Parting off problems

[John Wood]

I'm not yet very experienced and am currently battling with the usual problem of parting off. I have a 3.5inch Warco WMT300/2 lathe and have been trying traditional parting tools in the usual holders with mixed success. Not too bad on smaller diameters but awful on larger work. The main problem is of digging in. I have experimented with tool height etc as usual.

I do realise that the recommended approach is to use a rear mounted toolpost which I don't have yet so I am thinking of getting the kit from Hemmingway which seems about right. I have also been considering using one of the new, deep parting tools such as the Kit Q-Cut from Greenwood Tools would this help and are they as good as they say?

As I understand it the main reason for a rear tool is that the direction of rotation is upwards rather than down thus reducing the tendency for tools to dig in. Correct me if I'm wrong but could you not mount a parting tool (upside down) in the conventional front toolpost and simply run the lathe in reverse! Would this not be the same as using a rear toolpost and therefore save the cost?

Any thoughts from you experts would be very welcome.

Thanks, John

[Deleted]

John, you are not alone in this. Problems increase in proportion to the square of the work diameter! I've had a fair amount of practice, but I regularly part off 2" material using a 3mm blade: I grind the blade front edge concave, there is a slight top rake, but not enough to reduce the tool width, as this will promote drag and ultimately digging-in. Spot on centre height or very slightly less works for me: if it is over height you will find yourself putting more pressure on at the low end of the diameter, and the tool will do what? Dig in!
Set the blade up against the chuck jaws, using a piece of white paper below to ensure perfect alignment. If you have a power cross-feed, and have the courage, a sharp tool will sail down the cut under fine feed and lots of lubricant.
Try not to emulate the production guys by putting an angle on the front edge to leave no 'pip': the blades aren't normally rigid enough to support this function, and the angle will only cause the tool to deflect to the right and possibly dig-in some more!
Try to make the cut as close as possible to the point of work-holding: I can't normally get a fag paper between the tool and the chuck jaws!

For myself, I've never used a rear mounted toolpost: I believe that they are a useful addition when you don't have the luxury of a QC toolpost, but otherwise they get in the way, and they shouldn't usurp a properly selected, ground and set-up front mounted tool. Members with greater experience or knowledge than me will offer more useful advice, I just hope that you find this basic stuff useful.
Regards
JB
PS: When you do get the tool exactly right, don't lend it to anyone 'cos they won't want to part with it!

[steam4ian]

G'day John

I have a Chinese 7" (3.5" UK) lathe and do not have many problems parting off.

My blade came with a peak on the top surface. The pundits said grind it off, so I didn't! It works a treat even though the blade is 3mm thick. Previously I was using a 2mm thick, 10mm deep blade which gave no end of trouble.

The tool holder may be important, mine clamps the blade into the turret post very well. The tongue was too thick so I milled away the foot of the slot in the turret.

I suspect having the tool just a bees whisker below centre helps, I get a tiny pip. I use tapping oil on steel, nothing on brass or aluminium. I have parted up to 40mm steel.

BTW I put an ammeter in my motor circuit so I can check the loading as I go.

Good luck
Regards,
Ian

[dva]

John Wood,

re,

...Correct me if I'm wrong but could you not mount a parting tool (upside down) in the conventional front toolpost and simply run the lathe in reverse! Would this not be the same as using a rear toolpost and therefore save the cost?...

With a screw-on chuck this would cause the chuck to undo itself.

Dave

[Deleted]

After trying many combinations I find that a Greenwood tool mounted upside-down in the rear toolpost is just about ideal in the Myford. Part off using power cross-feed on 'top normal' speed on the Super 7. Keep a drip of cutting oil (or paraffin for aluminium) aimed at the top of the tool and the swarf drops away with no tendency to jamb in the slot. I get the 'frying bacon' sound so eloquently described by LBSC.

[SteveW]

John,

There have been a number of parting off threads here in the past and it might be worth you (others) looking them up.

