Tool dig in. Part, slide and moral totally shattered.

[jpeter]

That tool looks like it might have some "hook" ground into it. The cutting top rake surface ought to be horizontal or ever so slightly negative so it won't grab and pull itself in. There's lots of rpm charts available on the Internet. Generally when parting the spindle rpm is kept well below the recommended turning rpm. When parting on not so ridged lathes it helps to place a small prop under the tool as close to the work as reasonable/possible down to the slide. Use any old piece of scrap cut to the proper length to jamb under the cutter. That takes quite a a bit of load off the slides and firms up the setup. Try it.

 

[jpeter]

If the work piece had any significient length between the parting tool and tail stock and any significient diameter there's no way the piece between the cutter and tail stock wouldn't after partoff jamb between the tail and the spinning piece causing it to fly off into space. Really small diameter pieces might be ok cuz they'd drop off before jambing, maybe.

 

[tel]

Nope, no 'hook' - that is just a piece of 3/16" square HSS, ground only on the sides and face of the tool. Run at the lowest direct speed on the ML7.

 

[hobby]

When parting on not so ridged lathes it helps to place a small prop under the tool as close to the work as reasonable/possible down to the slide. Use any old piece of scrap cut to the proper length to jamb under the cutter. That takes quite a a bit of load off the slides and firm up the setup. Try it.

Thanks for that good tip.

 

[coopertje]

In addition (maybe completely unnecessary) to explanation of JPeter below some pictures of such a set-up (took the pictures from a workshop practice series book I was reading last week)

Makes sense to me!

Regards Jeroen

 

[Hauk]

Too bad to see your Wabeco brake down Haul, I would expect better from them, they are not cheap machines! Hope to see you back in business soon! By the way, what type do you have? I had a 6000D and was quite satisfied with it.

I have a 4000D, and I am hesitant to blame the machine for my crash. Wrong tool, far too high RPM and operator inexperince is to blame.
In the end, I think Wabeco handled the repair of the lathe very satisfactory. I would not hesitate to buy a machine from them again.

Regards, Hauk

 

[purpleknif]

Once again, as a full time machinist for 30 years I gotta tell you the MOST IMPORTANT thing whether you're boring, turning, fly cutting, or parting off is TOOL CLEARANCE. If you don't believe this try cutting something without it.

 

[chrsbrbnk]

I've also been a tool&die maker for 30-some years( actually I work with Mike N) ,still have all my digits( so does Mike). When parting off I try to use the t style cutoff tool the relieved sides help reduce the drag and heat ot the tool tip. There is a version of these that has a slight vee in the top of the bit this also helps get the chip from dragging on the par. t on brass I try to be at or alittle bit below center line. Using a cutting oil will improve your chance of sucess by like 10x stick to the standrad FPM cutting speeds for the material your cutting.

 

[rake60]

I remember parting sequences in programs if the CNC turning centers I used to operate.
The program would maintain the programed SFM up to a maximum that you would set in the program.

For a 2" part, we would program it to part down to a 1/8" diameter.
It would be spinning pretty fast by the time it reached that finish size.
Then you could open the machine up and snap the part off with one hand.
A few quick licks with a burr gun and you couldn't even tell it had been parted off from a larger piece
of stock.

I had that go all wrong one day.
I was running an untried program that had been written by the guy on 3ed shift.
Program looked good to me, so I hit the cycle start button. When it reached the parting sequence, all seemed
to be going well. Then an 18" long pin broke loose and almost came through the containment doors of the machine.

It SHOULD have been programed to have the parting tool come in and cut the 1/4"X45° chamfer, the part down to 1/8"
diameter. He had programed it to part down to 1/8" diameter, THEN cut the 1/4"X45° chamfer. That won't work! LOL

A quick change in the program and I was able to reach in after the program had finished and break the remaining 38 pins
in that order off by hand.

Rick

 

[MyfordMan]

This is my first post and like a lot of people here I wasn't trained as an engineer I have had the trial and error education and like several here are saying parting off isn't the easiest thing to do on a lathe.

if you look a Hauk's original pic of the failed piece you can see in the main groove on the right that chatter has occurred starting with the tool marks close together then gradually getting wider as the chatter worsened and eventually digging in causing the failure to me this looks as though it was due to a lack of lubrication but as others have said the things to remember are:

Sharp tool.
As little overhang as possible.
Cut as close as possible to the chuck
Lubricate
Not to fast

I haven't tried using a live centre to part and to me personally I can't see the logic but as Greener has been so adamant about this then he must be having good results this way so I'm not commenting one way or the other!

Lee

 

[Ken I]

Without reigniting this particular fire, I often bring the live centre in for parting on my mini lathe - It definitely helps with overall rigidity and therefore helps to eliminate chatter.

This is something I would rather not do - but needs must when the devil drives - If I had a more robust lathe I would not need to do this.

That said, I don't part off as its likely to end badly - but instead part almost to off and then remove the centre and either twist the component off, saw it off or complete the part-off depending on circumstances.

