PDA

View Full Version : Surface Plate



jr45acp
12-22-2002, 09:53 AM
I recently was fortunate enough to pick up (free) a 18x22 inch surface plate. A tap company was shutting down and it was headed for the dumpster. Being the consumate scrounge, I offered to take it off their hands. It's pink granite and has appraently had some misuse and abuse, but generaly appears to be in decent shape. My question is: How can I check it out for flatness without re-inventing the wheel? It will be used for home shop projects. Any advice/recommendations will be appreciated. I did check for calibration services but everything I've found to date is on the west coast and I'm in SE Georgia.

------------------
John B

Forrest Addy
12-22-2002, 10:31 AM
Granite is not malleable. Unless your scrounged surface plate is cracked it's OK.

Crask test: suspend the granite flat with a rope or stand it on edge on two pencils each about 1/4 the length from opposite ends. Tap the flat's corners gently in succession with a small wood or plastic handled mallet. You can also use a husky screwdriver handle if its axis is perpendicular to the plate's surface.

The flat should have a clear ring at all four corners. If it has the slightest crack the difference in ring will be easily detectable.

Scratched, dings and craters may be annoying but if cleaned up they will have no effect on accuracy.

If you're still worried you can send it off to a calibration service and spend two prices for fair quality import flats getting one free flat calibrated. Life is like that.

gizmo2
12-22-2002, 04:28 PM
If I were so fortunate, this is what I would do. I'd take the granite and set it on a piece of carpet, then I'd get my longest ruler(an 18" Starrett). With the ruler on edge on the stone, I'd eyeball it with a strong light behind. If any gaposis showed up, I'd make a mark on both the stone and the ruler, then wander around doing this from different angles. If the marks stay the same place on the ruler then I'd suspect the ruler. If they stay at the same place (places) on the stone I'd say it was the stone. If they both blacked out all the time I'd be pretty stoked. If they didn't, I would just avoid those places on the stone when doing layout work, and keep my investment at the Happy level.

Uncle Dunc
12-22-2002, 04:50 PM
I just did a Google search for

"surface plate" calibration (atlanta OR ga OR georgia)

and got 66 hits.

SGW
12-22-2002, 07:40 PM
Or put a dial indicator on the arm of a surface gage or other solid base, put the base in one corner of the plate, and reach out with the indicator and pivot it across the plate at various distances. Move the base to the other corners and repeat.

kgarver
12-23-2002, 06:12 PM
I was a QC Manager at a plant in northern NJ in 1989. We had all of our surface plates reground by Starrett. They had to have at least 100' sq. before they would come out and do the job. If memory serves me they charged $17.50 per sq. ft. Contact any of the big/small shops around you and try to have your plate included in their regrind cycle. All they can do is say "No" and you haven't lost a thing. Worth a phone call at least to Starrett to see what the $$ vs. sq. ft. is today.
Regards, Ken

crossthreaded
12-23-2002, 09:27 PM
We had all of our surface plates calibrated with an autocollimator and a mirror every 6 momths or so. An autocollimator is just a telescope that sends a parallel light beam out to a mirror and views the returned image on a reticle. You could make one from the pieces of an old telescopic sight or binocular. There was a construction article on building a simple one in the Nov. 2002 "Nuts & Volts" magazine. There is a good picture & description of a simple one at "www.micro-radian.com". To do the measurement you level the auto collimator to read zero along one side, and then compare the angles on the other sides and the diagonals. There is a picture of the basic concept in "Machine Reconditioning" in Ch 13 on "Squares". I acquired the book thanks to the efforts of Ken Garver of course. Good luck!

Thrud
12-24-2002, 02:24 AM
jr45acp:

Clean the plate carefuly with plate cleaner and wipe dry with a clean towel first. I like Starrett's (cheapest here) - it is formulated not to rust tools. I use a foam backed doormat to cover the plate when not in use, this potects it quite well from most hazards. You want to keep it covered to keep dirt off. I always clean it before and after use. The biggest hazard plates face is items dropped on them or stuff banged into them.

If you have any dings or craters you can use JB weld to fill them and then hand tools to finish them. I did this with my plate (the reason I got it for the regular price of the included stand) to protect the tools surface, but I do not use these areas for measuring.

