[andy_ck87028]

Hi

I think I have used the right term.

I have purchased an engineer's precision square and want to ensure that the plate that I stand the item I am measuring is flat.

I checked with my local Engineer's tools supplier and the standard plate is made from granite or cast iron and is ground flat to a high level of precision. Unfortunately they are also very expensive and start at $300 +.

I was thinking that float glass my be a cost effective solution but I do not know how flat is flat when it comes to it.

Can any engineer tell me if a piece of 10mm thick float glass at 500mm by 500mm will give me a cheap practical alternative.

I am just starting out in model engineering and I want to improve my quality as much as possible but also don't want to bankrupt myself in the process.

Thanks

Andy

[Zumba]

The 10mm glass you mentioned is too thin and will flex. A typical 500mm x 500mm granite plate has similar elastic properties to glass and is 6-8 times as thick, making them 100-500 times as stiff.

[Stepper Monkey]

I don't know much about glass plates or even surface plates in general, but there is some interesting info in the ongoing polymer concrete discussion about epoxy surface plates, which are cheap and really frighteningly accurate.
Its kind of odd I know, but if you want a good base to make them on, you can get large, flat polished granite slabs really cheap or free if you look at a stonecutters, as there are a decent number of second hand tombstones that suffered the "Y2K" problem that no one has any practical use for.

[walter]

Epoxy could work:

http://www.cnczone.com/forums/showpo...&postcount=861
_

[handlewanker]

Hi Andy, you could always glue 3 or four pieces of 1/4" plate glass together and box them in a frame.
Only trouble with glass is it doesn't hold blue very well.
Using the time honoured way of making three plates and indicating each to the other to reveal hollows or high spots wouldn't cost an arm and a leg compared to cast iron.
You wouldn't scrape glass to get it flat, if needed, but you can lap it with grinding paste like they do for making telescope mirrors, and this would matt the surface to hold the blue as well.
Ian.

[Geof]

You must have, or be intending to get, a lathe. If you bought a fancy square and are worried about getting a really flat surface then you should be buying a good small lathe. Once you have the lathe machine a cylinder square, then get a sheet of glass and check how flat it is using your cylinder square and your bought square. If you cannot detect any irregularities with these two pieces of equipment then it is flat enough for your needs.

[lerman]

See: http://www.use-enco.com/CGI/INPDFF?P...MITEM=640-0140 for some reasonably priced surface plates.

A 12 x 18 plate is $28, and weights 85 lbs. So, if you get in on one of the free UPS shipping deals (you need to spend $50 in total), you can get away pretty cheaply.

A 24 x 36 is still only $140, but at 460 lbs, you would be spending real money for shipping.

Ken

[Zumba]

Unfortunately, forming the epoxy composite is not a trivial process (as we've all found out), and Enco doesn't ship to Australia.

In the end, $300 for a precision ground plate from your local Australian distributor will probably be your least expensive and quickest solution.

[handlewanker]

Drat it and double drat it, I just went onto the Enco website and drooled over the granite plates and stands.
Oh well, I think I'll have a look at the headstone makers and see if they've got a bit of granite that could be recut to my size, probably 450mm X 600mm or in imperial sizes 18" X 24" would be good, and make a stand from 30mm square tubing.
There seems to be a bit of a drought when it comes to engineering supplies at reasonable prices in OZ, that is unless you buy Chinese or Asian stuff.
I can import ER 32 collets from USA with shipping cheaper than buying at the local machine tool dealers, and that's not buying cheaper Chinese stuff.
To get back to surface plates, I once met a bloke in UK who used to do model engineering, making small steam engines etc.
He had a piece of timber with a plastic laminate covering, about 2ft square that he used as a "surface plate".
His only comment when questioned over it's accuracy was to say that "it beats using the end of the lathe bed".
If all else fails I might just get a couple of pieces of 1/4" plate glass cut to size and glue them together.
Then if I make two of them at 600mm X 600mm I'll be able to finish off scraping the drain cover I'm been having a go at for some years now.
All you need is three surfaces to get one really true, and the other two just get done as well in the process.
I'd do this just for the love of making it but the sands of time are running out and who knows when the final whistle will be blown.
At the moment I'm trying to get my Ajax turret mill working from a rotary phase converter I'm making from a 3HP motor and a bunch of capacitors.
The motor starts and runs using the capacitor configuration as laid down elsewhere, and it also starts and runs a 2HP motor, but when the mill is plugged in nothing happens.
Today I'm going to get into the back of the mill and have a look at the contactors, fuses and relays as I could hear a bit of humming above the sound of the converter motor, when I connected it up, which also needs tuning, as after running it for 1/2hr it gets a bit hot, that is running without anything connected to it.
Ian.

