I have had my Grizzly lathe for 5 or 6 years and have been very happy with it. I changed the motor to 3 phase and added a vfd. It has a dro pros dro with magnetic scales. I mainly use the lathe for chambering barrels for my Benchrest Shooting addiction. I have 4 guns, 3 sporter class and 1 unlimited class that I use in competition. These guns eat barrels at an amazing rate. 1500 rounds (sometime less) and most of them turn into stainless steel tomato stakes. Some barrels will not shoot at a competitive level so they too become tomato stakes. I cut the threads on the barrel to fit the rifle action very precisely. The rifle action is the thread gauge I use mostly. I cut threads until the action fits snugly. (no wobble at all) About a month ago, I was threading a barrel, 1.060 major tenon diameter, 18 threads per inch, 1.023 Pitch diameter (BAT Action) I was using an Arthur Warner HSS stand up threading insert and viper's venom high sulfur cutting oil. The threads were sharpening up so I stopped to check the fit. The action went on about a half a turn. I went in another 4 tenths. The action went on about 3 turns. This is where my problem showed up. I did two more cuts a 4 tenths. My experience with the 416 stainless Kreiger barrels I use is that those last two passes should make the action fit perfectly. I got out my Greenfield ring gage. It would go on 3 turns and stop. At this point I realized that I was not going to get the fit I want and since this was a new barrel blank, (26 inches long) I could cut the tenon off and start over. I called a buddy that is a world class shooter and Benchrest gunsmith. He said he starts his BAT barrels at 1.059. I chopped the tenon down to 1.059 and repeated the process with the same results. I kept increasing the cut until the gage and the action screwed on. They both flopped around on those threads. (There was a lot of wobble - not like it shoud be.) I got out my thread wires and discovered that my pitch diameter was 1.018 ?? My Thread mike said just about the same thing. I did a couple more test cuts on some old barrels with the same results. On a 1 inch threaded tenon, the best that I could do was about 5 turns of ring gage. My conclusion is that I have variations in the pitch. I checked to see if there was end play in the lead screw. There was not. I made sure that the threading tool was square to the barrel. I used a fish tail and visually inspected it. I cut the tenon off of an old barrel and matched up the threads with the barrel in the lathe. I can see some light between some of the thread shoulders. Not much, but some light comes thru on some of the threads. I think that this verifies that the thread pitch is changing. I am out of ideas as to what to check or what the problem may be. I thought that perhaps the headstock to bed alignment was off. I have an Mt3 test bar and an Mt5-ro Mt3 adapter. I measured just under a thou at 15 inches from the spindle with that test bar and the 2 adapters. I believe that the acceptable standard for variation is 8 tenths per foot. I believe that I meet that specification. Any ideas would be greatly appreciated.
I forgot to mention that when cutting threads, both the compound and the cross slide are locked solid. I use a slight amount of carriage brake tension to take up any backlash in the lead screw / half nut connection.
