Grizzly G0709 threading problems

Earl Horgeshimer

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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.
 

Mitch Alsup

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Have you checked that the leadscrew (and half nut) is prefectly clean and then freshly oiled/greased? (unlikely to ge the issue)

You can verify pitch variation by coating the threads with prussian blue and screwing the action on and off where it binds, unscrew and look for places the blue was worn off. If you can isolate where the cut is mis-pitched you can then isolate where on the leadscrew/half-nut to investigate.

Good luck.
 

benmychree

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I'd be looking for wear on the leadscrew. Barely trace 16TPI on a piece of stock for several inches, then deburr and use an accurate steel rule against the part with the 16th graduation and with a good magnifier compare the two, this should be done in the same axial position where your barrel threading is done. The half nuts should not effect lead variations, nor should lead screw end play (within reason).
 

Earl Horgeshimer

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The lead screw and half nuts are clean. I replaced the thrust bearings at the end of the lead screw and lubed and adjusted them per the manual.
I will blue up the threads tomorrow and see if that shows me anything. I received a test piece from the action manufacturer (BAT) it is a 2 piece , male and female thread that is the same material and cut with the same program that they use for the action.
 

Mitch Alsup

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Did you resolve your macining issue ?
If so, what was the root cause ?
 

Earl Horgeshimer

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I have not yet resolved the issue. Bluing the threads was an unsuccessful effort. Where the ring gage stopped, the removed blue also stopped. The 4 threads until the stop all showed contact on the pitch diameter. I did manage to get the threads to fit very snugly (the fit that I wanted) on the ring gage and the action. I did that by increasing the depth of cut a couple of tenths at a time until the gage fit. the problem is that the major diameter when the gage fits, is 1.0556. The major diameter of the plug gage that I got from the action manufacturer was 1.060. (the specification for a 1 and 1/16 inch, 18 tpi thread) It also fit snugly on the ring gage. I checked some old barrels that I chambered on this lathe and they all measured 1.0585 to 1.0595. (digital micrometer)

I took the lead screw off and inspected it carefully. No problems. I checked the half nuts by closing them and shutting off the machine. With the machine off, I watched the DRO. I saw 12 to 13 thou of movement of the carriage when I tried to turn the handwheel. Pushing the half nut lever down a bit harder resulted in 5 to 6 thou of movement. $73 dollars later, I have a new set of half nuts and way wipers on the way from Grizzly.
I am not convinced that is the problem.

In the past, I use a 3/8 HSS threading tool that I ground. My threading technique then was to use the compound to advance the cutter. With the Warner insert, I plunge straight in. I think I will re-do this experiment with my old tool and technique.
 

Mitch Alsup

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I have not yet resolved the issue. Bluing the threads was an unsuccessful effort. Where the ring gage stopped, the removed blue also stopped. The 4 threads until the stop all showed contact on the pitch diameter. I did manage to get the threads to fit very snugly (the fit that I wanted) on the ring gage and the action. I did that by increasing the depth of cut a couple of tenths at a time until the gage fit. the problem is that the major diameter when the gage fits, is 1.0556. The major diameter of the plug gage that I got from the action manufacturer was 1.060. (the specification for a 1 and 1/16 inch, 18 tpi thread) It also fit snugly on the ring gage. I checked some old barrels that I chambered on this lathe and they all measured 1.0585 to 1.0595. (digital micrometer)

I understand about the tight tollerances of benchrest barrel fits.

Are you familiar with the technique told in "Houston Warehouse" where instead of lapping in the threads getting a complete and perfect fit, Virgil put a chamfer on the innermost edge of the action and on the outer edge of the barrel to guarantee that the barrel and the action ended up concentric at both the innermost points and the outermost points of the actioin--and this concentricity was not guaranteed by the threads, but by the champhers ?

I took the lead screw off and inspected it carefully. No problems. I checked the half nuts by closing them and shutting off the machine. With the machine off, I watched the DRO. I saw 12 to 13 thou of movement of the carriage when I tried to turn the handwheel. Pushing the half nut lever down a bit harder resulted in 5 to 6 thou of movement. $73 dollars later, I have a new set of half nuts and way wipers on the way from Grizzly.
I am not convinced that is the problem.

Generally, the half-nuts can have as much as 40-thou and still cut decent threads. {But good luck anyway}

In the past, I use a 3/8 HSS threading tool that I ground. My threading technique then was to use the compound to advance the cutter. With the Warner insert, I plunge straight in. I think I will re-do this experiment with my old tool and technique.

Perhaps cutting on both sides of the carbide tooling is causing the point to drive for and aft and the barrel goes round and round.
I would try an old barrel using the compound to dial in the cuts.

Good luck.
 

Earl Horgeshimer

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I am indeed aware of the exploits of Mr King working the night shift down in Houston. There are a lot of truths and a lot of myths about what they did. In my opinion concentricity is important to accuracy no matter how it is done. At the end of the day, though, I have 3 friends(Benchrest Hall of Famer's) that are of the opinion that it all comes down to bullets, barrels, and wind reading. Most of the serious competitors make their own bullets and chamber their own barrels - the reason being that they believe that they can do it better than anyone else. :)

I really don't believe that the half nuts are bad enough to cause the problem that I am having but that extra push on the lever that reduced the backlash bothered me. The new nuts will give me the opportunity to pull the carriage apart and see why that happens.

The Warner inserts are HSS not carbide. I am a little suspect of cutting both sides of the thread at once too. I have a lot of "drops" from barrel blanks that I can use to practice on.
 

Earl

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Did you ever get this issue squared away?
Perhaps. I decided to go thru the entire lathe. I replaced the half nuts. That resulted in some improvement. I found significant "slop" in the leadscrew nut on the cross slide. That was a combination of Lead screw and nut. There is a backlash adjustment on the leadscrew nut. It was already maxed out - came from the factory that way. I ordered another nut from grizzly and they sent me one that did not have a way to mount it. (no threaded hole to mount it to the cross slide) That nut was just as sloppy as the one that I have. The cross slide lead screw is very soft and worn. I can't believe that I wore it out chambering barrels. I am in the process of chopping off the threaded section of the cross slide leadscrew and replacing it with an acme LH threaded 304 stainless threaded section (1/2 x 10 tpi). In addition I am making an "anti-backlash" nut similar to the one that Randy Richards did on his you tube channel. The left hand acme tap will be here this week. The keyway broaches arrived this morning. I also contemplated getting one of the Precision Mathews 1440 lathes. A friend has one and I went over and looked at his. Nice Machine. The wife said to buy it. I almost did. Then I realized that I had made the changes to my lathe that were features that I wanted on a lathe. - I have a 3 phase motor with a VFD for variable speed, a DRO with magnetic scales, a tachometer with SFM calculator. Aloris tool post, D1-5 set tru 6 jaw Buck chuck with the outside jaws EDM'd off just about flush with the chuck face. (Modified to hold barrels), a barrel fixture with d1-5 mount to hold shorter barrels, an inboard d1-5 spider for barrel holding. All I have to do is fix this problem and I will be happy. Until then, I am chambering at Bruno's shop. He has an Enco turn pro. The only problem there is that his lathe is low. Makes my back hurt to bend over it. My lathe is up about 4 to 6 inches off the floor. Big difference. I will report some more when I get the nut and feed screw done.

I neglected to mention that I did get some improvement by going away from the "straight in plunge" with the vertical insert tool and went back to the "compound advance" method of threading. Still had issues with ring gage fit.
 
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