Surface Plate

[WilliamSpiret]

Has anybody used the 3 plates method to make a surface plate from granite? If so what materials are required

[Toolmaker51]

Depends on size you wish to attain.
If you have a Habitat for Humanity ReStores | Habitat for Humanity Int'l or other means for recycled materials, granite counter top is a good start. It cuts with abrasive blade in a circular saw. Don't use your best Milwaukee etc. Harbor Fright!! It's too thin beyond 12" x 18" or so, so bed it to a laminated wood base.
1/2" plate glass isn't too bad either, sliding gauges around though, the surface needs to be very clean to reduce deep scratches. Lap glass with wet silicon carbide [wet or dry] paper. Shiny is not related to flatness, air pockets in the surface are what help gauges slide. I hone and lap on a glass plate, probably have it 30 years.


https://www.google.com/search?q=lapp...utf-8&oe=utf-8

I found a suggestion for scrap aluminum cylinder heads, cracked but flat as carriers for lapping compound. Paste was applied with a little plastic wallpaper roller. Lapping is dependent on the lap softer than surface being lapped, so abrasive embeds there and cuts the surface. Water based abrasives are easiest to remove, cleaning has to be 100+%. Also found tips on reducing artist oil paint for hi-spot blue, less expensive and auto paint rubbing compound for fine grit abrasive. I'd probably start around 320 and work down from there.

Thickness and ability to support weight are related, being supported evenly is substitute. My A grade 3' x 4' with a tee-slot is 7". A B grade 5' x 6' will be about 10". Thicker granite is found at plants that produce headstones. Black granite is softest, lighter colors to pink [higher % of quartz] at other end of scale.
The real issue will be from securing 3 good flats, sufficient abrasive and Prussian Blue. Historically, cast iron plates go back I don't know how far, but a long time. They could scrape and flake planer beds, ways and rails to half-tenths or less [.00005], using the 3 plate method in the 1700's. The ideal surface is said to average 18 points of contact per square inch. It will be difficult to measure overall flatness [corner to corner] without collimators and such.

Or wait for free shipping at MSC, Travers or Enco. 12 x 18's will be in the $40-$50 range.

I will monitor and edit this as information or responses occur.

[PJs]

WilliamSpiret, I have not used the method but it was inspirational to do some research as I do enjoy metrology and scraping. I had forgotten that Whitworth was who developed this method back in the day. I found a few links that may be helpful.

On Practical Machinists.

On Whitworth.

An Interesting Power Point.

I got my B-grade 18 x 24 x 3 from Grizzly for a great price <$150 to the door. Most remarkable is how flat it is according to the test sheet that came with it (better than spec. 0, +.0001) and every thing I could measure it with. I also have a bunch of tiles up to 12 x 18 x ~1 I keep around from Habitat ReStore like Toolmaker 51 indicates. I look at the surface for nicks and dips holding up to the light, checking them when I get home as they are only a few buck each. Most of mine are within a ~.001 bilaterally. To lap them any better would be time consuming and expensive for the abrasives but doable. Glass is Always good but 1/2" is hard to find unless you go to a glass place and pay a price. Over all I think it would be the best for referencing back and forth with. All of them need support to keep them from warping over time and pressure...Hi laminate count Plywood works for me or old laminate counter top withstands the wet. I like the Toolmaker51's point on the cracked cylinder head as a carrier.

Thanks for the post to get the juices going! ~PJ

[Paul Jones]

Toolmaker51 and PJs,

I have a funny story about ordering a 12"X18"x3" Grade B black granite surface plate from ENCO. I wanted to take advantage of the free shipping and discount price offer expiring at year-end so I order the $46 plate in the morning of 12/31/2013. A few days later UPS delivered the undamaged double boxed boxed package to my front door at home. It was suppose to weigh 80 lbs so I got my hand cart before moving the package. I pick it up so not to hurt my back and think this is a lot lighter than what I was expect. The perfectly wrapped undamaged package had an inner and outbox but no surface plate. I called ENCO, they laughed and said they would immediately send a replacement and it arrived a few days later weighing the expected 80 lbs. Nobody knows what happened with the first surface plate.

[Toolmaker51]

OK, that's funny.
First image I had was, box had a dry flattened sponge in it - 'just add water'!
At one time dem guys were my mail order go-to. Until they ran a special on Kant-Twists, special pricing and shipping. My package arrives with quite a few less than expected. No back-order, out of stock or discontinued, but a bill for additional $$. ?, I called him (Mr. N.U.) personally. "Not on those..." from him, shock on my part. Still had catalog, page DID NOT specify which models and sizes.
Only buy one and lesser is half price.
Still, should have bought up all the Italian/Swiss QC holders, had I known large lathe was in my future. Yes, very same as avatar pic. Aloris pattern very good, 'cept 1.9" dovetail can't compete with parallel vee's 4" apart.