cruddy finish makes for cruddy attitude

[zeeprogrammer]

I've been learning/practicing with my new lathe and I'm struggling to get a decent finish.
More fiddling and practice will no doubt improve things...but still...

I'm turning aluminum with a insert cutter. I've tried different inserts as well as a couple of ground HSS.

For the most part, what I'm getting is a spiral cut under power feed.
Manual feed was not much better...spirals were simply more variable.

I've tried moving the cutter slightly lower than center, various speeds, wet and dry, making sure all parts are locked down (gibs, etc.)
I've measured the lathe, and while it's not level...I detected no twist to speak of.
The lathe seems really solid to me. Even with untightened gibs, I can detect no movement when I push/lift on the carriage, cross slide etc.
Not like my mini that seemed to want to cross the county line.

I finally got a somewhat decent finish on manual feed but still not what I remember achieving on my mini. Not good enough in my mind.
The problem with manual feed is that as soon as I have to move my hand in order to continue turning...I get a machine mark (and some squealing...if that's a clue).

But power feed still gives me a spiral finish.

I'm aware that too fast a feed rate can result in this. The ratio of spindle speed to feed rate has to be 'high'. Too low and the cutter moves across the material too fast.

This is the PM1030 lathe. It has two feed selection knobs. One is for direction and the other...well the other has been a struggle learning too.
It is marked A B C. When the lathe is off, I sometimes cannot turn that knob. The lathe has to be on (making sure it is running slow).
I had expected 3 settings but I find that between A and B the lead screw does not turn. Same between B and C.
What I find odd though is that A appears slow, B is faster, yet C seems slow as well. I haven't measured, but it seems the same as A.
I've tried all 3.

I would expect the fast speed would probably result in spiral cuts. But the slower speeds also result in spiral (just smaller).

I talked to a fellow at the show about this and one suggestion is changing the gears in the gear box.
I haven't tried that yet. Seems odd I'd have this problem with a new lathe out of the box.

If/when I do go that route, the questions will be 'how' and 'to what'?

Any thoughts my friends?

In the meantime, I'll try some other material and keep trying different spindle speeds as well.

Currently at the 'bummed to almost frustrated' level. Got a few levels to go before I lose it entirely. 

[wagnmkr]

Sorry you are having problems Zee.

For the aluminum, what speed are you turning at and what diameter stock?

That lathe has a quick change tool post ... silly question ... is the tool holder locked in tight, and is the post locked down good? You mentioned that you got a squeal and a mark if you stopped feeding by hand. That can mean flex in the tool post/cutter. I would not expect that to happen on that lathe.
The spiral mark shows the lathe thinks it is threading something ... is there a separate knob to engage threading versus straight feed?

Have you contacted the seller? That would be my main suggestion.

Tom

[Jasonb]

Question is Zee what gear train have you got set up on the banjo? as this will affect the input speed into the feed gearbox.

Also you should be using the smaller lever in the middle of the apron to engage feed, if you use the big one on the right you will be cutting a very fine thread which could explain your spiral

You should have a speed table on a plate on the front cover of the machine and that should also be in the manual, set the gear train for the finest range and then use the ABC lever to give what you need for the job in hand. That is all I have ever needed on my slightly larger 1128

C is the finest feed, A medium and B fastest

[Dave Otto]

I was wondering about the half nut vs the feed lever also?

Zee it would really be helpful to see some pictures of the tools that you are running, If the insert tools that you are using happen to be negative rake you are fighting a loosing battle on that small machine.
It sounds to me like your tool may not be sharp and also has too much nose radius; which is why it digs in a sequels when you stop feeding. Could you post a link to what you are using?

The technique that I use when hand feeding to use both hands, when your right hand has turned the handle as comfortable the left hand takes over with out stopping the handle from turning. With a little practice you can get a nice fluid motion; much easier to do on the cross feed than on the longitudinal feed.

