1) I have a boring bar which uses a CCMT060204 insert. The bar is tilted about 10 degrees negative. I use it with the tip about 0.1mm above center and it cuts fine.
I also have a set of these boring bars:
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which I have been using in my mini-mill. To use them on a lathe boring holes in steel would you:
a) Use them exactly on center b) Slightly above c) The blade horizontal d) Blade slightly negative e) Blade slightly positive?
2) Traditionally the lathe cutting tools are to be positioned "exactly on centre". What in this context is "exactly"? Is there an accepted tolerance and if so, what is it? Does this vary with the tool? In which direction is it better to err?
If one is turning the exterior of a bar and the cutting tool is on center or below center, any deflection of the tool brings it away from the surface being turned. If it is above center any deflection pushes the tool into the surface being cut. This leads to more deflection and more digging in.
If you are boring, the situation is reversed. Slightly above deflects away from the surface. Slightly below and any deflection makes it dig in.
For steel (assuming a mild steel, not something like stainless or a hardened steel) I would go for horizontal. For other materials, I might go for positive rake -- and with a more rigid machine than you have, I might consider negative rake for some materials.
When boring, you want to err slightly on the above center position, because when the shank deflects, it will move the cutting edge away from the workpiece. Below center can cause it to dig in, and even
*on* center can do that if the machine is not rigid enough.
When turning, you want to err on the slightly below center side for the same reason. Deflection makes the cut more shallow, instead of digging in.
In either case -- above or below center, you introduce some error into the diameter -- and the smaller the hole or workpiece diameter, the greater the error.
What Dan and DoN said. Beyond that, gae ye to thy lathe and discover what works for yourself. Try things, see what happens with various materials in various situations. There's no substitute for making chips and occasionally some eau chitte scrap. You may discover approaches different from conventional wisdom that work for you doing what you want to do with your machines.
The tools do make a difference. I have two brazed carbide boring bars, one with 3/8" shank and one with 1/2" shank, each of which cost more than your entire set. I also have a couple of sets like yours. I have no idea why the more expensive ones work better, but they sure as hell do and have been doing so for more than a decade.
Don't be obsessed with carbide. Carbide is great for high-speed production in very rigid massive CNC machinery but HSS takes a keener edge and often does a better job in non-CNC situations with materials like plastics, ally, brass, mild steel and some SS alloys like 303. Carbide works better for abrasive materials like glass-filled resins, some cast iron, harder stuff like CTA (Chevy truck axle), some SS alloys, heat-treated sockets, etc.
You can make boring bars with bits of HSS silver-brazed onto shanks and ground to shape. All of my internal threading tools are made that way.
The textbooks say to place the tool slightly above center. The reason being that the tool will not tend to dig in when it flexes downwards due to cutting forces.
I am, however, not able to see how this is supposed to work. I see it this way: The boring bar has equal stiffness in all directions. When moving the tool higher, you are also changing direction of the cutting force, and therefore also the direction in which the tool will flex. What you achieve by setting the tool high, is to induce a slight negative rake. This negative rake will, of course, create a force that will try to push the tool away from the work, thus reducing the tendency to dig in. In effect, setting the boring tool high with zero rake is exactly the same as setting it dead center with a little negative rake. The difference is that when the tool is set high, the rake angle changes as the diameter of the cut changes, which is a bad thing.
My personal recommendation: Set the tool dead center, and start with a slight negative rake. Depending on the material you are cutting and the stiffness of the boring bar, you may have to play with different rakes to get a good cut. Remember that when you have changed the rake, the tool height needs to be reset.
Just out of curiousity how do you set center when boring? When I'm turning I place my scale between the tool and work piece and look to shee how it is leaning.
Do you do the same trick with the boring tool against the od?
I turn the boring bar to a slight positive rake, rotate the holder and set it to height against the tailstock center. It's easy with a 40 position Multifix post, and IIRC I got it pretty close that way with a Dorian. You could make a setting gage from sheet aluminum by placing one edge across the ways and sliding it against the tailstock center.
But the bar's stiffness is not the only factor. There are the stiffness of the compound and the cross slide -- both of which (or at least one, depending on the angle of the compound at the moment) are likely to tilt towards the headstock, allowing the cutting edge to move lower. And this *does* produce a dig in.
The dovetail and the gibs wear, so you get more flex in them after some time in service that when you got it new.
Most of the advice is consistent with what I have been doing. I was not too worried when I was using indirect methods to set the tools but since I bought a height gauge I got caught up in the figures and started wondering...
Against the back side of the OD -- assuming that you have enough travel in the cross slide. Against the back side of something of smaller diameter before you put the main workpiece in place otherwise. (One of the benefits of a good quick-change toolpost -- you can set the heights of all the tools before you start making chips.
I made a boring bar holder that has the center of the boring bar slightly above the center of the spindle. So just use that boring bar holder and do not have to think about it.
Back side? You using a screw on chuck? I like my boring tools set up so I cut feeding out from me just like for turning. I don't like subtracting if I can avoid it.
Get one of those 6 inch digital calipers from HF when they put them on sale for 9.95. They are accurate to less than .002 an check right on the money with a set of gauge blocks. Set it to your finished size and zero it. The resulting measured reading will be the cutting distance to go and eliminate the calculations.
Another trick is to make a hight gauge for the centerline of the lathe. That would consist of a block of metal and a bent rod sharpened to a dull point that is the height of the center line. IF you are using quick change tool posts, set all your toolposts to centerline hight and if you put in a smaller tool holder use shims to get the proper height and don't adjust the quickchange unit. ie. if you are using a 3/4 inch tool and change to a half inch, put in a 1/4 inch shim under the tool and the height will still be right on.
By "back side" -- I mean the side of the workpiece facing away from you. By gauging against that side, your boring tool will be facing the right way against the curvature of the workpiece and you can move it forward with the cross slide to be able to bore without having to shift it.
Except perhaps dividing the number by two -- depending on whether your cross-slide reads in radius or diameter. (Mine reads radius, so I have to divide the difference by two to get the right amount of material removed.
With insert tooling this will work fine. With HSS or similar, ground to make the proper cutting faces, the cutting height is likely to be lower than the size of the shank, so you will still have to make adjustments.
I, personally, like to have enough tool holders so I don't need to switch between tools in the same holder. That way, you can set it right and *leave* it set. :-)
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