Parting off on a mini lathe

[Andy BrocklehurstParticipant @andybrocklehurst85292]

I’m learning turning and parting off on my mini lathe just using some 6mm brass bar, turning and facing is ok but when I part off I get left with a stalk.
The part seems to snap off before it’s cut, I’ve got the insert centralised (the best I can by shimming) running at around 350rmp with cutting oil using the insert shown.
Any advice appreciated.

[roy entwistleParticipant @royentwistle24699]

Don’t use cutting oil on brass. Is your tool too high? and slow down a bit.

Roy

[roy entwistle]

On roy entwistle Said:

Don’t use cutting oil on brass. Is your tool too high? and slow down a bit.

Roy

Ok thanks, I’ll try and get the tool a bit lower.

[NealebParticipant @nealeb]

The parting inserts I have bought always have a tip that is parallel with the lathe axis so that they produce a flat-bottomed groove. Inevitably, this means that the work will break off when the remaining material is too weak to support it and almost always leave more or less of a pip. In the days of HSS tools, the parting tool would often be ground at a slight angle on the tip so that the RH corner cut slightly more deeply than the LH corner. Get it right and the work would drop off leaving any pip on the material left in the chuck. However, my own experience was that this slight angle could also cause the tool to go in to the work and progressively move to the right, leaving a concave face on the parted-off work. These days, I use standard inserts and accept that I need a second operation to clean up the face. I believe that inserts with angled tips are available, although I have never gone in search of them.

I cut brass dry, and probably faster than you are, although my own lathe is a bit heavier than a mini-lathe. Main thing is to keep a steady feed but at least the chips tend to fly off and not jam, as they can do with steel.

[DiogenesParticipant @diogenes]

Yes, I find that can also happen because some of the cheaper tips seem to need a bit of pressure to keep them cutting – I try and ease-off a bit when I know it’s very close to the middle, and let it just ‘shave’ the last bit for a few turns, any residue left should be foil-thin and can be scraped off, much as Nealeb says.

‘Sharper’ inserts can help, also, as said, making sure that c/height and alignment are as good as you can get them.

[Peter Cook 6Participant @petercook6]

Looking up those inserts suggests they are primarily designed for grooving, so the cutting edge is square to the lathe axis. ARCEurotrade’s website shows parting off with those inserts with what appears to be a slight angle to make the RH corner of the insert do the main work.

On my micro lathe, the parting tool is ground with an angled tip as Nealeb suggests.

[Andy BrocklehurstParticipant @andybrocklehurst85292]

Thank you for all your replies folks, I’ll try all your suggestions.

[Bo’sunParticipant @bosun58570]

I realise it will be difficult to do, but I’ve often wondered if an inverted V profile on the face of the parting blade would work.  It would balance the lateral cutting forces caused by a single angled face, and help remove the pip.

[JasonBModerator @jasonb]

The ARC ones are Neutral inserts hence the N in the code, just the image shows the “ideal”. Right and left bias are available but can have the cut wandering off if the setup is not rigid.

MGGM200 inserts are available and sharper so will  put less pressure on the material and not cause the unsuported part and bit at the bottom of the cut to break off so soon but you are still likely to get a foil thin piece attached but flick it off with a finger nail or pull with pliers and then deburr the hole.

You could happily double or treble your speed with brass of that sort of diameter

[cogdobblerParticipant @cogdobbler]

At 6mm diameter,  brass with carbide tooling, rpm ideally should be about 3,000. Knock that in half because parting can be more difficult than plain turning and you are still at 1500 rpm.

Running at 350rpm on such a small diameter in brass with carbide tooling is such a low surface speed the carbide will struggle to cut, as you have found.

 

 

[cogdobbler]

On cogdobbler Said:

At 6mm diameter,  brass with carbide tooling, rpm ideally should be about 3,000. Knock that in half because parting can be more difficult than plain turning and you are still at 1500 rpm.

Running at 350rpm on such a small diameter in brass with carbide tooling is such a low surface speed the carbide will struggle to cut, as you have found.

 

 

Ok, thanks for that I’ll try speeding up.

[old martParticipant @oldmart]

I like that type of parting off insert, and have them in 2 and 3mm widths. Having larger lathes, I have found a source of 26 and 32mm blades which suit me better than the style that smaller lathes require. Inserts are available in steel and aluminium grades, and for brass, I recommend the aluminium HO1 grade. I get a small stalk on the part cut off, you can minimise it, but probably not totally eliminate them. Only experimenting with height and speed as advised will you find the best settings which may vary with different metals. Parting is better if there is a hole in the workpiece. Slow down the feed rate as the diameter left gets smaller or you get near the hole diameter.

[VicParticipant @vic]

Nealeb’s comment is what I’ve found. The effect can be somewhat reduced by grinding only a slight angle. I very rarely use inserts for parting since I tried the T shape HSS blades. They work great in all but the hardest materials and can be used on plastics and softer materials as well. They work fine on brass.

[old martParticipant @oldmart]

I have a 7 x 12 lathe at home and have found it works well with the blades like Vic uses. That is because that is the only parting tool I have for it. When I bought the lathe second hand, the original owner had had trouble parting off with the blade upside down in the holder, I didn’t mention that to him when I bought the lathe.

