Surface plate support

[Howard Lewis]

On Howard Lewis Said:

Never knew that hardboard was hydroscopic!

[…]

< Pedant Alert >

For the few that might care:

https://corecheminc.com/hygroscopic-what-it-means-what-you-need-to-know/#

Hygroscopic (or Hydroscopic): These two words may be confused …

MichaelG.

[JasonBModerator @jasonb]

Does depend on your hardboard though. Oil Tempered will be far less affected by moisture than cheap DIY shed stuff or what Ikea back their kitchen carcases with

[Andrew Johnston]

On Andrew Johnston Said:

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

This is in accord with my Metrology book.  Three feet to stop rocking, positioned at two corners and mid-centre on the opposite side.  No mention of Airy or Bessel, but it’s emphasised that Surface Plates must be ‘of deep section and properly ribbed so the plate can take it’s own weight, as well as heavy objects placed upon it, without appreciable deflection‘   The support under the table has to the same, which is why commercial surface plate stands are remarkably beefy!

The need to avoid rocking means that Taf’s arrangement could be improved, 3 pucks good, 4 pucks bad, not that he’s noticed this is an issue yet:

The most likely reason Taf hasn’t noticed rocking, is that the weight of work and type of measurement he’s been doing so far haven’t combined to highlight the error.   But I think it would show if a heavy block and the DTI measuring it were moved around the table such that the same dimension was measured from several different positions.

Moving on, a few other hints on usage that might help readers differentiate between whether they need an ordinary flat surface or an actual surface table.   In surface table world there’s a lot more to worry about, and if this list doesn’t ring any bells, the readers need is probably ordinary.

• ‘Slip gauges form the basic standard units for this class of work’
• ‘A set of slip gauge accessories can add considerably to the usefulness of slips’
• DTI essential.  Recommended to use a 0.01mm dial rather than a 0.002mm and estimate 5ths.  This because having less gearing makes the coarser DTI respond more sensitively.
• ‘When using a Dial indicator on a surface plate for the accurate measurement of gauges and fixtures, it is quite a good plan to tap the plate with a rubber mallet when the indicator is set and just before taking the reading.  However good an indicator is being used there is always the possibility of a certain amount of stiction in the movement.’
• And more…

What fun – to get accuracy out of a surface plate, the owner has to beat it with a mallet!

Dave

[Andrew Johnston]

On Andrew Johnston Said:

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

It is becoming increasingly obvious [to me] that the specified locations are are only applicable to simple cuboid blocks of homogenous material [e.g. Granite surface plates]

Iron plates with cast-in webs are a rather different beast !

… There may well be an optimum location for each supporting foot, but it will be specific to the weight and stiffness distribution !

The best approach therefore, would be for the user to support such a plate at the the same points where the manufacturer did when making it.

… and this is presumably what Andrew has done.

MichaelG.

[Martin KyteParticipant @martinkyte99762]

Of course you could always float it in a mercury bath.

🥴

regards Martin

[Kiwi Bloke]

On Kiwi Bloke Said:

I’m relieved to find that no-one’s mentioned the possibility of floating the plate in a bath of mercury. No need for anyone’s points then…

I guess Kiwi Bloke is no longer relieved, Martin

MichaelG.

[Dave SParticipant @daves59043]

If you actually care to learn about accuracy and surface plates I would recommend reading The Foundations of Mechanical Accuracy by Moore. It’s still available from Moore special tool if you have a lot of money, but pdf of it is available if you search.

as for using a 0.01mm DTI to read to 0.002mm I would disagree. If you need to read to 0.002mm spend the money on a decent tool.

[peak4Participant @peak4]

A few thoughts from a non formally trained engineer; well I am trained, but in telephony and networks.
If there is a need to support a surface plate by any particular points, surely use the same ones from when it was originally produced and calibrated.

