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

[Hauk]

I am repeating myself, but I am a total beginner on the lathe, but I just have to ask anyway...

I was happily machining a little flywheel on my lathe, and things went well till I was going to make a 2-3 mm deep slot into the 25 mm diameter round stock. I had already machined the surface, so everything was nice & concentric.

The attempted cut was made maybe 25 mm out from the 3-jaw chuck with a 3 mm wide cut-off/slotting tool. Speed around 1500RPM, very slow feed. No center in the tailstock.

Suddenly the tool dug in, ripping the part from the chuck and locking up the machine.

Part totally ruined, and some damage to the lathe as well.


As you can see, the groove was not very deep when the dig-in occured:

The damage done:

The tool with the quick change toolholder:

This leaves with me with several questions:

1) How stupid was the operation I tried to perform? Could you expect to pull off something like this?
2) Is that dovetail breakage something to be expected, or an indication of a inferior part?
3) What is the most likely cause for the crash? Lack of rigidity on the tool or/and toolholder, deflection of brass bar? All of the above?
4) Are crashes like this a part of a machinist life, or and indication of a total lack of skill?

Regards,
Haavard H

 

[Tin Falcon]

Well lesson learned.
parting and grooving can be tricky . I was professionally trained in the basics of machining and I to have had parts torn from the chuck. I am fortunate no broken lathe parts.
Observations: What I see.
1) rpm way too fast as chuck pointed out.
2) tool out probably 25 mm keep tool stick out to an absolute minimum.
3) part stick out check you look good there.
4) not a lot of meat on the dovetail weak design.
what I do not see:
5) how was tool height set ? Always set up tool to deflect away from the work. Parting tool should be on center line or slightly below never above center line.
6) was tool honed honing helps a lot . use dead sharp tools for parting grooving.

IMHO cause of crash RPM too high poor setup and poor lathe design/weak metal on dovetail.
curious what lathe do you have?

tin

 

[Hauk]

sorry to see your lathe broken :'(

Me too, but if that was all the damage done, I sort of consider myself lucky.

the first thing you mentioned that would cause this to happen is the rpm you were turning the work piece!!!!!!!!!
1500 rpm's is waaaay to fast you should be parting off at about 2 or 3 hundred rpm.

Ooops...

if the part is a flywheel you should have drilled and reamed to hole in the center of the part before you parted it off.
that way the center hole will be concentric to the o.d. of the flywheel.
secondly you should always part off as close to the chuck as possible, this helps retain rigidity.

The flywheel was not meant to be center driled, it shold have taps at both ends for small ball bearings, like this:

.

Here we can also see the small pulley wheel that was to be fitted to the flywheel. Btw, these parts were made by a very skilled machinist, not me!

crashes like this do happen but they are not the norm, they are usually caused by poor set up and not having the tool sharp and on center.

if you were parting off at a much lower rpm chances you would not have broke the cross slide.

hope this helps

chuck

It really does. Thanks for taking the time to answer a complete rookie!

Regards, Haavard H

 

[Hauk]

IMHO cause of crash RPM too high poor setup and poor lathe design/weak metal on dovetail.

Thanks for the feedback! Much appriciated.
The major bummer is having to wait for the replacement part. I would like to get back in the saddle as soon as possible!

Regards,
Haavard H

 

[Tin Falcon]

Another observation is there may have been a sharp score line on the inside corner of the dovetail this would have concentrated the stress along that line and cased a clean break .
when you get the new one make sure there is a smooth radius on that inside corner.
Tin

 

[Blogwitch]

Hauk,

It looks like the broken bits could be stuck together and measured up OK, and if it was me, I would see about getting someone to knock you up a replacement part out of a normal steel, it's not really a big job.

That really looks to be a very weak spot, and could cause you the same problems in the future. The top part of the unit is much beefier, so shouldn't cause any problems.

I don't know where you are located, but if it is the UK, then contact me and I will make you a new one. I'm sorry, but that offer can't apply if you live outside the UK, it just takes too long and expensive for bits to be shipped back and forth.

If you can't take me up on my offer, maybe someone on here from your part of the world will step in with the offer of help.


John

 

[Tin Falcon]

As a student I had access to a CNC-milling machine, and every now and then I could make some parts for my engine:

You could consider making a replacement part for the lathe. Make it beefier steel instead of aluminum perhaps and add a radius to the inside even that will add strength. Look at this as a learning opportunity.
Tin

 

[kvom]

What part of the work was chucked? If you were holding by the small diameter opposite the groove then the part was stuck out too far. Given the other gouges that seems likely. For a do-over, chuck on the large diameter, and as said above cut the RPMs way down, and also feed very slowly esp. if you hear chatter.

