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71CJ5
10-25-2009, 02:54 PM
Can someone please explain to me why I would want to turn a piece between centers using a lathe dog instead of just using a center in the tailstock and say a 3 or 4 jaw chuck to do the driving? What are the pros/cons to using this method? Thanks! :cool:

Jack F
10-25-2009, 03:00 PM
Better accuracy than the 3 jaw and faster set up than using the 4 jaw.

Jack.

juergenwt
10-25-2009, 03:25 PM
Like Jack said - better accuracy plus the benefit of leaving less material to grind off if you need grinding after heat treating.
Also gives you a chance to straighten after heat treating and the option of being able to regrind a shaft at a later time. Best to use a protected center - a center drill with a countersink or recess the center of the shaft so it will not be damaged.

dp
10-25-2009, 03:29 PM
Can someone please explain to me why I would want to turn a piece between centers using a lathe dog instead of just using a center in the tailstock and say a 3 or 4 jaw chuck to do the driving? What are the pros/cons to using this method? Thanks! :cool:

You can remove the work from the lathe to do another operation on say a mill, and get near perfect repeatability when placing it back on the lathe.

steve45
10-25-2009, 03:48 PM
You can use it to cut tapers, too.

71CJ5
10-25-2009, 03:49 PM
Thanks for the clarification guys!

However turning between centers will only work if you know your center holes on your workpiece are truly centered on each end, correct?

websterz
10-25-2009, 03:53 PM
Start with a large enough OD that when you get the part turned to size it won't matter of the holes were off a bit when you started. :)

71CJ5
10-25-2009, 04:10 PM
Start with a large enough OD that when you get the part turned to size it won't matter of the holes were off a bit when you started.

This is the reason why I like this board...keeps me from over thinking things :rolleyes: :D

darryl
10-25-2009, 05:56 PM
If you use a center in the tailstock to support that end of the workpiece, there's another problem that can crop up. You mount the workpiece in the chuck and turn it on- the outboard end wobbles a bit. If you then dial in the tailstock to center the workpiece, you'll be transferring the wobble motion to the chuck jaws. Something will have to continually flex to accommodate that, and one likely outcome is that the workpiece will become loose in the chuck and will back out of the TS center. Even if that doesn't happen, the workpiece will not come out perfectly true after machining.

One way to lessen this problem is to first mount the workpiece that way, then take a clean-up cut for an inch or so at the tailstock end. Then reverse the workpiece and clean up again at the tailstock end. Reverse it again and carry on with your machining. Of course you'll need a center hole in each end of the piece, centered as best you can.

An interesting way I came up with to accurately spot a center hole is to first machine a plug that can be temporarily inserted into your spindle bore such that the chuck jaws will still clear it if they are closed. Spot and drill a hole accurately centered in that piece while it's still in the chuck. You'll need a long drill bit, long enough to come in from the outboard end of the spindle and go through this jig. That's the size hole to drill through the jig. Then put the jig in place in the spindle bore, mount the workpiece that you need a center hole in, then put the long bit in a cordless drill and bring it in through the jig to start the center hole in the workpiece.

For the best use of this method, you'd want to make a second jig as well, one to insert into the outboard end of the spindle to center the long drill bit at that end.

A way to improve on this is to make your own long drill bit. Since it will be used mainly to accurately spot the end of a workpiece, it won't need long flutes. Use a length of drill rod, silver steel, or music wire, and grind it to a point. You could stop right there, since what you now have is a centering jig and a long center punch. Just tap it into the workpiece to mark the center. Or carry on and make a drill bit out of it. Grind two flats opposite such that they come very close to the point on both sides. What's left is to grind a relief angle on both lips, and there's your spotting drill. It will remain centered in your jigs very well.

With some imagination you can go further with this idea. You can use a rod which you drill out at one end to insert a regular short drill bit into, maybe securing it with a setscrew so you can replace the drill bit in the jig when required. The holes in the jig pieces would be sized for the rod you use, and you could probably make it in such a way that the entire jig stays in one piece. Just insert it into the spindle bore before mounting the workpiece to be spotted.

tattoomike68
10-25-2009, 07:11 PM
If you use a center in the tailstock to support that end of the workpiece, there's another problem that can crop up. You mount the workpiece in the chuck and turn it on- the outboard end wobbles a bit. If you then dial in the tailstock to center the workpiece, you'll be transferring the wobble motion to the chuck jaws. Something will have to continually flex to accommodate that, and one likely outcome is that the workpiece will become loose in the chuck and will back out of the TS center. Even if that doesn't happen, the workpiece will not come out perfectly true after machining.

One way to lessen this problem is to first mount the workpiece that way, then take a clean-up cut for an inch or so at the tailstock end. Then reverse the workpiece and clean up again at the tailstock end. Reverse it again and carry on with your machining. Of course you'll need a center hole in each end of the piece, centered as best you can.

An interesting way I came up with to accurately spot a center hole is to first machine a plug that can be temporarily inserted into your spindle bore such that the chuck jaws will still clear it if they are closed. Spot and drill a hole accurately centered in that piece while it's still in the chuck. You'll need a long drill bit, long enough to come in from the outboard end of the spindle and go through this jig. That's the size hole to drill through the jig. Then put the jig in place in the spindle bore, mount the workpiece that you need a center hole in, then put the long bit in a cordless drill and bring it in through the jig to start the center hole in the workpiece.

For the best use of this method, you'd want to make a second jig as well, one to insert into the outboard end of the spindle to center the long drill bit at that end.

A way to improve on this is to make your own long drill bit. Since it will be used mainly to accurately spot the end of a workpiece, it won't need long flutes. Use a length of drill rod, silver steel, or music wire, and grind it to a point. You could stop right there, since what you now have is a centering jig and a long center punch. Just tap it into the workpiece to mark the center. Or carry on and make a drill bit out of it. Grind two flats opposite such that they come very close to the point on both sides. What's left is to grind a relief angle on both lips, and there's your spotting drill. It will remain centered in your jigs very well.

With some imagination you can go further with this idea. You can use a rod which you drill out at one end to insert a regular short drill bit into, maybe securing it with a setscrew so you can replace the drill bit in the jig when required. The holes in the jig pieces would be sized for the rod you use, and you could probably make it in such a way that the entire jig stays in one piece. Just insert it into the spindle bore before mounting the workpiece to be spotted.

Much repect but thats way too much work, I dont think he is making parts for the space shuttle.

In the real world we chuck it up, pop it with a small brass hammer till it good and go for it. takes a minute or so. when you charge a dollar a minute you cant dick around.

MrSleepy
10-25-2009, 08:31 PM
hi

this is a pin under construction that connects the boom to my schaeff HR14 mini digger.

I was given some suitable bar by a friend who runs a punch and die co. in Rotherham, I first used my L5 and its fixed steady to drill the holes where the live and fixed centers will go , and to clean it up abit.

It then went into my maximat to be turned and threaded..


http://i897.photobucket.com/albums/ac180/MrSleepy123/IMG_1212Medium.jpg

I use a test bar and a dti to make sure that theres no taper between the centers.

Rob