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Tools and Surface Plate Questions

Bob Korves

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We asked the team who came around from Standridge Granite to do our plates what they thought about the Chinese surface plates sold in the US. They told us that the plates are often very accurate and quite usable, but that a small percentage of them are WAY off, some could have been vetted with a yard stick. Beyond that, the process that they use to make the plates is different than the process used for making surface plates here, and the Standridge team is not even able to make the Chinese plates correct in the field without excessive labor, so they will only test and certify Chinese plates, nothing else. Obviously, like many things imported from China and some other countries, quality control and quality assurance do not enter into the equation for inexpensive products shipped to the US. It costs a lot to send that stuff here, and they still want to make a profit while charging fire sale prices to the US distributor. Part of that something is quality control and quality assurance. I have seen it many times with their products.
 

darkzero

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Funny, just last night I was watching Tom Lipton's video where they came to his shop to resurface the plates. I had no idea it was done like that in the field with not much equipment. Pretty cool.
 

Bob Korves

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Funny, just last night I was watching Tom Lipton's video where they came to his shop to resurface the plates. I had no idea it was done like that in the field with not much equipment. Pretty cool.
The same team that came to Tom Lipton's shop also came to my shop. I think they are the only team that does field work for Standridge. Nice guys and hard workers, and aim to please their customers. They bend over backwards to get things done that they probably really shouldn't. The three of them did 5 plates at my shop to AA tolerances in less than an hour and a half. Great team.
 

astjp2

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I have a 48x72x10 plate that is grade b that I would love to get to grade a, but not really worth it for the shop I have it in or what I do with it, paid 400 for it used and I feel lucky to have bought it.
 

projectnut

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I know you mentioned you rarely do parts over 12", but in the scheme of things do you expect to do larger pieces in the future? I ask this because I started out with a small 12"x 18" surface plate that did the job for a short period of time. However in less than a year I found I was doing parts large enough that I needed to consider a larger plate.

There are always "large" plates available on either Craigslist or eBay for nearly scrap prices. In most cases you'll have to find them locally because shipping wouldn't be cost effective. I happened to find a Challenge 24" x 36" plate that was recently certified for $100.00. In my case the plate came from the shop I worked in. When the company closed down the shop it was sold to a friend of mine who also happened to purchase it and several other larger Challenge and Starrett plates. When he decided to his shop move to a different building he was willing to sell the "smaller" Challenge plate. Even at it's "small" size it weighs nearly 600 lbs. It was a beast to load it into my truck and get it on to the stand in the shop, but now that it's there I use it far more often than I ever used the 12" x 18" plate.

A lot of people seem to prefer the Granite plates over the cast iron ones. I on the other hand prefer cast iron. I have no idea whether one is better than the other, but for some reason I always seemed to gravitate to the cast iron one in the shop rather than the granite ones. Now I have the one I used at work in my own shop.
 

Bob Korves

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Cast iron surface plates are usually quite a bit less accurate than granite surface plates, because the cast iron ones typically have never been calibrated, or it was so long ago that it is meaningless. Cast iron most definitely moves over time due to internal stresses. If it it properly heat treated to remove the stress, life is a lot better, but it still moves. The skill and patience of the person who scraped in a cast iron plate is also a real question. Cast iron plates that are Blanchard ground or milled flat are not accurate enough for precision work. Granite is much more stable. Both wear with usage.

Rule #1. Guessing the flatness of a surface plate is hopeless and worthless. Vetting or creating a flat surface plate in your shop to a known standard is highly unlikely, but barely possible if you want to make it a long term project to prove a point, given enough time and effort. It can be calibrated by pros quickly, fairly cheaply, and reliably to a known standard. "Close enough" is a term that is used quite loosely and often, but is essentially meaningless, without answering both questions -- "close enough for what?" and "how do I know for sure what I actually have here?"
 