However, has I've outlined before...

The GREENWOOD parting tool is excellent if a bit expensive. On a light lathe (Myford) it works best in the rear tool post. These are dead easy to make with a bit of 50mm square length of steel, a 10mm bolt, a couple of allen screws and an afternoon of swarf making. The down side is the tool-tip is quite wide and you can loose a lot of valuable metal to swarf.

The thin blade/holder type are a lot more primitive in action. The moment you sharpen them the tip becomes narrower then the blade. Also any swarf cut tends to be a bit wider than the slot getting cut and this is one vector to jamming up. If the tip doesn't cut equally it'll get pushed left/right and trigger more problems. I have found that by using a Dremell to grind a bowl shape just behind the cutting edge to provide the top rake it causes the swarf to roll slightly and exit the groove cut cleanly. This is very similar to the action of the GREENWOOD tool but loads cheaper.

[ettingtonliam]

John
Yes you are right in theory about mounting the blade upside down and running the lathe in reverse if it has a bolt on chuck. In practice, normally the toolpost dimensions won't let you get the tool up to centre height, or anything near it when its uopside down. I had poor results parting off on my 'Chinese' 3 1/2" lathe when I was using quick change toolholders, but got much better results when I changed back to the 4 way toolpost, because its more rigid. I got even better results when I fitted the Arc Eurotrade roller bearing conversion, and can quite happily part off from the front using 'ordinary' (i.e. cheap) parting off tools, providing I obey the normal basic rules re centre height etc.

Richard

[John Wood]

Thanks JB for the details of your method. It all makes sense to me and I will have a go along those lines, I will have to get a 3mm blade as all mine are 2mm and I see from Ian that he has had problems with that width.

Thanks to Ian for your experiences on cutters and the grinding thereof, it's all been noted.

Good point Dave in that reversing the lathe would tend to undo a screw-on chuck, that hadn't occured to me however my Warco has a bolt-on chuck and backplate so shouldn't be a problem. It looks as though I am better off not trying that method anyway.

If all else does fail and I do eventually go for a rear toolpost then it's good to hear from ulster about his set-up with the Greenwood tool and also from SteveW. I have noticed the sharpening problems you describe Steve but had put it down to my inexperience so I will certainly get the Dremell out and try as you describe. I'm a bit new to these forums and had not thought to trawl past threads for similar information, obvious really, can we put it down to age??

Many thanks to one and all - John

[John Wood]

Sorry Richard - missed your reply earlier. Yes, of course using an upside down tool in the front toolpost will give severe height setting problems - should have tried it first! I thought the conventional 4-way toolpost might be better than a quick change and am interested to learn about the Arc Eurotrade roller bearing conversion, that seems like a good idea so I will follow it up.

Thanks again, John

[greenbat]

I have always been told never, ever part off under power feed. Wind it in by hand, you get a better feel for when you need more or less pressure.
I managed to get hold of a nice carbide insert parting tool from a works clear-out. This works a treat, and my lathe is at best a little ropey! Though that probably isn't much use to you.

[teakfreak]

I bought a Greenwood Q-cut tool and have never looked back. They are expensive but do the job perfectly. Chronos sell a cheaper version now.

[John Wood]

Yep! All the very useful advice had produced a nice parting tool which works a treat. I ground it myself in the end using the advice given which has saved the price of one of the new posh ones. Most of the success is, I suspect, attributed to rigidity and such points as thicker parting blade, good grinding method, shortest tool projection, cutting close to the chuck etc.

Thanks to one and all for the invaluable help

John

[greenbat]

Another thing you may find handy: If you look at an insert tip parting tool, you'l see that it has a little noggin behind the cutting edge. This is shaped to curl the edges of the swarf, so that it is narower than the slot you are cuting and so will flow out freely. If the swarf comes out as a flat section, it will jam in the groove.
You may also see the cutting edge is slightly angled, to reduce the pip left on the parted off bit.