Cutting tool edge shape, cutting tool angle of approach, lathe speed

[Stephen Nelson]

Hello,
I'm brand new to this forum, and pretty new to wood turning in a practical sense.
I build and restore drums, and want to use a use a wood lathe to turn stave drum shells to specific inside and outside diameters.
My cutting tool is made from high speed steel, ground to a shape that I hope will cut effectively. That cutting tool is held on a steel boring bar. The cutting tool edge is set at exactly 8" high -- right on the same plane as the center of the lathe's spindle. This all rides on a tool post, cross slide, and carriage that I fabricated. This is all the long way of saying, I'm not holding the cutting tool against a tool rest, as in traditional wood turning. Rather, the cutting tool is held in the rough equivalent of the metal lathe cross slide and tool post. See the attached pictures.
My immediate questions--
What is the correct/most effective cutting tool edge shape/profile? I'm doing very basic outside and inside diameter turning. Again, the cutting tool is being moved VERY slowly across the cutting length of the shell, held by a steel boring bar, moving along a dovetail rail, driven on a powered carriage. Currently my cutting tool edge is AO wheel ground high speed steel, square with a slight radius. I was trying to emulate the profile of carbide cutting tools I've seen. Or, should the cutting tool edge profile be more of a point?
Can I use the same cutting tool shape/profile for both inside diameter and outside diameter turning?
Should my cutting tool edge contact the work piece at exactly the same height as the center of the lathe's spindle? Or, should it contact slightly below? Slightly above? Should the cutter have positive rake? Negative rake?
What speed should I be turning? Currently I'm at 670 rpm; my lathe's lowest setting. Should I be slower? Faster? The shell in the pictures is black walnut. I'll also potentially be turning maple, cherry, birch, etc...
I do realize that I'm fighting a potentially losing battle by having the cutting tool way out at the end of a boring bar. Lots more opportunity for flex and vibration. But, assuming I win the battle to make my system as rigid as possible, my previous questions remain!
Thank you

 

[Brian Gustin]

I think the biggest problem with your setup using that boring bar is that your cutting tool is essentially a scraper, which might work great going with the grain , but not so much going across the grain - so what I think you might want to look into is to raise your boring bar to be cutting a little ABOVE center , and then profile your cutter to actually have a skewed angle with razor sharp edge and a bevel to match the cutting angle (You're basically spindle turning, so you want a slicing cut - studying how a skew is used and works might go a long way to showing how to achieve what you're after) However - I'd strongly suggest to simply forego the boring bar setup for outside cutting and learn to use a skew chisel on tool rest- properly done, a skew on spindle turning can leave you with a finish so smooth you might be hard pressed to improve it by sanding. Boring bar might work to hollow the inside, however, but if your aim is for a nice shiny smooth finish on the outside of the drum, you would be better served to learn to use a skew chisel and turn the outside traditionally..

 

[Stephen Nelson]

Thanks for the information. This is EXACTLY the type of info I'm looking for. I'll look into the skew cutting tool.

 

[john lucas]

45 to 60 degrees should give you a pretty good cut. A hunter cupped carbide tool might leave a better finish. It will certainly hold an edge longer. He sells cutters a 6mm cutter mounted on small different shafts that may fit directly in your bar. Give him a call and discuss what your doing. Huntertoolsystems.com

 

[hughie]

Basically you want a slicing action rather than scraping. Consider something like a Woodcut Pro- Forme head, your current boring bar set up could easily modified to carry a Woodcut head. With the shield adjustment you can control the cut very well.

 

[Bill Blasic]

If that bar you use is capable of holding a 1/4" or 3/8" square tool you could use the Hunter Tool Systems #2 or #1 retrofit carbide tool. Take a look at it.

 

[John K Jordan]

I agree with others about the Hunter tools. With your setup, I'd probably continue to use the scraper on the inside where the surface doesn't matter much, and mount a bar for a tool rest and turn the outside by hand. If not comfortable with that, replacing the outside scraper with a Hunter tool could give a much better surface. (Have to experiment to get the angles right for a clean cut.)

The Hunter bits can be used like a scraper but can give a better surface if angled so it works more like a gouge. If your sides are always straight that would be an experiment and set once thing. I'd probably get the large Hunter Hercules for this. (Note that the Hunter carbide tools are nothing like the flat-topped carbide tools some use.)

