surface plate use

[jcs0001]

I picked up a 12 in. x 18 in. x 3 in. granite surface plate a couple of days ago.  Was in the area and decided that for $40 plus tax it was worth getting.  Also saved quite a bit on shipping since I was in the area anyway.

As we were loading it into the car the store employee mentioned that they were selling quite a few of them as gravestones.  I didn't inquire to see if they were also selling the small vibratory engraving tools along with them.

I suggested to my wife that when I depart she may wish to re purpose the surface plate.  My brother in law suggested an epitaph "frugal to the end".

John.

[PTsideshow]

Around here, they use them as pet grave stones in the backyard or as memory stones, Covered with contact shelf paper,or duct tape and then the wording and design cut out with an #11 Exacto knife if you don't have the proper media blasting material a thick gauge rubber. You can blast a great looking head stone.

Some of the marker guys will layout a simply design and cut it on the computer for a fee and some will even blast it. As the surface plates are cheaper then the smallest slab stones cost.

If you don't mind a thinner one some of the counter top places have the sink cut outs and broken pieces that make nice ones to.

[Lew_Merrick_PE]

Actually, those in the "industry" never call them "gravestones," they are monuments.  A good friend of mine makes the CAD/CAM system for the monument industry.  It can be quite entertaining to listen in on their discussions.

My first personal "surface plate" was a piece of marble purchased from a "monument maker."  It was a 32" X 50" X 6" piece of marble that the wrong coloring for whatever its intended use was to have been -- it cost me less than $10 in 1968.  I spent 40-50 hours lapping it flat and checking it against an optical flat.  It worked pretty well for about two years until is was worn out of flat by the things being slid around on it.  I still have the other (24" X 24" X 1") piece of marble I purchased at the same time that I use as a pastry board.

[jcs0001]

Lew:

That must have weighed over 1000 lb?  A little more than I'd want to be moving around.

How did you lap that down to being flat?  I've read a bit about such things but it is a bit beyond me as to how it is done.

The 12 X 18 in. "slab" is big enough for my use and is still light enough to be moved around if need be.  I plan to mount it on a fairly short wheeled cart and store the cart and all under a bench.  May end up making a levelling system for the cart so that I can use a good level for some measurements and setups.

My wife has a pizza/baking stone that is likely similar to your smaller piece.

John.

[Lew_Merrick_PE]

That must have weighed over 1000 lb?  A little more than I'd want to be moving around.

How did you lap that down to being flat?  I've read a bit about such things but it is a bit beyond me as to how it is done.

John,  It ran about 1200 lbs.  The frame I made for it would collapse or expand with a compound lever similar to those used to cock crossbows.  The collapsed frame fit on a pallet jack.  I have not been able to make a move without a hydraulic liftgate truck since I was 15.  I have two 2500 lb shop cranes and a pallet jack.  If all goes well (i.e. I win the bid), I will begin making an automated assembly line in a few weeks.  Each "station" will weigh about 1800 lbs and there will be nine "stations" in the total assembly.  I will probably rent a forklift when it comes time to load everything on a truck.

Starrett used to have a video about how they made surface plates.  My process was about the same, just more handwork.  I started by lapping the entire surface with a CI lap and (220 grit, as I recall) carbide paste.  I worked up through 1200 grit paste checking my work with straightedges and .0005 feeler gages.  When everything checked out that way, I used a 6 ft X 1 ft CI certified plate to control a split coherent beam to an optical flat.  At the time, I was limited to a 2 inch diameter flat (since then, I managed to get NASA to buy me 8 inch optical flats), so I was quite limited in the area I could directly verify (I told you there was a lot of hand work involved).  However, I was really only going for a .000050 RMS overall flatness.

In terms of lapping, I made a 4-bar driver mechanism that I hooked up to an electric motor (5 HP as I recall) for gross lapping.  I applied counterweights to the lap to correct wear patterns.  It's the same way that Johansson gage blocks are made -- just turned upside down.

You then set your certified flat next to the plate.  You have to pick your reference points carefully to minimize hand work, but once they are tied together, you count and mark interference fringe patterns relative to the leveled reference plate (where the split mirrors are mounted).  The rest is like any other scraping/grinding job.  You whittle away at the high spots until the fringe patterns all go straight.  I polished the whole thing with a piece of felt stretched over the (plastic coated) reference plate and imbued with one of the "ultra-bright" type toothpastes (the least expensive find abrasive you can find).  When the surface is completed, check the reflection of something with a long straight pattern to it.  As I was learning how to do monochromatic light measurements at the time, I got to use this for part of my required time for my inspection license.

http://emtoolbox.nist.gov/Publications/NISTMonograph180.asp shows some of the information collected by NBS (later, and worthlessly, NIST) on gage blocks that might help if you haven't been lucky enough to work in such industries.  Optical flats and monochromatic light help, but fantastically good flats have been generated with nothing more than making a really true reflection in the surface.  It is a bit more complicated than that, but that is the basis.

Does this help?

[andreas]

I use a small piece of granite 4.5" X 8.5" for lapping small parts for more than 10 years!!. the best part? it was free(a cutout of a kitchen bench)!

cheers
Andreas

[jcs0001]

Lew;

Thanks for the further info.  Lots of interesting things to learn.  I am happy with having a known flat surface and happier that I don't have to make it that way.

One thing about machining as a hobby is that there is always something else to research and learn.

John.