Granite Surface Plate Size ?

[JPigg55]

Looking to buy a granite surface plate for use it mill head tramming.
What size would you recommend for use on a 24" X 6" table ?
Looking at Grizzly, I see 2", 3", & 4" thick granite plates. Is thicker better ?
I see some have ledges and some don't. Are the ledges used for anything other than clamping down the plate ?
What other common uses are there for them ?
Found a couple excellent post during my search, but no answers to my questions.

 

[Ulma Doctor]

not to discourage you but,
you can measure directly off the table when tramming the mill unless the table is questionable.

i use my surface plates for metal scraping reference and for layout.
if you are going for accuracy get a grade A Black surface plate,
if you aren't as picky you can get grade B Pink for a lot less money.
i would make consideration for the thickest granite i could get my hands on, but my needs may differ from yours.

for most the pink grade b is more than sufficient.
the pink granite is softer than black granite and subsequently needs to be replaced more rapidly than black granite plates.
neither you or i are building parts for the space program, so get what fits your means-
knowing that either pink or black/A or B, is a good choice

 

[John Hasler]

Looking to buy a granite surface plate for use it mill head tramming.
What size would you recommend for use on a 24" X 6" table ?
Looking at Grizzly, I see 2", 3", & 4" thick granite plates. Is thicker better ?
I see some have ledges and some don't. Are the ledges used for anything other than clamping down the plate ?
What other common uses are there for them ?
Found a couple excellent post during my search, but no answers to my questions.

I wouldn't use a granite plate for tramming. As far as I know the bottom is not especially flat and is not guaranteed to be parallel to the top. I use a disk from an 8" computer disk drive.

 

[JimDawson]

I don't think you will get the result that you want using a granite surface plate for tramming a mill. Surface plates are not ground for parallel surfaces. The working surface is very flat, but the bottom is just rough ground. A parallel ground steel or cast iron plate would be a better choice. Disk brake rotors have been used with good success in this application.

The ledges are not used for clamping, they are used for supporting the plate in the stand.

Surface plates are normally used for providing a flat working surface when doing layout and precision measuring of parts.
.
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[Kernbigo]

use the table

 

[John Hasler]

use the table

The trouble with using the table is that while flat on the scale of interest when clamping down work it may be rough on the scale of the tip of the DTI. A smooth, flat plate with parallel surfaces averages out the roughness as well as carrying the DTI tip over the slots.

 

[Big Rack]

I think I read some where about using a new disc brake rotor sounded like a good idea I wish the post included what model it was because he mentioned about $20 new.

 

[Tony Wells]

Black granite is harder......yes and no. Black has a higher MoE, but the pink has a quartz component that gives it better wear characteristics. In practice, the best comination of properties is a pink plate thicker than the same size black plate. Kind of the best of both worlds. I know that lapping a black plate is waaay easier than a pink plate, so it's a good thing that the pink usually has smaller "holes" in them.

On topic though, good points are made. But the deciding point is the lack of parallelism of the top and bottom of the plates

 

[projectnut]

Rather than using a surface plate try using a pair of parallels equally spaced from the center of the quill. Here's a U Tube video of Tubal Cain tramming a Bridgeport head.

Note at about the 4 minute mark he begins the process using a dial indicator and a couple of matched parallels.

 

[JPigg55]

My old Clausing has a few digs in the table and probably due for a good scraping, but until I can ge it done...
The digs along with the slots can make tramming a pain. Thought I'd seen a Tubal Cain Youtube video where he'd used a granite plate for tramming, maybe I'm wrong.
The Grizzly site lists under the description of their plates as: "tool room grade "B" Granite Plate has a bilateral accuracy of plus or minus .0001".
Took this to mean both sides were ground parallel. I thought with a larger sweep, tramming could be done with greater accuracy and faster since I wouldn't have to mess with the indicator to move it over the slots inducing potential error.
I'd considered using glass, but figured I'd break it and cut myself cleaning it up. Plus not sure of flatness of glass either.
Got the idea from reading "Interesting Information On Surface Plates" on this forum here: http://www.hobby-machinist.com/threads/interesting-information-on-surface-plates.33830/#post-299827.