Help w/ shaft too long to turn btw centers

This particular project is just a wooden dowel for a non-critical application. If it looks straight and round to the naked eye, it will be good enough. But I am treating it as a learning experience, and wondering how I would do this if it were a shaft or other application where accuracy mattered.

I need to turn a 37" long dowel to round from square stock. It will fit through the headstock of my SB 10L, but I can only turn about 28" between centers. Any particular tips or advice? Any potential pitfalls if I grab it midpoint and indicate in my 4-jaw, and catch the end with a live center, then reverse to do the other half?

Thanks,

Jeff

I saw my father do a task similar to this.
He needed to "clean up" the tail of an inboard-engine cabin cruiser's propellor shaft.
This was longer than any lathe we had access to at that time.
He lined up two lathes! He set the end to be cut on the headstock of his metal lathe.
The far end was supported by the tailstock of his WOOD lathe, which had its headstock removed for the occasion.

He had done a line-up with a taut wire. This is something you should look up, but it basically involved a single-strand steel wire which he painstaking made parallel to the metal lathe's headstock spindle. Then he made the wood lathe's tailstock spindle parallel to the wire.

This is one of those jobs that takes longer to line up than it does to actually do the job.

There are several ways to make extemporaneous wood lathes - use you head. Remember that the WAYS do not have to be metal! For this, you can use hardwood.
I've seen erstaz lathes made using an electric drill motor as a headstock and the "Tailstock" was a block of wood with a big nail in it. (I think that would actually be correctly called a Poppet.) Use your head! Post photos of how you get it done.

JRR

dont grab on with the whole jaw length, bore a step in the middle of your jaws
ahout .125 wide and your center should line it up pretty good as long as your cd are in line. If you grab with the whole jaw you will machine a bow in it.

Take off the tailstock and use a fixed steady with rollers or some sort of lube might work. I have seen this in a number of lathe books. If you cannot make it round at the point where the steady is then you use a tubular collar with 4 or 8 screws to center it on the outside of the square and then run the outside of the tube in the steady.

Chris P

With due respect toward the earlier posters, you're all wrong.
Often times the simple answer is best.....you NEED to buy a bigger
lathe. We know you have wanted one, well now you actually need one.
Don't miss the chance to fill this gaping hole in the tool collection.
Dave P.

SouthBendModel34 - Problem is, I need to machine the entire length, not just a section near the chuck.

Jrod - Yep. Learned that the hard way on my first try. I got a bow of about 1/8" over the last 15". Why is that? Rather than bore a step in the jaws, I used a couple of wire inserts to achieve the same thing, which seems to have worked.

bcstractor - I hadn't thought about using a collar and steady.

Good stuff. Any other ideas?

Wood does not behave like metal. It is more flexible, has grain, more internal stresses, and expands with increasing humidity.

You have a long slender piece

If you want it straight, you will need to turn part of it off- THEN LET IT SIT FOR SEVERAL DAYS- to warp and allow for changes in humidity. Then turn off a bit more- and again let it sit.

As for as the lathe part- put centers in both ends, and O ring in the middle, and the tailstock out as far as you can get it. Use the 4 jaw on the o ring and center the stock. The O ring helps with the springing. Turn off the corners-but undertand that it will flex so much in the middle that if it catches instead of cuts it may twist and break. You will need a very sharp bit. It will turn much better near the chuck or the center due to the support. You will then reverse it and do the same thing- but with the O ring on the turned part- and having as much length as possible over the bed- even though you are just turning down the last foot or so. ( this will hep keep the entire length straight) Then set aside as described above.

When you get close to the final size- if at ALL possible, set up a traveling steady with a "hole diameter the final size. Run this right behind the portion you have just cut. This will allow you to cut the unsupported center section to the final size- because the traveling steady will be supporting it when the cutter gets to that center section.

True about the wood warping. These pieces are actually laminated from thinner sections, so the effect is minimized, but still a consideration.

Ahh. I like the o-ring idea. Displaying my ignorance here, but what is a "travelling steady"? Is that different from a "follower rest"?

I did this some years ago to turn a few 38" long replacement bannister uprights in a metal lathe. I drilled and tapped the end of my lathes bed to carry a bracket which had a roller bearing in it which a metal shaft ran. The shaft was welded to a small round disc with 4 small holes in it, through which it was nailed into the end of the wood. The square piece of wood was held by a 4 jaw chuck. The wood flexed a lot and I needed roller steadies running on a taped section of the wood to get a good cut and finish.The pulleys from/to the motor were reversed, boy did it spin and kick up a racket! Job was a good'en. When I came to install these new uprights, I realised that the others (60?) were basically turned by eye as the pattern was the same but all the dimensions were +- 10% (or more)
Frank

A travelling steady is the same as a follower rest. The idea is to provide support where it will do the most good.

