Sharpening Lathe Tools

There is plenty of advice on grinding lathe tools but what about resharpening them.

Do you grind the front face, the side face or the top face?

Then where ever you grind you are going to make the cutting area smaller. At what point do you decide enough is enough and start over again?

I'd be interested in hearing what others do.

Peter

Hi Peter,

I assume you want to resharpen HSS tools. I mainly use a tangential HSS tool in addition to HSS parting off tools. I grind both as when they were new and then touch up the cutting edge with a stone or diamond hone. On other HSS tools I grind the edge that get worn, but eventually you will have to grind to shape tool.

Thor

If it worked when new then aim to regrind to keep the angles the same. If grinding on the top face then you will need to reset the tool height to centre. Use a loupe or magnifying glass to study the edge, its size is unlikely to have a bearing unless it has become VERY small. There are numerous books on the matter, a good start and cheap the workshop series! The tool MUST cut, NOT rub, a few degrees of clearence on all faces will work as a VERY rough guide ! Noel.

+1 for both Thor and Noel's advice.

If the tool has the right clearance, (usually not absolutely precise ) and is on centre height, it should cut and cut well.

At least you can regrind HSS rather than bin a chipped carbide tip.

Carbide has its uses, and again must be set with the cutting edge on centre height.

Howard

Yes to all these replies, But what I was really after was at what point would you stop regrinding to sharpen and start afresh.

I've got several HSS tools that cut well but have been sharpened so many times that the cutting edge is getting quite small, Or if the side face has been reground have quite a shoulder limiting their use when facing.

And when starting over how do you remove the old - they are going to take a considerable amount of grinding back. Cut the end off with an angle grinder?

Hi Peter,

When your ordinary HSS tools has been sharpened so many times that the cutting edge is almost gone you have to grind "the shape" back. And yes, it takes a considerable amount of grinding, if you are handy and careful with the angle grinder I suppose it is possible. That amount of grinding it takes to get an ordinary tool back to shape is not necessary with a tangential tool, and is one reason my tangential tools get used a lot. I tend to use carbide tipped tools for roughing and the HSS tools for the finishing cuts.

Thor

IMO it depends on the work balance between grinding the tool anew and setting up the over-sharpened one.

If there's a particular form to the tool, like a specific radius or 55/60 degree screwcutting point, you tend to sharpen on the top to preserve the form, and that can make it a bit fiddly getting centre-height - unless you've got a QCTP with lotsa holders. Eventually you'll shrink the form through side clearance and have to regrind complete.

Knife tools made from a solid HSS blank, you can extend the cutting edge back and grind back the plan trail angle to match, so you don't lose top cutting edge height. Parting tools just grind the front face.

Edited By Mick B1 on 20/01/2022 15:32:24

One thing that doesn't get mentioned too much is the size of HSS tool blank you are starting out with.

It's easier to shape and sharpen a smaller HSS tool than a large one. I generally use 3/16th section these days and for some applications, hold them in a simple steel holder. Doesn't work for every application but worth considering as makes tool maintenance much quicker...

Regards,

IanT

+1 to that. The balance depends considerably on the form of the tool. At which point does wear and resharpening force the whole form to be reground, rather than just touched up? It's when the tool stops cutting properly even though it's sharp.

Old skool:

Modern set:

Sharpening a modern Type 1 is easy-peasy. Type 6 is rather harder, and Type 8 difficult. Though sharpening HSS is usually not too difficult, I find it hard to make any of the traditional forms from a square HSS blank and the extra skill needed to correct a Form problem is one of the reasons I prefer inserts! Could be my fault - I'm cack-handed!

Dave

You answered your own question. Regrind (grind away, reform, remake) when it gets too small! Or needs too much effort to get it up to centreheight and still be able to see it cutting

Touch up grinding is done exactly the same way ( face or side) as the initial grind…….

Edited By Zan on 20/01/2022 20:36:14

When the cutting edge has become too small to grind, I just cut it off with a thin cutting disc and then shape the blank roughly with the disc before grinding to correct shape.

Quick/simple and it works. No need for "Exact" angles, we are not making rocket parts.

I'm a big fan of 'good-enough' engineering, so Larry's point is well made.

I wonder how many traditional Knife Forms are still used?

Writing in 1950, My hero Lawrence Sparey says 'modern experience has shown that a great number of the old tools are redundant, and that quite as good work can be done with only a few of the accepted types'. He also says: 'Never grind a tool in a haphazard manner; have some definite shape in view, and work carefully to this end. Happy-go-lucky grinding methods account for most of the weird looking tools - with as many facets as a diamond - which one so often sees in the amateur workshop.' Also, 'correct shaping of the tool is a necessary and vital part of doing the job.'

Sparey recommends six HSS Forms for general work:

Of Sparey's six, how many do forum members use? I get the impression that much home-ground HSS turning these days is done with a few simple shapes, and that many grinding problems are avoided by not making the more difficult shapes.

Can we do a show and tell? What HSS forms do you grind? Please provide pictures.

