Draw lines (Witness marks)
A few days ago, there was a brief mention about draw lines, sometimes called "witness marks" on barrels and receivers, specifically on the Luger pistol. These are the short lines on the bottom of the barrel shoulder and the receiver 180 degrees from the sight line. Apparently some writers on the Luger have stated that these were put on after the barrel was installed.
That was not the case; the draw lines were put on before assembly because they were used to ensure both the correct position of the front sight and to ensure that the barrel lined up with the receiver in assembly. As the name indicates, they show the point to which the barrel must be "drawn up" (screwed in) to allow the front sight to line up properly.
I am taking certain liberties here; the order of the operations may have varied, but this is basically the way the draw lines were used in building a Luger.
When the receiver was partly finished but faced and threaded for the barrel, it was placed in a fixture and a special jig holding a chisel shaped tool was used to make the receiver draw line at the exact bottom of the receiver beside the barrel hole. All receivers were so marked. The line was marked deeply enough that it would not be obliterated in polishing and bluing
After the barrel was partly finished but the front sight base had not been machined out of the "ring", the shoulder was cut and the barrel was threaded. Then a special female gauge was run onto the threads until it stopped. The gauge had a slot into which another chisel shaped tool was inserted and then struck, making the barrel draw line on the barrel shoulder. This draw line was also struck deeply enough to survive polishing and bluing.
The barrel was then set up for machining of the front sight base, using the draw line to ensure the base was in the right place (180 degrees away from the draw line). The barrel was then finish machined, drilled, rifled and crowned, the breech end was finished and the chamber cut. The extractor cut was then made, again 180 degrees off the draw line.
When both receiver and barrel had been finish machined, the barrel was clamped in a fixture and the receiver screwed on using a special wrench until the two draw lines matched perfectly. The assembly was mated to a grip frame, fitting was done, and all three parts numbered to ensure they remained together during finishing and bluing. Other parts were fitted and numbered also while the gun was "in the white". The gun was then disassembled (all but the barrel-receiver assembly), given a final polish and rust blued or tank blued, depending on the period.
Spare barrels also were fully finished and had draw lines, so replacing a barrel became a simple matter of screwing it in until the marks lined up. Usually, the line up was perfect. If it was not, the barrel shoulder could either be trimmed down a bit at the rear or "rolled" to widen it. Either way, the barrel would be of the proper tightness when the draw lines matched. I believe spare barrels may have been short chambered to allow headspace to be set with a reamer, as was done with rifles, but I have no confirmation of that, and tolerances should have allowed pre-chambered barrels to work without a problem.
Jim
|