[Andy Brocklehurst]

On Andy Brocklehurst Said:

On roy entwistle Said:

Don’t use cutting oil on brass. Is your tool too high? and slow down a bit.

Roy

Ok thanks, I’ll try and get the tool a bit lower.

As you are learning to use your lathe, try to establish the basics, such as making sure the cutting edge or point of the tool is exactly on the centre of the work.

This is one way

or

[samuel heywoodParticipant @samuelheywood23031]

I usually find any “stalk” can be removed with a sharp blade, eg a scalpel~easier with brass than steel 😉 my stalks are usually wafer thin, so check tool centre height if they’re not.

[PeteParticipant @pete41194]

Most industrial parting I’ve watched videos of are using at least medium to high pressure flood coolant, enclosed machines, wider and very rigid carbide parting tips, tool & tip holders, and extremely rigid lathes as well. When there tail stock weighs more than my whole lathe, the results are going to be a bit different. 🙂 And many are using the slightly angled parting tip so there’s nothing left on the finished part. The usual parting tool blade deflection with our usually delicate parting tools may not be acceptable to some. All that contributes to usually acceptable surface finishes. Were still trying to do the exact same thing as they are, but there’s huge differences in how it’s being done. A second operation clean up cut to improve the parts surface finish where it’s been parted off or to remove any pip would be highly undesirable in an industrial environment due to time and extra cost.

Few of us are using anything like that. Sometimes I can get a nice clean parted surface without any minor grooving from the parted swarf, but it’s not predictable. For myself I’ll usually part off slightly too long and accept that need for a second operation clean up cut to the parts finished length. If that surface finish is unimportant and there is any pip or torn material left as Samuel already mentioned, that few swipes with a file or a rub on some emery paper will remove it. What industrial part production considers easy and normal, most of us can’t even dream of doing. And for those high volumes of part production, they also spend a lot of time and expense to optimize every single cutting tool, carbide grade down to the brand and coating type, rpm etc until they can get exactly what there looking for in the absolute shortest time possible for every part. That’s something rarely even mentioned. Even when parting, the most basic of cnc lathes today are going to have constant surface speed (CSS). So the surface cutting speed remains constant until it’s parted off. Many here may not have or for there own reasons, may not want to use power cross feed while parting off, so that’s another variable.

[Neil LickfoldParticipant @neillickfold44316]

You want a neutral parting insert, one for Cast iron, that is ground sharp. The pressed inserts have a rounded cutting edge. They work because they rely on high rpm and feedrate.

Hss parting blades work great for brass and especially some of that brass bar that is not free machining. Use oil or something with the tool, only needs a drop or so, doesn’t have to be flooded.

to get a cleaner part off, you can grind a relief on the front edge, and so looks like a scallop in the front. Then make the LH side slightly behind the rh side. SO it will drop off the part and then drop off the tip. The curve will make the chip curl in a way that it does not bind in the slot.

I only buy ground inserts now, and don’t use any Pressed inserts. That way I can run the normal speed etc and get great results.

Make sure that the slides not being used a locked if possible and that there is no excess play in the cross slide and no excessive backlash in the cross slide nut assembly. Sometimes a below centre parting tool, can draw the cross slide into the workpiece. It wnats to be on centre or very slightly above. Slightly being less than 2 thou or 0.05mm.

Neil

[SillyOldDufferModerator @sillyoldduffer]

Parting-off on small lathes is notoriously difficult, so mini-lathe owners shouldn’t beat themselves up!

Lack of rigidity is a huge problem: The broad tipped cutter applies a lot of force, and it’s likely to bind or choke on swarf in a deep slot.  The tool-post towers over the saddle and bends!   The parting tool is usually extended such that it bends and levers the tool-post, causing the cutting edge to dip.  As slack gibs also allow the tool to twist sideways,  the chance of the cutter rubbing or digging-in is high.   Also important to maintain a steady feed-rate, which is hard to do manually, especially when trying to clear swarf.  Tool height, feed-rate and RPM are all critical.  Much skill required, especially if the material is awkward, or the job hard to hold firmly!  There’s a tricky moment at the end of the cut, when there isn’t enough metal left to support the weight of the job and/or the cutting force – the job likely to snap off rather than cut cleanly.

Palliatives:  sharp tool, accurately set to height; free-cutting metal, not grippy Bronze or work-hardening Stainless; clear swarf; minimise tool overhang; snug gibs down; lock unwanted traverses; use power traverse; fit rear tool-post, ideally with the lathe in reverse and the cutter upside down; reduce RPM; practice, practice…

My WM280 is much heavier and more rigid than the mini-lathe I started with.   Parting-off  is 100% reliable with a rear tool-post, and the front 4-way rarely fails – mostly on deep cuts.  Automatic feed makes a big difference.  It’s almost idiot proof!  In contrast parting off on my mini-lathe was much more difficult.  Certainly possible to part-off Brass and free-cutting Aluminium provided everything was ‘just so’,  but steel was high-risk.   I decided the failure rate was too high and switched to sawing and facing-off.  That wastes time compared with a clean part-off, but trust your experience with your lathe in your workshop – parting-off on a mini-lathe may be more trouble than it’s worth.

