lathe bit sharpening

[Piero FranchiParticipant @pierofranchi37209]

Back rake

[Piero FranchiParticipant @pierofranchi37209]

Hi Guys

Got a question for ya.

keep in mind, this is NOT for a carbon insert and its for a old ish lathe with a max rpm of 1600

My question is, how come they used to sharpen tool bits (and have tool holders) with a lot more back rake than we see now days???

Tell me if I am wrong in my thinking,

but try and scrape a stanley blade on some wood at a 90 deg angle to the wood,

now tilt it some, its going to cut into the wood now where as before it would only be scraping

is that not going to be the same on a lathe?????

If not, why did they use a lot more back rake in times gone by???

[BazyleParticipant @bazyle]

Less power. With higher rake the tool is 'sharper' and cuts more easily with low power, maybe only flat belts to transmit that power which are less capable than v-belts. However the side effect is less strength and less ability to conduct away the heat which dictate lower speed and more small cuts need to be taken.

[VicParticipant @vic]

If you look at the old books you'll see lots of different lathe tool shapes and several different angles for each tool depending on the material to be turned. I can't remember for sure but I think if you wanted to cover all likely turning operations you'd need something like 120 different lathe tools. That was for production turning using carbon steel lathe tools. Since the introduction of HSS though all those tools simply aren't needed, especially in a home workshop.

[Piero FranchiParticipant @pierofranchi37209]

just trying to learn as much as possible being a relatively new machinist

[mechman48Participant @mechman48]

Hi, & welcome Piero.

​Might I suggest you purchase 'Model Engineer's Handbook' by Tubal Cain, usual disclaimer, it is the ideal book for all level of model engineering enthusiasts. I have been in Engineering all my life & have still had need to refer to it on occasion, I think today's price can range from £6.95 to £8, well worth it. Have att. pics of extracts from it…

I purchased mine from Argos books… ISBN 085242 890 1… you should be able to get it from Amazon easily enough… Usual disclaimers.

Hope this helps
Cheers
​George.

Edited By mechman48 on 26/08/2017 11:40:51

[Roderick JenkinsParticipant @roderickjenkins93242]

Piero,

Bazyle has summed it up very nicely. Don't discount the old methods, they are sometimes more suitable for our low powered amateur machines. If you want to shift a lot of metal quickly then a tool like this works very well:

 

Might not last all day in an industrial environment but dead useful at home.

Rod

Edited By Roderick Jenkins on 26/08/2017 12:19:14

[HopperParticipant @hopper]

And yet, LH Sparey in his excellent book "The Amateur's Lathe" recommends grinding tools for Myfords and the like with no back rake, a practice I have followed since revivifying the old Drummond several years ago with great success. It will take a cut .100" deep in steel under power feed with no chatter using Sparey's tool geometry (much like Rod's pic above). Yet as an apprentice 45 years ago I was trained to grind tools with back rake, and indeed the tool holders on the Hercus (South Bend clone) lathes in the training centre had back rake built into them as you say – and they worked just fine too!

All you can do is play around and try different approaches and see what works for you and your lathe. One advantage of no back rake is you dont have to grind away the end of the tool and start again after resharpening a few times, so its more economical on HSS useage.

Edited By Hopper on 26/08/2017 12:19:50

[Roderick JenkinsParticipant @roderickjenkins93242]

I'm not sure what the backrake does apart, perhaps, from steering the swarf away from the job.

Rod

[Mick B1Participant @mickb1]

Posted by Roderick Jenkins on 26/08/2017 12:22:03:

I'm not sure what the backrake does apart, perhaps, from steering the swarf away from the job.

Rod

If you're using the same tool at a different horizontal angle in the toolpost for facing, backrake helps.

[Neil WyattModerator @neilwyatt]

Dare I say it, but Tubal Cain's diagram is rather misleading.

The tool tip moves sideways as well as the work rotating (unless you are parting off), that means that the 'effective rake' is a combination of side + back rake and varies with the feedrate. When depth of cut exceeds feedrate (i.e. most of the time except for light finishing cuts when forces are low anyway) the chip is peeling sideways across the tool not forwards as in the diagram and back rake is pointless.

