Granite surface plate tool

I made a granite surface plate tool, primarily for flattening cylinder heads, manifolds, sealing surfaces, etc, out of a recycled piece of granite countertop.

The old recycled piece of granite countertop was about 1" thick. Using a precision machinist's straightedge, I looked all over it for a flat area of my desired dimensions. (My desired dimensions were, large enough to surface a typical 4-cylinder cylinder head or intake manifold.)

I couldn't find *any* perfectly flat spot on the polished face, but I did find an area that was pretty darned close. I cut it out with a 5" "turbo style" dry-cut diamond saw, mounted on my 4.5" angle grinder. (It's a very dusty process needing a lot of personal protective equipment... to protect eyes, hearing, and breathing!) I did it outside on my lawn because it makes a huge dusty mess.

Anyway after I got that piece cut out, I realized the unpolished back side of the granite slab was perfectly flat! So I use the unpolished surface instead of the polished surface.

I have spray-mounted two pieces of 80-grit open coat (that is load resistant and long lasting) sandpaper to the surface plate. I used it to successfully flatten a 4-cylinder intake manifold. It's not a very fast process for removing a lot of material, but it gives very accurate results.
Attachment 5406

The dimensions I picked for the granite surface plate are 8" x 19". Shown, I have cut a typically sized sheet (9"x10") piece of sandpaper in half, and spray-mounted the two pieces right next to each other on the surface plate.

It's important to set it on top of a flat surface. This cast iron table on my drill press is very flat, and suitable (also height adjustable, which is kind of nice) . If I put it on my wooden welding table, since it's not perfectly flat I need to add shims underneath so that when I clamp the surface plate to the table, (and press down as needed while sanding), it stays flat. For a large surface plate like this, I like to have it remain stationary, and move the work back and forth across the surface plate, (or in circles if the work is small enough) to sand the work perfectly flat.

If you use a smaller surface plate like can be hold with your hands, (like a perfectly flat sanding block; I have a big chunk of aluminum with a perfectly flat surface that I use for that), clamping your work and moving the surface plate across the stationary workpiece works well.

Attachment 5407
This shows what the drill press table shown underneath the plate looks like as well it's width.

Using this process, I can make pieces as flat (or even flatter) than a flycutter or a mill can. I had a local machinist flatten an intake manifold and he put a warp into it by stressing the part as he mounted it in his machine. Using abrasive surface to flatten a piece, there is no need to stress the part during flattening, like is required to flatten it using a flycutter or mill. Of course, aluminum is a lot easier to flatten than steel or stainless steel using a technique like this.