New Granite Surface Plate

[jimmy bParticipant @jimmyb]

Mines just sat on the bench, I put some bits of plasticine under it to stop it rocking. Doubt it’ll warp.

Jim

[Geoff TheasbyParticipant @geofftheasby]

Neil:

Oh dear!

I trust the sentimental value wasn't because it had 'In Loving Memoriam…' carved on one side!

Oh, that was funny! May I please use it in my Club News column?

Geoff

[egaParticipant @ega]

Posted by Michael Gilligan on 19/01/2018 11:16:02:

Posted by not done it yet on 19/01/2018 09:13:25:

Should they not be supported at three points?

.

For "anti-rocking" stability, three-point support is ideal … but that does permit the risk of the plate sagging under its own weight [minimised by selecting the right three points, but never zero].

Otherwise; distributed support, as provided by James's cork sheet, should be good.

MichaelG.

I believe three-point support is fairly standard for cast iron surface plates.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by ega on 19/01/2018 13:03:53:

I believe three-point support is fairly standard for cast iron surface plates.

.

True

… They are also, typically, of webbed construction; and therefore not amenable to distributed support.

As an aside: Distributed support works very well for the manufacture of 'float glass'.

Regarding 'sag' it is worth noting that Joseph Whitworth made his set-of-three plates hexagonal in shape.

MichaelG.

.

P.S. Items 2 and 3 in these FAQs might be of interest:

http://www.tru-stone.com/pages/faq.asp

 

Edited By Michael Gilligan on 19/01/2018 14:13:37

[Michael GilliganParticipant @michaelgilligan61133]

Too late to edit my previous post.

It is worth reading, and understanding the implications of, FAQ_12

Note particularly that the three points should be prescribed at the production stage.

MichaelG.

.

Edit: deleted quote, to avoid a formatting problem.

Edited By Michael Gilligan on 19/01/2018 14:34:02

[Ketan SwaliParticipant @ketanswali79440]

Posted by Michael Gilligan on 19/01/2018 14:29:18:

Too late to edit my previous post.

It is worth reading, and understanding the implications of, FAQ_12

Note particularly that the three points should be prescribed at the production stage.

MichaelG.

An interesting link Michael. Thanks for that. Learn't something new today.

Ketan at ARC.

[SteveIParticipant @stevei]

Posted by not done it yet on 19/01/2018 09:13:25:

Should they not be supported at three points?

 

 

Yes. It is the standard approach for both cast iron and granite plates and in fact when scraping machine components it is standard practice to support the castings on 3 points. Results are much more repeatable and consistent when using 3 points. If you want to go crazy scraping a component have it rest "flat" or on 4 or more points.

 

Furthermore, you may see on all but the smaller plates the use of 5 points. This is actually still the standard 3 points. The 4th and 5th points do not actually make contact in normal usage. They are used to avoid the risks of a large component (or user) causing the plate to tip on the end with only the single point. So they are just a fraction lower than the 3 weight bearing points. For very large plates intended for large components I am told that further supports may be used and this will be calculated according to the components intended to be placed on the table. Having said that, for the model engineer scales, the only accepted way to support a surface plate is by the 3 (or 5) point method. You will distort the plate if you mount it any other way.

 

With reference to the fact this is a model engineering forum it is important to note that the effect of not using 3 points on small granite plates and with small components may be of little consequence. My point of view is that a surface plate is a precision tool and if looked after can last a life time. So why not support it correctly It is not hard to do and would in the OP's case require less cork.

 

I have not worked out how to do multiple quotes on this forum as I would like to address Michael Gilligan's posts. Which I do feel may be unintentionally misleading. I am not knocking Michael as I am most grateful to him in the sense that the forum is a better place with him involved. However the most important specification in a surface plate, aside from flatness , is stability, i.e. resistance to warping. To help produce this characteristic the 3 point approach is prescribed at production stage, in use and in storage. During production for a granite plate any "sag" is lapped out, although it should be noted that the location of the 3 points is calculated to minimise any instability or "sag". Hence why there is no problem mounting a plate on 3 points. In fact the risk is when you do not mount, use and store it on 3 points as it will certainly then "sag". I cannot emphasise enough that it is critical to the stability and hence flatness to store and use the plate on the very same 3 points it was lapped using.

 

Whilst any quality granite plate will have the location of the 3 (or 5) points marked. For plates that do not it is possible to calculate the location of the points and recommend to use those 3 points to mount the plate as the use will minimise the risk of "sag". As a practical example my 40" x 20" x 6" plate is a part out of a scrapped CMM. It was not clear how it was supported when lapped in as there were cut outs at the calculated 3 point locations for bolting it in to the rest of the machine. So I have had to compromise, however and after 3 years of use I am still very happy with it as it prints repeatedly.

 

The only reference I can find for this was from a post on the PM forum:

 

"The old US Federal Specification for granite surface plates — GGG-P-463c, dated September 12, 1973 — calls for the table supports for a rectangular plate to be " . . . located no less than one-fifth or more than one-forth of the length and width in from the ends and sides, respectively, with the exception that the single pad at one end shall be located in the center . . . "

 

For completeness I would note that in the case of a cast iron plate, the plate is rough machined, then stress relieved (aged) by heat treatment, then finish machined and finally hand scraped again all whilst it is supported on it's calculated 3 points.