So it's only when parting off that back rake becomes more useful, but I have some parting tools that use a long HSS blade operated at a shallow angle giving no more than about 5 degrees rake and they work fine.

Rod's 'bog standard' knife tool will perform 90% of turning and facing (as long as you don't force it), and is suited for finishing too with a decent tip radius, although my personal preference is a for a touch less rake (10 degrees-ish).

Rake (in the right direction) is more important in softer materials (aluminium alloys, plastic) where slicing rather than shearing is the predominant action, but side rake is still more useful than back rake for most cuts.

Neil

[Piero FranchiParticipant @pierofranchi37209]

makes perfect sence to me Neil

[duncan webster 1Participant @duncanwebster1]

I'm a great believer in tangent tooling for small lathes. I've got one of the Eccentric Engineering jobs,

**LINK**

there are loads of DIY versions on the interweb, for instance

**LINK**

[IanTParticipant @iant]

Piero,

Rake angles seem complicated but there is a simple way to imagine them when grinding a tool bit. A lathe tool always has a primary cutting edge (some have a secondary one too) which could be at right angles or parallel to the work – or somewhere in between – depending on its shape and intended use.

Looking at that (primary) cutting edge from above – the rake should normally run away from it at right angles. So for a parting tool – it will just be back rake and for a side cutting tool it will just be side rake. If the primary cutting edge is angled, then the rake will be a combination of both side and back. the cutting edge angle therefore sets the 'direction' of the rake and then the type of material defines the amount of rake (if any) required. Generally more rake gives sharper tools but shorter tool life.

Tools with just side rake are easier to sharpen in the sense that just the end needs touching up – but if some back rake is required, then an angled tool holder can also provide this without complicated grinding. Of course, a tool ground for use in an angled holder is ground differently to a tool held horizontally because the clearance angles are different too. The Tangential tool-holder is popular because it has a single face (giving both rakes) that is very simple to keep sharp by grinding.

Try Rod's (side rake only) tool and it will cut most things – but do use a flat topped tool for brass. Clearance can be ground by just using the curve of the wheel to give a slight undercut to the cutting edge…

Hope you can visualise all this – sounds complicated but it is easier once you get your head around it. As others have said, we are not in a production environment and the 'rules' can be bent quite considerably in the home workshop. Best way to learn is to grind a few tools and try them.

Regards,

IanT

[HopperParticipant @hopper]

Posted by Neil Wyatt on 26/08/2017 15:14:32:

So it's only when parting off that back rake becomes more useful, but I have some parting tools that use a long HSS blade operated at a shallow angle giving no more than about 5 degrees rake and they work fine.

And I run all my parting tools dead flat on top, zero rake. Never a problem. The old Drummond will part off 2" steel bar like this. Once you start grinding back rake on parting tools (as opposed to the angled holder you mention) as soon as you sharpen the front face of the tool after a bit of use, the tip of the tool is narrower than the widest part of the blade behind it and if that part ends up in the parting groove, a jam up is certain.

[Piero FranchiParticipant @pierofranchi37209]

To all the replys and perticuly IanT

many thanks

I am dyslexic and find it difficult to read, most of my efforts are going into reading and hence I dont take in what I have just read,

BUT I must say, I found IanT explanation very easy to follow,

maybe it just ticked the boxes that where missing at my point in time, NOT wanting to take away from every ones help.

many thanks everyone

[Niels AbildgaardParticipant @nielsabildgaard33719]

My homemade tangential carbide is ground top flat and eats everything.

Even hardened steel.

At around 5 degree negative it starts to protest.

**LINK**

[IanTParticipant @iant]

Piero,

You are very welcome – I am pleased that you found it useful.

I used to really worry about rake, feeds, speeds and all the other technical stuff (and tables) that you find in books on the subject but much of it is designed for use within a commercial production environment.

Hobbyists have very different needs and I think it's better to try and understand the basic underlying principles and then try different approaches and see what actually works (and what doesn't) within your own workshop – given perhaps limited personal manual skills & work experience (certainly in my own case) and often much smaller & less rigid machinery. Provided we manage to get the end result we need – then I think that is probably all we need…

Anyway – glad to help.

Regards,

IanT

P.S. Hopper – I've never seen anyone mention that 'tapered' parting tools only have side clearance at the tip – and a small clearance should really be ground (both sides) back from the tip….