Turning by hand is not too hard to learn - woodturning clubs in your area often have experienced volunteer mentors that could help if needed. They can teach sharpening as well if needed. Some may be well acquainted with the Hunter tools and could advise or help with setup for that.

The Hunter bits don't need sharpening but do wear after some use. Mike Hunter recommends loosening the screw and rotating the cutter some random amount occasionally.

I also like to round and polish the "heel" of the Hunter tools to avoid the chance of burnishing the wood. This is a side view of a Hunter Hercules and a view showing the cutters of some of the other similar tools (the large and small Hercules at the top)




I personally would probably do as Brian suggested and use a skew - easy once you know how! For that diameter a wide skew is best, say close to 2". A skew sharpened with a curved edge is often better and more stable. Another variable is the included angle of the two beveled edges where they meet to form the edge. A larger angle can be easier to learn. (say 45-60 deg instead of 25-30 deg). I teach the skew a lot and often start beginners out with a 60-deg curved edge skew.

A picture, the one at the bottom of this set of old Craftsman HSS tools:


I suppose a skew could be mechanically mounted but I've never seen that. Use it by hand is not that hard and could provide a way to do interesting profiles and detail.

This video shows some different skew profile designs. I grind sort of like the second from the right about 7 seconds into this video:

View: https://www.youtube.com/watch?v=yhuEVVSkDtE

This might be a good introduction to how a skew is used. You will be doing a "planing" cut for a smooth edge, perhaps after starting with a "peeling" pass if you have a significant amount of wood to remove.

Note that there are other ways to get a smooth, outside cylindrical shape.

JKJ

 

[john lucas]

These are the cutters I recommend. I use them on a homemade boring bar. The one pictured has a 1/4" square hss bit. The 3/16" one on the bottom can be rotated to give s clean cut in either a pull or push cut. The top cutter is designed to work as a pull cut and leaves a very good finish. The middle cutte will cut in any direction but doesn't leave as clean of a cut.

 

[Doug Freeman]

I think you can make a lot of improvement with the tools you already have. Stay with the boring bar/auto feed setup. I assume finished ID & OD dimensions need to be accurate. I see no reason to change to hand held tools. This is a true production setup, no reason to introduce the variability of hand held tools, where dimensional control is much more difficult and slow.

I cant see the exact shape of the current cutting edge. Initially I would try a shearing cut by rotating the cutter to 45*, lower end of the tool edge leading. Grind the tool with a “flat top” and a ~ 60* angle beneath, with a radius that keeps the ends of the edge off the wood while cutting. The hss tool edge needs to be burnished to create a “hook”. Assuming you are using M2 hss. M42 is a good step up. This approach means cutting can only be done in one direction, ID or OD. It may be that it is most efficient to rough cut with the current cutter orientation (with edge as described), then finish cuts with the cutter edge at 45*.

For the ID, set the CL up ~1/8” from drum center, for the OD set ~1/8” below center. This will reduce the force on the cutter, removing the force from the wood pulling the cutter into the wood.

Proper rpm is determined by surface speed at the cutter, determined by work dia and rpm. Depth of cut, feed rate, and surface speed determine chip load. Some experimentation will be required to optimize the settings. There may be rough and finish cut settings. A larger boring bar, and potentially beefier mount, will increase chip load capacity, but that lathe you are on doesn’t have much HP, so it may be a moot point. You also have limited speed options, so test to optimize. Vibration will be your guide.

The cupped carbide cutter with square mounting bar from Hunter (and others) may work very well in this application. There are different size cupped cutters available, and I’m not sure which might be best for this application.

You dont mention how much material needs removed from the ID or OD. Depending on the situation it may be that you run roughing passes and then 1 or 2 finish passes. With hss or carbide tooling, it will be a non-bevel supporting shearing cut, which should not be an issue with fixed tooling like you have. The carbide tooling is not that expensive, so it’s probably worth giving it some test time. The time savings of not having to resharpen (and change out tool tips) could be worth a lot in this application.

Different and interesting application for this forum. You may also want to find a forum for production cnc lathe with wood as the medium. Those folks probably have more experience with this type of application. Using “fixed” tooling is much different vs the hand held we work with.