Graver turning, face up or face down?

[karlmansson]

Hello!

I know this to be a matter of preference and I've heard good arguments for each. I'm just now thinking that I've not yet understood how to do both in a way well enough to be able to choose one over the other.

I started out turning with a small watchmakers lathe and hand held graver and had some success. I started out using the graver "right side up", that is with the ground diamond of the graver facing the work. Had some issues with gravers dulling, not getting any real chip formation and pressure polishing. So I tried switching to having the ground diamond facing away from the work and using one of the flat sides of the graver as the cutting edge. This allowed me to get better cutting action, especially in brass, but also resulted in the work climbing on top of the graver and bending/snapping more often than before.

Lately I've done far more turning on a larger toolmakers lathe with carriage and cross slide. Coming from that perspective and looking at tool geometries I'm reassessing this again. Using the first technique, with the diamond facing the work, I would almost need to angle the graver so that the handle is positioned below the work if I'm to keep the T-rest anywhere close to the work and to keep a 7 degree negative rake as is best for steels. It also means that I would more likely than not have to have a pretty substantial gap between the work and the T-rest with the rest below the work as most T'rest designs that I've seen look much like wood turning lathe rests where the tools are actually held inverted. They slope towards the work.
The risk of slipping off the work when turning and the often times resulting negative cutter rake (which in retrospect probably explains quite a lot of my issues with dulling and pressure polishing) makes my current use of this method less than ideal.

On the other hand, the inverted method also calls for a gap between work and T-rest; the height of the graver blank that is.This creates a pivot point in an awkward place that makes the cutting edge difficult to keep on the center line of the work, leading to the aforementioned tendency for the work to climb onto the cutter. The cutting geometry is however much easier to adapt to the material being turned. Neutral for brass and 7 deg pos. for steel.

My thoughts are that there are three main things to consider:
1. The graver needs to be ground so that the "diamond" isn't symmetrical to be used in the "face up" fashion, but with a thinner leading edge towards the work, effectively creating a more negative rake from the start.
2. The intended way of turning steel this way IS actually to use a negative cutting geometry and I just don't have the technique down.
3. There is something here that I'm missing or misunderstanding completely...

Help?

Regards
Karl

 

[measuretwice]

Hi Karl,

Where did the negative rake for steel come from? I would fairly strongly disagree that its best for steel, but am curious of the reasoning.

A bit of theory. ALL metal cuts by the pressure of the tool setting up a shear plane ahead of it. The lower the rake, the larger the shear plane, and the greater is the force put on the work and tool. The image below captures it perfectly - note the difference in the length of the shear.

Every tool would huge positive rake except for two limiting factors. One is at some on some materials the tool will dig in (e.g. brass with rake >0) and most importantly tool strength. You typical keep rake for steel (with a hss cutter) at say 8-12 degrees positive because if you went 30 or 40 it wouldn't reduce the shear plane that much, and tool edge becomes very acute and does not last.

The only tools for steel I've seen that use negative rake are some carbide cutters. Carbide is excellent for wear but brittle....the larger included angle with negative rakes gives the edge more strength. Even that might be somewhat in the past; stronger newer carbides seem to be more commonly neutral or positive. Also, there is no advantage to taking a given included angle (clearance and rake) and rotating to create negative rake (and hence increasing clearance). The increase in strength is from increasing the included angle (i.e. leave the clearance where it is and reduce rake)

On to gravers. Someone more experienced with watchmaking might have some great points that negate the following....but from the perspective of tool geometry, the graver is sub-optimal. What it does right is makes for very easy to sharpen tool. The problems with it are the angles are too acute to have an edge with lasting strength, especially carbide. And other than easy to sharpen, why? There is no need for the excessive amount of clearance this geometry creates (or least that I can see).

imo the graver should be used lozenge shape down; the other way there is no support under the edge. I'm fully prepared to be wrong if there is some technique whereby the typical graver geometry simply must be as such. I'm coming at from the mechanical and machining position of optimizing tool life and minimizing cutting forces.

I've used gravers like everyone has, but thinking this through I want try grinding a more appropriate (imo) geometry on one. It would basically be similar to a threading tool, flat top to a point with 7-10 clearance on each side

 

[karlmansson]

Hi Karl,

Where did the negative rake for steel come from? I would fairly strongly disagree that its best for steel, but am curious of the reasoning.

A bit of theory. ALL metal cuts by the pressure of the tool setting up a shear plane ahead of it. The lower the rake, the larger the shear plane, and the greater is the force put on the work and tool. The image below captures it perfectly - note the difference in the length of the shear.

Every tool would huge positive rake except for two limiting factors. One is at some on some materials the tool will dig in (e.g. brass with rake >0) and most importantly tool strength. You typical keep rake for steel (with a hss cutter) at say 8-12 degrees positive because if you went 30 or 40 it wouldn't reduce the shear plane that much, and tool edge becomes very acute and does not last.

The only tools for steel I've seen that use negative rake are some carbide cutters. Carbide is excellent for wear but brittle....the larger included angle with negative rakes gives the edge more strength. Even that might be somewhat in the past; stronger newer carbides seem to be more commonly neutral or positive. Also, there is no advantage to taking a given included angle (clearance and rake) and rotating to create negative rake (and hence increasing clearance). The increase in strength is from increasing the included angle (i.e. leave the clearance where it is and reduce rake)

On to gravers. Someone more experienced with watchmaking might have some great points that negate the following....but from the perspective of tool geometry, the graver is sub-optimal. What it does right is makes for very easy to sharpen tool. The problems with it are the angles are too acute to have an edge with lasting strength, especially carbide. And other than easy to sharpen, why? There is no need for the excessive amount of clearance this geometry creates (or least that I can see).

imo the graver should be used lozenge shape down; the other way there is no support under the edge. I'm fully prepared to be wrong if there is some technique whereby the typical graver geometry simply must be as such. I'm coming at from the mechanical and machining position of optimizing tool life and minimizing cutting forces.

