Parting Off (2)

Like a hot knife through butter - well just about when you have the right setup - read on !

Parting off  probably gives beginners to model engineering more sorrow than any other aspect of turning and facing – it certainly did with me.

A while back I called upon my friend and mentor John ‘Bogs’ Moore to prepare an article to help those who struggle with this aspect of machining and ‘Part One’ has already been published and covers the various tooling options whilst ‘Part Two’ was planned to describe techniques to employ for trouble free parting. Unfortunately those of you who know ‘Bogs’ will be aware that John has been increasingly dogged with ill health over recent times and this has prevented him from completing the second part of his article. Whilst preparing these notes there is hopeful news that the medics may have found a treatment that will relieve John of at least some of the considerable pain he has been enduring – I am sure many others will join me in wishing you well John.

Meanwhile, as an interim measure, I thought the following ideas and suggestions based on my own experience might be of some practical assistance to those of you who may be struggling with that much dreaded parting off procedure. I am not claiming that my techniques are the only ones that work but they work for me and may work for you too.

One thing to aim for before you start is to get the setup as rigid as possible. Try tweaking up the tightness on the gibs and lock down the carriage to the bed if your lathe has this facility.

Ensure that you have the absolute minimum amount of cutter blade protruding from the tool holder to minimise flexing of the blade.

Similarly avoid too much workpiece overhanging from the chuck. In other words part off as close to the chuck as is practical – and safe. On some occasions when machining a longer workpiece it may be impossible to reduce overhang to a sufficient degree and if that is the case Bogs has a few words of advice.

Don’t be tempted to use a tailstock centre for support. As you get deeper into the cut, the job will start to flex and will grab the cutting tool. If support for a long piece is needed, it is better to use either a travelling or fixed steady mounted very close to the cutting tool. For cutting off a long piece you could even use a fixed steady at the tailstock side of the saddle, and a travelling steady mounted close to the tool. But any type of support you use is liable to trap the tool as it gets towards the centre. Just that some methods grab a lot quicker than others.
In fact, I would part most of the way thru and finish off with a hacksaw, ensuring you have a piece of wood under where you are cutting, to prevent the hacksaw cutting into your bed as you come to the end of the cut and the metal and saw drops very quickly.

The other area to concentrate on is to reduce the likelihood of the tool ‘grabbing’ as you progress with your cut. Standard HSS parting blades are tapered down from top to bottom to reduce the grabbing potential. You can further improve this by grinding the sides so that the cutting tip is the widest part of the blade. Grinding the sides of the tool to reduce ‘grab’ could overcome the main cause of any parting off problems which you may be experiencing.

You must also ensure that the cutting tool is perfectly ‘square on’ to the workpiece – if you go in, even at the slightest of angles the parting tool blade will eventually start grabbing. Believe me it is worth using a DTI (dial test indicator) to get this spot on. It may take a few minutes to set up but the chances are it will save much more in machining time and probably avert you from a disastrous outcome.

As for cutter height this should be precisely on centre or the minutest amount ABOVE. When parting, you should never have the tool low, it must be exactly on centre or minutely above, as the forces tend to push the tip downwards, so if slightly high, it will end up on centre. If low, the tip will tend to be dragged down and under the job, with the likelihood of damaging the tool cutting tip. And we don’t want that. If you haven’t already seen it take a look at http://start-model-engineering.co.uk/begin-with-bogs/top-tips-from-bogs-setting-cutter-height/

Cutter speed – I can’t give you the definitive answer to this. I find that running at around half the speed I would normally use for turning and facing works for me. You may find it helpful to nudge up the speed as you near the centre. More important, in my view, is to try to maintain a steady and constant cutting pressure rather than taking ‘stabs’ at it. Try to achieve a continuous ribbon of swarf rather than chips which can jam.

Lubrication – for aluminium I find that WD40 applied with a brush is an excellent cutting lubricant. For brass use a cutting oil or even 3-in 1 will smooth the way. For steel, other than free cutting stainless, transfer the workpiece to the bandsaw then face off on the lathe !