 

[dalem9]

If you are in the states pm me and I will see what I can do.Dale

 

[mzetati]

You could probably replace the top slide & dovetailed guide with an equally thick block of metal where You will mount the the QCTP, to make use of the lathe while You're waiting for the replacement part.
That would all add rigidity to the setup, easing the parting jobs.
Marcello

 

[Ken I]

I think I can safely say we have all lost the occasional part out of the chuck - it shouldn't happen but it does.

Apart from damage to component and tooling, damage to the lathe is rare - sorry to see what happened.

Some tips :-

Have at least the length of the chuck jaws clamped in the chuck (ie the chuck jaws bear fully against material - the moment you deviate from this the jaws can start to tilt slightly - so your work is effectively held in a "taper" and can wiggle out) - the same goes for collets. Remember it will wiggle into the side of the part off tool and go bang before you can react to it.

Plan to part off as close to the chuck as possible.

Don't chuck a smaller diameter than you are parting.

If you have to deviate from the above bring a live centre to bear to support the work and prevent walkout. Withraw the centre only for the very last part of the cut. If the setup is dicey - hacksaw through the last bit and face off.
If you have to grip a length shorter than your collet / jaws - put a piece of identical diameter stock at the rear of the jaws / collet to prevent "tiltiing"

Part off tool to be sharp and on centre (or slightly below - never above).

Part off tool stick out from toolpost to be the minimum for the job to hand.

Use the narrowest partoff tool that will do the job.

If you have the facility, part off from a rear toolpost in reverse.

Keep the speed down and the feed rate up.

If it chatters - stop and make sure you are not doing someting wrong. If you are happy your setup is righteous try increacing the feed to stop the chatter.

Regards,
Ken

 

[mzetati]

- apply cutting fluids copiously: I'm using oil for steel (applied with a toothbrush, with all the bristles rows removed but the central one, held directly into the groove) and petroil with aluminium.
- have the tool perfectly square to the workpiece axis.
- retract the topslide so You will have the tool as near as possible to the pivoting point.
- widen the groove by moving the tool sideways half of the tool thickness, to ease chip clearance.
- retract the tool and clear the groove anytime You see the chips have stopped flowing outside the groove. Changes in the noise produced during the cuts are a good reason to retract and check, too.
- use T-blades.
- The lathe lowest speed is usually too fast: someday I will make an oversized pulley right for the parting jobs.
- reduce the tool overhang to the minimum: on deep grooves You'll start with the minimum overhang and increase it in steps.
- a dig-in / stall is much likely to have modified the way the workpiece is held in the chuck: re-tighten the jaws, or re-chuck the workpiece.

Marcello

 

[Arnak]

Hi,

Personally I use a TCT parting off tool, one with a replaceable tip.

It is much thinner than the tool you are using so much less cutting force.

You can get them in varying shank sizes from Chronos.

Martin

 

[Tin Falcon]

- apply cutting fluids copiously

Yes I forgot tat one on my list.
Tin

 

[Hauk]

Thanks for all the feedback! Almost overwhelming... Especially the offers to make me a part. Unfortunately, I am located in Norway, so I might just have to wait for a replacement from the lathe manufacturer. If it takes too long, I might be able to have the part made locally.

Btw, what do you consider the best cutting fluid for brass? From what I have read, most people machines brass "dry".

Regards, Haavard H

 

[Herbiev]

A bit hard to tell from the photo but it looks like the rake on the cutting tool is set up for cutting steel instead of brass. And as mentioned use heaps of cutting fluid. Good luck. Its all part of learning curve.

 

[lordedmond]

I will second or third the tool was far to wide try and get one that is no more that 1/16 inch or 1.5 mm

Stuart

 

[lensman57]

Hi,

Sorry to hear about the misfortune.
If funds allow get a tct tip parting tool, they are not cheap but the price is well worth paying. I used to have a lot of problems parting off even small diameter stock on the little Taig lathe but things are a lot better with the new tool. I got mine from Chronus with a couple of spare tips, 2mm I think.
Make sure your cross slide is trammed and secure, the tool height accuracy is a must, part as close to the chuck as possible, use the right speed for the diameter and material and lock all the gibs that are not required. Rigidity is the name of the game here.

Good luck,

P.S : make sure the tool tip is absolutely perpendicular to the work piece.
A.G

 

[dalem9]

Try to bolt it back together ,about 10 small screw would hold for light work or until you get a new one.Dale

 

[Hauk]

I finally got a replacement for the broken part. Wabeco sent it to me for free together with a new collett chuck (see separate thread), so I am quite happy with their service.

A little disencouraging was the fact that I have to enlargen and deepen a hole for an indexing pin in the replacement plate. As you might guess, I am not too eager to screw things up again...

So any advice on drilling in cast iron? I have a half-decent drillpress and a decent drilling vice. I tried drilling into the the broken part, but was not able to produce chips, only dust. Progress was painfully slow. Should I try to increase or decrease speed? Pilot hole?

Regards,
Haavard H.