bottom, the accuracy would change to .000275" convex!!! Therefore, ordering a plate with a tolerance tighter than Laboratory Grade AA should only be considered if there is adequate climate control.

---
I'm trying to work the numbers out and i'm coming up with a seemingly impossibly low value of on the order of 5 millions of an inch deflection between the 3 point supports on the plate.

so for instance, an 8x8 granite beam 48" long: if you support it at the ends it will sag about 1.5 tenths in the middle. you move the support points in, 1 foot from the ends and that middle section will sag just 5 millionths.

maybe i did something wrong but, i'm finding out the wikipedia grossly exaggerated image indeed captures the ratio between the beam deflections for different support points.


Airy points - Wikipedia
 
A while I ago out of interest I did a quick an dirty test for deflection on a 3x2 Granite. Not that thick and 3 point supported out near the edges (not uncommon in the UK it seems), a 50 millionths indicator set up the hot way out at the end of the beam could see no change when the plate was centre loaded with 60KG (it kept this zero after unloading).
Granite plate deflection

Ive seen the same 3x2 move due to environment change tho, ie enough to see with a 10 second level. Being in a room with a twin wall opaque plastic roof on a sunny day was enough to dome this previously concave 0.0001" plate something close to convex 0.0001" (measured with an AC). That might be enough to get you going if you were trying to final up that prized SE :D
 








 
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