Calibrate a larger plate with a smaller?

Hello,

Is it possible to calibrate a larger surface plate with a smaller one? I have a granite surface plate that is 640x400 mm that is flat. I found a larger cast iron surface plate that is 1000x700 that I'd like to buy but I'm not sure how to recalibrate it. I've always scraped from a larger master to a smaller, never the other way around. I considered using a small cast iron surface plate that is 400x400 mm to transfer from the known flat diabas plate to the new larger one, and then try and measure the high spots. I assume with enough overlap that the larger plate would eventually get flat but it feels like a brute way of doing it. Is there a proper way of doing it?

Best regards

corlm said:

Hello,

Is it possible to calibrate a larger surface plate with a smaller one? I have a granite surface plate that is 640x400 mm that is flat. I found a larger cast iron surface plate that is 1000x700 that I'd like to buy but I'm not sure how to recalibrate it. I've always scraped from a larger master to a smaller, never the other way around. I considered using a small cast iron surface plate that is 400x400 mm to transfer from the known flat diabas plate to the new larger one, and then try and measure the high spots. I assume with enough overlap that the larger plate would eventually get flat but it feels like a brute way of doing it. Is there a proper way of doing it?

Best regards

Click to expand...

Yes. One uses a spherometer. With some attention and a good lapping strategy a larger plate can be calibrated ( whatever that means...) to half the deviation of the smaller one. This can be later scraped with the special pattern for printing but I think it's way too much trouble.

It's definitely possible. Member Cash had his big surface plate (maybe 5' x 10'?) Lapped in 2017 and they used much smaller plates to do it.

Maybe he'll chime in.

Sent from my SM-G930R4 using Tapatalk

corlm said:

Hello,

Is it possible to calibrate a larger surface plate with a smaller one? I have a granite surface plate that is 640x400 mm that is flat. I found a larger cast iron surface plate that is 1000x700 that I'd like to buy but I'm not sure how to recalibrate it. I've always scraped from a larger master to a smaller, never the other way around. I considered using a small cast iron surface plate that is 400x400 mm to transfer from the known flat diabas plate to the new larger one, and then try and measure the high spots. I assume with enough overlap that the larger plate would eventually get flat but it feels like a brute way of doing it. Is there a proper way of doing it?

Best regards

Click to expand...

my 2c

Theres two numbers you want to concern yourself with for a surface plate. Global flatness and local flatness.
Providing your granite plate is good (theres no such thing as flat), scraping your 400x400 and overlapping the print on the larger plate should get you pretty close, but id think youd still need to keep an eye global flatness.

If it were me, after having a measure up to get the general shape of the plate, id probably go about getting the main lines (union jack style) in first, starting with the diagonals. Id use a straight edge for the print and something to keep an eye on the global number, eg:- autocollimator, repeat meter (using the best line on the granite to zero from), level/s.
When im looking good through all eight lines id can concentrate on the areas between them, mostly using the 400x400.

If the above is close to an accepted approach, its a tough gig next to lapping granite imo.

This video might help. No english, but if you forward through the action shots it gives a look over the shoulder of how these guys approach it. Notice theyre applying the red to the work and polishing tops with the reference. Youll also see use of ACs , and electronic indicators and smaller plates for the local, nice work imo.

匠の息吹を伝える～"絶対"なき技術の伝承～　（７８）平面を制する　～精密大物きさげ加工～ - YouTube

corlm said:

Hello,

Is it possible to calibrate a larger surface plate with a smaller one? I have a granite surface plate that is 640x400 mm that is flat. I found a larger cast iron surface plate that is 1000x700 that I'd like to buy but I'm not sure how to recalibrate it. I've always scraped from a larger master to a smaller, never the other way around. I considered using a small cast iron surface plate that is 400x400 mm to transfer from the known flat diabas plate to the new larger one, and then try and measure the high spots. I assume with enough overlap that the larger plate would eventually get flat but it feels like a brute way of doing it. Is there a proper way of doing it?

Best regards

Click to expand...

Pragmatically, one just doesn't DO that s**t.

That "larger" plate is only roughly 30" x 28". Hardly shirt-pocket size, but small as such things go. Small "enough" you can carry or ship it to a firm already equipped to refinish and calibrate it.