Dabbler

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I usually consider smaller cast iron surface plates as 'shop plates' - useful for work that doesn't require a lot of precision, like marking out, or lapping. For really accurate work I've always used granite surface plates, lovingly cared for, as they are very expensive and easily damaged.

projectnut, you can buy an offshore A grade granite plate very cheaply, and use it to check your cast iron ones, and do some basic corrections with a scraper. The only cast iron plate I used regularly was .002 out in the centre, and .002 low in one corner, with respect to the other 3 corners.
 

projectnut

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My Challenge surface plate is an older one that was hand scraped. According to their website the tolerance was within .0005". At the time I got mine hand scraped was still standard, but was more expensive than the ground plates. The 24"x x26" size was a little over $3,700.00. I see now they offer only ground plates as standard and the hand scraped are special order. Also I was mistaken on my last post saying the plate was nearly 600 lbs. The plate itself is only 475 lbs. The stand is another 170 lbs.
The plate was last certified in 2016. It's hard to read the label, but I would assume it was certified to the original factory standard.
 

Dabbler

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Cast plates do move, but it takes time. If your shop gets warm and cold during the seasons, it will mover farther and faster. A stable environment, temperature and humidity keeps the pates closer to spec.
 

Bob Korves

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My Challenge surface plate is an older one that was hand scraped. According to their website the tolerance was within .0005".
A plate with an "tolerance" of .0005" from the factory (when?) would likely be suitable (then!) for vetting work to around .003", and that is good enough for a lot of home shop work. What was it certified to when it was last certified and what was is it's history since certification? My 18x24x4" granite plate is recently "certified" to an overall accuracy of .00015" and repeatable to .000030". I take care to keep it so, and no one else but me has used it or had access to it since it was certified...
 

Janderso

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While we are on the subject, thinking of the cast iron surface plates.
Once they are scraped flat, properly supported and stored, will they retain their flatness or will it need to be rescraped down the road?
I can see why you would want all the stress worked out of the iron.
Thinking of the cast iron plate Rex just finished, it would be a shame to have to repeat the process.
 

Bob Korves

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While we are on the subject, thinking of the cast iron surface plates.
Once they are scraped flat, properly supported and stored, will they retain their flatness or will it need to be rescraped down the road?
I can see why you would want all the stress worked out of the iron.
Thinking of the cast iron plate Rex just finished, it would be a shame to have to repeat the process.
Hand scraped surface plates which are done by hobby machinists are usually ongoing works in progress, improving them as personal scraping skills improve. You have the plate heat treated to remove stresses, and then ring the thing well before starting to scrape it. It will probably move some more over time and need an occasional refresh of the scraping to deal with wear and any residual stresses being let out over time. Not having a solid and stiff mounting to properly support the plate will tend to make it move around as well. Bottom line, it will need to be tested at intervals, against a KNOWN accurate granite surface plate or by professional certification techs. I have 2 nice cast iron surface plates, one 8x16" the other 14x18", both works in progress. I have the smaller one fly cut to remove imperfections, and the larger one is a nice hand scraped Brown & Sharpe that has light surface corrosion from decades of proper storage. I need to put together some sort of crane in my shop to move the cast iron plates back and forth from the scraping bench to the granite plate to mark them up for each scraping pass. They are too heavy for moving them back and forth by hand and poor back for the number of times it takes to get them properly scraped in.
 

StevSmar

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Could you purchase three used granite surface plates and use the three plate method to level them?
 

Bob Korves

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Could you purchase three used granite surface plates and use the three plate method to level them?
If you work at it for a LONG time and have the correct knowledge, skills, and patience -- and you stick to it -- you can get it to grade A or AA accuracy, maybe even better, but how will you know when you get there? The three plate method requires strict care in how the stones are properly rubbed together and then carefully tested for improving results, and that is not easy when "lapping" very heavy chunks of granite. It can certainly be proved to be possible, and perhaps actually has been, not just hyped, but it is in no way a quick, cheap, and easy way of acquiring a accurate surface plate -- it is the exact opposite of that.
 

astjp2

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I bet my plate weighs about 1000 pounds, the forklift sure grunted when I lifted it up. Tim
 

Fitter Bill

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I bet my plate weighs about 1000 pounds, the forklift sure grunted when I lifted it up. Tim
Assuming its granite and my math is correct. 4' X 6' X 0.833 = 20 cu ft @ 170# cu ft = 3400 #
 

Toolmaker51

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Cast iron plates have one distinct advantage; being ferrous, they accept magnets and wonderful surface for assembly or mock-up work.
 
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