How badly do I need a surface plate?

[andrew lynerParticipant @andrewlyner71257]

I have watched a number of YouTube movies showing how to improve the various 'flat' surfaces on a mini lathe. They mostly seem to involve a surface plate, which is not a cheap item.

What is the alternative? I do have a number of thick steel plates. Would that be a good place to start – or perhaps some glass? Failing that, I guess I could start with the 'best' surface that's on the lathe and use that as a reference. Is that easy to measure? I have a budget dial indicator to help me.

Many people must have improved their mini lathe performance. How have they done it?

Edited By andrew lyner on 21/07/2019 10:35:23

[andrew lynerParticipant @andrewlyner71257]

[Michael GilliganParticipant @michaelgilligan61133]

A decent piece of 'Float Glass' is probably your best bet.

MichaelG.

[Mike PooleParticipant @mikepoole82104]

Many people have used plate glass as a surface plate, float glass is pretty flat as well but will actually have a curve of 8000 miles radius which would be near impossible to measure and certainly of no consequence to normal model engineering, for all practical purposes it is flat. Some care needs to be taken on supporting the glass as it can be distorted if heavy items are placed on it when it is poorly supported.

Mike

[BizibilderParticipant @bizibilder]

The old favourite is a piece of plate glass which is flat enough for amateur use as a surface plate. A modern alternative is a polished granite cutting board – under £20 from a well known auction site and many supermarkets. I checked mine with a new good quality 300mm steel ruler and feelers – I cannot get a 1.5 thou feeler between the ruler and the board in any position. More than good enough!

[Nick WheelerParticipant @nickwheeler]

I didn't do a lot to my mini-lathe, but bluing the sliding surfaces and gently removing the high spots made a huge improvement. Pinning the gibs and aligning the lead screws is a good idea and takes no time.

the bearing mods for the screws are worthwhile.

A cam lock for the tailstock is necessary if the lathe is old enough not to have it already

[jimmy bParticipant @jimmyb]

I have had a variety of granite work top off cuts over the years.

Finally treated myself to a 450mm x 450mm x100mm one a couple of years ago.

 

It appears I now use all of them to gather dust……..

 

Jim

Edited By jimmy b on 21/07/2019 11:11:20

[Former MemberParticipant @formermember32069]

[This posting has been removed]

[ChrisBParticipant @chrisb35596]

Posted by andrew lyner on 21/07/2019 10:34:39:

I have watched a number of YouTube movies showing how to improve the various 'flat' surfaces on a mini lathe. They mostly seem to involve a surface plate, which is not a cheap item.

Do you mean scraping? If so I think you would need a proper surface plate, but then again it all depends on the level of precision you're trying to achieve.

I use a piece of granite worktop, a 0.05 feeler will not slip under a straight edge – good enough for my needs, but could be way off for someone else.

[Tony Pratt 1Participant @tonypratt1]

Never fancied the glass option & you can pick up decent surface plates fairly cheaply.

Tony

[Pete RimmerParticipant @peterimmer30576]

Glass and granite worktops are fine for marking out, but not much good as a reference surface for scraping. The main reason is that whilst it MIGHT be flat enough it almost certainly isn't and then any bend twist or dish that's in the plate will be scraped into the part.

Edited By Pete Rimmer on 21/07/2019 12:39:06

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Pete Rimmer on 21/07/2019 12:38:51:

Glass and granite worktops are fine for marking out, but not much good as a reference surface for scraping. The main reason is that whilst it MIGHT be flat enough it almost certainly isn't and then any bend twist or dish that's in the plate will be scraped into the part.

.

Pete [and Tony]

Doubtless you are correct … but Andrew was quite specific when he asked:

I have watched a number of YouTube movies showing how to improve the various 'flat' surfaces on a mini lathe. They mostly seem to involve a surface plate, which is not a cheap item.

What is the alternative?

[my emphasis]

.

Incidentally: I have three small 'proper' surface plates [all secondhand, and far from pristine] two iron and one granite … but the flattest thing I own is an 'Optical Flat' made of [you guessed it folks] Glass.

MichaelG.

[old martParticipant @oldmart]

At the museum we now have two plates, the first one is a 12" square of float glass stuck onto a 14" square piece of chipboard with some 1/4 round beading glued round the edge. It does the job admirably , especially for small jobs.

The second is a 2 foot square surface table which I bought on ebay for £50, which I collected from about 30 miles away, it just fitted in the back of my Corsa. It is heavy, my left arm is still bad after four months.

I have no plans to retire the glass plate because it is still very useful. If anyone gets a glass plate then I recommend not sticking it down too permanently, as it can then be turned over when the first side gets scratched. I had the cross slide of the lathe on the glass sitting on two 123 blocks. When I lifted the slide off, one of the blocks stuck to it and then fell 6" onto the glass without breaking it. It took a chip out of the glass surface, the plus side of this is there can be no burrs unlike a cast iron surface would have.

[Paul LousickParticipant @paullousick59116]

You can make your own surface plate for little cost and a lot of elbow grease useing the 3-plate method of lapping the plates. See link below. (other examples of this method on the web)

Paul

Surface Plate

[andrew lynerParticipant @andrewlyner71257]

Posted by Paul Lousick on 21/07/2019 13:59:23:

You can make your own surface plate for little cost and a lot of elbow grease useing the 3-plate method of lapping the plates. See link below. (other examples of this method on the web)

Paul

Surface Plate

Many thanks for all those thoughts from everyone. Much appreciated, as usual.