[andy_ck87028]

Thanks Guys,

That's given me lots of info.

I appreciate that flex is an issue for thin plate glass and sandwiching will improve strength.

Let's say I were to glue multiple pieces together, just how flat is plate glass to start with?

Would I really need to employ grinding processes to improve the surface?

I guess I am asking how flat is flat when it comes to plate glass?

Andy

[handlewanker]

Hi Andy, given that plate glass is prety good for flatness anyway, if you were to laminate several layers the top layer is the only one to worry about.
I'd make up three plates and using the grinding process for telescope lenses just lap them to one another 'till flat.
This would be a lot cheaper and quicker than having 3 cast iron plates cast up with the aging, machining and scraping required for stability and accuracy.
Even a complete novice can hand grind a reflecting telescope mirror.
Ian

[andy_ck87028]

Thanks Ian

Andy

[hdj80]

Hi guys
Interesting discussion. I have a large granite off cut from our kitchen bench (48"x20"). It is 40mm thick and I was thinking of getting it cut into something a tad more usable in size.
Would the grinding process for a kitchen benchtop be of a suitable standard for a surface plate.
Craig
Brisbane Australia

[atifeh]

Hello,

I also use a 40x600x900 mm decorative granite stone which I guess should be a step or used as a kitchen counter. I bought it quite cheap. One side is polished and the other side is crude. I lay it on a steel frame table which I made exclusively for it and shim it to be level using a 0.02 mm precision bubble level. Its thickness varies +/- 1 mm but the surface is within 0.015 mm which I checked it with several means and been using it for quite some time. Hope that will help.
Nader

[davidmb]

what about floating the plate glass on some epoxy resin?

[andy_ck87028]

Floating plate glass on epoxy sounds like a very good idea.

Would surface grinding then be required? (Thinking of a short cut to perfection )

I also like the granite suggestion using a precision bubble level.

Also, if grinding of plate glass is still required, how does the problem of glass not holding blue get overcome?

Andy

[ckelloug]

I think that the ideas of either floating a glass plate on epoxy or gluing glass plates together will have a much worse effect than simply buying a piece of something like 10mm plate glass. With thermal expansion effects, I'd worry that the glued glass will "wrinkle" like a cheap suit to use a bad analogy.

Epoxy is pesky in thick sections but not unbearable to work in thin ones. See http://www.precisionepoxy.com for epoxy surface plate kits which is the source of this belief I think was alluded to by Walter. A properly poured epoxy plate will be good to something like .003 mm for a 50 cm square plate IIRC.

The poured epoxy plate is probably better than a cheap granite plate or the plate glass although for cheap and easy, the 10mm plate glass will likely meet specs for light parts on anything but critical aerospace stuff. (This paragraph is purely an opinion as I haven't checked this personally). I would also figure that using the thick glass plate so that it is supported on three rubber feet would be good and then set the feet without glue or anything like that on top of another like sized glass plate on top of a table. If it is rigidly connected to anything, I suspect it's likely to have some sort of thermal or other warpage issue.

The idea of putting 1/4 plates together is also good but I'd do it with no glue for the warpage reason outlined above.

--Cameron (from the epoxy granite thread)

[handlewanker]

Hi all, no matter what you use for a flat top, you still have to verify the truth of the surfrace.
Iron plates are made very accurate by nature of the means that produced them, that is making three together or comparing one plate to an existing surface plate of known quality.
If you haven't got the means to check your plate's flatness, you may as well use a paving stone.
On the subject of thermal expansion for glass, all inspection surface plates are only accurate at a fixed temperature and if the ambient temperature is fluctuating then you just haven't grasped the fact that everything will expand and contract if the surrounding temp fluctuates, which means you have to maintain a fixed temperature environment, otherwise don't get tied up in silly notions that your plate is going to be any good, even if it is made of cast iron, epoxy, glass or exotic marble.
The fact is you will never be able to detect whether or not your plate is moving about, once it's fixed into a metal frame and standing in the corner of a cold in winter and boiling hot in summer workshop.
The other side of the coin is weight, and if you've ever tried to lift a decent sized cast iron plate of approx 18" X 18" with all the ribbing it needs then you'll know that it's impossible to quote ANY figures for accuracy with regards to surface flatness.
For the average person a flat surface is anything that looks flat and is conveniently sized to slide various tools about without falling off the edges.
More than that then you'll need a temperature controlled environment of 68deg F, or 20 deg C, which is most unpleasant to work in for any length of time.
Ian.