New Axminster surface plate?

I just bought a granite surface plate from Axminster. Being a metalworking noob, can someone confirm that the cleaner/less dusty surface (second image) will be the ground one?

It all looks blemish free except for this small knick on the edge - half way along the top edge in the third image.

Obviously I would never be taking precision measurements on the very edge, but I'm concerned whether it's blemish for mishandling? Or is this a remnant from the calibration process (it comes with a certificate give the 9 point measurements)?

Any help gratefully received.

Edited By choochoo_baloo on 22/06/2017 22:48:30

Edited By choochoo_baloo on 22/06/2017 22:55:04

No expert, but I would think that if only one side has been ground flat, the other will not be (fairly obvious, that?).

In that case, a straight edge would sort out the two?

Thanks. Didn't make myself very clear; I assume that the 'rough' face would still be very flat and difficult to detect with say a parallel; and thus indistinguishable with basic equipment.

(I don't have an engineers' straight edge). Would a steel rule show the difference?

I think you will find that the polished face will be the measuring surface. If in doubt contact Axminster.

Brian

Really tricky problem without a second surface plate to check against:

• I'd give Axminster a ring. They may know which way up their plates are packed.
• It may be possible to see imperfections by angling light to bounce off the surfaces like a mirror. The top may be more highly polished than the base.
• Use a steel rule on a rough grid checking for light escaping under the edge. With luck one side will be obviously worse than the other.
• Failing that, a long, long session with the rule looking for light escaping under the edge. Divide the each surface into, say, a 1" grid and use the straight-edge to check each line. It may be necessary to repeat this with a diagonal grid. The side that lets through the most light is the bottom. Don't forget that light consistently from the same points on the rule are probably the surface plate telling you your straight edge is wrong.

Maybe if you can't tell the difference between top and bottom it doesn't matter. (Unless you intend to calibrate other flat surfaces!)

I don't think the nick matters at all.

Dave

Edited By SillyOldDuffer on 23/06/2017 14:24:19

Just out of interest is that the cheap Axminster £60 one?

I've been wondering about one of those for a while.

Jon

To check flatness you need a straightedge longer than the longest side to measure flatness.

Clean surface and ensure no dust on it and place straightedge along one side and start trying with a feeler gauge, repeat further across and from experience record your readings. Then try corner to corner which will just about show any distortion present. I doubt you will even get any penetration of the feeler gauge under the straightedge.on the table.

Mounting the Granite plate on three points, 2 of which should be able to be levelled. Place felt on top of pads. When not in use cover with a plywood cover that only touches around the edge of the plate. I have never put oil on the granite plate. Cleaning with Meths is OK. Note, rulers are NOT flat edged! Strait edges are not too expensivve and secondhand ones are never straight!

One other method is Auto collimatiom where you use a surface silvered mirror and a Theodolite with an Auto collimation eyepiece fitted, readings are related to Secs. over distance plus or minus. This method is very accurate and readings of half the thickness of the Graticule line can be resolved.This being less than half a sec.

Hard to believe, but they flex!

Before I retired someone at work leaned a thick 4 x 6 feet, granite table against a wall for a few days. When installed, it was no longer flat. Some £3K= gone to waste.

Suggest that you mount it on a substantial angle iron frame, with levelling screws at each corner, and then carefully level it, before use.

And make a wooden cover for it when not in use, unless you have a environment controlled site for it!

Lucky man!

Howard

Yes it is Jon - I've been on an Axminster spending spree lately. Would you like me to add a photo of the included calibration certificate? That way I feel like I'm contributing to the forum and not always asking questions!!

Edited By choochoo_baloo on 24/06/2017 01:34:37

Thanks all for the information. As ever very informative.

I take on board what you've all said about correct mounting. Can someone elaborate (in beginner friendly language!) on how to built such a mount.

For example do I need to calculate the Bessel points (I'd never have thought I'd need to use multivariable calculus in home metalworking ). Or is this total overkill for a 30 x 23 cm plate?

Yes, but then most of this thread is - given the size of the plate and its presumed use.

Supports positioned between 1/5 and 1/4 in from each end/side should be fine if you don't want to put it on notionally 'flat' surface.

Me, I would put it on a layer of felt over a good solid, (normally) flat surface.

If you carve a suitable message underneath you can save your heirs the cost of a tombstone

Hmm, perhaps I should patent a kitchen that converts into an extravagant granite tomb...

Neil

Aha, someone who realises that the Bessel points are more appropriate than the Airy points. Congratulations Sir!

Be that as it may, just plonk it down where you're going to use it. It'll make bugger all difference for any work that doesn't need a UKAS certificate.

As ME's, we do get hyped up about accuracy, then park our cars with one wheel up the kerb. What is it, some kind of fetish? What you are going to use the surface plate for would be a more enlightening discussion I think.
BobH

Question for the learned, if they flex when stood against a wall why don't they sag when stood on 3 points?

Don't know what all the fuss is about I don't even have a surface plate and seem to get by OK

I have an offcut of polished granite, from the

undertaker's scrap heap. It's far more accurate than me.

It was free and can easily be replaced if necessary

Best, Daniel

Shouldn't the mounting points be marked on it as used when making it? By default for an cast iron plate they are obvious as part of the design.

The underside of your surface plate should still have
the three points marked on which it stood during manufacture
as in the photo .These marked places should be used to
mount the plate.Some of the better quality plates will come
with mounting pads fitted.

This plate came from Roto grip.

All of the answers you will need to know about your
plate can be found here

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

The second photo shows this plate mounted,it is not
necessary to have the plate level unless your
application requires it,see question 12.

John

Jason, my use of a Granite plate was for overhauling and repair of the swinging arms of a mapping plotter. These arms supported on ball bearing/swing points by ball races about 3" in dia. Not only had the plate be super level but also be even temperature. The bearing inner and outer were very thin and the track was filled with 80 x 3 mm balls.

The 3 point suspension was the easiest to keep level as it was also affected by nearby works like lorries going past now and then. Not ideal. The plotting arms had about a 500 mm arc of work and had to stay in one position without rolling back to a single position. Much like doing a super balance task really.

A very expensive repair as an exchange set was about £3000 a go and there were 2 sets per machine. All this plus replacement of high and low speed bevel gears.that traversed the plotting system.

As ME's, we do get hyped up about accuracy,

Some might, I don't unless a particular accuracy/precision is required.

Like most times to drill through a cylinder centrally it is close enough to 'graze' the cylinder with a milling cutter (or even a drill bit) and dot punch in the centre of the flat/mark. Simple, quick and accurate enough for larger diameter items. My aged surface plate is good enough for me and mostly gets used for flatting surfaces by laying on a strip of abrasive paper. A plate glass sheet would be more than good enough, but perhaps likely to get broken.

No point in having a real super-dupa surface plate unless you already have a good surface grinder, I reckon. But a good job we are not all exactly alike.