# What Size Surface Plate



## ddickey (Jul 15, 2017)

I'm considering a surface plate purchase. Looking at a 9" x 12" that Lee Valley has for sale.
Checked E-Bay and see a lot of 12" x 18" plates. What size do you guys have? Is 9" x 12" considered a very small plate?


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## Ed ke6bnl (Jul 15, 2017)

I put in a very good ebay bid on one I think 18x24  BECAUSE I could go there and pick it up BUT it took two people to load and unload safely.


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## Bob Korves (Jul 15, 2017)

What do you plan to use it for, Duane?


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## ddickey (Jul 15, 2017)

I would like to learn how to properly use one, layout & measuring parts to check flatness on various projects. I have a surface grinder now so think it would be a nice addition for checking ground parts. So far my projects would work fine on a smaller plate I think. The plate would be kept indoors.


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## Ulma Doctor (Jul 15, 2017)

i'd get the largest plate that's convenient for your shop space and scope of work.
there is not much worse than needing a larger anything! 
the cost difference between a 9x12 and a 12x18 are negligible if broken down over the lifetime of the plate.
unless you are doing high precision work a grade B will be sufficient


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## Bob Korves (Jul 15, 2017)

To check parts and tools you will probably need some kind of height gauge for dimensions, a surface gage with indicator to compare parts with each other or with gage blocks, and a cylinder gage or granite square for checking squareness.  For layout you will need the surface gage with a scriber and the height gage and some spring calipers.  Not all of that stuff needs to be on the plate at once (or even ordered right away), but there needs to be room for whatever part you want to work on as well as the tools for layout or testing.  You will run out of room pretty quickly on a 9x12" plate unless you only work on really small stuff.  I have a 18x24", it has comfortably handled everything I needed to do so far.  The biggest thing so far has been spotting my 8x16" cast iron surface plate for scraping it in.  I have also calibrated straight edges up to 24".  I assume you are looking at Chinese import surface plates.  They can be really good, but not always due to lack of quality assurance, and there is no way to know, and no reasonable way to test it yourself.  The small plate is fine for starting out, but at some point down the road you may want a bigger plate to handle bigger parts, and one of known and tested accuracy so you can calibrate your tools properly and with confidence.

Edit:  The smaller Chinese plates make good lapping plates for general shop work.


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## ddickey (Jul 15, 2017)

Thanks for the info. Exactly what I was looking for.


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## francist (Jul 15, 2017)

I have a 9 x 12 from Lee Valley. Nice enough plate and I'm not dissatisfied with it, but it is small. To give yourself an idea, put an ordinary piece of printer paper on your bench and play with your layout tools on it and maybe a dummy part. The paper is 8-1/2 X 11, the plate is 9x12. If you find the paper area is feeling cramped, so too will the plate. Using bluing (for scraping) would be a challenge too without covering the entire plate surface.

Like I said, I'm not unhappy with the 9x12, but it will not be my one and only.

-frank


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## darkzero (Jul 15, 2017)

I've got a 12x18, I don't inspect parts a lot but I personally wouldn't want anything smaller. Put a height or surface gauge on it & a lot of space is taken up. Mine has ledges so it's easy to pick up & move around if you need. Mine just sits on my workbench.

I do also have a 9x12, as mentioned it's basically the size of a sheet of paper which is why I bought it. I use it for lapping, perfect size to glue or tape a sheet sand paper to it. I have this one sitting in an aluminum baking pan to catch the water when wet sanding.

I'd really like to have an 18x24 but they cost a lot more. My 12x18 is a grade A that I got for $37 at Enco. My 9x12 is a grade B, $33 from MSC. Those are Chinese grades though.


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## Wreck™Wreck (Jul 15, 2017)

If you plan on measuring  parts that are 60" long then you will need 60" of surface, no less no more.
If you are a hobbyist you probably  do not need a surface plate of any size, what kind of work do you do anyway?


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## ddickey (Jul 15, 2017)

Just a hobbyist. Maybe someday it could turn into something more.
I just think it would be good to have a nice flat reliable reference.


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## projectnut (Jul 15, 2017)

I've had a 10"x18" for several years and have always thought it was too small.  Earlier this year a friend was downsizing a commercial shop.  He had a 36"X 48" and 24" x36" granite plate, and a 24" x36" cast iron plate.  He kept both the granite plates and sold me the cast iron (Challenge brand) one for an excellent price.  I now have a plate that can handle almost anything I need to measure.


