Found another problem

[Nvreloader]

I just started turning a 26" x 1.250" 7mm bull bbl,

After running a truing cut, turning between centers, the full length of the bbl,
I measured the end diameters of the bbl and find that the tail stock end is .005" smaller than the chamber end.

Now I have to recenter the tail stock,

I read on here IIRR, about using a dial gage to check the centering of the tail stock with a dead center, but I can't find the post now.

Any suggestions??

Tia,
Don

 

[larry 1944]

Put a indicator on the tail stock & move it over .0025. Take another cut & move it again if you have to.

 

[Zebra13]

Don,

Even if you dial in your tailstock dead nuts to your headstock, you will probably still cut a taper as tailstocks usually sit a bit higher. They are made that way to account for wear. If your tool is on centerline at the tailstock, it would sit above the centerline at the headstock and give you different diameters.

On a barrel, I don't think a 5 thou difference over 26 inches makes a difference. That's about .0002 an inch.

However, if it really bugs you, do what Larry says.

Justin

 

[shortgrass]

Don,

Even if you dial in your tailstock dead nuts to your headstock, you will probably still cut a taper as tailstocks usually sit a bit higher. They are made that way to account for wear. If your tool is on centerline at the tailstock, it would sit above the centerline at the headstock and give you different diameters.



However, if it really bugs you, do what Larry says.

Justin

Best way to induce deflection is to have the tool above center. Set tool center at the center of the bar in the chuck. About the only time I set any tool above center is if I'm deep grooving or cutting-off. And then, no more than .005-.006 thou.

 

[Zebra13]

If I'm following you correctly, your inference is that I'm advocating setting the tool above center, which I am not. What I am saying is depending on where you center your tool on a long cut, the tool will end up either below or above centerline due to the tailstock being (usually) elevated. On centerline at the chuck, low at the tailstock. On centerline at the tailstock, high at the chuck. This is what causes tapers (amongst other things). I set centerline at the chuck, as you mentioned, which does away with the deflection problem, but you are still going to turn a taper due to the tailstock being elevated.

It's kinda counter-intuitive...you have to offset your tailstock (if it's high) to turn something between centers straight. I first thought that if I swept my tailstock in, anything turned between centers would be dead-nuts. Not so. Only after I thought about things a bit did I realize why I was getting tapers. In the link, you will find a tool I made to deal with the high tailstock when trueing an action. http://benchrest.com/showthread.php?91625-Action-Truing-Justin-Style&highlight=

Shortgrass, if I haven't followed your post correctly, my apologies and disregard the above. I will try setting my parting tool above centerline, though.

Justin

Don,

Even if you dial in your tailstock dead nuts to your headstock, you will probably still cut a taper as tailstocks usually sit a bit higher. They are made that way to account for wear. If your tool is on centerline at the tailstock, it would sit above the centerline at the headstock and give you different diameters.



However, if it really bugs you, do what Larry says.

Justin

Best way to induce deflection is to have the tool above center. Set tool center at the center of the bar in the chuck. About the only time I set any tool above center is if I'm deep grooving or cutting-off. And then, no more than .005-.006 thou.

 

[Nvreloader]

Justin

Thank you for the info,

I also see that you use the boring head in the tail stock,
I have been using that method since 1986, for cutting tapers on bbl's, and is fitted with a live center,
which I found better than the dead center.

Tia,
Don

 

[riflewoman]

That is common with tail stocks. A difference in height or wear or a slight twist of the bed and you'll get these results. They used to sell "centering buttons" that you could use to align the centers, but that was only valid at that point. Add the clearance in the tailstock quill too. All you can do is try to eliminate as much taper as you can by cutting and trying. You can we ordinary bar stock for this. You could also use a lathe mandrel for initial setup but you'd have to remember that these have a built in taper.

 

[OldmanFCSA]

I figured where the tailstock and live-center needed to be when turning a 50 barrel 35" long between centers.
Then using a bar clamped in my chuck with an indicator on end, I was able to dial in the tailstock to perfection.
Test cuts indicated 0.001" error in diameter end to end which I was able to adjust out by tightening the quill lock on tailstock, but with an increase of 0.001" in diameter near center of barrel either due to deflection or bed wear on my vintage LeBlond tool room lathe. Used a high relief sharp edge carbide insert for final turning to reduce tool pressure on barrel.
Barrel finished 1.750" diameter on each end and 1.751" diameter near the center. More accurate than required, but a good feeling regardless.

