Surface plate support

[MikeKParticipant @mikek40713]

I have this surface plate that I got years ago…12″ x 18″ x 3″ black granite.  And because of its heft, and storage location on a shelf under the mini-lathe, I don’t use it as much as I could (or would like).  It currently resides on a particle board shelf only slightly larger that the plate itself and with modest metal braces under each of the four edges.

In recent days I’ve been thinking about making a better storage solution for it, possibly above a tool chest where there’s plenty of unused vertical space.

I know in amateur telescope making circles there is a desire for a uniform support for the heavy glass primary mirror to mitigate the mirror’s distortion.  And I seem to recall it mentioned in a machine tool book, many years ago, that it was recommended to do a similar 3-point support for surface plates.

Has anyone done this?  Or has anyone noticed problems with the lack of “proper” support?

Mike

 

[Taf_PembsParticipant @taf_pembs]

Hi Mike, I have just done a similar thing.

I have a 18″ x 18″ x 4″ black granite plate and after lots of hunting and reading as to why I could not find a commercially available ‘proper’ 3 point support stand for it I found a site that deals with plates and stands that stated that they will not produce such a stand as a plate that small is liable to tip due to its lack of mass.

It also stated that plates smaller than 600mm x 600mm ( I think, may have been 600 x 400mm) would not distort by any meaningfully measurable amount.

If I can find the site again I’ll post a link.

So I made a tool chest top stand – a 1/4 thick steel plate edged with small angle to keep it in place, 4 Ice Hockey pucks skimmed in the lathe to the same thickness so as to also remove the shiny surface and provide a little grip then a 20mm hole bored through the middle to provide a location for a locating ‘nipple’ on the steel. The Ice Hockey pucks are vulcanized rubber so have a little give in them – it is perfectly stable sitting on them, works a treat.

I also made a small angle iron frame to surround it so even if nocked it could not slide off or move (not that it would with it’s weight!) although this provides no support.

Easier with pics..

The under side of the top plate

 

I moved the pucks a little further out before mounting, 90mm in from the outer edge so 20% of the surface plate length.

 

 

I really like the fact that I can now move it around – away from where I’m doing anything dirty. and I’ve made a felt lined wooden cover for it too.. Proper posh!!

👍

[Chris CrewParticipant @chriscrew66644]

I have a cast iron 18″ x 12″ surface plate which lives on the top of a sheet metal workshop tool cabinet. It has never crossed my mind that, given the ribbing of the casting, it might be insufficiently supported. However, I don’t, and can’t possibly, work to NPL standards of precision and if I get things to fit, work and look right I am happy. I would imagine this applies to most back-shed workers, although I wouldn’t for one moment try to take anything away from the wonderful work that some people are capable of producing. So, I would say it depends on what you are capable of doing but, with a 3″ thick piece of granite, as long as any table or bench that is capable of supporting its weight, in back-shed terms of accuracy, you will not notice any distortion whatever it sits upon.

[Michael GilliganParticipant @michaelgilligan61133]

For what it’s worth:

http://www.precisiongraniteusa.com/images/4919-INSTALLATION-INSTRUCTIONS-FOR-SURFACE-PLATES.pdf

Practical chaps and chapesses will probably choose to ignore the warnings of hell-fire and damnation.

MichaelG.

[MikeKParticipant @mikek40713]

Thanks Chris.

Taf…You did just what I want to do!  I want to make mine a little higher, as that will give me some storage underneath as well as preventing me from constantly bending over (although I’m only in my 50’s, I have brutal back arthritis).  And I even have hockey pucks!

[MikeKParticipant @mikek40713]

Thanks Michael.  That dimensioned diagram is what I remember seeing.

[Anonymous]

My 36″ by 24″ cast iron surface plate has three feet cast into the sides. The feet are not spaced according to the diagram but are almost on the outer edges. Mind you it is only Grade B. I The sides, and internal webs, are quite deep, about 5″, so I doubt the table will sag much. A big advantage of three feet is that the table will be stable with no rocking or induced stresses due to over constraint.

Andrew

[old martParticipant @oldmart]

In our fitting shop where I used to work, we had a 24 x 36 granite surface plate simply sitting on a steel benchtop on a sheet of rubber. It was calibrated every six months and was one grade lower than inspection. You people don’t know when to remember that you are not working to microns.

[Kiwi BlokeParticipant @kiwibloke62605]

Robin Renzetti (‘Robrenz’), posted a You Tube video, some years ago, describing, in his inimitably thorough way, his perfectionist approach to surface plate support. Watch, learn – and then do something simpler…

[MikeKParticipant @mikek40713]

Great, thanks…Something to watch while I have dinner.  I suspect, as mentioned above, that I’m worried about nothing with such a small plate.

https://www.youtube.com/watch?v=yPhzp-OG67s

[SillyOldDufferModerator @sillyoldduffer]

That Mike doesn’t specify the accuracy required suggests his need is ordinary.   The table simply provides a flat surface, from which vertical measurements can be taken with a height gauge.   Chances are, in a home workshop, the height gauge won’t be better than ±0.02mm,  suggesting any reasonably stiff support will do.   No need to fuss.