I've used gravers like everyone has, but thinking this through I want try grinding a more appropriate (imo) geometry on one. It would basically be similar to a threading tool, flat top to a point with 7-10 clearance on each side


View attachment 715191

Ah, dang it. I meant positive rake, that's what I tried to explain with the handle positions and such but I got the terminology mixed up. Seems I can't edit the post now so sorry for that red herring...

Thanks for your input! I know that there are also several members here that advocate for just getting rid of the graver altogether. I do have a cross slide for my Lorch 6mm and I use it with my face plate mandrel headstock. This wasn't the question I'm asking though. The question is how to get actual chips forming and an appropriate positioning of the T-rest with proper cutting angles using either the face up or face down position of the graver.

Regards
Karl

 

[measuretwice]

haha, no problem...I was thinking Karl knows what is doing, what the heck am I missing?

I wasn't thinking getting rid of the graver. The slide rest would have a hard time being as accurate; imo you can infeed more accurately with magnification that the clumsy dials on the slid rests....I was more thinking grind a different end on the graver vs the 45 degree lozenge so you had a stronger and longer lasting edge.

 

[gmorse]

Hi Karl,

I do have a cross slide for my Lorch 6mm and I use it with my face plate mandrel headstock.

Ditto, but it's only used occasionally, and usually for boring out plates for bushings; the mandrel is excellent for ensuring that arbors stay upright.

I've always found it easier and more natural to use the graver with the diamond downwards, towards the work. I seem to recall Roger Smith using a graver this way in one of his videos, and I think it was also sharpened to a much blunter angle than the 'normal' 45º, which echoes the point made by measuretwice in his post #4. I have a couple of my carbides sharpened this way.

Whether I find this comfortable because of the lathe configuration I've had for ages, (a 6mm like yours), and in particular the shape of the tip-over T rest, or something else, I don't know. It's certainly most important to set the T rest in the right position, both vertically and horizontally. Of course, if you're turning something small from larger stock, the horizontal position has to be constantly adjusted.

Regards,

Graham

 

[Betzel]

This is interesting. I've been trying to cut "the other way" with hand gravers to see what I'm missing as well. Thanks, Karl.

I wonder if most of us developed a habit (diamond up or toward the work) based on the tool rests we had when we started? All of my tool slide rests are old, so they are vertically short, flat, and thick, with only a slight downward (toward the operator) tilt making it hard to use the other way round, because the handle has to be held much farther down to get the upturned face flat and at a good angle. And, my shop-worn rests also pulled the edge away from the work when the handle was pushed down. Awkward. And, I liked the firm contact of the graver flat down on the edge of the rest, so a habit was born?

I may also have learned to cut this way by watching Steffen Pahlow with a Lorch handwheel between centers back in his videotape days. I was always fascinated by that meticulously slow and steady handwheel, thinking it was as close to Saunier's classical methods (bow and turns) I would ever see. With no "live" school for me, he became my virtual mentor. Maybe I'll make that Glashütte pocketwatch someday? His graver diamond faced the work, so I tried it. But he was doing final touches there on some hard pivots, not removing a lot of material. When I went to reduce material, especially on brass, I found that same tool/technique would cut unevenly with the slightest bend of the long tip into or out of the work. This did not mattter as much with slow speeds, but anything faster than a handwheel (80 RPM?) would grab the point or dig in with the heel. So I started doing "beaver toothed" work with a hand graver for roughing.

it was also sharpened to a much blunter angle than the 'normal' 45º

I discovered this by running out of carbide on some old Waller style tips ("nubs"). I found (as usual through the school of hard knocks) that it was much easier to cut well with, mostly for roughing. It cracked less often and held it's edge longer. Another happy accident from being frugal and playing through? So, the blunt angle helped me stop leaving beaver tooth marks when roughing at speed.

After all these years, I have still been curious to try to use the diamond face up, to see what I'm missing. But this seems to need a very tall and thin-edged tool rest that lets the handle come down lower, and get the edge close to the work. This is the tool rest "style" I see on almost all wood turning lathes. and some rests I see on auction sites are shaped like this.

Worth a try? Dunno; have not tried it yet.

Still, nothing makes beautiful chips like the slide rest with a nice cutter. It's rigid, the angle is constant, and for roughing or getting close on straight cuts, it seems less stressful in every way. But, for tight work or radiusing stuff, I would still use the hand graver...

 

[praezis]

Most times I use my crossslide, I must confess.
But small corrections are made with the hand graver: diamond to the work. IMO the only reasonable position from a statics view.
With diamond up and horizontal you will get a reasonable component of force that makes the graver slide backwards from the rest: an unstable situation.

Frank

 

[glenhead]

In my experience "it depends". A good cut with a well-honed hand graver is purely a matter of angles - how is the cutting edge presented to the surface of the stock. I use a graver both ways and have taught myself how to adjust the tool rest and how to hold the graver to accommodate the angles required. If my hand gets tired, I turn the graver over. There are some instances (I can't bring an example to mind right now) where I'll mutter "Ah, having the graver the other way around will help." It also provides a fresh cutting surface so you don't have to touch up the graver as frequently. Yes, learning to use a graver both ways takes more practice, but I find that it has been well worth the time and effort. (And all the different kinds of metals that have been turned to chips.)

Glen