[SolarSystemsLLC]

You need to be careful here. It's possible the vacuum bag will seal the edges of your glass to the granite, leaving a pool of the IPA under the glass.

True. The error introduced by the trapped IPA has got to be on the order of the error in the surface plate (I have a 0.00075" Toolroom B plate). If so, then there might be just as much error again in the consistency of the mirror plate glass.

I'm leaning now toward my 0.0005" mylar as a release film for the above reason. If the film has an max error of 50% of its own thickness (not likely) then that error isn't more than 0.00025", which is much better than the master surface plate itself.

Thom

[SolarSystemsLLC]

HOW ABOUT TWO CHUNKS OF KITCHEN COUNTERTOP LAPPED AGAINST EACH OTHER WITH GOOD SUPPORT UNDER THE BOTTOM PIECE. YOU SHOULD BE ABLE TO GET THEM PRETTY FLAT. A BIT OF ABRASIVE AND A BACK AND FORTH SETUP, MOTOR WITH ECCENTRIC ARM OR WHATEVER, SOONER OR LATER THEY SHOULD GET FLAT. MAYBE PUT THE SLABS IN A PAN OF ABRASIVE SLURRY.
AMPLEXUS

Amp,
Yeah, you got a lapping machine that big?! :cheers:
Besides, I want to do this as an exercise in epoxy-granite technique...

Thom

[SolarSystemsLLC]

I have been to several auctions lately and when no one bids on the surface plates they end up giving them away for $20.00 or less just to get the heavy bastard out of the building.

I would rather spend my free time eating a good doughnut anyway.

JoeyB

Good point Joey. My original purpose was to do this in EG so I can help further the open technology and so I could cast complex shapes/arbitrary bolt patterns. I figure that will be easier to do in-house (literally) than machining the real thing.

But your point is well taken.

Thom

[SolarSystemsLLC]

Well, I was able to spend some time on this last night. Starting with a clean surface plate, I poured a little water on it and spread out the 0.0005" mylar. Using a rolling pin made from some ground rod, the water was spread out and the mylar stuck to the surface plate with surface tension from the water.

All was well, then I taped the edges of the mylar to the surface plate. Since the mylar was bigger than the plate, I wrapped the mylar over the edge of the plate and taped it down. This unfortunately gave air a chance to sneak back under the mylar since it wasn't under consistent tension. Over the course of an hour air displaced the water about three inches all around the edges. Next time, I trim the mylar closer to the dimensions of the surface plate and tape it down all around with packing tape, air tight.

[Zathras]

This is a very interesting thread to me. I'm at an additional disadvantage however in that I do not have a reference surface plate in the first instance.
My goal for the surface plate is not really as a reference anyway but as an air bearing surface. I'm at a point of almost sheer desperation now since I cannot find a used granite plate to save my life. New ones cost on the order of $3,000 here for the size (1000x700 mm) I'm interested in. You guys are very lucky, they practically give them away over there in the US!
Oh, I have tried several "self leveling" Epoxies for a surface without success. The ones I have found or can afford just don't work.(nuts)
No, I cannot get the US Composites stuff, that would be great if I could but can't import it.

So my alternatives for a poor man's surface plate;

1) Thick (4mm?) Carbon fibre panel with highly polished surface
Have no idea how flat these panels are. If they were made by pressing then they could be quite flat.

2) 10mm Float Glass
The cheapest, flattest surface I can think of. A little worried about cracking!

3) Thin flexible Carbon fibre sheet (bonded) on float glass
To help prevent potential cracking being a problem(?)

4) Hydrostone surface replicated from float glass


I'll have to make a decision soon and at least try one of these. Any ideas/comments?

[greybeard]

Hi Zathras.
In your list, you haven't got headstones from your nearest stonemason.
Is that a possibility ?
I know that the masons will grind and polish the surface flat before doing any engraving, but I've no idea how "flat" the surface is.
If the price from them was reasonable, but the surface not good enough, could laminating a 6-10mm glass sheet to the face with thin epoxy give you what you need ?
Regards
John

Second thoughts - el cheapo version would be to epoxy the glass plate to the surface of a concrete slab.

[Zathras]

Hey John,

Yeah you are right about headstones on my list. I unconsciously left it out I think. I did look into this last year but found surprisingly little willingness on their part. I also did see some pricing for headstones that was a bit much also. 300x300x25mm is around $200AUD whereas an actual surface plate of the same size is around $250AUD. Hmmmm .... not a great deal of difference!

Spot on, your comment of the glass on concrete was really #2 on my list. My appologies I didn't actually say what the base was. Yep concrete, here are a few pics of my mould with the epoxy layer on top. The kinda olive-sandy coloured stuff is the epoxy sealer in between.
I was thinking of using an RTV to bond the glass to the epoxy because I wasn't sure if the epoxy would bond well to the glass. Would I have to roughen the surface for it to take using epoxy?