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## Wreck™Wreck (Jul 15, 2017)

ddickey said:


> Just a hobbyist. Maybe someday it could turn into something more.
> I just think it would be good to have a nice flat reliable reference.


True enough, however there are probably better ways to spend money so buy a plate when you begin doing work that requires one.
If you are Hooked On Accuracy©  a plate may open a can of worms that you do not expect.


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## f350ca (Jul 15, 2017)

I have the Lee Valley 9x12. Its adequate but I plan on getting a 12x18.

Greg


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## mikey (Jul 15, 2017)

Duane, if you have a Woodcrafter store near you, they sell a Grade A 12 X 16 surface plate for under $100.00. It comes with a surface map. I just bought one of these the other day. I also have a 9 X 12 Starrett toolmaker's flat and it is very good but I use it mainly for laying out, not for gauging stuff on.


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## wawoodman (Jul 15, 2017)

I have an 18 x 24 x 3 that I got on the local Craigslist. Back then, I could move it myself, admittedly, with difficulty. Now, not so much.


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## Rex Walters (Jul 15, 2017)

I'm happy with my little 12" x 18" plate. It's large enough for pretty much everything I do and doesn't take up much room in my shop (clear flat surfaces are at a premium!). Any plate is better than no plate, but I'd be frustrated if the only one I owned was any smaller than what I have now.

I only have one thing to add to everything that's already been said: no matter what size your plate, you should rest it on three points. To go the extra mile, you can put the rests on Bessel points, but that's not nearly as important as ensuring the bottom is resting on a flat plane. Three points guarantees a flat plane.

Even a 4" thick piece of granite can eventually get out of flat if you just rest it directly on, say, a child's desk with a thin 3/4" sway-backed wooden top. But only a complete idiot would do that.  (I finally realized the error of my ways.)

For a small plate, up to say 18" x 24", any three small pieces of metal or wood placed somewhat evenly inbound from the edges will suffice. I've just got three chunks of scrap wood under mine, but have considered using some 1" steel balls with a flat milled on one side. If I ever do go that route, I'll probably put the flat sides against the bottom of the granite and the round bottoms against the table to avoid putting too much point stress on the granite.

For a larger plate you should locate the 4 Bessel points to support the plate. Two of the points should be supported directly. The other two should be supported by a rocker arm of some sort, and then the center of that rocker arm supported. That gives you three points of rigid support to define a plane.

I understand that better plates have the Bessel points already marked on the underside, but you can calculate them yourself pretty easily. Just measure each dimension, length an width. Multiply those dimensions by 0.5594 and that should be the distance between the support points (just measure half those distances from the center lines to locate the points).

A real support stand should also protect the corners of your plate to prevent anyone from knocking it off the proper support points.

Dale Derry has a nice series on the stand he built for his plate.


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## Moderatemixed (Sep 16, 2017)

I am just a hobbyist, and a newbie at that.  In the Starrett literature I read early in my initial steps into this hobby it was made very clear to get the best that you can afford.  I kept my eyes open and picked up a grade B Starrett 36 X 24 black plate for $200 Canadian (so like $130 US lol).  I am a hobbyist as I said, but I have not once though to myself, "gosh I wish that this surface was smaller" (No I don't move it).  My shop is only 12 x 14 so it does command a fair amount of real estate, but worth it, completely.  I started with the Lee Valley plate and it was just fine.  Happened across this Starrett and sold the LV for $50CAD.   I did pick up a 12 X 18 Starrett for lapping, again on Kijiji (like Craig's list) for $90CAD.  It is grade A and had I found it first would have made do, but again my point being that go as big as you can.  Just my 2cents.  


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## dlane (Sep 16, 2017)

My two 12x14 stepped sides have three 2" round 1/4" thick high density felt strategically glued under them


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## ch2co (Sep 16, 2017)

Like me and most of my stuff, my surface plate is rather ancient. Its a 12 x 18 cast iron plate with lifting handles on both ends, its easy for even this old coot to move around. Quite adequate for anything that I ever need it for. Picked it up from a precision machine shop that was shutting down to move elsewhere. Just happened to be in the right place at the right time. The surface is as close as I can measure to being perfect. No dings, dents, or major scratches. I think I paid $30 for it back in the '70's. Never really had need to use it until I finally started playing around with doing my own machining. It just occupies a spot on the back of my work bench with my stereo microscope and its accessories sitting on top of its well worn and oiled wood cover. Nary a spot of rust ever. Perfect size for all of my needs. I think that a 9 x 12 would just be too tight to easily use.


