Replies

1. 
   JohnWW | Feb 04, 2008 07:53pm | #1

   I tested table saws for Fine Woodworking Magazine for years and I never once found a table saw with a cast iron top that was flat. 

   They were almost always distinctly concave, dipping unevenly towards the throat plate opening.  Typically the saw's table was out by several hundreths of an inch over a foot, so using your table saw as a reference surface when trying to working to a few thousandths of an inch is only giving you bad information.

   I have come to the conclusion that the only reliable reference for flatness is a granite surface plate, which has been the standard in the metal working industry for a century or more. 

   A second parallel problem is that almost all of the surfaces recommended for for lapping a plane with abrasive paper have one of two problems, they are either not flat to start with, like the cast iron top to a power tool, or they are flat but not nearly stiff enough to stay flat when downward pressure is applied to them, a common problem with even thick glass plates. 

   Again the only solution is to use a granite machinist's plate for the lapping.  Fortunately the price on these has become very reasonable since they are now being imported from China.  Plates are available from a number of sources, Grizzly has them in several sizes at good prices. 

   If you intend to do a lot of plane tuning, I'd buy a large plate, 18" by 24" which will only cost around $50.00.  Unfortunately, because of the weight, it will probably cost more than that to ship it to you if you can't find one locally.

   John White  

   
   

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   Edited 2/4/2008 12:08 pm ET by JohnWW

1. 
   jeffsech | Feb 05, 2008 08:51pm | #2

   Thanks, John, that's a good item to have around.  I'll look for one locally--you are right about the shipping.

   Do you have any opinion on the actual tolerance I should be working to for a No. 7?  Wouldn't 0.004" mean two different things when you're talking about a jointer plane versus a smoother.

    

1. 
   JohnWW | Feb 05, 2008 09:17pm | #3

   If you are using a granite surface plate, you should be able to get the bottom of the plane pretty close to perfectly flat if you want to spend enough time at it.  In theory, the area around the mouth opening should be brought flush with the sole for chip control, beyond that as long as most of the sole is flat the plane will work fine.

   I never got that cranked up about sole flatness, although I do try to flatten my soles to some degree.  When you are planing a large surface, the flatness you achieve is mostly dependent on how you handle the plane, rather than how flat the plane's sole is. 

   When planing edges for gluing up, flatness counts a bit more, but, in the end, finesse with the plane and fine adjusting the edges so they fit tightly against each other, rather than their being perfectly straight, is the best approach to achieving a tight joint.

   John W. 

1. 
   jeffsech | Feb 05, 2008 09:46pm | #4

   Thanks, John.