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First post after one week with a metal lathe; looking for some guidance.

prinzen

Plastic
Joined
Apr 28, 2019
Hello All

I am primarily a hobby woodworker, Engineer by education (but certainly not a skilled machinist), and decided I would try my hand at metal machining, specifically lathework. I decided on a Sieg SC2, and got my self some 6XXX Aluminum, 1 " diameter and some HSS pre ground cutting tools. My first few attempts at turning down the diameter went pretty well, although I could not get any better than 0.007" runout on the workpiece in my 3 jaw chuck.

I decided to remove the cross slide, clean and oil up the ways. Back to cutting, and things appeared even worse. I had a poor surface finish on a section (for about 0.5" length), and as I got closer down the workpiece length towards the headstock, the finish got considerably better.
Then, I changed to a parting tool and attempted to part off. Disaster; about 1/8" into the diameter, the workpience cocked out of the chuck jaws and the lathe stalled, while making some strange noises. So, I reset the workpiece, and luckily the motor started back up again with no apparent problems.

For the second time, I tried to finish off the parting, but again, the workpiece was dislodged from the jaws with some nasty grinding noises and the motor stopped. I thought better than try this again, and ask the experts. From a good start earlier in the week, I have lost a lot of confidence, so hopefully somebody here might be able to help with these questions:

1. After the stalling/ work piece dislodgement; would there be any damage done to my lathe (there is no obvious damage to me, and motor still works)
2. It looks like I have a runout problem, will a 4 jaw independent chuck help?
3. How can I try parting off without stalling the motor and damaging the chuck?
4. Any other advice for me?

Thanks
P
 
Bottom line: that's a very light hobby lathe. I doubt if you will ever be able to part off with it--just not rigid enough.
As to finish you've got to have your tool geometry right and the workpiece must be well supported. If you have more
than 1-1/2"-2" of stickout then you need to use a centre in the tailstock...
 
3 jaw chucks always have some runout, how much depends on mfr and how used it is. As stated above, you need the end supported to work any distance away from chuck. Not sure why so many people decide to "teach themselves" how to run a lathe, would you consider teaching yourself to fly? Either take a course if you can find one, or find a machinist willing to teach you.

Yes a 4 jaw independent chuck you can get .001 or less runout, but not a 4 jaw scroll.
 
Thanks Keith. Noted, I'll stick with the hacksaw for parting. Any thoughts on the damage that I might have caused?
 
Hey P Rinzen
First, don’t be discouraged- the initial curve is pretty steep, but with advice and a few adjustments you will have a lot more success.
Keith had a good point about the size of your machine but I don’t agree that you can’t part off. Sherline machines are the same size and you can part off with them.
I WILL say that you shouldn’t do any more parting off until you get basic turning down. Not perfect, but smooth and uneventful.
Also, you have to be sure you have a VERY thin parting blade (although too thin at it will get squirrely) for your machine to manage a cut like parting-off with much imparts greater forces than basic turning.
When you say “I can’t get much better than .007 runout”, what do you mean? So you mean after your turning cut you still have .007 runout? There should really be no runout AFTER your cut. If you have something loose, or you are hanging the part out too far out of the chuck, that may happen.

Bernie


Sent from my iPhone using Tapatalk
 
As regards damage, Are you concerned about your run out came from the damage? Or did you have that run out with you before the parted off? It’s hard to say, depends on how fast your chuck was spinning at the time and how quickly and abruptly it came to a stop.
You have a lot of factors to get adjusted first, what are you measuring runout with? You could try to measure up to the back plate of the chuck, if not, the front of it? The problem with asian machines isn’t that you can’t do anything good with them, it’s that until you get used to it and get it tuned in, you really cannot make assumptions that certain things “should be fine because it’s new” etc. I heard the Seig machines Or higher quality than others in that category, so I would think you should be able to measure the run out from the outside of the chuck or the back plate and see how it goes. Then put WORK pieces of different diameters into the chuck and measure the runout. Then release them and grab them more than once. When you grab the part, you need to wiggle the part gently while tightening the chuck around it until it grabs solidly.
Just describe what you mean by measuring the runout

Bernie


Sent from my iPhone using Tapatalk
 
Don't use a 3 jaw chuck. Learn to dial in a 4 jaw. Parting off using a 3 jaw is prone to problems such as you encountered and it is easy to damage your chuck and maybe other parts when the work shifts. A 4 jaw is much less prone to this type of problem. 3 Jaws always have some run out, a 4 jaw can be dialed in as close as you like. If you use the tailstock to support work during parting, make sure that you back the centre out before you part all the way through, otherwise you can have a really nasty crash.
 
Is the tip of the cutting tool At the same height as the tip of the centre in the tailstock. Could explain poor finish.
 
Thanks all. When I say run-out, I mean that round aluminum stock wobbles visibly. When I setup a DTI, it measures up to 10 thou TIR. This happened after I turned down the piece by a few thou and removed the piece from the Chuck and then re chucked it.
 
If you must remove work from chuck and then put it back be sure to mark how it was in the chuck so it can go back in the same way.
In your case it is possible the bar had .005 runout before you turned it, if you put it back in 180º to where it was it will have .005 in the opposite direction hence your .010 runout on the turned part.
 