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## Bob Korves (Sep 16, 2017)

Moderatemixed said:


> I am just a hobbyist, and a newbie at that.  In the Starrett literature I read early in my initial steps into this hobby it was made very clear to get the best that you can afford.  I kept my eyes open and picked up a grade B Starrett 36 X 24 black plate for $200 Canadian (so like $130 US lol).  I am a hobbyist as I said, but I have not once though to myself, "gosh I wish that this surface was smaller" (No I don't move it).  My shop is only 12 x 14 so it does command a fair amount of real estate, but worth it, completely.  I started with the Lee Valley plate and it was just fine.  Happened across this Starrett and sold the LV for $50CAD.   I did pick up a 12 X 18 Starrett for lapping, again on Kijiji (like Craig's list) for $90CAD.  It is grade A and had I found it first would have made do, but again my point being that go as big as you can.  Just my 2cents.
> 
> 
> Sent from my iPad using Tapatalk


With a used surface plate, Starrett, Lee Valley, or God's Only do not mean anything at all.  It is just a rock until proven otherwise.  If it is not in current calibration and certification to a specific grade within the last couple years, and then carefully taken care of under your supervision, it is a complete unknown.  If you are using it for layout and other non precision work, then fine, don't sweat it.  But if you are using it for accurate work, or calibrating other tooling that must be correct, or making parts that must meet accepted standards, then you are just guessing.


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## Bob Korves (Sep 16, 2017)

ch2co said:


> Like me and most of my stuff, my surface plate is rather ancient. Its a 12 x 18 cast iron plate with lifting handles on both ends, its easy for even this old coot to move around. Quite adequate for anything that I ever need it for. Picked it up from a precision machine shop that was shutting down to move elsewhere. Just happened to be in the right place at the right time. The surface is as close as I can measure to being perfect. No dings, dents, or major scratches. I think I paid $30 for it back in the '70's. Never really had need to use it until I finally started playing around with doing my own machining. It just occupies a spot on the back of my work bench with my stereo microscope and its accessories sitting on top of its well worn and oiled wood cover. Nary a spot of rust ever. Perfect size for all of my needs. I think that a 9 x 12 would just be too tight to easily use.


If it is adequate for whatever you need it for, then great.  Just do not assume any accuracy level that is not proven and certified by a pro.  You do not have the resources to assert it is "perfect," it is what it is...


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## benmychree (Sep 16, 2017)

I think a 12x18 was the handiest size that I have had, but now I have a 14x14 and an 24x24, I use the smaller one for layout, and the larger one seldom.


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## Moderatemixed (Sep 19, 2017)

Bob Korves said:


> With a used surface plate, Starrett, Lee Valley, or God's Only do not mean anything at all.  It is just a rock until proven otherwise.  If it is not in current calibration and certification to a specific grade within the last couple years, and then carefully taken care of under your supervision, it is a complete unknown.  If you are using it for layout and other non precision work, then fine, don't sweat it.  But if you are using it for accurate work, or calibrating other tooling that must be correct, or making parts that must meet accepted standards, then you are just guessing.



I got both plates for "a song" so I paid the $300 to have the 24 X 36 calibrated.  Fortunately it was above standard for toolroom grade so we calibrated and certified for toolroom grade.  It will for my basement shop purposes be accurate enough and I don't see any need to have it rectified ever again.  Cheers. 


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## Moderatemixed (Sep 19, 2017)

Re-certified, not rectified.....  stupid autocorrect.  


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## Bob Korves (Sep 19, 2017)

Good on the re-cert.  Too bad it cost $300.  A group of us in this area had Standridge Granite drop by on one of their road trips covering California and western Nevada shops.  We had 5 plates in my shop from 4 owners, and that let us share the mileage fee and together we had a total larger than their minimum invoice amount.  The plates were all calibrated to AA specs and certified to A grade (less costly.)  They did put the actual achieved tolerances on the tags stuck to the plates.  My 18x24" ended up costing me around $102 total by doing it this way.  The 24x36" plates were more like $150-175, IIRC.  I agree that a properly certified plate can go much further between certifications in a home shop where one person oversees all usage of the plate and uses it properly and carefully.  OTOH, a plate bought used cannot be trusted even if it is in current calibration, because we do not know its history of care.  A plate can be trashed in a short time if abused, and we have no way to properly vet it ourselves.


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## zmotorsports (Sep 19, 2017)

I would figure out your scope of work and then go a size larger.

I have an 18"x24"x3" and it is perfect 99.9% of the time but there are a few times when I wish it was just a tad larger.  I picked mine up from a shop going out of business about 15 or so years ago and is a shop grade.

Mike


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