Parting off tool - straight or angled.

No. If you look at my pic above and as I said in the text, no angled grinding of the top of the tool blank is needed. I grind it dead flat and straight right along , ie zero back rake. Cuts steel just fine like that, despite all the theories.

If you start grinding the top surface back at an angle, the next time you remove some metal from the front edge to sharpen the tool, it is now narrower at the cutting point that at the point where it has not been ground on the top edge. So when that point gets to the groove in the job, it jams.

I use a small diamond ball cutter in a Dremel to put a dimple in the top face, just behind the cutting edge. This mimics the form of a parting insert & distorts the swarf, which comes off narrower than the groove, so no jammimg.

Nigel B

I use the T shaped blades in a straight holder, all you need to do is provide a little relief at the front. The top of the blade includes a chip breaker along its length in the form of a slight U shape.

A mini lathe is usually, in my book, something like a Seig C2 or C3.in size. Think Clarke Metalworker / Chester Conquest as similar.

Myford ML7, Boxfords and larger do not fit the description.

Being tight (to put it mildly) I use my HSS parting blade, horizontally in the rear toolpost, without grinding any top rake, and have done so for nearly 15 years. Dig ins are almost unknown, and do minimal damage,

(And I invite them by grinding the front edge at an angle, in the hope of leaving the workpiece without a central "pip". This makes swarf wider than the cut, but the swarf usually finds its way out without a problem, in small pieces; possibly because of the zero top rake?)

As long as you are finding your way, I would not advocate angling the front until you are a lot more confident. If you have a problem, you need to have the minimum of possible causes. Don't let me lead you astray!

In addition to all the help that you will get on here, join a Model Engineering Club, where you will probably get face to face advice and first hand demonstrations. Whereabouts are you, roughly?

Someone on here may well be near, and happy to help and advise.

Howard

The two photographs above show the parting blades I use. The blades are ground on the front edge only. 5 Degrees of front rake and with a 10 Degree angle to give pip-less parting. The Maximat blade is 2 x 13 mm section, while the Compact 5 is 8 x 1.5 mm. The dovetail section of the blade combined with the front rake turns the swarf on its side and this exits the groove without any problems.

Too much front rake is the root cause of dig-ins and the second being the rigidity of the set-up. The blade in the Maximat was made by Seco and purchased back in the 1980's. While this blade is nearing the end of it's useful life this blade has parted off 60 mm EN24T with no problems. 25 mm EN1APb, (free cutting mild steel), is regularly parted off at 600 RPM on this lathe. The compact 5 easily handles 16 mm EN1APb, provided the carriage is locked, (rigidity again) and the speed kept down, usually 200 RPM.

I hope this helps

Regards

Gray,

Nice tool post Gray, that looks very solid.

Graham Meek:

Inspiring work! and, perhaps, something of a homage to GHT?

His rear tool post featured built in top rake/ height adjust.

I have made a mental note of your point about too much top rake for when I next grind my blades.

Thanks very much for the compliments. Yes the toolposts are an adaptation of GHT's. I fear some may have thought the two photographs above are one in the same. For clarity I have shown below the two versions.

Going back to the front rake. If one considers a tool will zero front rake then it will not cut as the point cannot penetrate the surface. If it cannot cut it certainly cannot dig in, is my theory. Therefore as the front rake is increased the tool point is allowed to cut, but with some resistance. A small front rake thus gives a self limiting action to the tool and is I feel why it does not dig in. This theory may be unfounded, but 40+ years of metal turning have shown this procedure works. Take the front rake to an extreme and this resistance is lost, the tool can literally bite off more than it can chew, with a calamitous result.

One thing a reduced front rake does require is a tool set on centre height. If the tool is above centre then it has zero front rake and will not cut. Conversely when below centre then the front rake increases with the usual calamity. The parting tools in my 4 tool turrets are automatically on centre height as they were made to be so.

Just as a passing example, the Hardinge HLV lathe I once worked had a Multifix tooling system. The HSS blade in these holders had a "U" shaped groove ground in the top of the parting blade, with a small land either side. Obviously the cutting edge is now much longer than the width of the parting groove. The "U" had the effect of curling the chip in on itself. Not that dis-similar to the inserted tip blade profile.

The section of this blade might be found interesting given that the Hardinge is about 5" (127 mm) centre height. The blade was 2.5 mm wide and approximately 11 mm high, not what would be expected on this size of lathe. Yet this tool would handle all the tool steels used in my profession.

Regards

Gray,

Don't buy one of these! Got it off eBay from a Herefordshire tool seller. Used it an hour ago for the first time. There is simply too little metal beneath the blade, hence the result in my photo.

