lapping & accuracy
A common question I hear is how I flatten the sole and sides of a plane. Most people assume that I have a milling machine, surface grinder or some other mechanical process. I don’t - I lap them by hand. I do use a bench mounted grinder to grind off the proud parts of the dovetails and pins, but other than that - they are lapped by hand. The next question usually has to do with accuracy and there is often a level of skepticism about how accurate they actually are. The implication being that hand lapping could not possibly be as accurate as a surface grinder or other mechanical machinists process.
Here is the process I go through - I will use an A11 style mitre plane as an example.
Here is the mitre plane after being piened together but before the infills have been installed. It shows the dovetails before grinding. The photo below shows the plane with the infills installed and the dovetails and pins ground down from the bench grinder.
I have several different lapping surfaces in the shop. The one in the photo above is a piece of marble inset into the top of the cabinet. I use an automotive sandpaper made by Norton called A275. It is normally used to abrade paint and fiberglass but is by far the longest lasting, least loading paper I have tried. I use a 3M product called Super77 which is a low tack spray adhesive to hold the pieces in place. Once the pieces are worn out, just peel them off and re-apply new ones. Expect to pay about $1 per sheet. As a point of reference, I have used 12 sheets (of 80 grit) so far, and 4 more will finish it off tomorrow. $16 to flatten the sole and sides is pretty good value for money in my books.
In addition to the 2 marble lapping surfaces, I have two 4" thick, 12" wide, and 36" long granite surface plates. I use one of them with A275 attached to them, but only for the final lapping with finer grit(s). The second one is kept clean and is used for measuring and reference.
The other important items are accurate measuring tools - for square and flatness.
The square is one of a set of 3 I bought at the first Woodworking in America. They are in wonderful condition and have knife edges on the inside and outside of the blade. I am always on the lookout for more but have yet to find another.
The other tool is a Russian straight edge with a very fine knife edge. Here are a few photos of it.
It is frightfully accurate. Here is a shot of the straight edge on the sole of the mitre plane facing the sun coming through the window. The second photo shows the effects of a 0.001" feeler gauge under one corner. Click on the images for a larger view.
As I am lapping, I keep my Starrett square handy and check the progress often. I will use a Sharpie marker to mark spots that are high or if a side is out of square. I make adjustments to my hands, stance and pressure to bring things into square as I am lapping.
Once the lapping on the first (marble) surface is just about done, I still check using my Starrett in multiple places, but I also like to place the plane on the Granite plate and push the knife edge square up to it. This is just another way to cross check that things are in fact square. (It is also important to dust off the reference surface plate before placing the plane and square. Even one piece of loose grit can really screw things up).
The square pushed up to the sole...
... the square pulled back a couple thousandths of an inch to show the light gap (click on the images for an enlarged view).
I check the plane in several spots to make sure it is square across the length of the sole. The only light gaps should be the micro gaps left by the scratches from the individual pieces of grit in the paper. A knife edge square makes things so much easier to see and will reveal gaps that a flat edged Starrett will not.
Hand lapping is physically demanding work, and may not be for everyone, but it is fairly low cost, and with practice, patience and a deliberate watchful eye, is certainly capable of producing excellent results.
It is also good exercise.
(the almost finished mitre plane)