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tlfamm
03-08-2011, 09:11 AM
Poster Westline has some images showing a fairly substantial (?) off-center rotating mass mounted in a lathe chuck:

http://bbs.homeshopmachinist.net/showthread.php?t=39202&page=112

That doesn't seem to have been a problem for him - but what are some rough guidelines of speed and mass in off-center turning (beyond: keep it small; keep it slow)?


I've no experience to draw on, and would prefer not to learn the hard way :)

squirrel
03-08-2011, 09:43 AM
DO NOT duplicate that set up, its a widow maker.

If you need to turn off center use a 4 (independent) jaw chuck or a removeable jaw chuck with custom cut jaws for the part and very low RPM.

Westline
03-08-2011, 09:45 AM
Poster Westline has some images showing a fairly substantial (?) off-center rotating mass mounted in a lathe chuck:

http://bbs.homeshopmachinist.net/showthread.php?t=39202&page=112

That doesn't seem to have been a problem for him - but what are some rough guidelines of speed and mass in off-center turning (beyond: keep it small; keep it slow)?


I've no experience to draw on, and would prefer not to learn the hard way :)
I was a bit scared to do this but it worked fine.
Just to give you a idea of the size and speed the material was a 50mm long piece of 80mm mild steel round bar.
I tested on 125rpm . The of set was 15mm long brass round bar.
The hole I bored was 16mm ID.
Keep in mind, be very careful when you do this I had images of the thing popping out and crushing my foot running throught my head the whole time while doing this.

Richard Wilson
03-08-2011, 10:27 AM
Poster Westline has some images showing a fairly substantial (?) off-center rotating mass mounted in a lathe chuck:

http://bbs.homeshopmachinist.net/showthread.php?t=39202&page=112

That doesn't seem to have been a problem for him - but what are some rough guidelines of speed and mass in off-center turning (beyond: keep it small; keep it slow)?


I've no experience to draw on, and would prefer not to learn the hard way :)
I'm not sure there are any fixed guidelines, if there are I've never come across them. The larger and more rigid the lathe, the less of a problem its going to be. If you put the lathe out of gear, spin the spindle several times and if the eccentric mass always ends up at the bottom, there will probably be a problem. Start up at lowest speed and gradually speed up till you can feel the vibration. If this is too slow for turning, you need to put some balance weights in place. Its tricky with a chuck, theres nothing to attach it to, though you could try a big blob of plastecene or putty,or a lump of lead pipe on the edge of the chuck, held on with a good wrapping of duct tape. Just make sure you've tightened the chuck jaws finally before you put the duct tape on. Its easier with a faceplate, I just bolt a selection of change gears on till things balance up. Its very much a trial and error basis, well it is for me. Doubtless there are those who can (and will) give you the full mathematical analysis.

Richard

TGTool
03-08-2011, 10:51 AM
I'm not clear that 3 or 4 jaw makes any significant difference for safety. With my 4-jaw I can slide a T-nut behind a jaw that's far to center and that makes it somewhat more convenient for bolting a counterweight.

I've seen people make up a bearing set and stub spindle that they can mount the chuck to off the lathe and play with balancing. It seemed like an excellent idea but I haven't actually done it yet myself. Since it's only for balance and sees no significant load the bearings themselves don't have to be large and expensive.

JCHannum
03-08-2011, 11:04 AM
I have bolted weights to the faceplate from time to time when turning a heavy offset part. In the four jaw, it seems less problematic. It depends on your comfort factor. Spin it up and see how it behaves.

While I won't necessarily condemn Westline's set up, it can be a potential hazard if not carefully set up. If turning an eccentric with only a three jaw chuck available, some other approaches can be taken to avoid the long spacer. Use two outside jaws and one inside jaw, or remove the jaws and wind one in a couple of turns in first and then install the other two. If you play around a bit, you can find a combination that will avoid use of that scary looking long spacer.

Carld
03-08-2011, 12:58 PM
I certainly would NOT do that in a 3 jaw chuck. If you do an offset such as that use a 4 jaw chuck, it's much safer. Having that piece of brass in there with the chance of things shifting and the brass flying out is not a good idea.

It's hard to balance a setup like that, just run it at the speed that feels safe and start slow and work your way up.

Westline
03-09-2011, 02:56 PM
I certainly would NOT do that in a 3 jaw chuck. If you do an offset such as that use a 4 jaw chuck, it's much safer. Having that piece of brass in there with the chance of things shifting and the brass flying out is not a good idea.