Dave

I think I have too many to show the full selection, plus I don't always remember what they were ground for, except that sometimes it's easier to grind a new one than ferret through the whole bunch to find something usable for the particular feature being machined. They're all 1/4" square section, which I fiind the best balance of size and grinding effort needed for my Warco WM250V, and for the Myford Speed 10 and Emco Unimat 3 I had before that.

There's usually another form ground on the other end of those in the pic, but these are my most-used variants:-

L-R:

Plain knife RH. There an LH one on t'other end. I try to keep 2xRH and 1xLH operational.

60 deg small screwcutting. Likely to have to work close to the chuck, so it's on that side of the piece.

Small parting and face-recessing. Tight radius clearance on chuck side. Might work for trepanning too, but it'd have a light radius on the front face if I'd ever used it for that.

Thread undercut tool for shouldered screw forms. Dunno if it's 55 or 60 deg or thereabouts, but can't imagine me making anything where it'd matter.

Small, medium and larger radius groove tools.

Hooky shoulder or thread undercut tool where room is very limited. Used it maybe a dozen times and it's somehow survived.

3 outside radius tools - middle one where central stem has to be kept, like on the little bronze peg-dolly drivers I put in the small toy cars I sometimes make.

Tiny bore finishing tool.

Of course, some can be used ad-hoc for other purposes - for example, the bigger radius grooving tool works nicely for facing too on most materials.

I try to keep one or 2 1/4"x 3" HSS blanks available in case of a sudden need for some form I haven't already got, or got something usable as a starting point.

I tried out something that I once saw, but cannot remember where, but it's a wizard tool for the purpose.

Take a round bar of HSS and grind a top flat on the end and below the edge, grind a 5° clearance. Clamp in toolholder with cutting edge at 45° to vertical and small swivel to the toolholder so that "top" flat is at small angle, say 5° to work. Run tool in direction of chuck at usual turning speed with a light cut, since this is meant to be a fine finishing cut. Mild steel shavings like pan scourer pads. Brilliant finish. Such an easy shape to grind. Use a D bit (same shape) to trial it.

Hi Peter,

JMTPW.

Once I've ground a tool to the shape that I want, I don't grind it again unless I have to ! Its far easier to simply stone the top or the face to put an edge back on and carry on working. Stoning only removes a few thou at most !

Unless you chip or damage a tool grinding it only reduces the amount of metal in the tool and it takes a lot more work to recover it, apart from changing tool heights.

In short the only time I grind a toolbit is to obtain the initial shape, after that stoning is the way to go !

Excellent Mick! I must try harder.

Ta,

Dave

The really big advantage of the tangential tools, is that as you regrind, the centre height is easily corrected. Ifanga used to make tangential threading tools too, but not sure if they are available any more.

As to the original question, cut off the end of the tool area that is no longer useful , then create the new shape that is wanted.

In the early days, the HSS tool holders came with the lathes , like Colchester lathes, the 1/4 square and 5/16 square holders, were inclined at about 5deg or 8 deg, and also had a side angle of about 2 or 3 deg. So the tools only required the font to be ground, and the side relief to be ground. The top already had the relief from the holder.

Neil

As long as the top rake is satisfactory, (Mostly, I work without it, certainly on the parting tool in the rear toolpost ) the only faces that need a regrind are the front and side clearances.

As soon as the sharp edge is regained, the regrinding process is complete. Further grinding just removes HSS that could have been used productively.

For a lot of work, I use a Tangential Turning Tool (Home made for 1/8 and 5/16, bought for 1/4 )

There is only one face to grind, and the jig sets the angles, so all it needs is a light skim to clean up.

(It has taken about 4 years to wear the short, 1/4" square toolbit supplied with the Eccentric Engineering Diamond tool, to the point where it needs replacement, if that is any indication )

Quite often a very light touch with a stone or diamond file will suffice.

Unless the shape is being changed, or it is chipped, very little,regrinding is needed to restore the edge.

When it eventually is worn so much that it is no longer strong enough for the task, then is the time to grind the end back to flat, and start again setting the clearance angles.

When the toolbit becomes too small to hold, then is the time for replacement.

For most purposes, what do you need? The list covers almost every possibility that you will ever need.

RH Knife tool

LH Knife tool For the rare occasions when needed..

Possibly finishing versions of the above with a small radius on the cutting edge. make sure that the radius is all the way down the edge, so that it cuts at the "point" rather than rubs below it.

Parting tool

55 Degree External screwcutting

60 degree external screwcutting

55 degree internal screwcutting

60 degree internal screwcutting

A tool to fit into a Boring bar.

Small boring bar.

Form tools for chamfering or radiusing; as and when you need them

If you were to make up all of these, some will see very little or no use.

In Front and Rear 4 way toolposts, my lathe carries

FRONT POST: Rougher, Tangential, and Boring bar; and

REAR POST Front Chamfer, Parting, and Rear Chamfer

For Screwcutting, Knurling, or Ball Turning, one of the Front tools is removed, temporarily, to make a space for it..

So almost AL:L my work is done by, at most, nine tools (For a half round thread form, I did use a type of Tangential tool using a 1/8 round toolbit. But that was a one off!

And I try to avoid internal screwcutting, if possible, or another two might be needed., to cover all eventualities.

Howard