If you’re going to use ice hockey pucks, buy 5, not three or four; support the plate on three of them, and thin the other two down a bit, so they don’t touch the underside of the plate, but will stop it rocking too far if you lean on it.

When I was playing and trying to teach myself to scrape, during the first Covid lockdown, in order to refurbish the little Herbert Junior grinder, I needed some sort of flat surface, both for blueing against, and also moving a dial gauge around to see whether things were parallel and flat and square to each other.

I already had a 12″ square plate, which I’d bought cheaply, when I went to collect some milling chucks several years previously.
Unfortunately, it had been re-ground but never re-scraped; it was fine for the purposes I bought it, marking out and a backing for wet and dry paper etc, but too small, and not flat enough for my needs here. I was however able to use it as a portable upside down plate for getting stuff roughly right.

I had a large lump of 40mm granite worktop, which I hoped to use, but that had several dips of a couple of thou or more, so not good enough for scraping in the knee and table etc.

I tried a sheet of 6mm glass on top, but the weight of the components flexed the glass into the dips. I even tried making shims out of a couple of different thicknesses of aluminium baking foil twixt the two, but that still wasn’t stable enough.

Eventually I found a chap selling a 24″ square plate, and was able to collect it between lockdowns.
Most of it still had the original scraping marks completely undamaged, though there is a small area which has suffered from water ingress; it has three feet for support, with others cast in, but not touching the bench for safety/stability. It lives on top of a filing cabinet, which itself has a sheet of 25mm quartzite worksurface on top to provide a stable flat base for the plate.
That did the job fine for working on the grinder, but I’ve hardly used it since. It will get used for checking motorcycle heads and crankcases etc.

On the main workbench, next to the vice, I have the original lump of granite worktop; it’s normally covered by a slice taken from the bottom of a cardboard veg box from Lidl, replaced as it gets dirty.
A clean forgiving cardboard surface, which can quickly be removed to expose a marking out table, or degreased for rebuilding brake cylinders or similar clean jobs. It’s flat enough for normal use so long as I remember where the dips are.

I did more recently pick up a freshly re-finished granite plate, not really a surface plate, as it has an embedded stainless channel section running down the middle, which is ideal for clamping a vertical column with a DTI etc.
It came with another lump of granite, about a foot long of a square U cross section about an inch thick, which is finished all over to a high standard, so can be used as a vertical square, height block etc.
That setup is probably the most used of my various flat bits; I’d miss it, but it’s hardly essential.

I had plenty of time watching videos post Covid, and discovered this one by the excellent Don Bailey, which covers most home workshop needs, re. checking surface plates.

Bill

[Taf_PembsParticipant @taf_pembs]

Hi Dave (SOD),

I totally agree 3 would be better, I was originally going to mount it on 3 pucks but as I said in my original post, I read a section of a website that explained about the potential for such a small plate to tip due to it’s lack of mass which to be fair, when I was setting up to measure the quill alignment in the gear head of the mill I had the plate supported on 3 points and it did tip when I made the stupid mistake of not lowering the gear head onto it far enough to the side supported by 2 points (I am kift like that!) so I went for 4 ‘soft’ feet in the hope that that would compensate.

Unorthodox – yes – but seems to be fine so far (he says with fingers crossed!) and I am only playing at this, muddling my way through as best I can.

I still can not find the site I read about the almost unmeasurable amount of distortion for such small but fairly thick granite plates even if they are just sat flat on an old bench etc on (it was talking workshop environment not lab obviously), I can only find the Eley Metrology site that produce stands that says they don’t do a stand for 600mm x 600mm or smaller at the stand is liable to fall over being top heavy for such a narrow base I was looking at their site for ideas. Not that my plate is anywhere near expensive or accurate enough to worry about that, it does what I need better than expected though.

Time will tell if I get any rocking but I’m hoping that the ‘give’ in the pucks will allow it to sit OK.. if not it’s back to the drawing board!

 

It’s turned into an interesting thread though!

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