Quartz Surface Plate?

[BaronJ]

Hi Pekka,

I'm sure that someone else will have said it, but it might pay you to have a chat with your local funeral director or headstone mason.  I've had a chat with one near me (UK) and he says that unpolished granite slabs 4" thick are relatively cheap.  However they are also quite flat !  The high cost is down to the amount of time and effort involved in polishing them.  He has machinery to do this although I've not seen it in action, mainly because it's always in use and inside a very large enclosure.  The fine slit that comes off makes a good lapping paste :-)  He also gets broken and damaged headstones when he has to replace one.  Some of these get cut down for plinths and other things.  So its worth investigating.

 

[PekkaNF]

I have contacted two businesses both do headstones and table tops. But they don't have information on how true the tops actually are.

One is local and I'll visit it if I get from my work any time when they are open.

I have tried two method home:

1) Support good straight edge on top of the 0,05 mm feeler blade from both ends. Straight sample should allow a same gauge feeler gauge to slide under the straight edge. I found that same gauge feeler blade is just a tad tight....as it should be. I have 0,03 0,04 0,05 0,06 and 0,07 mm feeler blades. This shows pretty easy if there are any big problems.

On real surface plate 0,04 mm feler blade slid sweet, I could feel a little constant resistance. Same thickness feeler gauge was tight and tend to move the straight edge...pretty sure the DIN 874/2 straight edege has some 20 um variation....

On granite kitchen top piece 0,03 was tight on some spots and couple spots could place even 0,06 mm feeler gauge under the straight edge that was supported 0,05 mm feeler gauge at both ends, but it was nearly as bad as I tought.

Hair staright edge did emphasis high spots and walleys pretty clearly. Looks like colorful table top granite is not really a best choise. It might have harder and softer parts in it and it seems to show on surface quality.

2) I place straight edge on sample plate and imobilize it with a long parallels on both sides, Then I slide a magnetic TDI stand w/ clock on the side and indicate from top of the straight edge.

This was interesting...Real surface plate showed practically no clear movement on the dial...which was good, because it showed that the measurement can be used to indicate straight line and instrument was good for it.

On the othe had the checkered granite surface plate showed regular waviness and raising/dropping error trough the whole travel. Indicator base is relatively short, it would pick up easily surface undulation. Another thing is that DTI was at offset from the base amplifying the error greatly. I did not do math, but on real surface plate the indicator hardly moved at all and my sample of granite table top indicated worse than 0,05 reading, growing to 0,1 mm at the ends and also showed definite dip on the middle of the plate.

Now I have a way of field checking the embryo surface plate in a somewhat reliable way.

VT's comment on using a building aluminium straight edge is haunting me. I know that there are some very good aluminium straight edges, but I can't get my brain around the oxide layer/truing abrasion when used against feelergauge atc problems. But it could be cheap, rigid and lightweight. Can't really protect the soft edge with a steel strip from feeler gauge roll.

Took two solid hours to get repeatable results, but I think I have a method for testing now.

Pekka

[vtsteam]

Pekka, an aluminum straightedge certainly isn't resistant to wear and abrasion the way a cast iron straightedge is, however, if new, and you are careful with it (as I was) there's no reason why it shoud show any wear at all for some occasional jobs.

Now if you were rebuilding machinery every day, that would be different.

No guarantees that you will find a perfect straightedge at the store -- but it seemed you had moderate expectations for the tolerances you would find acceptable on your mill. I don't remember the brand I bought 14 years ago, and I doubt I, or you, might ever find it again. And I did not test it either -- I was only building an 20" bed length lathe with it (12" carriage travel).

One nice thing about a store bought level, though -- you can always return it if you find it isn't suitable. And in my case, I've been using it as a builder's level ever since. Not suitable any more for scraping ways, but I now have better equipment.

Good luck, if you do try it!

[PekkaNF]

I have home two builders straight edge. They are good enough for building, general standard seems to be +/- 1mm/2m or better and they are somewhat better.

