Making parting off tools

Another beginner's question. On my lathe, if I make a tool from 8mm square HSS and clamp it in the tool holder with no shims or anything, then the top of it is about at the right centre height. I bought some 18 x 2 mm HSS with the intention of making parting off tools from it, but there seems to be no way of setting these at the correct height unless I grind them away to be 8mm high (unless I remove the compound slide and find a way to mount the tool holder on the cross slide). What's the best way around this problem?

Thank you - Rowan

It's usual for parting off blades to be held in a holder:

The one behind is for a carbide insert blade, but similar can be done with HSS. The lower is for a shop bought 2 x 8 mm HSS blade - the sides are wedge shaped to provide side relief, not parallel.

The height is adjusted by the shimming the holder in the usual way, but make sure the holder fits your lathe.

Dave

Parting on an old lathe with a home-made tool could be a recipe for frustration. Parting off is a rite-of-passage for lathe owners. I never had any luck with HSS parting tools. You can buy excellent carbide-tipped parting tools such as these:

https://www.rdgtools.co.uk/acatalog/8MM-Shank-Parting-Tools.html

**LINK**

which use inserts. I have a similar one actually made by Sandvik which I have used for years and it is excellent - in fact I just parted off a bit of 1" square steel using it running at 200-odd rpm.

In my experience parting off goes better with quite a high spindle speed and a fine X feed - I was feeding at 3mm/minute for my bit of square bar. This makes sure that the "tooth load" is small but there is plenty of heat being generated at the tool tip to soften the metal. The temptation for beginners is to use back gear and try to feed very slowly by hand - inevitably the tool digs in, the lathe stalls and the air turns blue.

That's the wrong approach; grind the tool to whatever shape is required and add shims to get the tool on centre height.

With regards to parting off tools one can buy 1/16" x 5/16" HSS blanks from Arc. I use similar blades on my repetition lathe:

With regards to insert parting tools i use one on my centre lathe. I found that they chatter badly if the feedrate is too low. I part off at several hundred rpm upwards and use a minimum feedrate of 4 thoui/rev, more usually 6 or 8 thou/ rev. I'm also in the part off using power crossfeed group.

Andrew

Absolutely!

This is my M Type rear parting off tool.

The tool holder is at the rear on the carriage with the blade upside down. It is solid with a 2mm blade and it has the original tip in I fitted when j made it at least 18 months ago. You can just unclamp the top and slide the blade to the right length for the job in hand.

David 

Edited By David George 1 on 04/07/2022 19:42:18

Can you still buy Eclipse parting blade? I don't know as I acquired a lifetime's supply of various sizes in an auction years ago so I have never tried to buy any recently. You need to put it in a good quality holder, like a Jones & Shipman, and mount it in a solid rear tool-post. You can either shim the tool or grind the blade to very slightly above centre height. Always use a copious amount of cutting fluid, preferably pumped and not dabbed and keep the tool cutting once it starts with the work rotating at a moderate speed. The reason the tool should be very slightly above centre height is that if it tends to dig-in the rotation of the work forces it away from the work. If it is slightly under centre height the rotation of the work tends to push it in further as the blade rises to the diameter of the work. I have parted 6" noggins of mild steel on my Colchester and some fairly hefty diameters on my Myford 7R with very little trouble. I can't say that I have never 'pinged' the odd blade, but not very often and it is literally a two-minute job to grind the broken tip off and reset it in the holder.

I personally dislike the George Thomas type rear tool-post for the Myford because, IMO, it puts an inordinate amount of stress on the T-slot and tends to lift along the central joint if the clamping bolt flexes, that's only my experience. Better to acquire a proper Myford accessory, or make your own to the same more rigid design. If you have a long cross-slide it can stay in situ for most of the time. I know some people will disagree with all of this.

I for got to mention that for parting larger diameters, it is better to use the 4-jaw chuck because I think you can hold the work-piece more rigidly than in a 3-jaw. Again this is open to dispute.

Edited By Chris Crew on 04/07/2022 20:10:49

I never managed to part off in my lathe(s) because my bench type lathes are just no rigid enough. Since I reversed the the tool position parting off has become an "easy job" using a 1.2 mm HSS blade or a 2 mm SP200 insert.

I make my own tool holders to keep the blade as close to the tool post as possible. I set the tool height by grinding the bottom of the tool holder until the height is accurate. I check the heigth by facing a piece of aluminium until the "pip" at the end has gone. Be sure to check the pip is removed by turning and not breaken off by a to low tool.
The top of the parting blade is supported by the tool holder to make it more rigid.

