measuring a plane sole flatness?



Yes, I did. But I am often surprised at how much knowledge and information actually exists that I am unaware of. ;-)
Don Young (USA)
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On Mon, 22 Dec 2008 11:20:48 +0000, bugbear

Not to sure of the necessity for extreme flatness of your sole plate. However a very convenient visual check is the ancient plate glass methylated spirits capillary test.
Lay a thick piece of glass on your sole plate with one edge of the glass propped up on a thin spacer chosen to give an airgap slope of about 1 in 100.
Carefully introduce a small amount of methylated spirits into this airgap. Very little is needed - the amount carried in a small artists brush is sufficient. Capillary action will draw the spirits to the touching end of the airgap. Add enough for a band of liquid about 1/2" wide. Surface tension causes the free edge to have uniform thickness so the height of the free edge is a direct indication of 100:1 amplification of the airgap at the edge height.
Because the meths is flowing in a very small gap it takes a little time to settle into its final position. The smaller the slope the greater the amplification and the longer the settling time.
The flatness quality of the glass can first be checked by the same method on your surface plate. The glass doesn't need to be very thick because it's fully supported along its length. 1/4" or more is OK.
The best glass to use is genuine plate glass as this has been ground flat and polished on both surfaces. This is not too easy to find.
Most current glass is float glass. This process inherently produces glass with two precisely parallel surfaces but the flatness is dependent on the precision of the mechanical alignment of the outfeed rollers. This may introduce a small amount of twist or bow. Its quality as a flatness standard is a bit variable so selection is necessary.
A much better bet is mirror glass (e.g. an ancient dressing table mirror). Twist or bow in a reflective surface is unacceptable so quality mirrors are either plate glass or float glass selected for minimum twist/bow.
The problem is removing the backing. The silvered surface is protected by a copper plate layer followed by a paint or resin film. The film can usually be removed by a methyl chloride based paint stripper but nitric acid is needed to shift the copper and silver coating.
For precision work with fine ground optical surfaces a 1 in 1000 slope can be used. For coarser work a similar method is possible but with a 1 in 20 slope and the meths replaced by a handful of identical smallballs.
The method is still usable with glass that is not dead flat. If the workpiece is stationary and the glass position is changed, workpiece errors stay put, glass errors move with the glass.
Jim
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snipped-for-privacy@yahoo.com wrote:

I thought I'd read several "older" books on precision engineering, but I've never read of this technique - thank you very much!
BugBear
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bugbear wrote:

http://www.leevalley.com/wood/page.aspx?sid=&ccurrency=3&page2526&category=1,43513

Then put a blade in it which is usually hand ground and stone finished and therefore imperfect. PS you can get a plane with a flexible sheet base which can be adjusted concave or convex what do you do then?
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F Murtz wrote:

Its performance, in terms of predictable behaviour and surface quality will be less good.
BugBear
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bugbear wrote:

In the end I did this:
http://geocities.com/plybench/flatten_practice.html#measure
Thanks to all for the input.
BugBear
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