Downsides to a large granite surface plate?

Hello everyone,

My question today is;
Is there a downside to having a larger granite surface plate (other than floor space and resurfacing/certification costs) that I may be overlooking.

With more customers asking for precision machine repairs every year I am starting to seriously look into a larger rather than smaller surface plate ( thinking 60" minimum to around 96" max length) I currently only have cast iron surface plates.

Are they:

supported differently to prevent twist/ sag?

Extremely sensitive to temp change?

Steal tools and hide them from machinists???


Let me know every down side!

Thanks,
Jacob

This will help answer some of your questions.

The federal specifications address support. IF the surface plate was supported at other points when manufactured -OR last lapped- those are the points you should use.

In addition to the length/width- look at the thickness. The standards give thickness for plate loads of 50 lb /sq ft and 100 lb/sq ft. All plates are going to deflect a little with any part- but the thicker plate will deflect less. You can do lighter parts on a thick plate- but very heavy parts on a thin plate could be an issue. If you anticipate very heavy parts- plan ahead.

Granite plates are not extremely sensitive to temperature changes. Due to their mass they are slow to warm up or cool down.

I would not say they steal tools- but they do tend to collect them, and tools can hide in plain sight.

J.R. Thiele, thanks that's what I was wondering about.

I know weight, but I can manage that, and cost of purchase obviously, but as for thickness, flex under weight ( even it's own) those are the things I hoped to find out.

Jacob

Thanks for the link, just read through it.
Pretty good information in there, I didn't know about silicon carbide stoning being necessary, but it makes sense.

Looks like I better go a bit thicker than standard.

Since cost of purchase will be of concern I will be buying used, anything I need to watch out for beyond obviously cracks and craters?

Also is it practical to attempt the repeat reading calibration? Or is that more of a "get a professional" type job?

I do have a gauge with the capacity but not the base, and no autocollimator. (Yet, still watching)

Sorry for the questions, couldn't find much in the archives.

Jacob

2 of my friends and students show how to test and lap a granite plate. Lance met a retired K&T Plate lapper and he showed Lance how to do it.

Lance is one of my best students ever!!

Given your location calibration is a concern. It is possible to do it yourself- but it is not a casual undertaking.
I would contact Campbell's Granite Surface Plate Service of Phoenix AZ. Their phone number is 602-697-5370. Their website shows as "unsafe" on my computer- but did not in the past. I got the phone number off the Calibration sticker on my surface plates. They specialize in calibrating surface plates- and travel a route that goes up the coasts. I am in MO, and they were able to come by on the way to the east coast. I know they used to go up the west coast. They travelled in an RV and on that trip were combining work with visiting with relatives. You are a long way from the coastline- but you are near areas people go on vacations. You won't know unless you call.

The flatness standards are based on contact of a 3/8 round disc. Plates will pass and be useful with divots smaller than that.

Given the challenges with certification one of the pink sones with high quartz content would be ideal as they wear more slowly.

hvnlymachining said:

Thanks for the link, just read through it.
Pretty good information in there, I didn't know about silicon carbide stoning being necessary, but it makes sense.

Looks like I better go a bit thicker than standard.

Since cost of purchase will be of concern I will be buying used, anything I need to watch out for beyond obviously cracks and craters?

Also is it practical to attempt the repeat reading calibration? Or is that more of a "get a professional" type job?

I do have a gauge with the capacity but not the base, and no autocollimator. (Yet, still watching)

Sorry for the questions, couldn't find much in the archives.

Jacob

Click to expand...

You might be surprised how cheap big ones are at auction a while back I saw a 6' x 10' x16' or so and they could not get a 25.00 bid. I bought a 24" B&S Cadillac gauge with an 8" riser and a 36" height gauge off the table for about 250.00.

moonlight machine said:

You might be surprised how cheap big ones are at auction a while back I saw a 6' x 10' x16' or so and they could not get a 25.00 bid. I bought a 24" B&S Cadillac gauge with an 8" riser and a 36" height gauge off the table for about 250.00.

Click to expand...

This is very true, I was curious once on how much the big ones go for, so I placed a $1 bid. 2 weeks later I get an email saying I have to remove a 3x6'x10" plate from the facility. $351 later it was on my floor.

Sounds like I better keep my eyes open for auctions! I had planned on taking a little time to find one anyway.

Mr King, thanks for the link, I don't know how I missed those videos, I watch both fairly often.
I'd sure like to be able to do that myself.

I bought a cast surface plate that looked about 24"x30" on a pallet in an online auction. When I arrived the 1x4's the pallet was made from had somehow transformed into 2x8's and the Challenge plate was 48"x60" and about 2 tons.

48x60 seems like a useful size to me without being too huge.

Up here in Minnesota we get a lot of auctions of machine shops that close. Here is one that is in about 30 days and they have a number of med sized granite tables. They also have all sorts of other bargains. I have a 74" granite straight edge and a medium sized granite now. I used to have an old CMM I used for a granite table. I bought it at a 3M sale they have every quarter. I stripped off all the CMM parts and used the granite plate. It was a 48 x 96" I paid more to truck it to my shop then I paid for it. I remember it was the minimum bid of $75.00. I ended up giving it to my friends at Kurt Mfg when I downsized to my home shop. https://www.machinesused.com/auctio...tion---Complete-Gear-Facility-Liquidation-211