Thanks for your comments.

Zathras

[greybeard]

I would go with RTV. If the worst happened, and the glass got damaged, you could replace it without too much difficulty. Not so if you had used more epoxy.
Regarding adhesion, I don't see that being a problem. Air pressure alone will hold the glass to the rtv, and that in turn to the epoxy. You could give the epoxy a light sand, but I don't really see any advantage.
If you go through your suppliers lists, there may be a grade, or additive, that will make it bond chemically, but I honestly don't think it's necessary.
With a 1000x700mm flat, you'll have ~7 tons air pressure holding it down

John
EDIT
I've just thought that any dust from sanding, or anywhere else, will promote air entrapment as you pour the rtv over the surface, so a tiny problem, but not worth worrying about.

[Zathras]

John,

Thanks for your comments.
Ahhh strange minds think alike. I was thinking the same with RTV if the glass gets damaged. It shouldn't be a huge drama to remove it.
Just to further explain;
Because the epoxy layer I have now is quite badly bowed in a convex manner. I obviously don't want to press the glass down too firmly, just essentially lay it on a layer of RTV taking up the gap. Actually I have just enough epoxy for one more coat (I have tried two so far) so I'm considering putting that on to 'fix' it but I could surely make it worse. I think I will cut my losses here and buy the glass and RTV. Yeah, I'll leave the glass alone too.
Just started searching for some good RTV.

Thanks!

[greybeard]

As part of my learning curve towards building my first cnc machine, one of my designs(still waiting to become m/c no.2) was based on air tracks.
They were made from Al extrusions like the ones that are popular for building small m/cs with, but they had glass strips epoxied to a pair of faces at right angles to one another.
Two such beams parallel and turned > < gave me a Y-axis support for an epoxy/sand moulded truck. The whole was then repeated for the X-axis.

As I say, still waiting, but the other component, the air diffusing sliders were originally to based on slicing up porous carbon block, ex water filters, and that was a complete no-no.
Now I have aquired some porous polythene block, I will give it another shot.
All the best with your plate,
John

[Zathras]

John,

Great idea using the 80/20 extrusions with glass facings and the porous plastic DIY bearings too. Probably need a bit of experimenting with porosity of the plastic etc but I would be very interested how those bearings turn out!
As you know, porous (possibly EDM stuff) Carbon was discussed in another thread but nobody knows exactly what type and porosity (which is a very important property to know) that's required. For the moment I have little incentive to experiment with Carbon since I have all my bearings now thanks to Neatman here on the 'Zone.:wave: Good luck with your machine.

I will be using some Granite parallels for X&Y axes that I got recently with 2 bearings loaded on each long face. Three bearings for the "truck" sliding on the glass. The arrangements of the axes is in a "T" fashion.
I will update with some pics once I grab my glass and whack it on the table.

:cheers:

[jashprecision]

John,

Great idea using the 80/20 extrusions with glass facings and the porous plastic DIY bearings too. Probably need a bit of experimenting with porosity of the plastic etc but I would be very interested how those bearings turn out!
As you know, porous (possibly EDM stuff) Carbon was discussed in another thread but nobody knows exactly what type and porosity (which is a very important property to know) that's required. For the moment I have little incentive to experiment with Carbon since I have all my bearings now thanks to Neatman here on the 'Zone.:wave: Good luck with your machine.

I will be using some Granite parallels for X&Y axes that I got recently with 2 bearings loaded on each long face. Three bearings for the "truck" sliding on the glass. The arrangements of the axes is in a "T" fashion.
I will update with some pics once I grab my glass and whack it on the table.

:cheers:

Waiting for your pictures.......

[Speed]

marble tablets are quite flat
they use it on the billiards tables

[headsmess]

i been off thinking about flate references for my lil mill so i can scrape it, when i realised i had a slab of pink granite handy...

at 800*400*50 its pretty substantial. no idea what it would have cost... or what it was for exactly..

polished side looks to be mirror perfect. speaking to the guy that gave it to me, you cant split em when wet and slapped together... i was thinking of checking them with blue but that sounds good enough... so im sorta going to go out on a wing and say its close enough to flat to check a tiny lil 8*4 inch long slide...


reading the whole thread, im confused. whats the whole goal meant to be? mass produce tables or something?

anyways! for me, im happy with parrallel reflections!

[Zathras]

FWIW, I completely gave up on all these alternatives to making a (large) surface plate. I took the risk and found a Chinese manufacturer and imported it through a freight forwarder. I posted my experience about this on CNCZone in fact. Cost me significantly less than 1/3 from suppliers here in Australia and it was delivered to my door. (We Australians get ripped off very, very, very badly in this area!!!)
It was not about the fun of trying or anything else for me. I needed to solve a problem with limited funds, otherwise I would not have bothered to be honest.

