Parting tool trouble

Oscar Wilde I believe.

To make a shallow groove in the top face of an HSS parting tool, a circular diamond rat-tail file does an excellent job.

Since I discovered parting off under power feed at sensible rotation speeds I haven't really had any problems.

“Sarcasm is the lowest form of wit but the highest form of intelligence,” wrote that connoisseur of wit, Oscar Wilde. (Scientific American)

And G H Thomas also recommended grinding a small groove in the top surface of the parting tool, not to produce two separate streams of swarf but to produce one ribbon of swarf that was curled inwards so was narrower than the width of the groove being cut/parted.

The Eccentric Engineering T section HSS blades have the top surface hollow ground slightly concave so the ribbon of swarf comes off curled slightly narrower than the groove it is from.

I tried a couple of the cheap Chinese version, neither did very well

Chris, is this your first attempt at parting off or have you been doing it before? If you did do it successfully before what tooling did you use?

I don't have problems parting off and never have, I use a bog standard HSS parting tool mounted in a Dickson holder on both my Super 7 and all other lathes I have. Not saying its wrong but I have never used power feed for parting off, I like to have the "feel" of the tool during this operation.

Worth noting Chris is using an ML7 and no power cross feed is available ! I do have a tool of the type you are using , tried it and it was OK but did not cut as well as the HSS, considerably more pressure required to make a cut.

Pretty much everything else has been said, make sure the machine is adjusted correctly, main bearings in particular, tool to be "tad" above centre, plenty of lube and a steady feed -- with hand feed do not dwell at the end of the cut -- feed in and back off, feed in and back off. If you leave the tool touching the work it will induce chatter and don't be too gentle with the feed - its all about "feel" !

To get the centre height correct a tool height gauge is well worth making but in the absence of one just face a piece of stock leaving a tiny pip at the centre to set your parting tool to.

Isn't this more a case of what is the centre height?

Is it the very top of the insert in which case you get two streams where the chip is forced to tear in half or the bottom of the vee, in which case you get the U shaped chip that also explains the 'you set insert carbide 5-10 thou above centre'.

I've tried both HSS and carbide parting-tools, in a rear tool-post, on my ML7.

The HSS seems the more forgiving.

Carbide inserts are made in a baffling array of types and some suit particular metals better than others, but they all have that central groove to fold the chip in on itself, recommended for HSS tools too, I see.

THE big mistake and one I made for years until learning better, is to grind the tool at a pip-breaking angle, because that forces the tool to try to cut a chip wider than the groove.

The tool needs not only be at right-angles to the work (parallel to the chuck face I use to align the tool-holder). It also needs be perpendicular to allow its side clearances to work.

Wherever possible I set the tooling to centre-height by turned pip, which will naturally be on the axis set by the spindle and chuck. Sometimes I use the tailstock centre but that might be optimistic.

"Above" centre-height? (Below for a rear tool-post). Not sure, but I would have thought 10 thou a bit too much.

'

It was Oscar Wilde who opined that:

Sarcasm is the lowest form of wit...

but that is only half of the sentence. It continues,

... but the highest form of intelligence.

QC Parting off Toolholder and HSS blade supplied by RDG Tools. "It's got their name on it," photo in my album. It's 3/32" thick x 1/2" deep x 4" or 5" long. I clamp it in the QC Tool post then loosen the post, run carriage up to the (stationary) chuck, hold side of the blade against the front face of the chuck, tighten the toolpost nut. Run carriage back to position for parting and lock carriage, half usual turning speed, plenty of good quality cutting oil, both hands turning the cross slide handle at a steady slow speed. Works perfectly every time. Conveniently, tool ready ground angle at front face ready to use in the front toolpost. Rear end of the tool is also end ground but the other way up, so it could be instantly transferred to the rear toolpost and immediately ready to use. One very satisfied customer of RDG.

The ten thou I mentioned was allowing for a Myford top slide to bow down under load. 5 Thou is normal and correct on a more rigid lathe. Nothing at all to do with two chips or one curved chip (two chips would indicate something was severely wron with the insert). It's to stop the work riding over the end of the insert at the finish of the cut and pulling the insert apart.

Others have said it already, but rigidity is key when parting off.
My nearly 60 year old Chipmaster parts off with a carbide insert tool from Glanze like a hot knife through butter.
Loads of coolant and a steady feed, it leaves a mirror finish. Never really bothered about precise height either, just the usual trick with a steel rule between work and tool to set the height.

If any subject is needed to stir the pot, parting off will do it !!

Too right! Iv'e Googled it and took a bit of interest in the subject, there is every conflicting opinion under the sun out there, some say HSS some say carbide is best, then there is front vs rear tool post, some say set under centre some say set high and some even say set on C/L Just do whatever works for you.