I'm thinking of buying pretty good 1 m DIN 874/1 straight edge. I also bought Russian? 400 mm long camleback, let's see how does it compare when I'll get it from the mail. At 40€ price range I don't know what to expect.

I'm including here some pictures that relate to my previous message. Hope they are more clear than my text.

Pekka

[DMIOM]

......I'm including here some pictures that relate to my previous message. Hope they are more clear than my text.

Both are perfectly clear - thanks for taking us along on the journey.

Dave

[vtsteam]

I was thinking about that 400 mm long camleback, and I was imagining bolting three of them to a piece of bar or square tube to make a 1200 mm long straightedge. Set them on your granite surface plate at work (or wherever) and tighten the bolts so the camelbacks are all in line along the bottom.with the surface plate.

Then I was thinking, well heck you could do the same with (not camelbacks) but just short lengths of bar stock, bolted to a full length piece. As long as they were reasonably straight three of four hundred mm long pieces, and de-burred, it should work for your purposes.

Hey and if it ever warped or anything, you could just re-straighten it with the bolts on a surface plate again. An adjustable straightedge.

[PekkaNF]

Hi Pekka,

I'm sure that someone else will have said it, but it might pay you to have a chat with your local funeral director or headstone mason.  I've had a chat with one near me (UK) and he says that unpolished granite slabs 4" thick are relatively cheap.  However they are also quite flat !  The high cost is down to the amount of time and effort involved in polishing them.  He has machinery to do this although I've not seen it in action, mainly because it's always in use and inside a very large enclosure.  The fine slit that comes off makes a good lapping paste :-)  He also gets broken and damaged headstones when he has to replace one.  Some of these get cut down for plinths and other things.  So its worth investigating.

I checkked the local one and the machine was a little antique...and rock plates lookeed fine, but measured badly. One was better, but it is still marginal.

Pekka

[jcs0001]

Pekka, Steve and others - thanks for the discussion about this.  I was wondering how to check such items for flatness and you've provided some answers.

John.

[BaronJ]

Hi Pekka,

I checkked the local one and the machine was a little antique...and rock plates lookeed fine, but measured badly. One was better, but it is still marginal.

Pekka

I haven't been down that way for a while, I'll have to call on him and talk to him some more.  Not surprised that the machine your mason has is a little antique, I suspect that the machine  that my local mason has is an antique as well.  I'll have to persuade him to let me have a look at it and maybe photograph it.  I'll let you know when I get chance.

[PekkaNF]

Plate is pretty, but looks like as a whole it is not straight enough.

But I got three weird knife straightedges and one camelback. They sem to be pretty nice.

Pekka

[sparky961]

So I just happened to be driving through a busy section of a larger city today and passed by a sign on a building that said "GRANITE COUNTERTOPS".  I kept driving along for a while before it clicked that I should go back and check it out.

I very patiently waited in the showroom while the salesperson finished up with some actual customers before asking about their scrap; sink cutouts, broken pieces, whatnot.... and I was a little surprised when she directed be to a large green bin behind the building, saying "take as much as you want; we pay to get rid of it".  It was filled with varying sizes and shapes of all different kinds of mostly 1-1/4" countertop material.  There were lots and lots of long narrow pieces that didn't seem too useful at the moment (though I got to wondering about straightedges), quite a few smaller rectangles about 8 x 10 or so, and .... GOLD MINE ... a handful of quite large pieces, both rectangular and irregular or broken.

Just goes to show that it never hurts to ask.  Especially when you ask nicely.

I'll try to get some pictures up for those who just like to oggle, but now I'm having to think about what to use as a cheap lapping compound.  What's cheap and easy to source, very fine, and harder than granite?  I'll do my own research in the meantime but anyone's experiences lapping their own plates would be a great addition to this thread.

[Manxmodder]

Sparky, I was just thinking about this kitchen worktop granite surface plate earlier this evening.