...This happened after I turned down the piece by a few thou and removed the piece from the Chuck and then re chucked it...

If you turning it and rechucking it and getting that kind of runout then your chuck is pretty crappy. It's possible that
one of your parting off incidents spread the jaws enough to cause the problem--could also be just a bad chuck. I'd
take a piece of ground shafting, chuck it up and run a dial indicator on it. That will give you an idea of how much runout
the chuck has without factoring in any machining. As others have said, try a 4-jaw; it's slower but with practice you can
get things dead nuts...
 
3 jaw chucks always have some runout, how much depends on mfr and how used it is. As stated above, you need the end supported to work any distance away from chuck. Not sure why so many people decide to "teach themselves" how to run a lathe, would you consider teaching yourself to fly? Either take a course if you can find one, or find a machinist willing to teach you.

Yes a 4 jaw independent chuck you can get .001 or less runout, but not a 4 jaw scroll.

Why teach yourself? In this neck of the woods, the nearest place to take a class was over 30 minutes away, and they were only available during the day when I had to work. I know a couple of machinists, but they're busy guys outside work. So I read a lot and played with a succession of lathes until things started to make sense and pieces started to look like what I wanted them to look like and wind up the sizes they needed to be. SB's "How to run a lathe" and other SB publications were a good start...my grandfather's book on running a W&S turret lathe was very good, and lots of posts written in this forum by pros have been extremely helpful as well.

Now I have enough experience that I doubt I'll take a class.
 
Parting off a piece is one of the more difficult processes in a lathe, particularly a light one such as you have. Near absolute rigidity is required. The cutoff blade is hanging out in front of the support. The chip curls as its cut and tries to expand in width. That causes binding and galling with the side walls. Lack of lubrication will allow the chip to weld to the side walls, putting high bending stresses on the tool that can stall the lathe, break the tool bit, knock the work out of the chuck or all of the above. The closer to the center the tool gets, the slower the work is turning (ft/sec). With the tool advancing at a constant feed rate, the chip thickness will increase exacerbating the tendency to weld and break the tool.

At best you will get a crummy cut surface.

My experience is high speed, careful grind and alignment of the tool and lots of coolant/cutting fluid done in a rigid machine is key to success.

Tom
 
Post # 1 said the OP is using pre-ground HS tools. That means they will be ground for general purpose turning, specifically for turning steel. Brass and aluminum want to have different angles on the tools than steel or each other.

That is why the first thing done in a lathe class, before the lathe is switched on, is to teach how to grind a tool bit, usually for steel. First come the various relief angles (side, end, top and back), then the modifications like hand stoning the edges and putting a radius on the tip. Later, the class will get into what angles work best for other metals. The beginner books will also go through those points. Obviously, you need a grinder at the same time as getting a metal lathe. You buy blank tool bits and grind the ends for each different material and turning job.

Someone said to use the tailstock for support. I want to warn that that advice applies to turning, not parting off, facing, drilling or boring.

I have seen a lot of lathes that have hacksaw marks in the tops of the bed ways and top slides. Keep the carriage away from the saw. It is a simple thing to make a wood cover for the ways that will protect them against a slip of the saw when the part is cut through.

The first machine repair lesson I taught myself was how to replace the spindle on the cheap little metal lathe that my father bought for me in 1954. It was a very thin spindle with a 0 Morse taper and a 1/2"-20 thread for the chuck. The end of the spindle bent when I tried to take too big a bite from a piece of steel hanging out of the 4-jaw.

By the way, you can use some (not the gouges and skews) of your HS and carbide wood turning tools for turning metal by hand. It is the easiest way to make curved surfaces and is a quick way to put a little chamfer of a corner.

And the cutting fluid for aluminum is different from what you use for steel, so you need two different products. Brass bar stock for turning does not need a cutting fluid.

Larry
 
Why teach yourself? In this neck of the woods, the nearest place to take a class was over 30 minutes away, and they were only available during the day when I had to work. I know a couple of machinists, but they're busy guys outside work. So I read a lot and played with a succession of lathes until things started to make sense and pieces started to look like what I wanted them to look like and wind up the sizes they needed to be. SB's "How to run a lathe" and other SB publications were a good start...my grandfather's book on running a W&S turret lathe was very good, and lots of posts written in this forum by pros have been extremely helpful as well.

Now I have enough experience that I doubt I'll take a class.

Growing up dad had a plane and taught me to fly, by the point I wanted a license I had read all the books and knew what to do, but none of that meant squat, I still had to pay for lessons and take tests. As for school on learning to run a lathe, it was 30 mile drive from work, then 60 to get back home. And to pass that class there were tests, and you had to turn multiple thread patterns all on 1 piece of stock.

Most of the self taught lathe operators I've seen have some bad habits, and can't grind a HSS tool bit for sh*t:D
 
Why do the smart ones always seem to have the scary stories?

Op is lucky that he has a toy machine and not one capable of doing some damage if the part comes out.

Next time a part comes out of the machine examine why it happened and not shrug and try again :D
 








 
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