If patience and time are more in abundance than money, and you don't need it that "good", it is a 24" to 36" straight-edge that will do more easily than a smaller SP.

Those, too, are small and light enough to be taken/sent away for professional refinish/calibration. If even they need such.

A professional shop earns their keep by already HAVING all the bespoke goods needed to do this sort of work, and rapidly. It is more economic to "rent" their skill and arsenal of equipment than to try to duplicate it the hard, slow, tedious way.

You probably don't need to do this but once or twice for the working life of your plate, anyway. Figure that third-party cost amortized over many, many years.

Then consider spending more for a plate that needeth NOT any touch-up at all.

Either way, you get to go do something more productive with your scarcest asset - TIME - (ever try to buy it back, once wasted?) that you CANNOT as easily farm-out to a third party.

2 1/2 CW

Demon73 said:

my 2c

Theres two numbers you want to concern yourself with for a surface plate. Global flatness and local flatness.
Providing your granite plate is good (theres no such thing as flat), scraping your 400x400 and overlapping the print on the larger plate should get you pretty close, but id think youd still need to keep an eye global flatness.

If it were me, after having a measure up to get the general shape of the plate, id probably go about getting the main lines (union jack style) in first, starting with the diagonals. Id use a straight edge for the print and something to keep an eye on the global number, eg:- autocollimator, repeat meter (using the best line on the granite to zero from), level/s.
When im looking good through all eight lines id can concentrate on the areas between them, mostly using the 400x400.

If the above is close to an accepted approach, its a tough gig next to lapping granite imo.

Click to expand...

With your system you'll be chasing your tail all day long - it's a shortcut of doing the job right and in order to work the plate must be "close enough". The RIGHT way to do it is to make the entire plate spherical and then straighten it. You push everything into one direction and retrace your steps into another ( single ) direction , into flat.

Monarchist said:

Pragmatically, one just doesn't DO that s**t.

That "larger" plate is only roughly 30" x 28". Hardly shirt-pocket size, but small as such things go. Small "enough" you can carry or ship it to a firm already equipped to refinish and calibrate it.

If patience and time are more in abundance than money, and you don't need it that "good", it is a 24" to 36" straight-edge that will do more easily than a smaller SP.

Those, too, are small and light enough to be taken/sent away for professional refinish/calibration. If even they need such.

A professional shop earns their keep by already HAVING all the bespoke goods needed to do this sort of work, and rapidly. It is more economic to "rent" their skill and arsenal of equipment than to try to duplicate it the hard, slow, tedious way.

You probably don't need to do this but once or twice for the working life of your plate, anyway. Figure that third-party cost amortized over many, many years.

Then consider spending more for a plate that needeth NOT any touch-up at all.

Either way, you get to go do something with your scarcest asset - TIME - (ever try to buy it back, once wasted?) that you CANNOT as easily farm-out to a third party.

2 1/2 CW

Click to expand...

Surely you must've noticed that most discussions on scrapin' , flattenin' and straightenin' here are amongst amateurs with a Biax ( or looking for one ) and not amongst people who refurbish optical flats, cyl squares and the like... In other words, we're talking discussions.

AlexO said:

Surely you must've noticed that most discussions on scrapin' , flattenin' and straightenin' here are amongst amateurs with a Biax ( or looking for one ) and not amongst people who refurbish optical flats, cyl squares and the like... In other words, we're talking discussions.

Click to expand...

It is, however, warm and gratifying as to the preservation of a craft skill to see how well the most determined and dedicated of them DO.

I'm too penurious, even impoverished, to go that route.

I find it cheaper to hire it done by better and FASTER men. Or just not buy as sad an item of "junque" to begin with that needs insanely tedious levels of "TLC".

Monarchist said:

I find it cheaper to hire it done by better and FASTER men.

Or just not buy as sad an item of "junque" to begin with that needs insanely tedious levels of "TLC".

Click to expand...

Better and faster men are in some shortage in my part of the World but I'm a long time subscriber to the 2nd bit. New beats junk each time, every time.