If you need to remove any high spots on the plate I reckon a steel backed diamond coated sharpening stone would be quite effective.

You would just need to keep checking with a DTI clock to keep track of your progress.

I have a couple of diamond coated steel backed stones that I bought a while ago on Fleabay.

Or may be one of these multi faced rectangular blocks would be worth considering.
http://www.ebay.co.uk/sch/i.html?poi=&adpos=1s1&ul_noapp=true&geo_id=32251&MT_ID=11&crlp=71010014297_807&keyword=diamond+sharpening+block&rlsatarget=aud-105106656306%3Akwd-6907918083&_nkw=diamond+sharpening+block&device=c&crdt=0&treatment_id=7&clk_rvr_id=894874226903

[PekkaNF]

So I just happened to be driving through a busy section of a larger city today and passed by a sign on a building that said "GRANITE COUNTERTOPS".  I kept driving along for a while before it clicked that I should go back and check it out.

I very patiently waited in the showroom while the salesperson finished up with some actual customers before asking about their scrap; sink cutouts, broken pieces, whatnot.... and I was a little surprised when she directed be to a large green bin behind the building, saying "take as much as you want; we pay to get rid of it".  It was filled with varying sizes and shapes of all different kinds of mostly 1-1/4" countertop material.  There were lots and lots of long narrow pieces that didn't seem too useful at the moment (though I got to wondering about straightedges), quite a few smaller rectangles about 8 x 10 or so, and .... GOLD MINE ... a handful of quite large pieces, both rectangular and irregular or broken.

Just goes to show that it never hurts to ask.  Especially when you ask nicely.

I'll try to get some pictures up for those who just like to oggle, but now I'm having to think about what to use as a cheap lapping compound.  What's cheap and easy to source, very fine, and harder than granite?  I'll do my own research in the meantime but anyone's experiences lapping their own plates would be a great addition to this thread.

Very nice, have you measured any of them yet? I'm eager to hear how are the results?

Pekka

[jcs0001]

Another possible source of granite countertops:

I was in our local re store (Habitat for Humanity runs them) and saw three pieces of granite countertop - two about 18 x 36 in. or so and one about double that size.  As I recall they wanted $20 for the smaller pieces and $40 for the larger one.  I was tempted to buy one of the smaller ones as a glueing table surface (it would definitely be flat enough for glueing wood pieces up).  Don't have room in my shop for it however.

John.

[Pete.]

I might have some very nicely finished slate slab down here in Kent. I'll run a dial over them and if they are suitably flat I'll put up a photo.

[PekkaNF]

Good or bad, we need to know!

I haven't found any dead sure way telling the good and bad appart. They all look good and most of the ones that looked like a real surface plates were pretty bad.

Pekka

[Pete.]

A dial gauge and stand will tell you how flat they are.

[DavidA]

Pete,

How would you set about doing that ?

I would have thought that you would need to know that your dial guage stand was precisely square to the slab under test and that the vertical shaft was also dead on vertical at any position of the base.

Any error at all and the dial guage will move relative to the slab.

Dave

[sparky961]

Funny, I was just going to pose a similar question.

I've done a fair bit of reading and watching videos on this subject.  It seems that there are two checks, one for local flatness that uses a height gauge and DTI (or similar contrivance) and a more elusive overall version.  The former I can do, but how do I know that the whole thing isn't concave or convex?  Add to this the requirement that it be within the reasonable limits of a home shop budget and environment.

I did try some off-hand "lapping" of just two pieces; a smaller one on a larger one.  I did this mostly to test the"feel" of the operation, which was interesting to say the least.  When I was using water with toothpaste (yes, you read that correctly) it was smooth in some spots and then just sort of "stopped" as though a corner dug in, or the friction or suction became strong very quickly.  It didn't slide smoothly as I expected, rather in a bit of a "jerky" motion.  Dry was considerably better, but my selection of readily-available fine hard powders is very limited.  I tried using talc, but I know this is very soft.  I was thinking it would be more of a dry lubricant and the two granite surfaces would wear each other.  Maybe it would work just completely clean and dry to start even?

