That's why I like this Mortar and Cannon board so much, this bunch and other bunches of members can fathom a fairly obtuse problem and figure out a solution to it without biting each others head off. In this particular case we have one element of the best, most complete answer provided by Double D coming from each and every member who posted on #6, the score marks on the 15" Rodman Gun.
Specifically we have this from our savy East Coast moderator:
#6 - I'll guess that the 'circular scratch marks' are a measurement from one end or the other that was put onto the tube when being turned.
Quite so, before placing the first form tool in a carriage mounted, special mono-block fixture, the machinist would place a special locator gage over the trunnions and it's other end would point to a spot where the right edge of the first form tool would go. A light groove would be single pointed there as a guide for the right hand spur on the form tool. The left side had a spur too and this would guide the second form tool to a correct start position, etc, etc.
Those "circular scratches" look a lot like something I used to do when turning parts on a lathe. At various points I would cut into the stock to the desired diameter, then cut away the metal between the scribes.
Although I don't remember reading that this was specifically done, you could not be too far from the truth if you surmised that this was done on the very first section to be cleaned up with the first form tool. The reason is that if you merely formed that first surface without setting some specified depth, you might go through almost all of your set-ups, only to have the last form tool fail to clean up the rough, as cast surface. Doing all that work over would make the set-up man look like a fool. Most likely he wouldn't let that happen. In this case an ounce of prevention is worth a pound of cure.
# 6
could also be from when they changed from the sraight cylindrical shape and started to make the radius .
Dan is correct here, where the cylinder or straight taper is tangent to the first part of the breech radius, you find a score mark. You don't find any of these marks within the cylindrical area OR just after the radius starts. They are exactly WHERE the radius is tangent.
6. The dismounted 15” Rodman Gun at Fort Knox in Prospect, Maine displays 5 or 6 of these circular score marks on the breech and first reinforce. What made these tool marks? What purpose do they have?
I think Terry is on the right track. Those are the incremental machine marks from where the cutting tool was started and stopped to cut each increment of the radius. At each stop the pattern for the tracing attachment was changed for the next increment in the radius. If you can't dazzle them with footwork, then baffle them...
Sorry fellows, but I did not see that drift into "Tracing Attachment" land. I've never seen one of these used with a form tool DD. They are used to single point a profile into a surface. We have seen chatter marks on the 10 and 15 inch Rodman tubes too in various places, a very good indication of a broad form tool being used on a relatively lightly constructed finishing lathe.
HOWEVER, the very essence of an understanding of what these marks indicate is found within his sentence I have converted to bold Italics.
T&M
since the marks are only at the rear end, were put there for the convenience of the guys doing the boring operation so they would be able to mark their tool length to know how far from the muzzle they would have to make the depth of internal features (steps? chamber depth?) inside.
They could well have been used for this purpose as a secondary, added benefit. Actually, this seems very logical to us.
DD, you correctly answered this one with that very succinct, one line answer. Professional writers could not express this concept in fewer words.
T&M
