Bore Centering

[Henryrifle]

So many good thoughts above. Can't wait to dig into many of them. There was a question about putting the barrel in bind with the outboard spider. I am sure that I am putting the barrel in bind--likely a lot of bind. I have bent some L-shaped aluminum shims that I use to hold the barrel in the 4-jaw. The shims extend about 1/2 inch under the jaw and are clamped down on the barrel.

Adjusting concentricity with the outboard spider is absolutely putting the barrel in bind. I could shorten the part of the shim contacting the barrel and 4-jaw to lessen the bind but it seems like this method is always going to induce some bind.

With no machinist background, it is not immediately apparent why this might be bad to me. The barrel, even if in bind, is being held statically within the spindle. The bind seems like it would remain the same under threading and boring/chambering.

I do know that there is a lot to be said for letting something be where it wants to be (the opposite of bind) but, is bind bad in this case with regards to indirectly (using range rod) measuring concentricity?

 

[epanzella]

I make a small cat's head to both protect the surface of the barrel and to limit the bearing contact in the 4 jaw to avoid barrel bending. The other end is in the spider w/brass tipped bolts.

 

[Tozguy]

The bind seems like it would remain the same under threading and boring/chambering.

I agree that it is very difficult to avoid bending the barrel using the inboard/outboard spider approach.
But keeping the bend to a minimum will improve the chances of it not moving during machining.

I like to zero the bore with a range rod using only finger snug screws. During this process if a screw is turned without a corresponding movement showing up on a dti then some screws are too tight.
Once the set up is stabilized to my satisfaction then I go around several times slowly tightening the eight screws to final torque.

Whatever fastener style is used, it has to hold the barrel well enough to prevent slippage during machining. Therefor it has the potential to stress a barrel. A good test for any magical mechanism is to set it up tight ready for machining then back off all four outboard screws. Does the outboard end of the barrel stay suspended in the center of the spindle or does it fall?

 

[eeler1]

So, as I understand it, after dialing in the bore by hand, you are then using the dti under power? And how much runout are you getting then? Not sure yet what's bugging me here, I might make a dummy barrel and see what the effect of the increased runout is. But you've got some good ideas above.

 

[mickri]

Henryrifle I am curious what your intended use is for the rifle that you are building. What caliber and what action. There seems to be some debate on whether chambering through the headstock is any better than chambering between centers for anything other a benchrest match rifle.

 

[Henryrifle]

I compete in F-Class, F-T/R specifically, and the caliber is .224. The barrel is chambered for the Berger 90 VLDs and really has to shoot in the .2s to .3s to be competitive.

Runout between hand turning and under power: This is somewhat variable but I can get it down to no movement on the .0001 gauges near the chamber and 6" away at the end of the range rod but as soon as power is applied I'll see 3 to 4 10,000ths at the muzzle and maybe as much as .0008" at the far end of the range rod.

I've chambered several of these for my personal use and they all shoot well. Maybe it doesn't much matter but I am more than a little confident that most people here that have set up a lathe at their home have a higher-than-average passion for precision.

I wasn't allowing the lathe to warm up before starting this thread but, I do now. I also did relax the jaws and spider on the barrel I am working on and reduced my shim's chuck jaw-to-barrel contact surface down to 1/4". I made an effort to center the muzzle in the spider but left the spider screws loose (not contacting the barrel) until the chamber end was very close to perfect. Based on the muzzle position in the spindle, I know I am 'bending' the barrel less between the chuck and spider.

As soon as I get this barrel finished--still have muzzle end to work on--I am going to take a look at the preload on the spindle bearings and measure runout on the spindle face and nose. Cutting the shims has helped increase barrel's ability to gimbal in the chuck and I know warming up the machine will contribute to more consistent measurements and cuts.

Will report back as soon as I take some measurements and will reread all response to see what else I can work on.

Thank you all again!
Henryrifle

 

[mickri]

I am impressed. To compete at that level you definitely need a very accurate rifle. And you have to believe in the accuracy of your rifle. Any doubts about your rifle will effect how well you do. So do whatever you think is needed to build the most accurate rifle you can.

 

[RBaker]

I compete in F-Class, F-T/R specifically, and the caliber is .224. The barrel is chambered for the Berger 90 VLDs and really has to shoot in the .2s to .3s to be competitive.

Runout between hand turning and under power: This is somewhat variable but I can get it down to no movement on the .0001 gauges near the chamber and 6" away at the end of the range rod but as soon as power is applied I'll see 3 to 4 10,000ths at the muzzle and maybe as much as .0008" at the far end of the range rod.

I've chambered several of these for my personal use and they all shoot well. Maybe it doesn't much matter but I am more than a little confident that most people here that have set up a lathe at their home have a higher-than-average passion for precision.

I wasn't allowing the lathe to warm up before starting this thread but, I do now. I also did relax the jaws and spider on the barrel I am working on and reduced my shim's chuck jaw-to-barrel contact surface down to 1/4". I made an effort to center the muzzle in the spider but left the spider screws loose (not contacting the barrel) until the chamber end was very close to perfect. Based on the muzzle position in the spindle, I know I am 'bending' the barrel less between the chuck and spider.

As soon as I get this barrel finished--still have muzzle end to work on--I am going to take a look at the preload on the spindle bearings and measure runout on the spindle face and nose. Cutting the shims has helped increase barrel's ability to gimbal in the chuck and I know warming up the machine will contribute to more consistent measurements and cuts.

Will report back as soon as I take some measurements and will reread all response to see what else I can work on.

Thank you all again!
Henryrifle

Old thread I know. But looking at it I thought well why measure the range rod 6" from muzzle as opposed to advancing the rod in or out in the muzzle, keeping your measurement inst at the same place? I use a spider mounted to a back plate, with eight adjusting screws. Move range rod with bushing in and out in a 4 inch range adjusting spider screws until i achieve less than 0.0002 runout. I do shoot benchrest I am new to machining. My mentors who are much better than me indicate this works. Has for me so far

 

[Tozguy]

advancing the rod in or out in the muzzle,

A range rod is a tapered rod and with the right bushing on the end will slide into the bore until the taper fits snug at the crown. Thats the only position where it is aligned with the bore. It won't go in farther and if you slide it out from there it will be too loose in the bore. So I don't understand how sliding the range rod in and out of the bore would give you reliable readings.
If you sliding the dti on the rod once it is inserted to a snug position that is different. But the range rod has to stay put.

 

[RBaker]

A range rod is a tapered rod and with the right bushing on the end will slide into the bore until the taper fits snug at the crown. Thats the only position where it is aligned with the bore. It won't go in farther and if you slide it out from there it will be too loose in the bore. So I don't understand how sliding the range rod in and out of the bore would give you reliable readings.2 increments
If you sliding the dti on the rod once it is inserted to a snug position that is different. But the range rod has to stay put.

Maybe we are talking about two different animals. I use a "Gritters" type rod with interchangeable bushings in .0002
increments. The rod fits snug but moves within the bore. Move in adjust, move out adjust. Your method is just different than mine. i am trying to indicate the bore in more than one position