I haven't put any measuring devices on there yet, save for an angle plate with a cross-hatch pattern drawn on it with a sharpie.  The pattern didn't wipe off as consistently as I'd have liked, but this is all far from scientific. 

I did notice that the formerly shiny surface has begun to take on more of a matte finish.  I hope this indicates something beneficial, and not just some residual toothpaste haze...

[Pete.]

You can spot dishing or twist in a plate by trying the stand and DTI at different angles and directions across the plate.

[krv3000]

well a piece off glass will let you no watts hi and watt is not put engineers blue on the table then place the glass on top remove the glass the glass that has blue on is hi and the glass that has no blue on is lo if the blue is all over the glass then the top is flat   

[sparky961]

well a piece off glass will let you no watts hi and watt is not put engineers blue on the table then place the glass on top remove the glass the glass that has blue on is hi and the glass that has no blue on is lo if the blue is all over the glass then the top is flat

Hmm... indeed, but a bit of a paradox, isn't it?  If I had a piece of glass I knew to be flatter than the plate I'd just use the piece of glass.  

That said, are you talking about regular thin "window" glass, or thicker "plate" glass?  If window glass is flat enough I'm sure I could rustle up some of that.  The thicker stuff could be an issue.

[sparky961]

So, I had a chance to take a closer look at my haul tonight.  Spoiler alert: Don't get too excited.

Good news is that I was able to measure flatness deviation.  Bad news is that it wasn't difficult to measure.

To start off, my setup is far from ideal so take everything here with that in mind.  I have a shiny rock (could be granite, but how would I know?) about 26 x 26 x 1-1/4 sitting on a relatively rigid small table made of MDF.  Between the rock (we'll call it "granite") and the hard place (the table), there are three strips of minicell foam about 1" thick and maybe 12" long, placed somewhat evenly to support the plate.  Why?  I had some left over and it seemed this would allow the plate to "float" the best way possible with available materials.  With the indicator set up I pushed in various places and with a lot of pressure could move the indicator by about 2 tenths.  As you'll see in the next paragraph, proper support is the least of my concerns at this point.

After cleaning the slab, I proceeded to jury-rig my DTI to my height gauge.  Yet another tool/adapter thing to make.... The gauge base is about 5" long and my first setup put the indicator point about 6" from the end of the base.  Moving this around the plate in no particular pattern yielded a rough average of +/-0.005" deviation over 6", with a maximum of about 0.008".  The gauge base didn't slide nicely in many spots like you'd expect on a surface plate.  This probably explains the "grabbing" I noted earlier while doing my lapping trial.

One of the pictures below shows the granite floor tile I picked up at Home Depot.  It was more difficult to measure because there isn't a lot of room to move the gauge around.  I shortened the reach to about 4" and deviation seemed a bit better, maybe +/-0.003".  Given the smaller distance, this may or may not actually be any better.

So, in summary I don't think granite countertops are a useful surface plate as supplied.  If you're ok with +/- 0.010 or so then go for it, but I'm looking for better than that.

I'm going to continue working with some of the pieces I have to see if I can't find an effective way to improve the flatness.

[sparky961]

Man, I really should have gone to bed after that last post but I had an idea as I was writing it that I had to try.  Yes, I'm sure lots of you here know the feeling.

So it turns out it was a great idea!  Don't give up on granite.... more to follow.

[Pete.]

well a piece off glass will let you no watts hi and watt is not put engineers blue on the table then place the glass on top remove the glass the glass that has blue on is hi and the glass that has no blue on is lo if the blue is all over the glass then the top is flat

The only problem with that is if you put a piece of glass on a surface plate it might well be flat but if you put it on anything else - especially with a chunk of iron on it - it will bend and not be as flat as it was when you checked it.