The lap used by the toolmakers was quite a simple affair. A piece of half-hard brass was turned identical to the dimensions of the associated D-bit. The actual lapping diameter, of course, had to be no greater than the cutting diameter of the D-bit, otherwise, it would not enter the hole at the start of the lapping operation. If it were a thou. or so smaller it was of no consequence.
When he got it, the toolmaker proceeded to hacksaw a longitudinal slot in the working diameter of the lap. Holding the shank of the lap in the chuck of a drill press, and running at about 1000 rpm, he coated the lap with some coarse Carborumdum paste, and commenced lapping. With the bush held in his hand, he ran it up and down the lap, which was removing minute amounts of metal. It was imperative that he use the whole length of the lap, moving up and down all the time, otherwise, he got a hole which was not parallel.
When he felt the lap was not cutting as he wished, he stopped the chuck and with a screwdriver inserted in the end of the slot he had cut previously, gave it a little twist thus enlarging the effective diameter of the lap. The enlarged-dia. lap was sometimes a little too big for the hole but the saw cut permitted it to close down to the proper size, whilst exerting a radial pressure on the sides of the hole being lapped. He usually finished by cleaning off with paraffin, (kerosene?), and using a fine grade of Carborundum paste to finally attain the size he desired.
Author: Cyril Collins