BTW, a sheet of float glass with reasonable thickness 1m sq was around $750AU+. Not even worth considering.

-Mark

[headsmess]

yes, we get ripped off big time here in the land down under. typical. people think we like being walked over.

so, who is this magical supplier? cus now i got a big mill to scrape in... dang namit!

[Zathras]

Headsmess,

My post about this supplier and freight forwarder I used is here; http://www.cnczone.com/forums/calibr...ut_ripped.html


Please Note: The person I had been in communication with at Fuminstone has now retired. The new person is "Cherry Liu".
Also, I have found in the last few days that their webpage is not responding. This worried me for a bit but they have assured me it's OK at their end. However I have just informed them I cannot see their webpage at work either.
I'm sure they will look into it.


-Mark

[handlewanker]

Hmmmm, the quest for a dead flat surface is a relative one.......whatever you use for your dead flat surface you need to check it's flatness.

You can get a dead flat surface by the three surface method as has already been noted.

That's a tall order for a plate at least 500mm X 500mm.

One solution I tried and am still working on is a cast iron drain cover measuring 600mm X 600mm X 30mm.

The cover is used to block off manholes that appear in the road to service sewers and the like, and are designed to withstand the weight of a 5 ton bus or lorry etc.

Now that is the raw material....the next is the preparation to render it to a pristine surface condition and as flat as a pancake.

I used a large Webster & Bennet vertical boring mill by taking a couple of facing cuts to remove the diamond pattern that is on the surface to prevent cars etc skidding on it when wet.

You can also use an angle grinder, it takes some doing, but by constantly checking with a 600mm ruler on it's edge you get to stage 1 when the diamond pattern is gone and you have a flat metal surface that is ready for the next attack.

That's it....you go from one step to the next using the 600mm steel ruler as a straight edge....they are amazingly true within a couple of hundredths of a MM across the entire length.......I have a 600mm Rabone Chesterman combination ruler, that checked out almost dead true on a granite surface plate in our metrology department, from end to end, so it became my straight edge.

Once you have ground.....very carefully to prevent digging in hollows etc.......the entire surface, you then map the surface by dividing it into 50mm squares and using your "straight edge" map the hollows and use the angle grinder to touch up the high points....that's as far as you can go with the angle grinder.....rough cut.

The next step is to use a 100mm belt sander to lightly go over the entire surface to remove the hills that got left by the angle grinder attack.......checking constantly to see that you aren't digging a hollow.

I marked the surface with a black marker and checked constantly with the straight edge.....left to right, top to bottom, corner to corner at 50mm intervals and using a feeler gauge to map the hollows.

At every check point you will get a high point and a hollow.....rub the high point down with the finest paper on the belt sander until you get to some semblance of overall flatness.

The rest is a mix of a flat plate and scraper, both used in combination to reveal the high points.

The flat plate can be an old lathe faceplate, or chuck backplate, used dry to reveal the high spots as a shiny mark, which the scraper at the early stage can remove better than the belt sander.

It's not easy, but by degrees you eliminate all the high spots and also keep the plate flat by constantly cross checking the run of the surface contour.

A very determined person would make 3 Camelback straight edges and that is the final stage with the scraper.

If I can get some, I'll post some photos of the plate as it presently is.....still some work to go but getting better everytime I have a go at it.

BTW, if you use steel for the plate, it is not very satisfactory, as it burrs up if you drop anything on it, and it rusts from perspiration and condensation too unless kept oiled up.

I found glass for a plate to get sticky from body grease from the hands etc, same for a granite plate too, and makes any tools drag when a sticky patch is met.

If you acquire a piece of cast iron big enough and hand work it.......initially rough work on it outside as the cast iron dust will make everything turn rusty.

When you get to the final stages of bent file and scraping etc, I use plenty of cloth on the bench and a hand vacuum cleaner to remove the fine dust......this can be done inside the workshop.

Making a reference plate of even modest accuracy is not an impossible task, it just needs a certain amount of "go for it" attitude.

It won't happen today, and it won't happen tomorrow, but it will happen.
Ian.

[Jam67es]

cus now i got a big mill to scrape in... dang namit!

[handlewanker]

Hi......."a big mill to scrape in"?????....where do you intend to start......what are you going to scrape it in to?

The purpose of scraping is not to scrape one surface true to another, but to get one surface true to a reference surface, IE one that is guaranteed and certified flat etc, and then make the other one true to it, otherwise you can get two surface true to one another but end up with a banana shape.

If you haven't got the necessary straight edges etc for the job, do not attempt to scrape any surface ....to one another.....you'd be doing irreparable damage.
Ian.