 

Thanks,

Steve

 

Edited By SteveI on 19/01/2018 18:42:18

[jimmy bParticipant @jimmyb]

My new plate came with no marks to indicate how it was supported for calibration.

It was shipped in a box….

Jim

[Neil WyattModerator @neilwyatt]

Posted by James Wilkinson 3 on 18/01/2018 22:11:52:

I purchased a selvyt cloth

I assumed that was a typo for 'velvet' until I saw the photo!

Neil

[Neil WyattModerator @neilwyatt]

Posted by Geoff Theasby on 19/01/2018 12:59:42:

Neil:

Oh dear!

I trust the sentimental value wasn't because it had 'In Loving Memoriam…' carved on one side!

Oh, that was funny! May I please use it in my Club News column?

Geoff

I'll sue!

Go ahead!

Neil

[VicParticipant @vic]

There are instructions somewhere on the net on how or rather where to support a granite surface table. I followed the instructions carefully and given its modest size I can’t imagine it would warp.

[SillyOldDufferModerator @sillyoldduffer]

Posted by jimmy b on 19/01/2018 19:01:01:

My new plate came with no marks to indicate how it was supported for calibration.

It was shipped in a box….

Jim

I wouldn't worry about it – I believe only high grade plates (ie better than needed for tool-room use) are so marked. You might be more concerned to know that tables made to support surface plates cost roughly twice as much as the plates themselves. Levelling, stiffness and so forth matter when you're serious about flatness.

Steve's point : 'With reference to the fact this is a model engineering forum it is important to note that the effect of not using 3 points on small granite plates and with small components may be of little consequence' is well made. I use a bit of kitchen worktop and occasionally put a sheet of glass on top as a treat. It's my Bodge Standard Surface Plate. You may well need better, and I'm sure your boxed granite is a considerable improvement!

Dave

[Douglas JohnstonParticipant @douglasjohnston98463]

Are we really going to see any difference between a 3 point support and a distributed support of cork or (in my case ) a sheet of foam carpet underlay on a granite plate of modest size. Is it just me or are we being a tad theoretical here and chasing such tiny differences as to be meaningless?

Doug

[SteveIParticipant @stevei]

Posted by Douglas Johnston on 19/01/2018 20:45:01:

Are we really going to see any difference between a 3 point support and a distributed support of cork or (in my case ) a sheet of foam carpet underlay on a granite plate of modest size. Is it just me or are we being a tad theoretical here and chasing such tiny differences as to be meaningless?

Doug

Based on my limited experience I would say that for hobby layout work this is purely theoretical. I.e. of no real consequence, hence this is not of interest to everyone. However for scraping I have first hand experience that it is much easier and efficient with a top quality surface plate. The reason being that when bluing results are much more repeatable which does make progress quicker and avoids wasted effort.

Over on PM they suggest 3 ice hockey pucks, or similar sized aluminium. discs. I.e. soft enough to deform such that the load is somewhat spread out but robust enough to provide the required support. I've used 3 slices of Ø2" nylon. I've never found out what the appropriate British standard is that covers granite surface plates. I would be very interested if someone had a reference.

With respect to Dave's glass plate, chapter 8 "the surface plate" of "machine tool reconditioning" by Connelly cover the following types; cast iron (scraped), ground metal plates, hardened steel flats, glass surface plates, and granite. Up until now I have never read the section on glass plates but it turns out the glass plate was first developed during the war to offset the shortage of metal and were made in standard sizes upto 18". All the plates types have different pros and cons but glass has a lower co-efficient of expansion so is less affected by temperature variation and will not warp due to a warm hand like cast iron. Perhaps an ideal choice for the hobbyist chasing microns in the unheated garden shed!

Steve

[Pete RimmerParticipant @peterimmer30576]

I bought some cheap cork tea coasters and glued them to the bottom of my plate. Thee coasters 25% in from each respective edge. Two at one end and one at the other.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SteveI on 19/01/2018 18:40:17:

… I would like to address Michael Gilligan's posts. Which I do feel may be unintentionally misleading. I am not knocking Michael as I am most grateful to him in the sense that the forum is a better place with him involved. However the most important specification in a surface plate, aside from flatness , is stability, i.e. resistance to warping. To help produce this characteristic the 3 point approach is prescribed at production stage, in use and in storage. During production for a granite plate any "sag" is lapped out, although it should be noted that the location of the 3 points is calculated to minimise any instability or "sag". Hence why there is no problem mounting a plate on 3 points. In fact the risk is when you do not mount, use and store it on 3 points as it will certainly then "sag". I cannot emphasise enough that it is critical to the stability and hence flatness to store and use the plate on the very same 3 points it was lapped using.

.

Steve,

I take no offence at what you have written, but I must confess to being bewildered.

Almost everything that you have 'clarified' was either explicitly stated, implicit, or referenced in what I posted.

… Right down to the Federal Standard.

