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.

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

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…

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

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

On 7 July 2024 at 21:34 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

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.

On 8 July 2024 at 08:44 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

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

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.

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