Machining between centers

Glmphoto said:
I set out to make a new arbor for an old table saw I am currently refurbishing. I have done some between centers work but I haven't yet had to thread both ends of a shaft. I know a part can be removed from the lathe and then returned to the lathe with great accuracy. My question is once one end of the shaft is turned and threaded is it OK to remove it and swap end for end,, removing and reinstalling the dog end for end as well. Then go about finishing up the other end of the shaft. Am I thinking right here or am I headed for trouble. Any comments/suggestions appreciated.
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Use a piece of soft sheet copper or aluminium between the the bottom of the set screw and the finished threads. That is how I did it. Good Luck.
 
I teach my students to turn a piece of round stock and thread both ends between centers as a project. I have them, first, chuck a piece of 1/2" stock, about 3/4" out of the jaws, and taper the end to 60 deg. to use as the center and use the chuck jaws to drive the dog. If you have a center for the spindle and a dog driver, it's the same thing. Wrap a piece of 14 or 16 ga. alum around the thread and clamp on the dog. Use a piece of alum long enough that can be doubled up under the dog bolt to protect the threaded end.
I do all of this so the students can cut their first thread in one class and the other in the next and keep the piece concentric which is what you need for an arbor.
I hope this helps.
 
Don B said:
Just one bit of advice I'd give you is make sure you do all your roughing first on both ends, just leave a little for finishing, I'm not sure how large a shaft your starting with and how much material you'll be taking off for steps and such, I've seen people rough and finish one end then turn the piece and start roughing the other end, by the time they get to where they should be finishing the second end there watching what looks like a cam spinning on there lathe, flipping end for end for threading purposes should not be any problem just remember a typical driving dog will not have the same holding power as a chuck, nice light cuts when threading, it's a bugger when the lathe dog slips and you unintentionally start cutting a double start thread.:))
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Don B (or anyone who knows). In the post quoted above, you said "I've seen people rough and finish one end then turn the piece and start roughing the other end, by the time they get to where they should be finishing the second end there watching what looks like a cam spinning on there lathe". I'm very new to this, so, please explain this statement further, I'm trying to understand what your saying, about looking like a "cam spinning", and what would cause this? Thanks JR49
 
drs23 said:
What is the reason to be turning between centers with a dog? I just finished the same job an hour or so ago. All I did was chuck it up, face, turn to nominal, single point to my check nut, ran a file over the threads and wire brushed. End for ended it and repeated. Removed from lathe and installed.

What did I do wrong?:thinking:
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The runout in your chuck will prevent it from turning true.
 
Wow thanks for all the comments.
I prefer to do this job between centers. after some roughing I will be pressing a part onto the shaft. Then it will be returned to the lathe for further work. The centers will keep it all true as it is removed and replaced. The centers would allow you to find center again later if a repair or alteration were needed. Perhaps some of you have machines that are so spot on that you can chuck it, flip it, and everything is perfect. I do not, and find it hard to believe anyone does. Turning between centers is a tried and true method of maintaining concentricity. I believe a table saw arbor needs to be true if you expect it to run vibration free.
All this aside, my question wasn’t whether or not to turn between centers. I was asking if, whilst turning between centers if the part could be turned end for end. I had never done that before and it seemed OK. Sometimes things aren’t as they seem so I threw it out there. Tony Wells and a couple others seemed to get the question I was asking. I will try to make a photo record of this project and post it here for those who are interested in working with this method.
I want to thank everyone for there time to share there comments and opinions. Only through open and spirited exchanges like this can we all learn and therefore raise the bar.

Best Regards
 
Another good habit (machining between centers) is to mark the chuck to the drive dog (tape/ sharpie). Sometimes will be a need to remove the
part especially threading. Removing a chucked part and returning it, is nearly impossible, it can be done, but I would'nt trust it. between centers
I prefer.....
 
JR49 said:
Don B (or anyone who knows). In the post quoted above, you said "I've seen people rough and finish one end then turn the piece and start roughing the other end, by the time they get to where they should be finishing the second end there watching what looks like a cam spinning on there lathe". I'm very new to this, so, please explain this statement further, I'm trying to understand what your saying, about looking like a "cam spinning", and what would cause this? Thanks JR49
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What I was getting at is that metal relieves itself as you're cutting material away, usually the more material removed the greater the chance of distortion, just for an example, your making a shaft 16 inches long and for most of the diameter it's going to be .875 but right in the middle the diameter is 1.500 for 2 inches of the length, and to make things worse all you have around is 2 inch diameter stock, lost of material needs to go, if you where to finish one end completely (on centers) then flip it and start taking meat from the other end, by the time you get close to your finish size and the metal has relieved itself one end of the shaft will be going up while the other end is going down, I just meant rough out completely first before going for finished sizes, metal relieving itself throughout the machining proses is something to be considered whether turning or milling.
Sorry I'm not great at explaining this, distortion and how much material to leave for finishing is something you'll get the feel for after making a few tons of chips.:))

Don
 
I've said it before but, wow, what a great forum! Thanks to Glmphoto for the perfect question ( and I hope my question to Don B wasn't "hijacking your post", if it was SOMEONE PLEASE TELL ME, so I'll know next time). Don B, your explanation, although somewhat technical, for this beginner (stress and distortion stuff) actually answered me perfectly. Your original response had me thinking that it had to do with the centers wearing out or somehow becoming off center by the time he got to the finishing cut. So thanks for that, JR49
 
Well hopefully, there is a ball bearing center on the tailstock end, so no significant wear to worry about there and if that center won't repeat within a tenth or so, I'd dedicate it to rough rough work. Dead centers are harder than most material turned anyway, and the only non-uniform pressure they see is from the front of the lathe, as the tool pushes back on the material. Otherwise, the end thrust keeps any wear concentric anyway. On the headstock, or drive end, the center rotates with the work, so there theoretically is no movement between the center and the material.

The whole concern is material squirming during roughing, which is more pronounced with some materials than others. Correct material choice will minimize that effect, but all material will move some. If there are critical journals, then if I were doing this between centers, I'd rough out one end, flip it, rough and finish the other end, except for any critical journals, leaving ~0.010 or so, then flip it again and finish the first end entirely and hopefully have planned well enough to go ahead and skim the critical areas of the other end without flipping it.
 
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