Support pad locations on Starrett surface plate

[Yan Wo]

I just bought a seldom used and 20 year old (or so he thought) Starrett 24 x 36 x 6 pink granite surface plate.

I now want to build a stand for it. I haven't been able to look at the bottom of the plate. Do Starrett plates typically have support pads -- or some indications of where the supports should be -- on the bottom?

A Starrett web site says to place the three supports at 1/5 length from both ends with one centered and the other two at 1/5 width from the edges. Someone here on PM said the single support should be on the end with the precision label.

Any advice is appreciated!

 

[eKretz]

Have you had a look under the plate at the bottom surface yet? You might find the original support points are visible. On all of my large plates they were. There were "shadow" prints of the original support locations from having been there so long.

 

[Richard King]

I just bought a seldom used and 20 year old (or so he thought) Starrett 24 x 36 x 6 pink granite surface plate.

I now want to build a stand for it. I haven't been able to look at the bottom of the plate. Do Starrett plates typically have support pads -- or some indications of where the supports should be -- on the bottom?

A Starrett web site says to place the three supports at 1/5 length from both ends with one centered and the other two at 1/5 width from the edges. Someone here on PM said the single support should be on the end with the precision label.

Any advice is appreciated!

Thats the Specs they use. I use 30%. Google 3 point plate support location practical machinist as we have discussed this several times over the years. Call tru stone Starrett on Monday and ask them to send you a PDF on this.

 

[Mark Rand]

Federal Spec GGG P 463c says a third to a quarter of the way from the edge in both lenght and width, plus central for the third point.

When you make the stand, put three supports at the single point end. Make the outer two so they can be snugged up, but not taking any significant load. That saves you having a lump of granite on your feet when something heavy is put on the corner...

 

[Larry Dickman]

Installation Instructions

Industrial Granite and Services

standridgegranite.com

 

[sfriedberg]

When you make the stand, put three supports at the single point end. Make the outer two so they can be snugged up, but not taking any significant load. That saves you having a lump of granite on your feet when something heavy is put on the corner...

Let me expand on that just a tiny bit. Suppose the plate is sitting on three points. You put something heavy (for example, a 500lb milling machine saddle) on a corner at the one-point end and it can move the center of gravity outside the three support points, flipping the plate off its supports. To protect against that, your stand should have some strong additional supports, not normally in contact with the plate, roughly 1/4" below the bottom of the plate. On many stands this is a frame of angle iron that runs all the way around the plate. This additional support is for safety, not normal operation, and prevents the stone from falling off if it becomes unbalanced. (Perhaps obviously, you should not try to actually use the plate while it's resting on the safety supports. Move the heavy load so the plate is resting on only its three normal supports.)

 

[Yan Wo]

This is getting interestinger and interestinger!

Here's an image of the tag on the dual-support end of my 24 x 36 plate:



I was able to get a good view of the underside of the plate. Here's what I observed:

There are four pads. Each is 1/2" thick and 2-1/2" diameter. They are some kind of fairly hard black elastomer, somehow bonded to the plate.

The two pads on the single support end are tangent and the tangent point is centered on the 24" width and their centers are 9" from the end of the plate. That's 25%. I assume two pads were used so the weight on each would be the same as on the single pads on the other end.

The centers of the two pads on the double support end are 9" from the 36" length and 6-1/2" from the 24" width. That's 25% and 27%.

I phoned Starrett as Richard advised and they emailed this pdf of a hand-drawn sketch:


So my pads are close to half way between what Starrett currently recommends and what Richard recommends.

I am very curious as to why Starrett has apparently changed the pad positions over the years.

(PS: I'm happy that I can now post images that are full size.)

 

[Richard King]

Scroll down to page 5

https://www.tru-stone.com/pdf/FedSpecGGG-P-463c.pdf

Contact Tru-Stone

www.starrett.com