It's hard to balance a setup like that, just run it at the speed that feels safe and start slow and work your way up.
I posted this in a extreme case of " kid's don't try this at home" :)
It's was more a case of what can be done but shouldn't.
If I stop posting It just means Darwin was right and my tools took me out.:D

Carld
03-09-2011, 04:05 PM
:D well, I'm glad that was a demonstration of what not to do. To be honest I set up like that once and when I looked at it and thought about what may happen I switched to a four jaw. It probably saved me from getting hurt.

Lew Hartswick
03-09-2011, 05:59 PM
I certainly wouldn't offset anything THAT FAR in a three jaw. I have
done an offset of about 0.030 or a bit more making a cam for the
tool holder I built. That was a 0.500 round and I wanted a
pull distance of about 0.030 to 0.035 so it was not a big deal.
I even used a tailstock centre to steady the part.
BUT when the offest gets as big as that shown, NO WAY am I going
to do it in a 3 jaw. :-)
...lew...

John Stevenson
03-09-2011, 06:54 PM
Dis anyone read the part where he said he didn't have a 4 jaw chuck ?

Anyway South Africa is a long way away, should have stopped bouncing way before it reaches here, let alone the US. :D

Carld
03-09-2011, 08:30 PM
No I didn't see that. I would feel better if the spacer was welded to the piece in the 3 jaw and even that is chancy. He could weld a stub on the end of the chunk that is offset the distance he needs. There's a few ways to make it work.

fciron
03-09-2011, 09:17 PM
He did make it work and lived to tell the tale. :rolleyes:

Carld
03-10-2011, 12:01 AM
That may be true but why temp fate. To continue doing something that is dangerous will get Murphy's attention.

I learned long ago it's best to not do something that looks dangerous.

I don't use the, "Watch this" red neck approach anymore.

winchman
03-10-2011, 02:16 AM
You can do off-center turning in a 3-jaw by starting one or two of the jaws early as you put them in the chuck. Just turn the chuck key enough to advance the scroll the desired number of full turns before inserting the other jaw(s). That avoids having to use spacers.

You can also use a combination of inside jaws and outside jaws for holding irregular shapes and doing off-center turning.

It works, but a 4-jaw is better if it's available.

Westline
03-10-2011, 03:43 AM
You can also use a combination of inside jaws and outside jaws for holding irregular shapes and doing off-center turning.

Thanks did not think of that, pretty clever.:)

John Stevenson
03-10-2011, 03:55 AM
Anyone remember this thread ?

http://bbs.homeshopmachinist.net/showthread.php?t=25386&highlight=eccentric

I didn't get as much flack over this and it wasn't that different. :confused:

Westline
03-10-2011, 04:57 AM
John
I just checked your thread ... really nice job.
That one will go into my bag of tricks next time I need to machine a strange part.
I know the whole issue with my setup was safety and honestly I knew I was going to catch some flack and I'm fine with that.
Getting back to safety here is a interesting video that will let the people that hide behind "safety first" think a bit.
http://www.youtube.com/watch?v=IRVdiHu1VCc

fciron
03-10-2011, 08:12 AM
No I didn't see that. I would feel better if the spacer was welded to the piece in the 3 jaw and even that is chancy. He could weld a stub on the end of the chunk that is offset the distance he needs. There's a few ways to make it work.

Italics mine. I think what you meant was "There's a few safer ways to do it."

It's not a set-up I'm going to replicate, nor would I recommend it to anyone else, But I, too, wonder why this is getting so much flack. Any large offset mass is going to require the lathe to be slowed down and the cuts to be lighter. Packing pieces are used in all kinds of applications, both packing pieces and shims are going to reduce the grip between parts to some degree.

Carld
03-10-2011, 08:31 AM
Why is it getting more flack, I don't know. The reason John's article didn't get much flack is he used a four jaw chuck. Any time you put something larger than a piece of shim to protect the work surface you have a chance of slinging it out and getting hurt. With the piece off center as much as it was in the 3 jaw it would have to be run slow so the chance of it flying very far is slim.

Yes, I meant to say safer.

The reason I have said anything at all is because many of the members here are very new to machining and if they see something like that setup they may think it's ok and try it. Since they don't have a clue about the danger of it getting slung out of the chuck they may start the lathe at 300 rpm or more and the lever effect of the off center weight would/could sling the chunk out with a vengeance in the direction of the operator.

I have seen alleged machinists at places I worked do things just as dangerous more than once. You would think after the first failure they wouldn't do it again.

Not intending to offend anyone, just pointing out any dangers.

tlfamm
03-10-2011, 10:04 AM
"many of the members here are very new to machining"


exactly the motivation for the OP.



I reread the entire thread, and while there are definite opinions expressed, I don't see anything I would characterize as flack - to include a complete absence of personal malice, and insults against the sainted name of Bridgeport ... :)