cheapo flat surface lapping

[sorveltaja]

1: Take a finest grit wet sanding paper sheet that you have.
2: Then find the flattest material sheet you have (sacrificial).
3: Place that sanding paper sheet upside down to the surface.
4: Apply some abrasive paste evenly (be it toothpaste or something more fancy).
5: Take your time to lap (depends on paste and material you lap).

I have tested this only on aluminum with toothpaste so far, but the result was beyond my standards. Nice and even satin surface.

P.S Soaking that sanding paper sheet into water, that has some soap in it, to break water's surface tension, helps to keep the sanding paper sheet in place.

[bogstandard]

S,

I remember seeing some old photos of how surface plates were made, even great big ones. They were stacked on top of each other, face to face, with a lapping compound between, and then just lapped together. They wore each other totally flat.

I suppose they do it all by grinding nowadays.

The plate glass platens out of old photcopiers are perfect for what you are doing. But mine couldn't resist being tapped with a big lump of steel. It lasted for about 10 years before giving in.


Bogs

[John Hill]

If I am not mistaken any two 'flatish' surfaces rubbed together in a random fashion will form a portion of a sphere though the radious of the sphere may be very large and the plates then appear 'flat'.  Rub three plates together in the same say and you get flat surfaces.

[Baldrocker]

Hi sorveltaja
Not wanting to hijack your thread

John
What sort of out of "flatness" are we talking here, critical to our needs
or just a poofteenth or gnats wisker? A long term project relies on the answer.
BR

[John Hill]

BR, the home telescope people know how to grind surfaces to a fraction of the wavelenght of light! Quite a bit less than 1/1000th of a millimetre.

Google 'making optical flat', I am sure there will be a description out there!

[bogstandard]

I don't know which John you are referring to BR, but with a surface plate, just sliding a height gauge across it will instantly take it out of what would be called 'flat'. It all depends on how flat you want something to be.

The flatter your requirements, the more expensive it gets. There is a great deal of difference in the cost of a flat surface plate for general purpose workshop use, or one used in say a metrology lab used for calibrating instruments, or the flatness required by the flat earth society.

To someone in the distant future, John Hill's flat optics would most probably be classed as bent like a banana. It all depends on how well you can actually measure it for flatness, and how to get it there.


Bogs (John)

[John Hill]

John Bogs, I think the amateur optics people accept that what they make as a 'flat' is really a section of a sphere of many kilometeres in radius.  I do not know if a 'perfect' flat is physically possible, maybe by rubbing an infinite number of 'flats' together for an infinite length of time.

So no doubt what you say is true, the best flat of today will indeed look pretty poor in the future!

[bogstandard]

John,

We used to get our 4ft diameter 6" thick cast iron lapping plates flat by doing what you said. We used 3 free rotatings rings left running for hours at a time, eventually the wear curve was flatted out. But you had to keep a very close eye towards the end because it would soon 'go over the top' and curve the other way.

The smaller 2ft ones I would face off on the lathe, and the 9" ones I faced up on the surface grinder. When the 2ft ones were checked for flatness in the metrology lab, if they were within no more than 0.0002" deflection, that was good enough for what we were doing. It was easy getting within that measurement on the suface grinder with the smaller plates.


John

[jim]

i use a kitchen granite worktop saver from asda as a "flat surface", i couldn't afford a proper surface table and got one from asda for about £12. its about 3/4" thick and flat to within a couple of thou, grown attached to it now

[ozzie46]

 I got mine from a place that makes granite counter tops. Asked for the cutout piece that is removed for the sink. Price, FREE. 

  Ron

[websterz]

 I got mine from a place that makes granite counter tops. Asked for the cutout piece that is removed for the sink. Price, FREE. 

  Ron

+1 on the sink slug!

[krv3000]

HI ther is a plase wher i live that cuts granet and marbel i got a pece of marbel 18" by 18" by 1/2" thik for £5 wich i think is resnebel the name of the cumpaney is marbel arts durham  as per for give speling

[bry1975]

Ring Taylor Hobson and ask if they've got any unused or slightly marked flat surface blocks! 