When in my technical high school I was lent to some plant where I had to scrape bearings for some submersible pumps for around 4 weeks or so. The woman who used to do this was ill or just gave birth, something like that. I got INCREDIBLY good at it - really chuffed with myself. Then she came back to work and I had to wake up to reality - she was like 4 times faster and at least twice more accurate.

AlexO said:

With your system you'll be chasing your tail all day long - it's a shortcut of doing the job right and in order to work the plate must be "close enough". The RIGHT way to do it is to make the entire plate spherical and then straighten it. You push everything into one direction and retrace your steps into another ( single ) direction , into flat.

Click to expand...

Thanks for your opinion on that alex.

My 2 cent's.

As I read this I started to remember doing this years ago. If I were to do this I would locate the lowest area of the big cast iron plate. I would assume it's in the middle as most use the middle. I would use a level to scope out the plate by graphing the Iron Plate by drawing pencil lines on the plate approx. 4" apart and 2 sets 90 deg's to each other. Then set the precision level (.0005" per division per 12" or finer) and check every 4" and make a graph on paper to document the plate. A Repeat-o-meter, Planekator, height gage and gage block, etc could also work. I found these interesting idea's on You Tube. Repeat O Meter - YouTube

Then you would need to make a frame to wrap around the granite plate so you can lower that plate down from above the CI plate. Don't slide the G plate from the edge to the low spot as you would get a false reading. I have my qualms about using a granite plate flipped over as it could bend a small amount, but I would guess it's not more then .0005". The frame would have handles to either lift it by hand or using a jib crane.

Just blue up and scrape the area say 1" bigger then your reference G plate. Once finished then move the G plate out 1/2 it's width and scrape the high area until the first scraped area touches be sure to mark with a marker where that area previous scraped is so you don't scrape in the finished are. Use the repeat-o-meter or level or etc indicator as you scrape. Have to be careful not to dig a deeper area... Not the easiest way but it world work. If your near Horby Sweden (SE) I have a friend who hosted a class who has a bigger Granite plate and scrapes. I bet he would be happy to help you do it at his shop. Good Luck

Richard King said:

My 2 cent's.
.

(snip lots of work, and need of CAREFUL use of a granite plate, suspended..)
.
.
If your near Horby Sweden (SE) I have a friend who hosted a class who has a bigger Granite plate...

Click to expand...

There's the key. Larger granite plate, rather than smaller. No need to "own" it.

With a 30" x 48" grade A Herman granite plate here, I could turn that 28" X 39" Cast-Iron plate topside downward and attach lifting gear to it more easily than to try and grab an upside-down granite plate safely.

Too lazy, though.

So long as you had the autocollimator or electic levels to show that the 30"x48" grade A plate was still grade A. otherwise, all bets are off.

This year's project is to re-lap the 36"x48" Crown Windley lump in the shed. That'll be done using autocollimator and first surface mirrors, with 18"x24" and 12"x18" cast iron plates as lapping plates. The entire purpose being to ensure that it's good enough to use in the rebuild of the 8"x24" surface grinder.

I'm lucky in that I no longer have to make a profit for anyone. That's why I spent four hours today making two 6" milling vices identical (instead of 45 thou different in height)

Mark Rand said:

So long as you had the autocollimator or electic levels to show that the 30"x48" grade A plate was still grade A. otherwise, all bets are off.

Click to expand...

"in due course", as all three of my Autocollimators are in need of lamps I haven't gotten around to sorting. I do have the optical wedges and mirrors to calibrate THEM, though.

This year's project is to re-lap the 36"x48" Crown Windley lump in the shed. That'll be done using autocollimator and first surface mirrors, with 18"x24" and 12"x18" cast iron plates as lapping plates. The entire purpose being to ensure that it's good enough to use in the rebuild of the 8"x24" surface grinder.

I'm lucky in that I no longer have to make a profit for anyone. That's why I spent four hours today making two 6" milling vices identical (instead of 45 thou different in height)

Click to expand...

Aye. Retirement. Sad to report the major accomplishment today was emptying then breaking-down a s**t-metal Fort Steuben alleged-workbench to go to the recycler.

Whereinthehell did all that "free time" they warned us would bore us to tears or even borderline madness get to?

Oh.. wait.. seems to have WORKED as to the "madness" part anyway..