Master surface plate for scraping in a cast plate?

[Kealan O’CarrollParticipant @kealanocarroll32466]

Hi all,

I've recently won, but not collected, a 12" x 18" cast surface plate from the 'bay. I don't know how flat it is, & I'd like to hand scrape it in.

What's the done thing in terms of a reference plate? Do people borrow a master off a mate / go to a different workshop and scrape it in there?

Has anyone had a cast plate calibrated by a professional & can you give an indication of the ball park cost? Maybe I could farm out the plate scraping and then just scrape in some parts myself off the professionally done plate.

I'm based in the east mids

[Kealan O’CarrollParticipant @kealanocarroll32466]

[Howard LewisParticipant @howardlewis46836]

Whereabouts in the East Midlands? PM me.

I have a 12 x 18 plate that you could use to assess yours.

But be aware that anything that alters the stress on the plate may then send it out of flat.

I made a small Granite Surface Plate for The WaterWorks Museum. The base was 2.5" steel Angle with an intermediate cross pieces. On each corner was a levelling screw. On the levelled base was a layer of hardboard. The Granite was laid on a layer of moist plaster, on this hardboard, so that the granite was not stressed.

My former employers lost a brand new 6' x 4' x 6" granite surface table by leaning against a wall, instead of placing it on the steel base prepared for it. It sagged!

Howard

[Pete RimmerParticipant @peterimmer30576]

The most straightforward way is buy a granite plate (but that kinda defeats the purpose of getting an iron plate), or borrow one that you know is good and use that for a reference.

At the scraping class last year we had an autocollimator and a repeat-o-meter which we used to check several plates. My best one was pretty good showing a couple of .0001" dips in it, but the brand spanking new Chinese one that one of the members brought along was perfect to the limits that we were able to measure it. The repeat-o-meter didn't move in the slightest.

Iron plates are really nice to use for scraping work so you won't be disappointed if that's what you got it for. They give really nice prints from the engineer's blue.

[Brian HParticipant @brianh50089]

The correct way (though maybe not how you want to do it) is the scrape 3 plates in together. When they all show small blue dots all over, they are all flat.

Brian

[Speedy Builder5Participant @speedybuilder5]

Kealan – Seems like a lot of effort, what do you intend using the surface plate for ?

[Pete RimmerParticipant @peterimmer30576]

Posted by Brian H on 05/12/2018 15:27:06:

The correct way (though maybe not how you want to do it) is the scrape 3 plates in together. When they all show small blue dots all over, they are all flat.

Brian

That's only the 'correct' way if you have no reference surface to work to, and to be honest only undertaken if you really have no choice because it can be a hugely laborious task using the 3-plate method. Otherwise just scraping it to a known good reference i.e. another plate is the most time-efficient system.

[Kealan O’CarrollParticipant @kealanocarroll32466]

I'm planning on using it to make a straight edge, and then using the plate & straight edge to scrape in my compound & cross slides as they're both tight at the extremes of travel & loose in the middle. I'll probably scrape in the base & top of my rotary table & angle plate too but there's no rush on them; I'll see how I find the process first.

I don't have any flat surface for marking out at the moment so I'll likely use it for marking out too; is this considered bad practice with a cast plate? I can imagine a cast plate wearing more than a granite plate when marking out?

I did consider the three plate method but I don't think it makes sense right now in terms of cost of all the plates and the time needed to get the job done. I saw the thread about the scraping class last year, I've watched Stefan & John Saunders' videos on the Richard King classes & they look excellent. I'm amazed to hear of the granite plate sagging!

[David ColwillParticipant @davidcolwill19261]

Check out ROBRENZ on youtube. He has some good stuff on scraping.

David.

[IanTParticipant @iant]

I have a large (30" square) commercial cast-iron surface plate that I've never had checked but which I use as "my reference" Kealan – and it seems to be good enough for my needs (having admitted that I've never checked it I'm sure Stephen Gotteswinter will never speak to me again!). However, I don't use it for marking out.

For that I have a smaller plate (about 12" sq. – from a CES casting) that I machined and surface ground many years ago. Over the years it has accumulated quite a few scratches and other slight scuffs that happen when you place materials on it that might not be perfectly flat or perhaps have a burr that you haven't noticed. This plate is also small and light enough to be moved easily – an important consideration as I get older. I have a longer (24" x 9" ) float glass plate that is very useful for marking out longish things like (G3) engine frames without standing them vertically.

Of course many here will tell you that marking out is now obsolete because they have DROs on everything – but unfortunately maybe – I do not.

So my advice would be to keep your 'reference' plate for 'referencing' – and use something less valuable for marking out if at all possible.