I am happy to discuss this item-by-item if you wish; either publicly or privately. … But, for the moment, suffice it to say that I emphasised that three-point support is ideal, with the essential caveat that the three chosen points are suitably distributed AND prescribed at the start of manufacture.

The plates which are the subject of this thread do not appear to feature such prescribed points [i.e. they are decently finished, simple slabs of granite]; and therefore supporting them on three points MAY do more harm than good.

In Peace

MichaelG.

[Pete RimmerParticipant @peterimmer30576]

One of the guys brought a brand new in the box plate to the recent scraping class. We set it on three points at 1/4-1/5 spacings and checked it with an autocollimator/repeat-o-meter, and it was as close to perfect as you could measure in the home shop. Not even a tenth variation.

[Clive HartlandParticipant @clivehartland94829]

A note on Selvyt cloths, they must be washed often and by doing so become lint free. washing them you should use one of the soap solutions used for woolens. Mainly used for polishing/cleaning optical lenses and prisms.

A Jeweler will polish gold and silver items with one but he will load it with Jewelers Rouge, this will then retain any gold and over time the cloth can become quite loaded with gold. The cloth is then burned in a crucible and the gold recovered. Remember this is your gold that the jeweler has polished off your gold items.

Clive

[SteveIParticipant @stevei]

Posted by Pete Rimmer on 19/01/2018 22:55:57:

One of the guys brought a brand new in the box plate to the recent scraping class. We set it on three points at 1/4-1/5 spacings and checked it with an autocollimator/repeat-o-meter, and it was as close to perfect as you could measure in the home shop. Not even a tenth variation.

 

Good evening Pete,

Did you measure it when it was not correctly mounted on 3 points? What was the size of the plate?

 

 

My Chinese granite tri-square is square to better than I can measure. In that sense it compares favourably with a British made product. However perhaps I got lucky.

As far as I know this is the last UK manufacturer of granite surface plates and standards:

**LINK**

are there others?

 

Although my overblown sense of civic pride would ensure any tool from eleymet would give me great satisfaction to own and use in practice I agree that there are more affordable good enough quality deals out there.

 

A little googling has dug up: BS 817:2008 Specification for surface plates, which replaced the BS 817:1988.

 

If anyone has a copy of either of those for purely educational purposes please do get in touch.

 

Michael — I've sent you a PM.

 

Thanks,

Steve

 

Edited By SteveI on 19/01/2018 23:48:47

[James Wilkinson 3Participant @jameswilkinson3]

jimmy b I can see you are as happy with this plate as I am mine was also shipped in a box but I discarded it because it smelled of ebola. These granite surface plates wont deform, bend or distort to any degree that will prove problematic, it's just not the characteristics of granite to do so unless they are of a large size and then mishandled. These 450x250x80 plates will be more than stable enough.

Clive Hartland good to see some talk about the selvyt cloth. I am an ex Goldsmith as it happens thanks for the info on washing powder! there are many different selvyt cloths, some impregnated with different polishing compounds and some not. Mine is just a basic soft selvyt cloth, here is a link to the cloth should anybody wish to purchase one: **LINK**. I have many other selvyt cloths which I use to remove polishing compound with cellulose thinners off the brass and aluminium musical instruments I manufacture before I clear lacquer them. I use buffing mops on a polishing motor first with tripoli and blue and red "rouge" for final mirror finishing.

When I was a Goldsmith we used them after polishing, ultrasonic cleaning. We used the cloths only for handling purposes and to remove greasy fingerprints once the polishing process was complete.

[jimmy bParticipant @jimmyb]

I'm far from worried about it moving! I've checked it as best I can with a 2 micron clock, seams perfect. Easily worth the money, (to me!).

My day job is more precise than I will ever get close to at home.

My comment about the plate not indicating its mounting points, was to show that it won't matter. It will have been made simply placed on something. It was only £78 (delivered) after all.

They now show as out of stock, so I'm happier than ever now.

I had kept the box with my previous surface plate, the Axminster one, to keep it covered, but this box was very bad!

Not sure if it smelt the same as yours did James

JIm

[jimmy bParticipant @jimmyb]

I made a cotton cover to try and keep dust off it.

It fits well over the "Trimos" height gauge.

Mrs Jimmyb won't notice the spare bed is missing a sheet for ages ………………….

Jim

[Douglas JohnstonParticipant @douglasjohnston98463]

Some time ago I was doing a lot of work with making cabinet drawers in wood and obtained a piece of plate glass 800mm square and 12mm thick to lay out the drawer sides for glueing.

​ On checking the flatness of the plate I was very surprised to find it was anything but flat. Over the 800mm width it had a bow of at least 1mm. I later discovered that the glass had been toughened and I believe this involves heating the glass in some process, and I suspect this could have caused the warping.

​ A lot of people use plate glass as a surface plate, and a lot of modern glass will have been toughened to make it safer to use and as a result may not be suitable as a good flat surface. Just something to be aware of.

Doug

[SamsarandaParticipant @samsaranda]

James I thought Ebola was a virus, how can you smell it?

Dave W

[VicParticipant @vic]

There’s a diagram near the bottom of this page showing how to support a granite surface plate.

It’s not difficult so why do it any other way?

**LINK**

[Author]

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