I did consider the Three Plate method aamof, as it was mentioned in the Optics part of my Physics degree course. The lecturer (decades ago) seemed to imply that it was done by ancient old technicians in long brown coats and that it took days and days (he was talking about optical flatness of course).

I guess it would have the advantage that I could sell one or two of the resulting plates.

A cheapish solution seems to be the way to go. After all, it I get more fussy as time goes by, I can always step up in cost and quality.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by andrew lyner on 21/07/2019 14:08:44:

A cheapish solution seems to be the way to go. After all, it I get more fussy as time goes by, I can always step up in cost and quality.

.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Barrie Lever on 21/07/2019 11:20:51:

Once you own a surface plate or surface table you will wonder how you got on without one, look on Ebay for a good condition second hand item.

I purchased a second hand high quality Swedish made granite table 900x600x100 thick on a stand for £360.00 on Ebay.

B.

Not in my workshop! I feel it would be a waste of space and £360. I'd cheerfully buy one if it was going to be used, but I rarely need such a thing. If Andrew only wants one to sort out his mini-lathe, that's big money for one job.

When I need a flat surface a lump of kitchen worktop is generally 'good enough' or a sheet of float glass when better is needed. Float glass is close to being optically flat. The main disadvantage of worktop and glass sheet is they bend when overloaded, so care is needed in use. (Bending is probably the reason why the stands used to support granite plates are 2x or 3x more expensive than the plate itself.)

For precision work a surface plate must be calibrated. Read all about it here. More expense!

My main reason for needing a flat surface is to transfer a position accurately from one object to another with a scriber block or height gauge. Doesn't happen often. I can't remember ever needing to make a flat surface flatter than can be managed by my mill or by facing-off on my lathe, so scraping is off my agenda. Ignorance is bliss – am I missing any other uses for a surface table?

Dave

[pgk pgkParticipant @pgkpgk17461]

I freely admit to being a 'that's good enough' hobbyist and figure if the mill table isn't flat enough for reference then I'm in trouble. I'd like a surface plate just because I like tools but it'd be second to wanting other stuff more.

[Maurice Cox 1Participant @mauricecox1]

I can recommend the use of a piece of plate (or float) glass as a surface plate. To avoid the possibility of it bending under load, or indeed to make it stronger, the advice used to be to bed it on a layer of pitch. This stuff always remains a liquid, albeit a very viscous one, and will flow out to support the glass right across its surface. If buying a piece from a glass dealer, then for a modest charge they can remove the sharp corners and polish the edges.

Maurice

[Former MemberParticipant @formermember32069]

[This posting has been removed]

[mechman48Participant @mechman48]

i have used float / plate glass & either are more than 'flat' enough for marking out. I presently have an polished marble off cut ( sink cut out ) from a kitchen fitting firm, free gratis! Just went in & asked the foreman & after a short explanation of what I wanted it for, he produced a 18" x 18" x 1" piece; ran a digital dti over it & only a .001" fall off at one corner with slight chip on same corner, does fine for me.

George.

[Tim StevensParticipant @timstevens64731]

It is fairly easy to check the flatness of a slab of eg granite worktop, before purchase. Hold the slab at eye level and about horizontal, in a god light, and look carefully at the reflections in the whole of the surface. As you move the plate slightly, the reflection should not distort at all as it moves across the surface. Any wavyness or hollows etc will show as the reflection changes shape and/or size as the plate is moved.

This is a useful check on any bits of worktop left over or sink cutouts etc, when it is not possible to do a proper engineering check in a builder's yard.

And although worktops can be fairly thin (12mm rather than 50mm for a 'proper job&#39 a slab of thicker MDF glued on makes a good reinforcement.

Cheers, Tim

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Barrie Lever on 21/07/2019 17:43:12:

Granite table and plates do not suffer from bending and flexing

The problem with glass is that is way too flexible, it really conforms to what ever it is mounted or placed upon. Over a very long period of time glass also flows out, an old window is thicker at the bottom than it is at the top, maybe not too much of a problem for a plate but the flex definitely is.

.

It does, of course, rather depend upon the 'aspect ratio' of the plate

My Optical Flat is relatively thick in relation to its small diameter.

But 900 x 600 x 6mm [for example], whether in Glass or Granite, would be a very different matter !!

MichaelG.

.

P.S. … some useful information about granite, here:

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

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

etc.

Edited By Michael Gilligan on 21/07/2019 18:30:41

[Pete RimmerParticipant @peterimmer30576]

If a piece of glass or worktop is out of flat by 0.0002", a flat part will spin on it and you'll think that it's not flat. Part of the analysis of parts when scraping is a 'hinge test'. It's used for identifying the high and low spots on a part. If your part hinges in the middle then it's humped. If it hinges on the end then it's dished or bowed.

I have a small home-made dovetail straight edge that I use for scraping internal dovetails and it's slowly creeping into a bend over time. If I leave it 6-12 months I have to be sure to check it before I use it because it will bend a couple of tenths and spin like a top on my plate. It only takes a quick scraping pass over it to get a good hinge and make it useful again but if I didn't have a plate that was at least that flat I couldn't check it and any slide ways I scraped would be bent also.

You really do need a proper plate for scraping.

[Neil WyattModerator @neilwyatt]

Posted by Tim Stevens on 21/07/2019 18:16:41:

Hold the slab at eye level and about horizontal, in a god light, and look carefully at the reflections in the whole of the surface.

You should be able to tell if it is as smooth as the face of the waters…

Neil

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