Thread: Testing a second hand surface plate

Hi Michael,

Looking good, now we just need to tweak it until your plate passes

I've just been thinking(never a good thing). How much of an error will you get if the two single measuring points(for lack of a better name) arent dead center and square between the other two points?*

Stuart

*when the "two other points" roll

One thing I did notice while playing was that in a traverse (either pulling towards me or pushing away) I'm "only" getting say 60 microinches of movement (or less in some positions). When I move to another location though the dial reading will change. The hinge is a bit of spring steel with some 3/16 holes in them and M4 button head capscrews to secure. Using a crappy bolt as a measuring pad relies on always measuring in the same place on the bolt head. I'm wondering whether the hinge can shift slightly when changing direction, disturbing the CBP alignment (CBP = Crappy Bolt Position).
I may need to counterbore the hinge bolts slightly and fit a close fitting sleeve to make up a mini shoulder bolt to prevent that movement.

Michael

Originally Posted by Stustoys

I've just been thinking(never a good thing). How much of an error will you get if the two single measuring points(for lack of a better name) arent dead center and square between the other two points?*

*when the "two other points" roll

Not sure what you mean. Neither of the standards give a tolerance on things, just nominals.
The three feel on the body are just to establish a plane, the 4th foot is there measuring a relative distance from that plane. (Strictly speaking I guess you are establishing a second plane with the 4th foot and the hinge axis). With a perfectly flat plate (or spherical one too) the distance from the surface to the plane is going to be constant.
Therefore, it shouldn't matter if the three points on the base are not geometrically symmetrical because their only function is to define the plane. Provided that the fourth foot is the right distance from the others, that precise geometry should not matter either. If one of the 3 feet defining the plane landed in a hollow in the plate, that might affect the reading (the roll?) but as the divisions are 20 micro inches over a 4 inch base, the error induced is going to be tiny.
(I think)

Michael

Hi Michael,

A picture should help me explain what I mean.

The first is the ideal pattern. Rolling the two bottom points changes the reading, as do the next two(by even more)
The last one is (to all intensive purposes) uneffected.


I think lol
Maybe I am thinking to much. Given the amount of "roll" there is likely to be wi;; it even be an issue? I have no idea, but 110 microinches doesnt give you much to play with lol


Stuart

p.s. How about 4 pins in the hinge?

See attached.
Scan5 (Large).jpg

By my calculations (and by the way, I thought you and Ewan were the number jugglers around here?)

If a foot is in a maximum depth hole in the plate (110 micro inches) for the ideal configuration the dial will show a movement of half the depth of the hole. By comparison if the measuring foot is translated by ¹/₈" then the effect will be an additional (+ or -) 7 micro inches on the apparent movement shown on the dial caused by the hole. Roughly 12 ¹/₂ %, or a ¹/₃ of a division on the supramess.

Doubt if I'm losing sleep over that one...

Michael

Originally Posted by Michael G

By my calculations (and by the way, I thought you and Ewan were the number jugglers around here?)

I thought you let autoCAD do all the hard work.....

Nice work on the "MPT" Looks just like a Rahn, getting that Mahr was a really good score. I regard my 3 Mahr indicators as the best i have hands down......

Ew

Originally Posted by Michael G

(and by the way, I thought you and Ewan were the number jugglers around here?)

No No, I just come up with the ideas, I let others do the grunt work..... as I wouldnt really have a clue where to start.
I can follow your maths, but I'd be surprised if I came up with the same answer if I tried to muddle through on my own.

So as long as the marking out isnt done with a cold chisel.... it should be "close enough"

Thanks

Stuart

Originally Posted by Ueee

I thought you let autoCAD do all the hard work.....

Nice work on the "MPT" Looks just like a Rahn, getting that Mahr was a really good score. I regard my 3 Mahr indicators as the best i have hands down......

Ew

Well, didn't you see that AutoCad drawing in Michael's post?

Dean

Looks good, I'm still waiting for Josh to make one Without a 20 uinch indicator, we will be using the Sony gauge, it's got 20 uinch resolution, (1/2 micron) plus stores max min readings.

How did your plate measure up, did it pass the 60 uinch test?

Regards
Ray

Originally Posted by RayG

How did your plate measure up, did it pass the 60 uinch test?

Hard to tell, but I don't think it will. As I said above I think I have some hinge movement which is causing me some grief. If I can get the movement out I think I should make the class B requirement. Looking at the plate under good light it is pretty sorry - some small chips, a deep scratch and several lighter ones. The other thing I want to do is talk to some of the monumental masons around here. I don't know how good their equipment is but they may be able either resurface it for me or give me some advice on how to do it myself.
Stuart suggested some pins to locate the shim but it is not all that thick (15 thou), so a pin may not do the job. I did think of cheating and using some bearing glue. The other option is to remake the hinge with ⁵/₃₂" holes. As well I need to re-mount the gauge position. At the moment it is 3mm further out than it should be (adds roughly 3% error) and it irritates me!

Michael