How to: non-concentric turning on lathe

drcoelho · Jan 16, 2019

Assume I have a manual lathe (Weiler Praktikant VC Plus, german toolroom lathe, 12.6" swing over bed, 11 HP) and I am trying to make the following out of a solid length of steel:

6" O.D. cylinder about 1" long, then reducing to a 2" O.D. cylinder about 4" long

The tricky part is the 2" cylinder portion is not concentric with the 6" portion, it is offset from center with one edge co-incident with the edge of the larger cylinder.

How to make this part? I presume turn the entire length of piece down to 6" O.D., then mount it off center and turn the 2" portion down to size? But I'm concerned this much non-symmetrical metal might be a problem for the lathe? Any suggestions? How best to hold the work piece?

cyanidekid · Jan 16, 2019

To be honest, I’m a little concerned too, you have an 11HP lathe and you don’t know how to use it yet. consider taking a class if one is available, or paying a knowledgeable competent individual to give you basic lessons, with an emphasis on saftey.

An 11HP lathe is nothing to mess with, it can kill you. (if I “assume” you have it, or even are just operating it)

We all started somewhere, but most of us did on something with a bit less power that that, and a belt that would slip if we got in trouble. ( Especually if you are working on your own)

Safe and happy turning!

number 2 · Jan 16, 2019

Might be able to hold it in a 4 jaw offset with 2 jaws not one, so it’s not holding at a weird angle. Machine will take it, go slow, too fast and machine will wobble if it’s not bolted. You can remedy that by bolting counter balance to the chuck to balance the thing. Bear in mind pretty much all the cutting will be intermittent, so lots of thump thump. You could rough it out in a mill with a rotary table or similar first? Or just rough it to a 2” square plus finish stock. Gets you 90% of the way. Can rough mill to an octagon if you can be bothered. Scribe a circle and go sorta close to it ?

johnoder · Jan 16, 2019

And COUNTER BALANCE as needed. Big 50 taper mill tool holder does the trick here.

It helps to not run fast in such set ups - common sense

gbent · Jan 16, 2019

I would rough turn the 6" diameter, and then put the appropriate center in the offset end. Then I would rough that out in a vertical band saw or mill first. Then put in offset chuck and turn to size. Always turn spindle by hand and move carriage to check clearance before powering.

That is a lot of offset. If the lathe wants to walk around the shop its running to fast. If you start to think you are getting hypnotized by the turning offset, stop! Come back another day.

gustafson · Jan 16, 2019

You are going to have trouble offsetting a 6 inch piece 2 inches in a chuck big enough to hold it. IOW how far will the jaws stick out a, Say 8 inch chuck holding a 6 inch part?

might need to be fixtured on a faceplate

gustafson · Jan 16, 2019

and you will be turning reeeeeaaaally slow

if the lathe wanted to move, way too fast

APD · Jan 16, 2019

11 HP on a 12" lathe?

isaac338 · Jan 16, 2019

Make two cylinders and bolt them together. Machine a pocket for the 2" cylinder in the 6" cylinder if you want to position it better.

HuFlungDung · Jan 16, 2019

It is only a 1" offset. I'd just set it up and have at it. If it looks too extreme to offset it by shifting one jaw out, and the opposite one in, then split the offset midway between two jaws and adjust jaws in pairs. Use a flat end pin in the tailstock chuck to push against the part to keep it from falling out of the chuck while making the adjustments.

I'd definitely want the part against the face of the chuck, or its jaws, as the pounding of the interrupted cut may cause the part to move fairly easily. A counterbalance weight is an excellent idea, although you won't be running at too high of an rpm with something that large.

I prefer a large depth of cut and a fine feed on interrupted cuts like this. Use a large lead angle (tool tip angled towards tailstock) for the tool to make the chip thinner.