You can do a general check by using a .ooo1" dial indicator mounted to a good surface gage. Zero it on a relatively unused area. Move the gage in CW or CCW pattern to check measurements in various areas of the plate. If it is within .ooo1" to .ooo8" I would not worry about it - it is serviceable.

FWIW:
The machine grey and pink granites are more desirable because of their higher quartZ content. The black granites are stronger, but have less quartz which is what gives all the plates the ability to be finely ground and retain a precision surface. The Quartz acts as bearings, and the softer material between the quartz crystals wear faster, leaving "islands of quartz". The tops of the islands are the bearing points of the plate, the lower surrounding areas (from the softer materials) allow for an "air bearing" of sorts. This is why gages blocks do not wring to granite, and why it is the ideal surface plate material.

SCRAPING IS A FEEBLE ATTEMPT AT EMULATING GRANITES DESIREABLE PROPERTIES

jr45acp
12-27-2002, 01:19 PM
Thanks to all for your suggestions! I will now be better able to make a resaonably "educated" effort to get my plate "up and going" BTW, Thrud, I had acutally considered using JB Weld on the dings, but had visions of a big, nasty Canadian asking me just WTH I was thinking of! It does make sense though, doesn't it!

------------------
John B

Forrest Addy
12-27-2002, 10:58 PM
Thrud! Shame!

Granite and cast iron references each have their places. Granite is hard, dense, and durable. It's also very delicate and VERY heavy, expecially if you have to muscle it. It takes a real thick granite plate to resist nasty ol' gravity. My 2 ft x 3 ft granite flat is 4" thick and weighs 370 lb. My 2 1/4" x 8 x 48" straight edge weighs about 80 lb. If I want to check them or have the reference face re-worked I have to send them out to Stanbridge or NWC.

Granite is hard. While it will scratch its surface is harder than most any work or tools used on it. With ordinary care in every day open shop service its surface is good for a year or more between calibrations and it seldom needs truing oftener than every two to four years. OTH scraped cast iron references should be checked every month in regular service and most likely they need a re-scrape every six months.

As for delicate, I let my old 48" granite straighedge bump a steel table (don't ask, stuff happens) and broke it into three pieces. It was just a little bump. Geeze!! they're delicate.

Cast iron isn't as hard as granite but it can be cast into intricate shapes including those that are both stiff and light. Further more, one can gather together 2 more cast iron references of roughly equal size and nature and you can scrape them together in rotation and get a surface whose flatness is limited only by your patience, temperature control, and technique.

A well designed 2 ft x 3 ft surface plate weighs about 180 lb and a 48" cast iron fitter's straightedge weighs about 50 lb. About half the weight and easy to handle. I like that.

You can knock around a cast iron reference in handling and never sweat a minor ding. simply stone it down flush and go back to work. Because of their design it's easy to rig cast iron references from cranes.

Granite references have to be drilled in their edges for lifting hardware attachments if they're to be rigged for use as scraping references. They never are and I don't know why.

So. I'll take a set of each, granite and cast iron and use them appropiately. Thank you.

Thrud
12-28-2002, 02:39 AM
Forrest

The problem with Iron is when it dings metal is displaced and the surface is curved as a result. With granite it fractures leaving the surrounding area flat and still usable. Heavier granite pieces usually have air cushion assist for movement and careful placement. Iron ABSOLUTELY requires more attention in handling to avoid damage. Granite straight edges should be handled with just as much care, I am more careful with iron than granite - I have not had one break yet (knock on my head). http://bbs.homeshopmachinist.net//biggrin.gif

As to my wee 24"x36"x6" plate, it was a high end Mitutoyo Machine Grey Granite custom made for demonstrations. It received some dings on two edges in the years it was used in moving from show to show across Canada. When I pointed out to their national sales manager that having a plate on display that is damaged belittled the Mitutoyo image - he had me make an offer. I got it for the price of their HD Steel Stand ($500) with the stand included. (cocky bastitch, ain't I?) I filled the divots on mine to prevent accidental damage to tools (k.i.s.s.). I still have two good straight edges for layout.

And contrary to your statement of granite being brittle - have you ever curled before? Granite rocks, man! (good pun, eh?) It is quite elastic - what it cannot handle is blows from sharp edges such as straight edges, metal blocks, chisels. Even then, tungsten carbide chisels are used for carving granite - beautiful, and tough. Big divots only fall off a surface plate from severe blows.