Granite Surface Plate Size ?

JPigg55 said:
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.
Click to expand...
"Bilateral accuracy" refers to the "x" and "y" directions.
 
One other thing to consider is that every ding in the table also has a raised area around it. When something hits the table and creates a depression, some of the metal is displaced and makes a raised area around the depression. Placing a surface plate or piece of glass on the table will mean you are tramming off of the three highest points on the table, and will not guarantee any kind of accuracy. You can very lightly stone those high spots off until you are able to scrape the table true. Then, tram as usual, making sure that the point of the indicator is not sitting in a depression. You will also need to help the indicator point over the t-slots, but that is a minor inconvenience.
 
Even if it could work, my small 12x18 Chinese tombstone must weigh about 70 pounds. Using parallels is a lot easier. :)
 
easymike29 said:
http://www.standridgegranite.com/surface-plates

"Our granite surface plates are manufactured in three ledge types as well as standard sizes and grades.
Plates that have ledges are used for clamping purposes only".
Click to expand...


I stand corrected, thank you easymike29. But the ledges are not used for clamping the plate down to a table as the OP questioned. Maybe we are both right;)
 
Dont use this one.
uploadfromtaptalk1438772505846.jpg
For size comparison, thats a quarter on the corner!

Sent from somewhere in East Texas Jake Parker
 
JP,
I had a chance to buy one that size at a Auction one time for $50. No one would bid on it because of it's size. If I could have got it loaded I would have bought it.
3' x 4' 8 inches thick the owner said it weighed around 1200 lbs.
More than I wanted to try a load by myself. :)
 
Take up all the room in my whole shop. LOL
 
A little off topic, but since your goal is tramming your mill, I strongly recommend building a dedicated tramming tool / spindle square with two dial indicators. It's a quite simple and inexpensive project to build one, and it is amazingly easier and less work to tram your mill once you have one. I'll never go back. There's little reason to buy a commercial spindle square when it's so easy to build one out of scrap and a couple of cheap harbor freight dial indicators. There's a pretty good YouTube video available from "Metal Tips and Tricks" that shows how to build one:


Swinging a single indicator back and forth when tramming used to drive me nuts: "Lessee, off by 0.010. Whack the head halfway and re-zero. Swing around to the other side. What?!! I'm still off by 0.004. Wait, which way am I out?" When I finally think I've got it, the final torquing of the head bolts would knock it out of tram again, of course!

The headache (for me at least) with a single indicator was primarily due to two problems: "bilateral ambiguity" and basic trigonometry. The first problem is that the indicator faces you when you have it positioned on one side, and faces away from you when swung it to the other side. As silly as it sounds, this caused me enough mental friction that at least half the time I'd whack the head in the wrong direction as I was tramming. The other problem is that it's a non-linear process. Since the bottom of the quill is following an arc, when it's out of tram the high side will indicate a larger distance than the low side. If I re-zeroed each 180 degree swing I'd invariably confuse myself.

A spindle square with an indicator on each side of the quill makes it trivial: just knock the head about until both indicators read the same value. It's even easier to game the process of torquing the bolts down without knocking it out of tram (as you get close start torquing down the bolts — the last couple whacks take a fair of strength to move the head back into tram, but the final torque doesn't move it out of tram). Seriously, being able to see the deviation on both sides at once makes all the difference in the world.

It's trivial to calibrate the tool: just make a mark on your table next to the indicator plunger, zero the dial, turn the spindle so the other indicator is now over the same mark (without touching anything else), and then zero the other indicator.

With the tool, I've found no reason not to measure from the table directly (or from the reference surface at the bottom of my vice). Just move the table in Y so the indicator plungers aren't over a gap.

Honestly, if I ever lose or break this tool I'll build another before I ever go back to using a single indicator on an arm — even if I'm in the middle of another project.

Regards,
--
Rex
 
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