Dave

[Martin KyteParticipant @martinkyte99762]

The one factor that makes a huge difference and is not related the tool form or the size of the lathe is the confidence of the machinist. A timid approach seems to spell disaster to parting off whereas confidence usually leads to much better results.
I can only surmise that a confident operator will tend to part at a higher speed and keep up a decent feed rate both of which produce better results and improve confidence further.

So how do you gain confidence in the first place. My suggestion is that you obtain some nice free cutting bar say 1/2” diameter. Set a sharp parting tool at centre height and square to the work. Drill for a suitable size bolt and then slice of a number of washers with the lathe running at around 600 rpm keeping up a steady infeed.
Hopefully you start to think ‘hey this is not so bad’ and you can progress to more difficult parting jobs with confidence.

[JasonBModerator @jasonb]

Seems like a good excuse for a Sunday Morning Turning Trial.

 

Warco 280, 1000rpm, 6mm Brass, Fine power feed. All cut Dry

Nikcole-Mini-Thin 1.0mm N left a pip of 0.5mm. This is my goto parting tool for small diameters but not Cheap.

1.5mm HSS left a pip of 0.70mm

MGGN200 left a pip of 0.82mm(new insert)

MGMN200 Same as OP is using left a pip of 0.84mm(new insert)

GTN-2 in 19mm Glanze blade left a pip of 1.02mm

 

Not having a mini lathe to hand to see if the problem was due to a smaller machine I used the closest to hand with was the Unimat 3 Micro Lathe. Had to cobble something up to hold the large insert holder but got similar results to the larger Warco using the ground MGGN200 on 6mm brass at 920 & 1400rpm. Steel at 6mm and 10mm gave a slightly thinner pip of 0.75mm both at 920rpm.

So I would conclude that the thinner the tool, the smaller the pip or thinner the foil around a hole. Makes sense as the thinner tool is not pushing as hard against the ever decreasing diameter. There is a downside to this in that a thin tool will want to wander more so for deeper cuts increase the width but on small work a narrow tool is the best option and saves wasting expensive brass.

[Howard LewisParticipant @howardlewis46836]

If possible, make and fit a rear toolpost, with the tool inverted. Necessary with screw on chucks.

Lock up everything that you can.

Although with the flange mounted chuck on a mini lathe, the parting tool could be mounted inverted in the front toolpost, and the lathe run in reverse. Even make a toolpost specifically to take the parting tool.

I use no Top rake. Grinding the front face at an angle can leave the stalk on the raw material, BUT the wider swarf risks jamming in the cut, so only a small angle on the face.

The inverted parting tool has the advantage that the swarf can fall out of the slot reducing the risk of jams.

Obviously, the tool needs to be at, or only VERY slightly above (Being inverted), centre height.

Just maintain a slow, steady feed (On my big lathe I actually use power cross feed, 3/32″ HSS tool, and apply ordinary engine oil with a brush or a drip feed of soluble oil!), Don’t let the tool rub and get hot, or it will jam!

HTH

Howard

[not done it yetParticipant @notdoneityet]

My input would be the same or similar to Howard.  Rear tool post or inverted cutter with lathe operating in the reverse direction is best for most hobby machines.

My lathe is a little larger than a mini lathe (it’s a Raglan) – but it is rather older than any chinese mini lathe.  I have no (or minimum) parting problems with a simple HSS cutter mounted on the rear of my cross slide.  All parting-off is left to the power feed to just get on with the job.

Obviously brass would be cut dry at fairly high speed with a very sharp cutter.  One must expect a nib to be left on either the part or parent metal.  How large that might be is obviously down to the material, size of the part and other factors.  If the cutter is too high, the part will simply be ‘pushed-off’ when near to finishing the cut, or break away at possibly a smaller diameter, if dead on the ideal cutter position.  Clearly, if the cutter is set too low, ideally the cut would not be completed!  With the cross slide being forced down towards the lathe bed, the cutting forces must be at the most consistent possible for a lightweight machine.

When cutting through to a central hole, the cutter, whatever it might be, will break through at the thinnest point and grab (think here a perfect parting cut with a very slightly non concentric hole).  Finishing, after parting off, should be expected.

Your nib size may well be down to insufficient experience as well, so don’t give up!

The last point is always to part off with the cutter protrusion at the minimum necessary.  Any extra is just another unwanted variable, whatever is being parted off.

[JasonBModerator @jasonb]

Worth bearing in mind that most Mini-Lathes don’t have tee slotted cross slides so rear toolposts are not so easily fitted.

[old martParticipant @oldmart]

Assuming that the cross slide was long enough, it probably be drilled and tapped to hold a rear toolpost for inverted parting off tools. The cross slide would have to be removed first of all to acertain whether there was enough thickness ans also clearance for the modification.

I made one for the Smart & Brown model A which holds 26 and32mm blades inverted which is easily detachable and a set of slotted grub screws to fit in the holes to keep swarf out when it is not fitted. The blade in the picture is a 32mm Kennametal 3 or 4 mm.

 

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