 

[eww1350]

Remember to use a follow rest when turning barrels..

 

[Will Henry]

To compensate for the difference in tailstock height, turn on the lathe and let it run while you have a cup of coffee; This will warm up the headstock and bring it closer to the same height. In reality, the difference in height won't make that much difference in diameter unless it is extreme or the diameter small. Some deflection in the middle is to be expected. I have turned tubing up to eight feet long (6 inch dia. 3/8 wall thickness) and while the ends were perfect, I was happy enough to be within .005" in the middle and let polishing take care of it. To minimise deflection, the sharper the tool, the better. Less pressure, less heat. WH

 

[shortgrass]

To minimise deflection, the sharper the tool, the better. Less pressure, less heat. WH

So, ditch that carbide insert and use a properly ground HS tool.

 

[butchlambert]

You can have your tailstock dead nuts and still have taper. You know that you also have bedwear close to the headstock and not at the tailstock. How do you propose to handle that?

 

[Steelnecktie]

If tailstock end is .005" smaller, the backside tailstock end of machine needs jacked up. Try 1/8 turn on leveling feet. You'll know if you did too much, it will cut larger on tailstock end.

 

[Gundoktr]

You can have your tailstock dead nuts and still have taper. You know that you also have bedwear close to the headstock and not at the tailstock. How do you propose to handle that?

And not only that, you could have twist in the bed too.

 

[B Nettesheim]

Adjust your tailstock after your truing cut, especially if your machine is worn. Not a big problem just account for it. The tailstock can only follow the bed so dont be surprised if you have moved it to a different spot close to the chuck and another spot closer to the tailstock end of the bed.
A follow rest and a hss tool might give you better results.

 

[Gundoktr]

My opinion on setting up your lathe. Chuck a piece of aluminum and make a light cut a few inches long. Make sure you use a heavy piece, 1.25" or so to prevent deflection. twist the bed until theres no taper in the cut. Then turn a piece between centers and adjust the taper out by moving the tail stock. Even with wear in the bed you should be able to get some very good results. I do not use a level anymore, I dont care if my bed is level, I care how it cuts.

Alex, I agree, but, I level mine to make sure there is no twist or hump in the bed, After all, how did they get those on board ship lathes to cut straight

 

[Nvreloader]

Thanks Guys
For the info,
I got the bbl turned close enough for my use,
within less than .005", to add a muzzle brake,
and contour it to fit the bbl etc.

Tia,
Don

 

[climb-101]

on the ship lathes that i have seen they were mounted on 3 points, 2 at the head stock and 1 under the tail that could pivot. that way the lathe didn't twist with the ship. as far as a hump in the bed, i would think that bed would resist that given the thickness of it but then again the old iron i have is a lot different than the new stuff made from recycled kia's.

I think the bed thickness on mine is around 16" and the carriage weighs more than most lathes now days.

 

[Gundoktr]

on the ship lathes that i have seen they were mounted on 3 points, 2 at the head stock and 1 under the tail that could pivot. that way the lathe didn't twist with the ship. as far as a hump in the bed, i would think that bed would resist that given the thickness of it but then again the old iron i have is a lot different than the new stuff made from recycled kia's.

I think the bed thickness on mine is around 16" and the carriage weighs more than most lathes now days.

Place a precision level on the bed and a heavy pinch bar under one of the bases and watch the bubble move. Think of the stress you can put on it just by tightening one of the levelling screws. I worked in a shop with a big overhead door in northern Minnesota. We were leveling one of the bigger lathes and someone opened the door. You can't imagine how much that bubble jumped.

 

[Ggmac]

I live on a big sand bar , south Florida , you can't imagine how much my 12" slab ( machine shop ) moves . Water , temp , road traffic , etc , all make the bubble move . I need to recheck weekly , probably more .
4 houses up from me is all coral 6" below the surface . He can't even install a fence without great expense .