Surface plate refurb…

[BazyleParticipant @bazyle]

Very interesting about the plate glass Iain. I don’t think I have ever seen reference to anyone actually measuring it and its use stems from the time when Model Engineers could not afford real plates and made do.

[DiogenesParticipant @diogenes]

https://www.norfolkmarine.co.uk/10kg-mudweight

[Alan DonovanParticipant @alandonovan54394]

Diogenes.

Thank you for the link.  Now that makes sense, …….. every day is a school day!

All the best.  Alan.

[Howard LewisParticipant @howardlewis46836]

There is a technique mform achieving a perfectly (IF that is possible) flat plate, but it involves using two other plates. Each is blued and scraped to the other, until then”work” plate blues to a standard where the user is satisfied.

You might be able to use two marble slabs and blue and scrape until they are considered satisfactory, and then to use to blue and scrape the actual surface plate.

An iterative slow and messy procedure.  Just bdepends on how precise you want to be.

The support for the plate is important. A support which can distort, or allow the plate to distort is useless.

Youn can’t scrape the plate in part of the workshop, move it to another place and expect it to be as flat. Your final blue and scrape needs to be in the final location.

Howard

[Iain Downs]

On Iain Downs Said:

On Glass for surface plates, I bought a piece of about 15 mm thick 30mm x 50mm (or thereabouts) a few years back.

It was as flat as the Himalayas!  Well – clearly an exaggeration.  But it was out of flat by at least 0.25mm over the  surface.

A colleague of mine who used to be a glass quality inspector (who knew) and later a CNC engineer told me that the requirement for flatness of glazing glass was around 0.5mm per metre.  Which this bit met.

I ended up throwing my bit away as it was no use.

Clearly this is not a universal experience, but thought I should mention.

 

Iain

Same here, I got a 200mm x 300mm offcut from the local glass merchant but it was completely unusable. Part of the problem is that glass, it would seem, is quite flexible and bends under its own weight unless adequately supported. The trouble is, your support has to be sufficiently flat to provide the necessary level. So you might as well just throw away the glass and use the support (I did try various foamy type materials but they didn’t really help). I replaced it with one of the granite ones when Chronos had them on special offer.

Even if not fantastically flat, a piece of thick iron or steel like the OP’s can be pretty useful for marking out and assembling for welding, etc. And you won’t feel terrible about drilling a few clamping holes in it.

I once bought some slabs of steel from a scrapyard just as a source of 25mm thick material. They had been planed on both sides and were actually decently flat for the above kind of work, so I kept the best one; I’ve no idea what it had originally been for, it was about 500 x 900, with a shallow curved top. I used to tell people it was a biker’s headstone.

[Andy_GParticipant @andy_g]

On the topic of glass, it’s probably worth noting that the historical recommendation was for the use of plate glass.

Prior to the invention of the float process in the 1950s, and for some time afterwards, large windows were made by grinding and polishing cast or rolled glass to produce polished plate glass (ordinary window glass being drawn as a sheet – sheet glass). This was typically carried out in the ‘twin grinding’ continuous process that used multiple grinding heads, each spanning the width of the glass sheet, followed by a similar polishing section. This naturally tended to result in a finished product with a very uniform surface that, in small sections, was effectively ‘flat’ to a reasonably good accuracy.

Float glass *can* also be very uniform, but this isn’t automatically the case. There is a ‘selvedge’ at either side of the glass ribbon which isn’t uniform, and will generally be cut off when producing finished pieces – you don’t want this.

The flatness of the glass plate is dependent on the homogeneity of the glass – small chemical changes have large effects on viscosity. Homogeneity depends on the uniformity of the raw materials and how the furnace is operated. Financial pressures drive toward cheap materials, and high throughputs.

If you look at glazed facades at a shallow angle, you can easily see how shockingly bad the surface of some modern glass is.

In a previous employment, we used to use 25mm thick float glass as working flatness standards. These were checked / “calibrated” before use which was generally an academic exercise. I still have one such piece, and it’s as flat as I need it to be. Support points do need to be carefully chosen, or rest it on a partially inflated inner tube.

I think the best chances of getting a piece of flat glass nowadays would be to get a piece of 19mm (or thicker) cut from the body of raw glass made in an European plant.

(or go and break a shop window 😉 )

[Michael GilliganParticipant @michaelgilligan61133]

Very helpful notes there, Andy … wise words indeed

I would just add that, at the limits of Metrology, [transparent] glass is still a very useful reference surface, because it permits the use of interferometry.

https://www.edmundoptics.co.uk/knowledge-center/application-notes/optics/optical-flats/

Generally speaking, one face of an Optical Flat is the reference surface, and the other acts simply as a window.

Also, as you implied, the ‘aspect ratio’ of an Optical Flat needs to be high, and they are therefore typically of relatively small diameter.

MichaelG.

 

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