Ken

 

[hobby]

That said, I don't part off as its likely to end badly - but instead part almost to off and then remove the centre and either twist the component off, saw it off or complete the part-off depending on circumstances.

Ken

I don't think ,using a tailstock to support the workpiece during cutoff, is really the concerned issue, it's using the tailstock to support the workpiece, as it is being completely cut away from the parent material, that seems to be the caution flag for this method.

 

[AnneCourtois]

Also, as a self trained, hobby machinist, if you look at the scrapped part, you can see that before the illfated catch of the tool into the part, there has been multiple bites of the tool into the part, indicative of tool-flex. This happens very quickly because these scenarios develop exponencially. Look at the cut 340° earlier, you have a smooth cut but within 180-270°, critical heat is reached, your tool start to bend and eventually catches. Within 20-30milliseconds. Failure probability = Lack of rigidity X Lack of lubrication. Meaning you could get away without lubrication if your setup is quite rigid and vice-versa; you could lack a bit of rigidity(that happens easily with hobby level machines) but with plenty of lubrication, one could do just fine.

 

[dman]

i once saw a machinist of several years training a new guy on parting. the machine had a 16" swing and he was using a very rigid quick change tool post though it's been so long i forget the size/model. he was explaining that you dont want to let things rub or they harden, heat up, or gall. well it was a 6 inch aluminum bar he was cutting and quite aggressively to avoid letting it rub. well i guess the tool wasn't too sharp or chip ejection was a problem and the material started to heat up and gall anyway. next thing you know as he was mid sentence probably looking off to the side talking to the new troop there was a crash as the part pulled down on the tool so hard it broke the dovetail from the tool holder!

the trainer turned red as he stopped the machine and explained " that is what you don't want to do" lol.

 

[MuellerNick]

I have read some quite ... funny tips in this thread!

But first of all, I think that the dovetail broke, and then the rest happened. Brass is very forgiving when you turn it. It is really hard to make something wrong there.

Retract the compound to make it stiffer.
Part off very near the chuck.
Align the tool dead to the X-axis. The fastest way is to push the side of the tool against the chuck's face, then clamp the tool.
Never adjust tool height above center.

Cutting speed:
Brass with HSS is turned with 100…120 m/min surface speed. With a diameter of 25 mm, that makes 1270…1530 RPM.
I do not remember that I ever turned brass with coolant or oil. In fact, I would never use oil on brass to part off. Why? I'll come back to that.

Now the most common error done while parting off is a feed way to low. If you have an automatic feed along X, USE IT! 0.1 mm / revolution almost always works. If you feed manually, feed fast. Most people start very slow, tool will squeek (SP?, you get it by the sound) and they lower the feed. Wrong! Increase the feed and things will get silent and smooth. Watch for the sweet spot of the feed. If it gets to high, you will get a rumbling sound.
Why no oil for brass? That only starts the tool to rub. You feed in, nothing happens. The oil film breaks and the tool starts to cut and digs in. You then have a sequence of rubbing and digging in.
If you use oil, you should have an automatic feed. Coolant (water/oil) is more forgiving on that.

On my CNC-lathe, I part off at 150 m/min surface speed (1045 AKA C45 etc.) and a feed of 0.1 to 0.15 mm/rev. At 25 mm diameter, the RPM flats out, she only does 2000 RPM. I use coolant.

On aluminium, always use coolant! The deeper the cut, the more necessary it is. Al tends to bind to the tool. One chip that is torn between work and the side of the tool and the disaster starts. You can avoid that with coolant. Plenty of it.


Nick

 

[Brian Rupnow]

After 5 years of parting stuff off, I must admit it still terrifies me . I grind the tool correctly, I retract the top slide to a minimum of overhang, and lock the gib. I lock down the bolt on the cross slide that locks it to the bed. I make sure the tool is on center. I use lots of cutting fluid. I pray. I stand well to one side of the "disaster zone" if anything is going to fly out of the lathe and kill me. I wear my safety glasses, and sometimes wish I had a full suit of armour like Sir Lancelot had. Sometimes the part turns off cleanly. Sometimes it digs in, gives a large "SNAP" and the whole lathe jumps 3" off the floor and I crap my pants!!! Sometimes I turn untill the cut is about 1/8" deep, then chicken out and take the part over to the bandsaw to finish the cut.

 

[MuellerNick]

After 5 years of parting stuff off, I must admit it still terrifies me .

Aren't you the one who said: "I turn all my parts with 650 RPM"?


Nick

 

[Brian Rupnow]

Aren't you the one who said: "I turn all my parts with 650 RPM"?


Nick

No, not me. There are 2 or 3 other Brians on this forum though. My lathes lowest speed is 115 RPM and thats where I do my "parting off" otherwise it chatters terribly.

 

[MuellerNick]

No, not me.

I'm sorry, I got the RPM wrong, not your name
"I do most of my aluminum turning at 970 RPM." (Brian Rupnow)


Nick

 

[Brian Rupnow]

I'm sorry, I got the RPM wrong, not your name
"I do most of my aluminum turning at 970 RPM." (Brian Rupnow)


Nick

Turning yes, parting off, no. ---Brian