Dave

[Florian Eberhard]

Hi Zee

I am guessing its the spindle bearings which are not set correctly (too much play). I had quite the same when I rebuilt my schaublin lathe. I had to adjust the front bearing and i had heard it was quite a delicate thing to be done. The bearing needed exactly 0.002mm play. Now the thing is you almost can't measure that and I luckily heard from someone how to do it.
However, the point is before readjusting the bearing play, I had exactly the same kind of bad finish of what you described (at least as far as I understood it).
Although I have a different spindle bearing solution than your lathe, the effect of too much bearing play will be quite the same.

To test if that's the case, you can try to machine something supporting it with the revolving center. This will reduce the bearing play in the front bearing. (I think all those type of lathes have two tapered roller bearings)

If you get quite some improvement (but probably not perfect finish) you should disasseble your spindle, clean everything, probably lap the rear bearing seat on the spindle (in case the bearing sits very tight) and then reasseble everything. It may also be a good idea to put some good quality bearings in it (timken, skf or similar) instead of those chinese bearing of unknown quality.

Hope you can sort it out then,
Cheers Florian

[zeeprogrammer]

@Tom - I'd tried different speeds. Stock is 1/2" aluminum. Keep reading...I made some progress.

@Jason - well well well. I did not realize the little lever also engaged the lead screw. I was using the big-un which is what I was familiar with on the mini. That made a huge difference. And thanks for explaining the ABC differences. Yes, the tables are there. I haven't been able to figure them out. I'll work on that.

@Dave - One of the inserts I tried is supposedly 'matched' to what I'm doing. But what surprises me is your comment about 'too much nose radius'. I was trying what I believe is a bigger radius nose thinking smaller would be worse. I'd like to know more about this. As for the hand work...I implied I was stopping the wheel when changing hands...but like you I switch from one to the other. I got decent on the mini. Need more practice here.

@Florian - That was probably THE thing I'm hoping not to hear. Brand new lathe out of the box. If I have to deal with bearing play, lapping, etc....I'll probably need to bribe someone to visit me and help.

Back @Jason. So it appears using the little lever uses a different gear ratio between the lead screw and spindle than the big lever. Right? Why are there two levers? For that reason...to get a different set of speeds using the lead screw? Or is the big lever more relevant when threading.

Thank you all so very much. I've made progress. My level has moved from 'bummed to frustrated' to somewhat bummed (only in that I don't have it perfect yet).

  Thanks!!!

I need to think more about the squealing when I stop the carriage.
I have the cross slide locked so I can't simply pull the cutter back.
I first stop the carriage then stop the motor then can loosen the gibs to pull the cutter back.
The squealing occurs after stopping the carriage and before stopping the motor.
Should that happen? Am I doing it wrong? Or if I'm not doing anything wrong...is it about the setup?
Seems I didn't lock (tightly) the cross slide when I was using the mini.

[Jasonb]

Back @Jason. So it appears using the little lever uses a different gear ratio between the lead screw and spindle than the big lever. Right? Why are there two levers? For that reason...to get a different set of speeds using the lead screw? Or is the big lever more relevant when threading.

That sounds like the main problem as by using the big lever you were cutting a fine screw. When the smaller lever is used that goes through more gears in the apron so the carrage moves less for a given rotation of the leadscrew when turning along the lathe axis. It then goes through a further 2:1 reduction when the little lever is moved the other way for facing cuts so will move at half the rate. Generally if you set the gear train for the slower of the two feed groups shown ( should be supplied with that setup) then you will find that 99% of facing can be done in position A and turning along the lathe in position C.

When you get to the end of turning the work down have one hand on the carage handwheel and the other on the lever, disengage feed and move carage back towards tailstock end slightly this should stop you squeeling. most of the time the cross slide should not need locking.

[Florian Eberhard]

@Florian - That was probably THE thing I'm hoping not to hear. Brand new lathe out of the box. If I have to deal with bearing play, lapping, etc....I'll probably need to bribe someone to visit me and help.

