Granite plate refurbishing.

[Screwdriver]

dunno why some folk need all this critical accuracy not as if there into aerospace territory . can anyone on here show us piccys of what they are producing that need it

Not me! That's why I am quite happy to watch someone else going to all that trouble. I find it quite compelling for reasons I cannot fathom but some of the workpieces Tom Lipton and Robrenz put out are a joy to behold.

It is interesting that we can all tell just by feel if a lump of metal is some exotic hardened steel item or a dull chunk of mild steel. Don't quite know exactly how that works but there is a definite tactile difference. Similar strangeness with "perfectly" flat surfaces on metrology equipment, the way it glides (and sticks!) to other crazy flat surfaces.

It's just geeking out on the "ultimate" examples of things we commonly do and perhaps aspire to. I still have a hankering for a bit of scraping. Have a go at getting something as flat as I can just to see how flat I can get it...

 

[Parm]

Years ago I watched a father and son duo scrape in and calibrate a very large granite surface table in an aerospace place. It was kept at STP

 

[Pilgrim22]

im sure somewhere from one of the granite manufactures they said that smaller plates are not big enough to deflect an amount that is significant so the 3 point only apply to larger plates

i think it was mitutoyo but im probably wrong


my small cast iron plate had 3 points but thats relatively bendy

Smaller plates need not necessarily be mounted on the airy points but should be mounted on 3 points as they will take the shape of what they are sitting on (if you sit one on a flat steel surface plate or on a wooden bench there will be a difference).

You could lap any size plate flat on any 3 points , even if they are near the edges and as long as those points don't alter, the plate will retain its shape: Gravity is constant. The benefit of sitting the plate on airy points is that it minimises the effect that gravity has on distorting the plate. IE. you don't need to lap as much to get it flat!

 

[mtt.tr]

Smaller plates need not necessarily be mounted on the airy points but should be mounted on 3 points as they will take the shape of what they are sitting on (if you sit one on a flat steel surface plate or on a wooden bench there will be a difference).

You could lap any size plate flat on any 3 points , even if they are near the edges and as long as those points don't alter, the plate will retain its shape: Gravity is constant. The benefit of sitting the plate on airy points is that it minimises the effect that gravity has on distorting the plate. IE. you don't need to lap as much to get it flat!

Im sure you are right im just furthering the discussion.

In my mind ( probably wrong)if the
plate will not deflect under self weight and is able sustain the accepted and likely load with out noticeable deflection which is part of their spec then putting on 3 points or flat will make no difference.

I'll dig out my deflection calculations this takes into account second moment of inertia and modulus of elasticity and enter the details see what it comes up.


This isn't an argument just an attempt at furthering the discussion.

 

[Pilgrim22]

Im sure you are right im just furthering the discussion.

In my mind ( probably wrong)if the
plate will not deflect under self weight and is able sustain the accepted and likely load with out noticeable deflection which is part of their spec then putting on 3 points or flat will make no difference.

I'll dig out my deflection calculations this takes into account second moment of inertia and modulus of elasticity and enter the details see what it comes up.


This isn't an argument just an attempt at furthering the discussion.

No problem MTT im not arguing or saying you are wrong I was just explaining what happens with different scenarios and different plates / tables.

I've calibrated and Ive lapped plates from 12" x 12" up to 22m x 2.2m and everything in between and even small plates will distort.

 

[mtt.tr]

No problem MTT im not arguing or saying you are wrong I was just explaining what happens with different scenarios and different plates / tables.

I've calibrated and Ive lapped plates from 12" x 12" up to 22m x 2.2m and everything in between and even small plates will distort.

In that case you might know the answer , what sort of distortion would you expect to see for instance a 18" x 12" plate. Would for instance a plate suspended at its ends bend enough to go out of its spec?

 

[Robotstar5]

This thread reminds me of when the new Metro body build shop was built at Longbridge in the late 70's, the Metrology room had a gantry CMM mounted on a Granite block capable of measuring complete body shells. The block weighed around 40 tons and came from Canada, well 2 of them did as the transport company managed to break the first one!, no idea what they must have cost.