The angle of the blade in a QCTP holder is, at least in my opinion, immaterial. The top of the blade van be ground to produce zero or negative rake or to create a high positive rake.

I have never used a rear mounted parting tool so take my opinion with a grain of salt. I know from experience that a front mounted parting tool tends to deflect INTO the work. That can cause jamming, or worse. I think the rear mounter (inverted) parting tool deflects AWAYY from the work, tending to reduce depth of cut. Another factor is that rear mounted tool posts are very rigid compared with the tool post mounted on the top slide.

The problem with grinding on top rake is that when you come to sharpen the worn tool by grinding a bit off the front surface, the cutting edge moves down the ground slope to a part of the blade that is narrower than the very top. Thus the cutting edge is narrower than the full, unground blade behind it. This wider part will then jam in the groove.

You see various attempts to remedy this by grinding the sides of the blade etc but it rarely works, and weakens the blade. My experience over the years has been to never grind the top of the blade at all. If you want top rake - which I have never found necessary - use an angled holder.

Exception: Some parting tool blades are supplied with a top edge that is slightly angled or domed etc so I will grind that flat and square uniformly along the full length of the blade.

LOL. It's only for "hobby" use sir.

Had a similar thing with a conventional straight parting tool holder, except it simply bent like a banana under the toolpost clamping bolts until the HSS blade snapped. Finest cheese metal.

The angle blade holder that came with my QCTP is a heap of junk ( bottom left of lower pic ) and as the blade is only clamped at one end and it sits in a parallel sided groove the rear of the blade is free to wander wherever it wants .

Hello Hopper, I really like the look of your rear tool holder; is it one that came with your lathe or a shop made one?

Brian

Here's a picture of my rear part off toolpost I have just finished. It's the first time I have used one but the feeling of sturdiness is amazing and it gives you the idea that you are in total controle.

David

I think it was shop-made by a previous owner sometime in the 1940s before my Dad bought the lathe in 1952 or so. It is very solid! No flex at all. I've never seen another like it.

I've used one of these RDG blade holder for I don't know how long. It works on steel, titanium, brass, delrin or whatever I'm parting up nearly 2" diameter. I wore one out over the years and replaced it last year.

It's not always ideal, and very occasionally I've broken the blade. The only thing I can think of that's defeated it was some 2.4" OD x 1.75" ID EN8 suspension bushes I was doing for the railway - the stuff was so resistant in hollow tubes, and I couldn't stand the shrieking, ringing chatter that was making my hearing aid go into feedback loops.

Zero top rake is again not ideal for anything except CZ121 and suchlike, but for most materials - as long as you keep the tool well into the work - it cuts accurately and cleanly.

One problem I have had is the the last couple of blades I've bought have had side rake on the top surface, which deflects the cut and produces a concave surface on the cutoff face of the workpiece. Just touching the top with the bench grinder to take off that rake seems to have resolved that.

I've use blade-type partoff tools with carbide inserts and they're better in some ways, but relatively expensive and give you another item to maintain stocks of. The HSS blades are of course regrindable and leave you more self-sufficient. I don't wanna be sitting there stroking me chin and trying to decide which of a range of tools to be using for parting-off.

Hopper,

Thanks for setting me straight. I knew all along the parting blades are tapered. It never occurred to me that by grinding the top I was forcing the sides to cut as well as the end. No wonder I had problem sith the tool jamming in the cut.

Too bad somebody didn't enlighten me before I made an ass of myself in public. Now I know I have been screwing up for 60+ years.

The advantage of the flat parting tool, is that it allows for the length of the tool out of the holder to be adjusted and the centre height remains the same. The reason for the inclined parting tool holders was to allow the chips to fall down the inclined surface. The inverted rear tool post has all the advantages as mentioned by others.

I only have the normal parting off tool from the front face. I purchased a 2mm wide inserted parting tool with 5 inserts for parting and another 5 inserts for the multi turning function. I am still on the 1st parting insert. Until then I was always using the hss blades that came with the original Dickson holder and the spare.

On the insert and the hss blade, it has a curved geometry on the top face. It curls the chip as it comes off the work piece and as it curls, it creates clearance so it can clear from the groove. I always apply from a bottle with a small spout and hole, from the front and back towards the tool holder. The chips come back along the blade. You can use water based coolant or cutting oils, it just needs something to stop any buildup on the edge of the tool.

I have often thought of getting a rear toolpost setup, but have only ever thought about it.

Neil

Thanks Hopper for the info on the rear toolpost, it certainly looks sturday.

I shall have to knock one up as a project when I have time.

Brian