You might not have an indicator with 2" of travel to set the offset. But you can use the indicator in combination with some measured movement of the cross slide.

eKretz · Jan 16, 2019

That's a 2" offset actually (3" radius - 1" radius = 2"). I agree though, easy enough to just offset in the chuck. I'd leave the stock a little long and put dual centers in the end, one for each centerline. Then face the extra off at the end of the job. But then I like to take heavy cuts. If you want to be a little more careful the chuck should hold it just fine if you have it well secured - it's not a very long piece.

gustafson · Jan 16, 2019

HuFlungDung said:
It is only a 1" offset. I'd just set it up and have at it.

check yer math

6inch coincident with 2 inch, would be 2 inch offset, no?

there would also be no material to center it on, as the entire 2 inch diameter would be outside the center.

Rob F. · Jan 16, 2019

It sounds like an eccentric pipe reducer.
If not an off the shelf product weld it.

fciron · Jan 16, 2019

The offset portion could be turned between centers if your chuck won't handle the set over. (The "centers"would not be in the center of the 6" round.) The part would then be held at both ends, which is nice. The driving plate provides a convenient place to attach a counterweight.

eKretz · Jan 16, 2019

gustafson said:
there would also be no material to center it on, as the entire 2 inch diameter would be outside the center.

That's why I mentioned leaving some extra material. Put the centers in first, and leave the extra length at a large enough diameter so the center for the 6" portion won't get wiped out.

Alternatively, if the centers can be left in without causing any issue, just put the centers in the 6" diameter end. Turn the 2" diameter first, turn it around and chuck the finished 2", then turn the 6" diameter.

Matt_Maguire · Jan 16, 2019

eKretz beat me to it → If the diameter edges coincide then it’s a 2” offset (3”R – 1”R = 2”). I used to get stuck with this crap in the early 80’s… For this one I’d be 1” longer & use the waste end for driving. YES you’ll need some junk for balancing, and (trust me) take the time to mount the faceplate. For turning you cut the 2”dia length 1/4-1/2” longer & saw off the waste when done. If you have the stones you can mostly part it at the waste end. I always just blocked things up, sawed the end off & faced the part in a mill (hollow chamfer it also).

This part is short enough I’d do the centers on a small turret or knee mill. Drill centers on one end, plus drill & ream for dowel pins (good to have a nice V-block for this). Then flip & place the pins in the table slot to keep the angle of rotation right, clamp it & re-center the spindle over the end. On the waste end I’d either drill a shallow hole or mill a slot to drive it part with. (see comments bottom)

(comments) (1) The 6”dia face when place on a mill table will tell you (a lot) about how square your cross slide is to the ways… Be prepared to set on 3 pieces of paper if it wobbles on the mill table. (2) I tried to do this with 1 end chucked more than once, I quit it.

The double cutoff crank below had a 7” throw IIRC (centered both ends on the HBM) & the next one made on a 40” x 20’ american with face plate worked WAY better. I don’t miss them...

Good luck,
Matt

MattiJ · Jan 16, 2019

Also pay attention that the balancing is going to change a lot during the machining. When finished its almost balanced without counterweight compared to beginning.
Assuming your lathe came with independently adjustable 8-inch 4-jaw that might work but you need to get creative with the jaws. And go really slow if you have 30lbs of eccentric steel lump mounted with only 1/2" depth of grip in 4-jaw. Should be obvious but also note that counterweight mounted to chuck doesn't help for the tendency of the workpiece to fly away and split your skull.

HuFlungDung · Jan 16, 2019

gustafson said:
check yer math

6inch coincident with 2 inch, would be 2 inch offset, no?

there would also be no material to center it on, as the entire 2 inch diameter would be outside the center.

Yup, you're right, I was thinking 6" dia down to 4" diameter.

2" actual offset is a lot, 2 of the jaws won't be grabbing properly unless you affix the slug to a much larger diameter carrier if you go round, but a rectangular carrier would likely be better. I might tack weld it to the carrier if there is some extra length to be had so that the HAZ can be faced away. The one end of the carrier might also be convenient for affixing the counterweight to.

drcoelho · Jan 16, 2019

Terrifically useful set of suggestions, thanks!

garyhlucas · Jan 16, 2019

I am not much of a lathe guy. How about offsetting it a small amount to start, say 1/2” and remove that. Then offset again 1/2” and so on keeping the off center mass way down. Maybe use a live center in a flat cap just to make the piece isn’t coming out of the chuck.

How to: non-concentric turning on lathe

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