[Mick B1Participant @mickb1]

Posted by IanT on 27/08/2017 10:10:54:

P.S. Hopper – I've never seen anyone mention that 'tapered' parting tools only have side clearance at the tip – and a small clearance should really be ground (both sides) back from the tip….

I thought he meant bladed parting tools with a trapezoidal transverse section – and AFAIK he's right. I use one of these and vary the protrusion with the diameter of what I'm parting. It's laborious and largely pointless to grind side clearance on these, but if you put back rake on, you have to grind back the runout of that back rake every time you grind back the cutting face, if the radius of the piece is enough to interfere with the runout to the top edge.

Edited By Mick B1 on 27/08/2017 10:24:32

[HopperParticipant @hopper]

Posted by IanT on 27/08/2017 10:10:54:

…

P.S. Hopper – I've never seen anyone mention that 'tapered' parting tools only have side clearance at the tip – and a small clearance should really be ground (both sides) back from the tip….

Hard to explain without diagrams, but yes I was referring to teh usual tapered cross section parting tools. Never seen grinding the sides for extra clearance be particularly successful. What I was attempting to explain was that if you grind back rake on, say, the first half inch of the top, going, say, 1/8 deep at the rear of the ground notch, if you then keep grinding off the front face of the parting tool to repeatedly sharpen it, as is usual, the cutting tip of the tool moves down that sloping ground top surface and thus moves down the tapered cross section, thus becoming narrower than the top of the unground tool blank, which may end up jammed in a deep groove if parting say 2" bar etc.

If you then try grinding the sides of the blade to make it fit into the groove, you end up with a blade getting narrower and narrower as you keep sharpening it and needing to create extra clearance.

That's why I grind the top of my parting blades dead flat all along. To sharpen, simply grind the front face. No further mucking about needed, no jamming, and no need to grind a half inch off the end of the tool when it gets too ground down to be useful and start all over again.

[IanTParticipant @iant]

Exactly so – but a zero rake tool cutting into mild steel will impose much greater strains on the lathe – than one with some rake, which will be sharper. I suspect you must have a fairly heavy lathe. My Mk1 S7 would struggle to do what you suggest and it's one of the reasons I moved to inserted tooling (in a rear tool post) for larger parting cuts, the other being the shaped tip which folds the chips inward, something I never managed to grind on myself…

Regards,

IanT

[BazyleParticipant @bazyle]

It is sometimes easier to understand what is going on when you make things bigger. Cut a 2"x2" piece of wood to your planned profile and hold it against a big biscuit tin as though taking a 1/4" deep cut at 1/4"feed. You will then see that it has to cut on 2 faces.

[Anonymous]

As far as I know metal is not sentient, and will therefore have no idea whether it is being cut on an industrial or hobby machine. Hence the cutting process at the metal/tool interface is the same for industrial and hobby. So it would seem rather silly to dismiss 'industrial' techniques as not relevant.

Of course one has to make allowances for lower power, spindle speeds and rigidity. But there is no reason why standard tool shapes won't work in a home setup. None of the angles are particularly critical, and the tool finish isn't as important as is sometimes made out. I suspect that back when carbon steel and HSS tooling was used extensively in industry the tools were still mostly hand ground anyway.

Personally zero rake parting off in steel doesn't work for me; I definitely need a few degress of back rake. But of course that is on an industrial lathe (Britan), so may be my metal is more intellegent than I give it credit for.

Andrew

[IanTParticipant @iant]

I wasn't suggesting that industrial techniques were irrelevant Andrew – in fact they are a pretty good starting point. It was more that I don't think hobbyists need to be too religious in adopting them in great detail – basically for the reasons you go on to outline.

And whilst the metal may not be sentient – my machines might well be, given some of the strange noises they make if I'm unkind to them…

Regards,

IanT

[Anonymous]

Posted by IanT on 27/08/2017 14:15:38:

I wasn't suggesting that industrial techniques were irrelevant Andrew – in fact they are a pretty good starting point. It was more that I don't think hobbyists need to be too religious in adopting them in great detail………..

Quite so, I expect those in industry mostly used them as a starting point too. I'm not sure why some people take them as gospel.

Andrew

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