On the mini lathe, I also lock the top slide gibs for a more riged setup.

Edited By Huub on 04/07/2022 22:03:02

sorry Chris but you need to have a good look at GHTs rear tool post design, it does no strain the T slot as everything is sandwiched together making it an extremely rigid affair. Funny how there hasn't been any designs to supersede it!!!

To deal with the basics of lathe tools and other issues with older lathes, what you really need is one of these. LINK

Do what holds it down? I'm not suggestion that it is overstressed, but it must have some stress applied.

Yes thank you, I already have a copy of this. It answers many questions, but not all.

Rowan

That's good. His section on grinding tool bits is the best I know of. Very clear. His knife tool bit will solve your other brass turning problems in your other thread too.

But being a bit dated may not have the modern style parting tool holders in it . He does have a rear toolpost design much the same as the GH Thomas one discussed above. They work very well on these older lathes but it;s a bit of work to make one.

Edited By Hopper on 05/07/2022 00:47:34

I am a great admirer of GHT and have benefited greatly from the information he has provided in his various articles and books. However, because he was such a superb craftsman everything obviously came very easily to him and his tools probably always cut as smooth as silk and like a hot knife through butter. This is the the reason, and it is only my opinion, his designs are a little 'flimsy' and lacking the substance that much lower skilled workers, like myself, feel more comfortable with. I actually made one of GHT's indexable rear tool-posts and found it to be next to useless, because, a: you have to grind the angled blades very accurately in order to get anywhere near centre-height, b: unless the angled grooves have been machined very accurately this compounds the difficulty of achieving centre-height, c: having got the blade to a semblance of centre-height, if it dig's in the centre clamping bolt tends to flex and the turret lifts slightly.

This was only my personal experience. I am certain that many other much higher skilled operators than myself have had great success with GHT's rear tool-post and his other designs, it is just that I personally absorb his advice in principle but apply it to more substantial lumps of metal in practice.

If I remember correctly the parting blade slot on GHT's rear tool post is machined on an incline providing easy adjustment of centre hight. The slight lifting/flexing means that the tool is disengaged from the work slightly which prevents digging in.

regards Martin

No flexing in this baby, on my Drummond M-type. Made by a previous owner back circa the 1940s.

Everything flexes, just a question of how much and for parting does the flexing add to the cut or back it off.

regards Martin

If it possible, mount the parting tool inverted in a rear toolpost.

There have been explanations of why this works better; but it does.

The fact that swarf falls away, reducing the chances of clogging must help.

Again the greater the rigidity, the greater chance of success.

If it needs to be said, ensure that the cutting edge is on the centreline, and that the tool has front and side clearances.

So slack gibs and nothing clamped is a recipe for failure. If things can flap about, there are very likely to be dig ins, and damaged tooling.

With front mounted parting tools, both HSS and carbide, I have suffered dig ins and damage tooling.

With a back toolpost (Home made 4 way copy of the 75 mm square front) there have been very few problems, never a broken one!. The tool is an old (Upwards of thirty years ) 3/32" wide HSS.

Often used dry, or with either gravity drip feed of soluble oil, or brush applied lubricating oil. Works so well that I now frequently use power cross feed, (0.0025" / rev ) Lacking PCF, just keep up a slow steady infeed.

Get the machine, tool, ieed rate, and cut, right and there should be few problems.

Howard

Plus one for that.

Tony

"sorry Chris but you need to have a good look at GHTs rear tool post design, it does no strain the T slot as everything is sandwiched together making it an extremely rigid affair. Funny how there hasn't been any designs to supersede it!!!"

Bernard T, I know we are never going to agree on this because we all have our own individual workshop experiences, and I am genuinely pleased that you find the GHT tool-post performs satisfactorily. But just for the record I too made a rear tool-post of GHT's design and followed as closely as I possibly could all the dimensions and instructions as given by the Great Man himself. It never, for me at least, lived up to its reputed performance and the remains of it are still in my scrap bin after probably getting on for thirty-years. I think there that will be many more designs around that easily out perform it in terms of simplicity and rigidity and its not rocket science for anyone to knock something up off the top of the their head. When I acquired the OEM Myford accessory it was immediately obvious that the increased rigidity offered by this item exposed all the faults I found with the GHT item, both in design and performance.