 

[mtt.tr]

This thread reminds me of when the new Metro body build shop was built at Longbridge in the late 70's, the Metrology room had a gantry CMM mounted on a Granite block capable of measuring complete body shells. The block weighed around 40 tons and came from Canada, well 2 of them did as the transport company managed to break the first one!, no idea what they must have cost.

On eBay you see a number of big tables with lumps missing, Normally some fancy enporium.

 

[Pete.]

I didn't want the thread to turn into a discussion on theroetical issues TBH it was just to showcase my own progress. Everything bends regardless of shape, size or material. I'm going to run a few tests today to check the actual practical effects of shifting support points to the limit of my ability to measure then we (or at least I) will know the difference between what happens and what matters.

In that case you might know the answer , what sort of distortion would you expect to see for instance a 18" x 12" plate. Would for instance a plate suspended at its ends bend enough to go out of its spec?

No, since that's the size I suspended on it's ends with no variation. I'll rig up the autocollimator and get a more meaningful result. Then I'll do it with a bigger plate if I get time.

 

[Shox Dr]

I cant see it making any noticable difference, otherwise it would be specified how and where to mount it. Interested in your tests @Pete.

 

[mtt.tr]

I cant see it making any noticable difference, otherwise it would be specified how and where to mount it. Interested in your tests @Pete.

Agreed.

 

[daedalusminos]

Surely the only correct way to mount it would be how it was mounted when manufactured, I believe there is a spec that defines 3 points 20 - 25% L or W. With the single point centred and adjacent to the labelled face.

I dare say the size and load on my plate it wouldn't make any difference, in fact I just placed it on a sheet of expanded polystyrene.

 

[pressbrake1]

I’m prone to listening to the gentleman that used to relap and calibrate them

 

[mtt.tr]

I’m prone to listening to the gentleman that used to relap and calibrate them

What do they say for diddy plates? Does make a measurable in our personal use use?

 

[My Old Landy]

Mine came in a wooden crate from Chronos and now sits on a rubber mat on top of my Halfords tool box
I don't do anything that warrants checking it and if I managed to deflect it to an extent that I would notice, I will have probably broken it in two

 

[Pete.]

If you manage to put a permanent set in granite then it HAS broken in two

You ought to hear that stuff go when we burst it.

 

[pressbrake1]

What do they say for diddy plates? Does make a measurable in our personal use use?

Personal use? 99% of people could use a piece of mdf and it wouldn’t matter myself included

 

[mtt.tr]

Personal use? 99% of people could use a piece of mdf and it wouldn’t matter myself included

No you wouldn't be seen dead near mdf.



I have a log of ebony real ebony still as a log that's more like it.

 

[Pete.]

Can you get anywhere without an autocollimator?

I'd love to have a go at this (you make it look so easy ) but a used autocollimator costs more than a new plate.

I'm gonna change my mind on this to say 'Not unless you're very experienced'. It's very easy to make a face which is apparently quite flat but actually has a fairly uniform dish, as I did today.

This is my 12" x 18" test subject.



I didn't get a 3d graph sceenshot of it but the figures make it to be no better than a grade1/grade B equivalent. It's low in the middle up to 4.9um on two corners.

So I re-charged my lap and went over it again favouring the corners in an effort to bring them down without going too mad and rolling the edges. The result is much better.







This easily achieves the equivalent of grade 00 and just about makes the US grade AA for the size of plate. Very pleased with that.

Of course it's wholly unnecessary for my purposes but what the hell - it was my first attempt at re-finishing a plate. It doesn't even qualify for the specification which is why I say 'equivalent to' - there are stipulations that it doesn't achieve. That doesn't matter though I achieved my goal and got it from 20 microns deviation to 1.8um globally. I wanted to see if I could hit that spec and I did. I have found that it's quite easy to produce a Grade 0 plate but a 00 is a lot more challenging.

BTW it's so flat now that another plate placed upside-down on it will float right off the edge just from the air molecules not being able to get out from between the plates quick enough

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