Tony

Edited By Tony Pratt 1 on 04/08/2022 17:31:44

Given the number of queries regarding parting in this forum I must be the one of the few who has next to no issues and I have parted large noggins of mild steel to make chuck back-plates and that eventually requires a long blade over hang. I start with a small overhang and progressively let the blade out of a J&S type HSS Eclipse blade holder. I have no experience of the new type of tool design which I suspect is primarily intended for CNC machines. The 'secret' is, although it is not really a secret, is to mount an inverted tool in a rock-solid rear tool-post, use a moderate 'common-sense' speed commensurate with the diameter of the work-piece and flood the groove with coolant, just dabbing it on with a brush is next to useless in my experience. I know some people don't like the mess pumping coolant sometimes makes, but needs must etc. You have to be 'fearless' in applying the cut and once the tool starts to cut make sure you keep it cutting. Anyway, on steel at least, it works for me.

It does seem that parting-off is THE operation many of us to fear above all else.

Yes, you've successfully cut a difficult thread - oh, all right, if you're me you've finished it with a tap or die... now it comes to parting the work-piece from the stock.

The factors that seems to stand out are accuracy of tool alignment (perpendicularly both horizontally and vertically to axis) and rigidity.

I don't have a flood coolant system presently set up, so have to rely on brushing coolant on. In fact it's not acting as a coolant in that situation, but as a lubricant, and after a few revolutions it is anywhere but at the cutting edge. I mitigate the problem a bit sometimes by using cutting-oil neat, so it sticks to the cut a bit better, but it is still not ideal.

On a large, industrial, good quality, rigid lathe like the Chippy, you can part off with just about anything. But on flexible little hobby lathes like Myfords etc, it seems HSS with a good sharp edge works best. They just don't have the power or the rigidity to get the best out of carbide tooling with its relatively blunt cutting edge.

Another solution is buy a horizontal bandsaw. Handiest thing ever in the workshop, including for parting completed jobs from stock if you have trouble with parting tools.

Having deliberately ground a3/32" HSS parting tool with an angled front face, to hopefully produce a workpiece without a pip, the wider swarf did tend to jam in the groove, producing an occasional dig in, even with drip fed soluble oil coolant.

So reverted to grinding the front face square to the again. Since then almost no problems.

(Now so brave that I use PCF for parting, meaning that the need for facing to clean up, is minimal. )

There have been various arguments "explaining" why an inverted tool in a rear tool post works better.

To my mind, it just does! As long as you remember to follow the advice about correct adjustment of gibs, maximum rigidity, minimal overhang and sharp tooling, in combination with suitable feeds and speeds..

You tend to be either lucky, or get better as you gain experience!

Howard

Locking the carriage is essential too.

Hopper,

Reading your comment about locking the carriage, made me re-read my last post @12.40 on 4Aug, thinking that I hadn't listed it, but I have, in line 5. Hopefully, I listed every move in detail, of my method. I notice that I made no mention of centre height, but I think that should be something done automatically. As others have said, so many different reccommdations and methods of, just do whatever you find works best for you.

John

When it comes to parting off, it has all been said before.

Yes, but newcomers are often shocked to find how difficult it can be. The operator's ego is liable to receive a sharp slap, worse when the individual is over-confident and armed with the best lathe and parting tool he can afford.

My experience:

• Parting-off steel on a mini-lathe with the tool mounted conventionally in the tool-post is very difficult.
• Parting off on a WM280 with a rear-mounted tool-post and power-traverse is easy.

The main difference between the two machines is rigidity.

• A mini-lathe's tool-post is relatively spindly and there's no facility for locking the gibs. The heavy pressure needed to force a broad cutting edge into steel is likely to twist the set-up between tool-tip and bed enough to cause a dig-in. A steady hand is essential: only real men have the level of skill needed to part off steel with a mini-lathe! Easier to do with HSS than carbide.
• The WM280 is much heavier, the gibs can all be locked, the saddle takes a rear tool-post, and power-traverse eliminates problems caused by an unsteady operator. The operator doesn't have to be highly skilled - just competent enough to set the job up correctly. HSS and carbide both work.

Chris's machine in good condition is stiffer than a mini-lathe, but may be worn, and it isn't as hefty as a WM280. He's not using a rear tool-post or power-traverse, and is new to parting-off. So he has to do everything possible to stiffen up the job, and then develop the skills needed to feed the tool into the work at the exact steady rate needed to keep cutting without digging-in. Quite hard to do, because it's also important not to rub and to keep the slot clear of swarf. Practice and experiment. Start with thin HSS blades rather than broad carbide.

Dave

I think it was This Old Tony on Youtube who showed that a shorter distance from the headstock improved his parting off even on a larger lathe than the mini lathes. This was done by using two different chucks so that as much as possible all other criteria were the same. I know that parting close to the nose of the spindle using collets is far superior to using a chuck even on my solid old machine.

So here's a thought, Leave space to use a fixed steady close to the parting cut and see if that improves the result.

Martin C