Supporting the table properly gets much tougher as soon as accuracy and precision requirements are set.  Now the class of table matters, as does having it calibrated and certificated.  The rigidity of the support becomes critical, to the point that it costs about as much as the surface table; heavily built.

Surface tables are important in metrology labs, in tool-rooms, and to inspectors.  For amateur use I’m not sure they’re  worthwhile.  I’ll probably never get one.  Before owning a milling machine, I simply plonked a sheet of float glass on my kitchen worktop covered bench.     I’m confident this met my accuracy needs, about a thou, all that can be expected of my height gauge.  Now I have a mill, I use it’s table, because then it’s not necessary to mess with a sheet of glass.    May just be me, but I use the height gauge and engineer blue marking out methods far less than expected, I think because a CAD/mill/DRO combo provide a good alternative.

Question for surface table owners; what’s it used for, how often, to what accuracy, and can you prove it?

🙂

Dave

 

[bernard towersParticipant @bernardtowers37738]

Well SOD I have two that are VERY little used and then only to loose dims and how accurate they are I have no idea. DROs have brought about this situation I think.

[Howard LewisParticipant @howardlewis46836]

I made a small surface plate for The WayerWorks Museum.

It was 18″2 x 24″ x 3″ granite. The support for it was a frame made from angle iron, 2.5″ x 2,5″ x 5/16″with reinforicing “strips” of the same running symmetrically from end to end, and side to side.

The corner posts protruded vertically, to keep the granite in place, with levelling screws at the bottom end of each leg.

Onto the frame was laid a sheet of hardboard. Onto this was placed a slightly runny plaster mixture, and the granite then immediately placed on top of that, so that it would “float” on the soft plaster, being uniformly supported without stress.

It is important that the surface plate, particularly if it is relatively thin, is not stressed

At work a new 6′ x 4′ x 6″ granite surface plate (£3,000+) was ruined, by leaning it, almost upright against a wall.

It sagged and warped!

Howard

[BazyleParticipant @bazyle]

Howard, No doubt there will be other comments but it seems to me that water absorbing plaster and water absorbing hardboard are the worst things to bring into the equation. Most particularly the normal practice in anything ie lathes aswell is to target 3 points of support only so there can be no contention as to which is doing the work – kind of like a 3 legged stool may not be level but it cannot rock like a 4 legged one.

[SillyOldDuffer]

On SillyOldDuffer Said:

That Mike doesn’t specify the accuracy required suggests his need is ordinary.   The table simply provides a flat surface, from which vertical measurements can be taken with a height gauge.   Chances are […]
[…]

Question for surface table owners; what’s it used for, how often, to what accuracy, and can you prove it?

🙂

Dave

 

You make some good points, Dave … but it’s worth emphasising that there is a significant difference between a “marking-out table” and a “surface plate”

[We tend to use a range of convenient descriptive terms, but I think those two are reasonably indicative]

I couldn’t locate this Starrett document yesterday, but found it this morning, on the Raptor Supplies website:

https://cdn.raptorsupplies.co.uk/pub/media/catalog/product/datasheet/starrett-80601.pdf

Amongst other things, it identifies various grades, and addresses your closing question.

Barely relevant to most of us … but worth spending the few minutes to read it carefully.

MichaelG.

.

Edit: __ might as well link the price-list too:

https://www.raptorsupplies.co.uk/p/starrett/crystal-pink-granite-surface-plate

 

[JasonBModerator @jasonb]

Maybe I should get a plate/table some day as it could improve what I make 😛. Obviously this will depend on what the individual gets upto in their workshop.

While I agree that getting it out in the open where you are likely to actually use it is a good thing I wonder if rolling the tool chest to different parts of the workshop will be the variable rather than the mounting unless you have a very even shed floor. Beware of using teh cover as a convienient table to pile other stuff onto as you will then get back to the situation where it is not readily available to use.

My “flat enough” surface gets used for both the odd bit of marking out though I do a lot less since getting a DRO as well as a surface to do things like assess a casting on and shim it up to see what needs to come off where. . It also comes in handy as a background for photographing small parts laid on it. I think in most hobby workshops they will serve the dual purpose of marking table and surfac eplate as not many of us have enough space for both

[Paul LParticipant @paull58212]

Ideally a surface plate shoud sit on 3 points. These are called Airy (spelling?) points and are 22.6% of its length and width in from the edges. the 3rd point is on centre line. this minimises the distortion of the plate caused by its own weight. on larger plates there should be 2 ‘outriggers’ either side of the single point to stop the plate tipping.

On larger plates this makes a significant differance to the flatness.On smaller plates, in reality, it makes no differance, the wear on the surface will be of a greater significance.

if you turn your plate over Mike you may be able to see marks where the points were. If so I would just glue 3 M12 nuts onto those points.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, my small cast iron indoor surface/marking out plates have three points for support, as does my 24″ x 18″ cast iron one in my garage.