In normal operation the workpeice to traveling downward at the front -where it is contacting the cutting edge. The force of the contact pushes down on the bit. the workpeice will be forced back and up. It will be trying to roll over the top of the toolbit. Becasue of this, the traveling steady will usually have two flat adjustable pads- one at the back, and one at the top. To counteract the force- they need to be in contact with the work- which means they need a surface they can run on- meaning both smooth and of consistent diameter. The diameter is important because the work is trapped between the back pad and the cutting bit. If the back hits a bump- the peice is forced into the work and dip is cut out. If you have a consistent diameter, you -could- run the steady in front of the cutting blade. Usually the steady pad is just behind the cutting bit- so it is running on the "just cut round" surface. In your situation- you will not be starting with a round cross section. You can still use a steady- but instead of pads at the back and top- you need a cylinder for the rounded corners to run on. This also means a cylinder for each diameter cut- and a way to hold it fixed in place- which is why most steadies have the adjusing pads.

You will want the steady behind the cutting bit- and to start at the tailstock end for another important reason. Having the steady in front of the bit will allow you to reduce the diameter- but leave in any curves or warp, as the work is forced to run true right in front of the bit.

Such a thing as too long

Flipping the piece to get both ends is a common practice. The problem is the to cuts never meet perfect; leaving a little ridge. You'll have more error if you chuck the middles like that though it can be done. The usual way is to support the middle in the steady and the inboard end on a center with the faceplate. You start with the chuck and turn three short sections, all of equal diameter. One at each end and one in the middle. This gives a bearing surface for the steady and provides a size reference when it comes time to reverse the bar. Turn one end down to the sizing cuts then reverse the bar and do the same to the other end.

Without the tailstock you need to tie the driving dog to the faceplate with sufficient tension to ensure the bar is riding tight on the center.

Seeing as how your workpiece is wood I would just chuck it up and go over the whole piece with sandpaper clamped in split bush. You'll be able to feel where the piece is oversize and with plenty of spindle speed and the right grit you'll get the piece down to size in no time and it'll be smooth to boot.

Just another thought here:

The dowel that I want to turn reaches into, but not all the way through, my 10L spindle. If I position my tailstock as far out as possible, with a live center, the other end of the dowel is about 1" from the far end of my headstock.

What if I made a dead center to fit inside the end of my headstock (like a modified spider), and drove the dowel with a faceplate and dog? This way, I would be turning between centers the way shafts like to be turned.

It did occur to me that the lathe dog, not being a precision balanced piece of equipment, might cause a wobbling imbalance in the shaft, since it is farther from the dead center than normal. I think that could be reduced by making a round (symetrical) dog from wood or plastic, that would be lightweight, no larger than necessary, and closer to balanced.

On the surface, this seems like a sound approach. Have I overlooked something dumb?

Firstly, When you say that shafts 'like to be turned between centres', I think that's arguable.

Not just because they're not very talkative about their emotions :-) but there are provisos and caveats

The good things about turning between centres are: no chuck marks, and you can take the job out, do another process (eg milling, heat treatment) and pop it back in and it will be running true to a very high degree of accuracy.

In theory, you've come up with a really good idea, because the other good thing about BC turning is the ability to end-for-end the job and still have it true. If you did make a centre for the outboard end of the spindle, you could simply make it pyramidal rather than conical so it drives the work.

But now we come to the bad thing about BC turning:

In engineering terms, a chuck is like a 'built in' end (imagine a beam cantilevered out of a riverbank). A centre is like a 'simply supported' end (ie a beam sitting on a pivot on the riverbank)

Deflection is all-important in turning, and trumps many other considerations.

A bridge with pivots at both ends could stand a long span, but only if the girders are really deep and stiff.

What you've done with this latest idea in comparison with your proposal in the original post is make a skinny bridge much longer, and substituted a simple support for a built-in end as well. *

The deflections arising from cutting are going to be much increased, and the shaft will be fish-bellied.

If I was doing what you propose, except in metal, I think I'd probably turn half the shaft per your OP, then the other half hanging over the end of the bed, (remove tailstock) with a fixed steady running on the turned section right alongside the square, and the inboard end held in a 4 jaw which would also allow dialing in the extreme outboard end of the turned section.

This would be a relatively easy way to ensure the two turned sections ended up strictly coaxial. A fixed steady always makes a shaft run true to the section you're gripping, but it's up to you to adjust it so the centre of the shaft at that point coincides with the turning axis of the lathe, so you don't turn a taper.

This method would work in wood. However you'd probably have to make a cathead for the steady to run on.

The level of accuracy this procedure is designed to permit is not achievable or necessary in wood, but as you imply it could make a useful 'dry run' for metal. Otherwise I see nothing wrong with the entire sequence you lay out in your original post.

Spin the work as fast as your lathe will go and use a really sharp tool steel tool with plenty of top rake, but not so much that it will 'hog' and pull the work towards the tool rather than pushing away. The ideal is to hit the neutral point.


*Incidentally I personally think you've hit on a novel and sensible idea for the right application -- but that right application would perhaps be a lathe with a large spindle hole but very short bed - not often seen.

Thanks Troup for your detailed response. I think the idea of using a pyrimidal center inside the headstock, instead of a conical one, is perfect. That would allow me drive without any type of lathe dog that might cause imbalance.

Your comments about deflection between simple supports (as opposed to one end cantilevered) are duely noted. As I recall, it's not linear either, but a squared or cubic function. Definately calls for multiple passes and light cuts with sharp tooling as I close in on the final diameter.

I'll let you know how it works out!