[mechman48]

As with others; went to a granite worktop manufacturer,explained what I was after & what it was for..he came back with a 20"x 18"x1"thick piece of polished black granite from the scrap bin (sink cutout)..result     went over it with DTI.. .0005" (1/2 thou')runout at one corner    ..Free gratis.. no need to pay for marble/granite trivet from Asda,Lidle,Aldi etc.

cheers
George

[DavidF]

I have been lapping with sand paper and plate glass for years, it produces a very flat surface except on the corners which tend to get a little bit more.  I kinda think that as you are lapping on the sand paper there is a wave that is pushed up ahead of the stroke direction causing a slite "snipe" for lack of a better term...  But lapping in this way has given me the results I need without question.

[srm_92000]

Ring Taylor Hobson and ask if they've got any unused or slightly marked flat surface blocks! 

Hi I did my apprenticeship at Taylor Hobson and 12 years after!
We used to use a reference optical flat about 5" diamter for setting up interferometers that was just over of lambda/10
i.e. 1 tenth the wavelength of light IIRC.
Did you have anything to do with them? (Taylor Hobson that is)
 'oops just noticed the thread date '

[vtsteam]

To produce a flat surface for a reference, traditionally 3 plates were used.

Call them A, B and C.

A&B were lapped together first. This would tend to make a complementary pair -- convex and concave to a small degree.

Then A and C would be lapped together. C would will tend toward the form of B (as a complement to A).

Finally B & C would be lapped together -- since they have nearly identical irregularities rather than complementary ones, these irregularities  would be lapped away.

Presumably if the degree of flatness wasn't then acceptable, I imagine the process could be done by using 4 plates yielding additional flats after the first round, and then these lapped together to further refine it.

All of the above can be done with plate glass and corundum powder in the home workshop. The 3 plate method for glass is given in the Gingery machine building books.

[philf]

To produce a flat surface for a reference, traditionally 3 plates were used.

Call them A, B and C.

A&B were lapped together first. This would tend to make a complementary pair -- convex and concave to a small degree.

Then A and C would be lapped together. C would will tend toward the form of B (as a complement to A).

Finally B & C would be lapped together -- since they have nearly identical irregularities rather than complementary ones, these irregularities  would be lapped away.

For which we have to thank Joseph Whitworth - born 210 years ago about 3 miles from me in Stockport, Cheshire.

Phil.

[vtsteam]

Thank you, Mr. Whitworth! 

Also for a probably unrelated but awe inspiring bit of joinery, the Incas. 

http://travelblog.mtsobek.com/2012/03/14/the-craftsmanship-of-the-incas/

[Dawai]

OR used fixing a hillbilly with a "leaky harley" spraying oil out on the ground.

Using a trick learned from a old machinist, I laid a piece of "glass" down on a table, a piece of emery cloth upside down, figure 8'ed the warped oil pump housing around and around till it was as flat as the glass. A manual surface grinder.  dyekem stain showed it was evenly wearing off.  It fixed the pump.

I tried to study the ancient methods of cutting those blocks so close, or how they placed them, them being different sizes locked it together during earthquakes, making a structure that outlived the race of people who built it. MY research went into "sun and moon worship", "Levitation", "resonant magnetism", then "MOLOCK" worship where I drew the line and quit reading. (human sacrifices) I don't really want to move big stones that much.

[ibuildstuff4u]

My idea of a cheap flat surface is to go to Grizzly.com and buy one.  Their granite surface plates start at $19.95 with just $7.95 to ship it.  This is for a 6X8X2" plate but larger ones can be had for just a few bucks more.  I would never lap anything on my good surface plate for fear of damaging the surface, but would have no problem with a cheap one such as these.

Dale P.

[S. Heslop]

I heard a few years ago that the offcut from cutting out sink cavities in granite kitchen counters makes a good cheap surface plate, and that if you ask about you'd probably get one for free since they just get thrown in a skip.

Ever since then i've been trying to convince someone who owns a car that touring kitchen fitters would make for a great day out.