I don’t know the accuracy of any of them, as they are all second hand, but they are good enough for whatever I will be using them for.

Regards Nick.

[Nicholas Farr]

On Nicholas Farr Said:

…

 

I don’t know the accuracy of any of them, as they are all second hand, but they are good enough for whatever I will be using them for.

…

But what does that mean Nick?  I guess you have no intention using them for accurate work, but where do you draw the line:  ±1mm, ±0.2mm,  ±0.02mm, or ±0.002mm?

±1mm is woodwork – no need for a surface table.

±0.2mm is crude metal bashing – no need for a surface table, just a reasonably flat bench.

±0.02mm is basic precision metalwork, probably what most of us do in our workshops.  No need for a surface table: ordinary kitchen worktop may be ‘good enough’ provided it’s rigidly supported.   Sheet of float glass on a solid kitchen worktop, or a milling machine table will certainly be good enough.

±0.002mm is the real mcCoy,  and a surface table would be valuable.   But a point often ignored by hobbyists is that this level is only achieved when done properly.   Buying a second-hand tenths micrometer, gauge block set, and surface table off ebay absolutely isn’t good enough.  They all have to be in good working order AND calibrated.  Plus, the owner has to know how to use them: ±0.002mm is only achieved by understanding what causes tiny errors – not ‘common sense’.

On arrival, second-hand tools might be ‘good enough’ to do a shade better than ±0.02mm, but even that is uncertain.

The reason I asked surface table owners to say what they’re used for, is I suspect surface tables seem like a good idea, but turn out to be rarely used in home workshops.  For us, I reckon they’re mostly a waste of money and space.   Most of our close fits are achieved by ‘fitting’, not by accurate precise measuring.  The most likely exception I guess is a repair shop needing to get accurate dimensions off random incoming objects.

Dave

 

[Paul L]

On Paul L Said:

Ideally a surface plate shoud sit on

Ideal might be 4 points… 2 direct to plate and 2 on a subframe that has a single pivot in the middle … so three point of connection to ‘solid ground’ but 4 points touching the plate.

 

[JasonBModerator @jasonb]

Dave how would you “fit” a flat surface if you were not measuring it.

Surface plate is the ideal matching part to get your “mating” part flat. Be that by bluing the surface plate and rubbing that scraped machine part against until you get the pattern you want. Or it could be something you lay a sheet of abrasive on and rub the base of your model engine over as you can’t really do a lot more until you have a flat surface to work from or to mount the part for subsequent machining of the upper surfaces.

Assessing a casting. I’m only interested in what gauge wedges the part vertical not it’s actual thickness as they will simply be use da spackers when mounting on the mill.

Another that has been rubbed on abrasive, red Sharpie indicates where I have contact, not worth going further as the “hollow” is only a few thou and won’t affect how the engine sits

[Robert Atkinson 2Participant @robertatkinson2]

Dave (sod),

I have to disagree on the requirement for calibration. It comes down to the difference between precision and accuracy. Calibration only confirms accuracy. A fully tracable calibrated item with a 0.1% specification is unlikely to be as accurate as an uncalibrated, un-abused item with a 0.01% specification. in any case the 0.01% item is likely to be inherently more precise.
For most home users precision is more important than accuracy. That is because you are making all the parts and not expecting them to fit something made using different measuring instruments.
To clarify it’s better to have all your parts precisely 0.01% oversize than randomly < 0.1% different.

Even if only working to 0.02mm having a surface plate and better quality instruments will make acheving that much easier than using a mill top for example.

Robert.

[larry phelan 1Participant @larryphelan1]

Fascinating to read that there is so much to know about surface plates !

Mine is a piece of granite worktop [cut out for a sink to be fitted ], picked out of a skip. I was so proud of it until I read all this, now I regard it with scorn !

Really messed up my day, but then, I seldom need anything better than a six inch rule and a try square.

[Paul L]

On Paul L Said:

These are called Airy (spelling?) points

You question the spelling of Airy points. The correct spelling is “points of minimum sag”. See:

https://en.wikipedia.org/wiki/Airy_points#Other_support_points_of_interest

Airy points are used with an end standard, so that the ends of the standard are vertical. A surface plate is a standard of flatness so minimum overall deviation from a notional (mathematical) plane is desirable.

 

[Martin KyteParticipant @martinkyte99762]

If you want to spread the load on the support as per telescope primary mirrors then tripod supports centred on the Airy points with the ‘legs’ against the bottom of the surface plate work. For mirror mounts these are termed ‘teeter totters’. For large mirrors the tripods can be stacked to create 9 points of contact with 2 layers or 27 points with 3 layers. All loading will be equally distributed providing each tripod is free to pivot on its point.
However I would suggest this is all a bit unnecessary for a granite surface plate in a home workshop. I mention it only as a curiosity as telescope mounts were referred to.

regards Martin

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