Regards,

IanT

 

Edited By IanT on 06/12/2018 11:01:18

[Pete RimmerParticipant @peterimmer30576]

Posted by Kealan O'Carroll on 06/12/2018 10:23:52:

I'm planning on using it to make a straight edge, and then using the plate & straight edge to scrape in my compound & cross slides as they're both tight at the extremes of travel & loose in the middle. I'll probably scrape in the base & top of my rotary table & angle plate too but there's no rush on them; I'll see how I find the process first.

I don't have any flat surface for marking out at the moment so I'll likely use it for marking out too; is this considered bad practice with a cast plate? I can imagine a cast plate wearing more than a granite plate when marking out?

I did consider the three plate method but I don't think it makes sense right now in terms of cost of all the plates and the time needed to get the job done. I saw the thread about the scraping class last year, I've watched Stefan & John Saunders' videos on the Richard King classes & they look excellent. I'm amazed to hear of the granite plate sagging!

Iron plates are said to wear faster yes, but I have a couple that were scraped by a guy who scraped jig bores and the scraping is quite sublime. Very sallow which should wear away quickly although none of the scrapes are worn away yet. As with everything how you use it will have a huge bearing on how it wears. Rotate your plate and your part and don't forget to use the very edges and it'll last a lot longer before it dishes appreciably.

I also have a 'rough' plate which I always use first in case there are any burrs or ridges in the part to score the surface. That's another benefit the granite plate has – no burrs or embedded particles.

As soon as I have scraped any high spots I put it on my good plate.

Granite plates shouldn't sag if they are supported on their airy points.

[Anonymous]

Posted by Pete Rimmer on 06/12/2018 19:38:17:

Granite plates shouldn't sag if they are supported on their airy points.

That's not correct. When supported at the Airy points a beam will sag, but the sag is minimised while keeping the ends parallel (zero angular deflection) , which is important for a length reference. For a surface plate where a length measurement isn't important the support points for minimum overall sag are slightly closer together than Airy points.

Andrew

[Pete RimmerParticipant @peterimmer30576]

Well, technically that's true – everything that's suspended will sag. It's all a matter of scale/significant figures though and for the purposes of what we're talking about the sag is nil because nil is what you'll measure outside of a metrology lab. Heat-induced movement is of far more concern for the scraper.

[Clive HartlandParticipant @clivehartland94829]

Even stored gun barrels sag and have to be turned every couple of months. Your plate 12" x 18" is unlikely to sag! Too small. just use it evenly unless you want to have the scraping experience.

[Robert DoddsParticipant @robertdodds43397]

Talking sag of beams and the like I recall in the "Standards Room" they kept a cast iron beam that was bellied on the underside, It rested on Airy points (or very near) and was designed to even out stresses and get deflection to a minimum.

I see one of them still in the ITP group range of surface plates

Bob D

[Mark RandParticipant @markrand96270]

Posted by Andrew Johnston on 06/12/2018 20:26:24:

Posted by Pete Rimmer on 06/12/2018 19:38:17:

Granite plates shouldn't sag if they are supported on their airy points.

That's not correct. When supported at the Airy points a beam will sag, but the sag is minimised while keeping the ends parallel (zero angular deflection) , which is important for a length reference. For a surface plate where a length measurement isn't important the support points for minimum overall sag are slightly closer together than Airy points.

Andrew

To amplify Andrew's points, two things:-

1) The working face of a granite surface plate or table that is supported at the same locations (wherever they are) that it was last lapped on will not 'sag' other than when it has a load placed on it or where the surface is worn.

2) The implication, that others have raised, that a granite plate might take a permanent set due to improper storage/mounting etc. is bollocks.

3) The correct support points for minimum deviation from flat are the Bessel points, not the Airy points. The latter give parallel ends, which are important in end-to-end length gauges. Both are defined for two dimensional beams, not for three dimensional objects.

OK. Three things.

[Sam StonesParticipant @samstones42903]

Like this …

**LINK**

Sam

[Howard LewisParticipant @howardlewis46836]

As already said, anything supported on just two points will sag. The sag may be of the order of millionths of an inch, but it will sag. The granite surface plate that was scrapped was 4' x 6' x 6". The company concerned is a multi national engineering company, leaders in their field, with over 85 years experience of precision high volume manufacture. If the folk from the Toolroom, and the temperature and humidity controlled Calibration Room deem it unfit for purpose, I will believe them.

But it all depends on how high are your standards!

For our purposes,as a hobby, we do not need to work to such levels of precision. But that does not mean that using a 6" rule is better than a micrometer! We should always try to attain the best quality possible, with the materials and machines available, to achieve fitness for purpose.

Howard.

[Michael GilliganParticipant @michaelgilligan61133]

A couple of links that may be of interest regarding Granite surface plates:

**LINK**

http://doxcalibration.com/wp-content/uploads/2018/04/Federal-Spec-GGG-P-463c.pdf

**LINK**

http://www.starrett.com/metrology/metrology-products/precision-granite/precision-granite-faq

MichaelG.

[Author]

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