Lapping - how to (cheaply)

I've had a quick confusion of the subject on the interweb and wanted to check my assumptions and understanding with proper experts.

I'm talking about flat lapping here.

The proper way requires a lapping table and various sorts of grinding paste and I get how that is done. I suppose I could build one myself with a bit of steel and scraping to get it flat. However, that sounds tedious (albeit a proper piece of metalworking).

The other thing I've seen is using a surface table and sandpaper. the idea is to 'stick' the paper to the surface plat with some water and then do the lapping on that. I've had a bit of a go at this, but the outside of the lapped item seemed to get more attention than the inside. The sandpaper (800 grit) happily claimed to have velcro on the back (bought for woodworking), so perhaps this wasn't the best type.

Is this a reasonable approach? and if so are there particular types of abrasive paper which would work well?

Iain

Hello Iain,

Rather than run the risk of damaging a surface plate, I would confine your experiments to a sheet of plate glass or an offcut of kitchen worktop [in granite] which you should be able to pick up cheaply.

The paper needs to be the wet and dry type very fine indeed, no Velcro backing. The lapping medium also needs to be graded accordingly, working down through to fine grit which should bring up a surface with a polished appearance Do all the rough work first and 'lap' the piece on a fine oilstone, cutting in several directions to avoid rounding the edge to get it ready for the finer work to follow.

It is NOT a rapid process

Regards Brian

3M do some that is adhesive backed and available in all sorts of grades down to 1um or so. This is it

**LINK**

I found them to be a good place to buy a sample pack but haven't tried lapping with them. I have played around with something similar used to lap fibre optics. It can be held in place with a water film underneath but as this was on some flat formica water didn't matter. I also used it as a lubricant for lapping tools so being dead flat didn't matter. Oil should hold the film in place as an alternative.

Thick plate glass is generally pretty flat but not easy to find stockists now. It is about up to 25mm thick.

A wise old toolroom person once told me that linishers can get things very flat but having used the type of linisher that he would be referring to - long adjustable bed - I have never seen anything really suitable at the hobby end.

Wont be of interest but lapping plates when they were around were made of cast iron with a grooved pattern and usually ground. It is possible to make them flat by lapping 3 against each other and using blue to work out how they are shaped as the blue will only give flat results when all are flat. So by checking each one against the other it's possible to see which ones are convex and which ones are concave - after a fashion.

John

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Edited By Ajohnw on 12/06/2016 12:18:46

Edited By Ajohnw on 12/06/2016 12:23:46

A reasonably cheap source for a surface plate is scrap grave headstones. These are normally ground by the stone masons. We have a local gravestone refurbisher and they often have broken headstones they have to replicate. I popped into them a while back and asked if they had any small offcuts they would be willing to sell. When I explained that it needs to ground flattish and what I intended to use it for they were most helpful. Seems for a few quid equivalent they would supply a piece 75mm thick and 300mm square, but I would need to collect it!

Hi, Brian.

My surface plate is granite which will be resistant to water - which I presume is what you would be concerned about damaging the plate.

It seems easy enough to get wet and dry from amazon - and without paying too much.

AJohn - I'll put a lapping plate on my to do list. I'd work it from my surface plate and not using whitworths approach! life's too short...

Iain

Thick float glass (plate to some) generally has a flat surface.

I m not so sure about any kitchen related products as they are a product to use and not measure from. You can buy granite cutting boards but they are nowhere near being flat, you can see that by looking at the surface.

DMT products have a 10" x 4" "Dia-Flat" lapping plate 160 mesh, +/- 0.0005" intended for flating oil stones but they cost US$212.

Edited By KWIL on 12/06/2016 14:31:29

Another source of flat glass is from scanners and plenty of those in the recycling centres. It is toughened but only 1/4" or so thick so mount it on a piece of plywood to support it, with some baize interspersed.

Dave
The Emerald Isle

Although probably not really classified as lapping, I have used the abrasive paper method successfully to flatten items. I use wet'n'dry which is quite thin so does not deform much and paraffin as a lubricant, rather than water, so as not to rust my cast iron surface plate. There is a bit of knack to avoid rocking the item but press hard and push slowly and you should have success.

HTH,

Rod

When not using a dedicated lapping block, I glue wet and dry paper to something flat, like a piece of plate glass. I have found that the spray can of photo adhesive is good enough for this job. It has enough tack to keep the paper down flat , but can also be removed when the paper is dull.

Neil

Edited By Neil Lickfold on 12/06/2016 20:29:30

You can buy a Lapping plate, it is about 30 cm square and has slots cut into the plate of approx. 1 cm square.

You charge the plate with diamond paste with a roller and then you lap your work flat by traversing the work around the plate and rotating it. This is how we used to level the standards on the steel bodied Theodolites (T2) It also cleaned off any rust etc.

Clive

Import granite surface plates are so inexpensive that there is little point in searching for an alternative flat surface. I have not tried it yet but plan to use the abrasive film on a small granite plate.

I use a bit of 10mm float (glass) and wet'n'dry paper of various grades. Spray on carpet adhesive is handy to keep it flat and stop it slipping about. (maybe double sided tape?)
If it's going to be blued I then use some of my old mirror grinding grits and tin oxide or rouge on the glass for the final polish.

Hi Kwil,

Curious that you say kitchen granite boards are not flat, I bought one this very PM, 16X12 under £8, and looking at a reflection of a straight line it appears flat. I have not checked it thoroughly yet and I know it will never be inspection grade, but, for my sort of hack work it should work out fine.

chriStephens

Hi Francis,

Take a look at Oxtool's video "surface plate calibration and conditioning"

I got a cut out from a local granite worktop manufacturer 18" x 18" x 1.5" free gratis; checked it out from corner to corner both ways with a brand new digi dti... 0.000" all over apart from one corner that had a 1/2" thou' drop off right at the edge... it'll do for me, that's for marking out, for lapping I generally use a piece of plate glass with the emery sheet held on by suction & water, sometimes taped on with masking tape ... as I'm not lapping optical flats or diamond lapping safety valve discs/nozzles... more than acceptable.

George.

Edited By mechman48 on 14/06/2016 10:45:16

Thanks for this. I've a bit of glass handy that's suitable and I've had a play with some 1500 emery paper and it seems to work well enough, though I've not completed a project on it.

I shall find some MDF to mount it on and see if I can find somewhere safe to store it in my crowded shed...

Iain