I don’t find hex or square to be a problem with a front post, maybe run a little slower than I would with round until the tool is cutting solid metal. Surface may want a clean up skim anyway so you could just as easily saw it off.

For what it,s worth, I agree with Jason, the bandsaw first then just a simple skim to clean up. As it happens I had to make a number of unusual thread nuts recently for a local and I found it was quicker and simpler to employ saw and face tool, forget about parting off and all its hassle.

I keep a really sharp, double ended flat top symmetrical 45° round nosed tool in one QC holder for chamfering, edge breaking and fine cut cleaning up of not quite good enough surfaces. So simple and so useful.

If I’m parting off hex bar intended to be a nut of some description I run the round nose in until there is a chamfer all round across the flats, so a little deeper than the hex edges. Then I run the parting tool into the middle of the round end Vee. No issues with getting going on the sharp hex points and, after parting off, there is nice, blunt, chamfered edge just like a normal nut. Simply parting off always leaves a painfully sharp end to the hex corners.

Clive

I parted some mild steel Hex bar the other day with a T shaped HSS blade. As expected it cut fine.

Beauty is in the eye of the beholder. I mostly make single chamfer nuts ‘cos they look right on old engines. 😉

I do take the initial cutting at low speed and gentle feed until there is an appreciable arc between each flat.

One problem I suspect with intermittent cutting on lathes with screw-on chucks is the hammering action tightening the chuck still further.

If we reverse the process, making polygonal features on essentially cylindrical items, I use a different approach where possible, especially for batches of items. Here I cut grooves to the eventual A/F diameter at the parting-length, and add any chamfer at that point, then mill the flats before parting-off. It means two machine transfers but since I am more likely to make five rather than five hundred parts, that’s not a great problem. If the maximum diameter fits the lathe spindle the intermediate stage is a rod with a series of grooves round it. The end effect is that the parting-off is always on cylindrical stock.

I think when we are new to something we tend to try to look for the silver bullet for any unknown situation that will automatically solve it.  Unfortunately there are frequently too many variables to resolve with a single simple answer.

What experience will eventually teach you is to ‘feel’ when you are machining.  You can set the revs for the material and the diameter but then the feed is up to you and that is determined by the feel of the handwheel.  Only you can feel as you slowly rotate the handwheel whether it is too much or just right.  Too high a feed and the chatter will rattle the bed, saddle, slide, and your teeth and eventually break the tool.  Feed it slowly enough so that everything feels nicely under control.  As Jason mentioned earlier, you will find that initially the required feed is slower when the chatter is more noticeable but the feed can increase as the depth increases.  It is all governed by the feel.

On my centre lathe I part off using power feed. So feel doesn’t come into it, just knowledge of what feeds and speeds work.

Carbide parting off inserts need a minimum feed, otherwise they chatter badly and break. I use a minimum feed of 4thou per rev. If I need to machine an accurate groove, sneaking up on dimension, I use a HSS toolbit ground to look like a short parting off tool.

Julie

I’ve seen the machines working when we used to do work in Monument Tools factory. There is a rotating head with a cutter that is timed to the spindle depending on how many flats you want for say a square or hexagonal key the cutter would rotate 4 or 6 times faster.

That is much like I remember seeing, drive shaft came out of the headstock to drive the tooling and I suspect the gearing was also in the headstock. Quite a large diameter cutting tool flying round.

No CNC in there workshop in Balham but that was back in the early 90s, posh flats now. They also had cam opereated lathes with long bar feeders for the repetative items.

Slow and steady is the safe way, with a sharp tool on centre height. Rush the feed and, particularly on the flats and a dig in or broken parting blade is likely.

Saw and then face is the safest option, until you are confident in the tool, machine and operator

Howard