Turning a long, thin, straight bar

T

ttabbal

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I want to make a mandrel. I am leaning towards using aluminum, cheap, abundant, holds shape well. The goal is about 2ft long, though on the PM1127 that might be a bit optimistic. 1ft would likely be fine. It's only going to be about 0.55" diameter though. So I should be able to feed through the headstock, unless I need to run between centers for some reason.

I need it reasonably straight and free of taper. +/-0.005" is more than adequate.

I figure the first thing to do is ensure that the tailstock alignment is within that. I was thinking of using a ground bearing shaft to run an indicator along to dial that in. Perhaps with the 4 jaw if I need to adjust the chuck side.

The actual cut needs support. Is the follow rest the right tool here? I need to read up on setting it up.. :)
 
Yes, a follower rest may be necessary. That will be quite a worthy challenge. Consider a few approaches: perhaps half way from each end (flip it) and transition any bump. I think you'd still need the follower rest - even with a very keen HSS tool (and small radius tip). I suspect it will be near impossible to eliminate all the tool push. Still, a tolerance of 0.01" should be quite easy to hit. I'd give it a try without the follower rest - a keen pointy tool and slow feed (whatever you can think of to minimize the tool push).

If you have a proper follower rest - great. I've cobbled together supports out of bar stock clamped to the tool holder (over the top and over to the back side).

Let us know how you make out.
 
Should be able to cut that using the follow rest. All a follow rest does is prevent the work piece from deflecting back, away from the turning tool. It is usually used on slender or long work pieces that must be accurately sized along its length.

Not much to setting it up but the location of the support arms can vary with the tool post you're using. Bolt it to the cross slide and note where the arms are in relation to the tip of the tool. If the arms trail the tool as they do with most QCTP then you need to turn a short section of the work first, enough to get the arms in contact with the work piece. Just bring the arms into light contact with the work, lube lightly along the length of the work piece to reduce friction and start cutting. Make sure you readjust the arm positions after each cut.
 
It helps a lot to use a tool that cuts on the side, with the cutting edge 90 degrees to the center line of the work. Then all the push is toward the headstock, not sideways causing the work to flex. Make sure that the tool is SHARP(!) and has plenty of relief (edit: also a small nose radius) for effortless cuts. You will be amazed what that does for you. But the real technique here is using AXIAL pressure, and not RADIAL pressure. Joe Pi has a video on that............. well, actually two videos. See if you can keep your mouth closed during these videos... :eek 2:
 
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With that tolerance you can do your turning in stages close to your chuck. Blend as best you can. Once all your turning is done set t it up between centers and use a file then some emery cloth. You may not have a spider for your bar sticking out the back of the lathe. If not make a bushing for it. Delrin or aluminum. No need to be cute here. Drill the hole for your bar stock amd turn the o.d. For the spider bore. Doesn t have to be a super fit. Drill and tap two set screws in the bushing. Setup where your bar will be then set the bushing in place and move along until not needed. Be careful with filing and the emery cloth.
 
Thanks for the tips guys! I hadn't considered just doing it in sections close to the chuck.

I do have a follow rest that came with the lathe. I might try it just for experience. I've never used the follow or steady and it makes some sense to learn how to before I need tighter tolerance. As this is aluminum, my cutting tool of choice is a super sharp HSS.

Joe Pi does some amazing stuff. I've actually done the small diameter trick, mostly to see if I could. Made a nice 0.030" rod. I didn't think it would work on a long piece, but maybe it would.
 
I gave it a go. Didn't work out that well. I ended up with a bit too much taper and poor finish on the tailstock side. I decided to try to use the follow rest in lage part just to learn how to use it.

I suspect that the tailstock alignment drifted after using the test bar and moving it. I didn't consider that it might not be aligned after moving it. The taper is smaller toward the tailstock, so I suspect that it got small enough to lose support from the follow rest and the finish issues are from deflection. It looks like chatter, didn't sound like it though.

I discovered that the longest part I can turn this way is about 18". That's with the collet chuck just barely avoiding the rest and the carriage hitting the end of the travel. This is sufficient for what I want to make, but it's good to know for the future.

I also discovered that the lathe does have a clutch on the drive shaft. A bottle of tap magic got caught in the slot. Nice to know that it works, but I'm definitely not putting the can over there anymore. :)

I'll try it again after ensuring that the tailstock is aligned properly. Make sure I get the same diameter on both sides of the cut before starting should do it. Unless there's something else I should be checking?
 
Perhaps make a small test cut at the head and tail and adjust the tail stock based on those measurements. I've determined that my PM1340GT has a misaligned head stock (0.001" of diameter per 6" along the axis) which is very irritating considering the quality boast. I know how to adjust but really dread going though the process.
 
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