Well yeah, its a brand new lathe out of a Chinese Box

Setting tapered roller bearings is not difficult since they are not very sensitive on preload (like angular contact bearings are.)
All you have to do when mounting them is to first tighten them until the spindle starts to turn with resistance (only turned by hand!)
Then you loosen up the nut again and screw it against the bearing by hand. Then you tighten it again only a tiny bit, with very few force. If you did this, you can run your spindle with the motor and then the bearings should not reach more than 50°C after half an hour of running it. If they get hotter, you need less preload, if they stay completely cold, you can try what finish you get and if its ok, you can go with that setting, otherwise you should tighten the nut a little more.

You only have to lap the rear bearing seat if you cannot move the rear bearing by hand (ideally with a little resistance)
Lapping the bearing seat on the spindle is quite easy, all you need is a lapping ring (made from aluminium), some lapping compound, a micrometer to check the size of the bearing seat from time to time.
And depending on how much you have to remove quite some patience.

Florian

[Florian Eberhard]

The squealing occurs after stopping the carriage and before stopping the motor.

That should definitley not happen. And sorry to say this, but this is also a hint for too much bearing play.
I think what happenes is the following:
You machine a diameter and the machining forces push the spindle away from the lathe tool. When you stop the carriage, the spindle wants to move back into the centered position and there the workpiece rubs over your cutting edge and makes that squealing noise.

Florian

[Jasonb]

How much of your 1/2" bar is sticking out the chuck? too much and the bar will flex away duringthe cut and rub when you stop even with perfectly adjusted bearings.

It will also tend to rub as the "thread" you cut with the wrong lever is flattened out by the stationary tool

Are you cutting dry or with lubricant?

[zeeprogrammer]

@Jason - Thanks! I managed a pretty decent finish. My hopes for the future have risen tremendously.

@Florian - That's very helpful information. I learned quite a bit and will keep that information tucked away.

For the moment I am 

(You all know me by now...I get to know the valleys just as much as the peaks. Short visits to both...but frequent.)

Just saw your last post Florian. I'm not so sure. I can understand that more with deeper cuts where the cutter is really pushing but this occurs even on the lightest cut.
I'm thinking rubbing. Possibly not centered properly. Possibly cheap index.
I'll keep an eye on it.
I do want to investigate the spindle play though. Even if built well enough...shipping etc and cause changes.

Also Jason...A couple of inches (probably too long). I cut up to about 3/8 away from the chuck. It's near the chuck end that squealing occurs. Dry and wet (WD40) but less so with WD (which I would expect).

Going to play some more with it. Also want to try some brass, steel, and stainless.
(No fluid with brass. Some cutting oil for the steels.)

[10KPete]

Make sure there is a bit of a radius on the end of the cutting tool. If the point is sharp, it will show the feed rate. Radius wants to be much larger than the feed rate to minimize the height of the 'groove'.

Pete

[philjoe5]

Zee,
I own 3 lathes, 2 imports, 1 USA made.  They are 2, 10 and 71 years old respectively.  All give good finishes on steel, aluminum, brass and cast iron.  If I chuck a 1" round bar x 12 inches long and put a dial indicator against it and pull the bar toward me, they all deflect the bar 0.001" or slightly less.  That much play does not appear to affect finish.  If that's what you're getting I'd look elsewhere.

A reasonably sharp tool set on the spindle center should give a good finish with a feed rate of 0.002"/rpm, 400 rpm on a 1" round projecting NMT 1 inch out of the chuck.  Plus, if you take a cut of say 0.025" and wind the carriage back toward the end of the work while under power you should only just barely see a spiral cut.  If you get a pronounced spiral cut I'd check tool tip for sharpness, and height on centerline.

Good luck with solving this issue, we've all gone through this.

Phil

[jadge]

Zeeprogrammer: You might find this thread (on another forum) of interest:

http://www.model-engineer.co.uk/forums/postings.asp?th=51900

If nothing else it will explode the myth that inserts must be run hard and fast to get a good finish. They can be, and for some materials must be, but not for all materials.

Andrew

[zeeprogrammer]

Thanks very much my friends. I think I'm getting a handle on this.

Andrew...that was a very helpful and interesting link. Thanks!
Are you the same Andrew as the author?