I've read Mikey's Boring Bar primer (thanks for doing that - what a tremendous effort to do all that!). I have some inserted carbide bars, one of the cheaper sets, and have had some success using them , but they are too big for my next project, making some bullet sizing dies. I'm looking at the Micro 100 carbide bars, and I have the choice of nose radius or sharp (no radius at all). I've not been able to find anything so far explaining the differences between the two. I'm sure it's there somewhere and I've missed it. Thanks for any help!
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Boring bar cutting radius
- Thread starter mwestcott
- Start date
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- Nov 17, 2017
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The nose radius provides some things and takes other things away.
Compared to a sharp point, any significant nose radius
a) is less likely to chip
b) can take larger DoCs
c) can take higher RPM
d) can produce pretty good finishes when used with proper travel per rotation = 1/3rd of nose radius..
But
e) the nose increases too pressure (more cutting area)
f) requires stiffer holder (to deal with the higher cutting pressure)
So for cartrige dies, one would likely want a nearly sharp point (a couple thou of nose radius max) because you are reaching up inside a hard steel hole and trying to make precise cuts with a surface finish you can smooth out with fine grit emory. Thus, nothing about significant nose radii is helping in your task. About the only place a specific nose radius could be used to good effect is at the transition from the (rather) straight wall to the bent shoulder.....
Compared to a sharp point, any significant nose radius
a) is less likely to chip
b) can take larger DoCs
c) can take higher RPM
d) can produce pretty good finishes when used with proper travel per rotation = 1/3rd of nose radius..
But
e) the nose increases too pressure (more cutting area)
f) requires stiffer holder (to deal with the higher cutting pressure)
So for cartrige dies, one would likely want a nearly sharp point (a couple thou of nose radius max) because you are reaching up inside a hard steel hole and trying to make precise cuts with a surface finish you can smooth out with fine grit emory. Thus, nothing about significant nose radii is helping in your task. About the only place a specific nose radius could be used to good effect is at the transition from the (rather) straight wall to the bent shoulder.....
Ok, that helps me understand some more. I would be cutting mild steel, not hardened. These are sizer dies to reside lead bullets from their as-cast diameter a couple thousandths smaller. Max bore length would be less than 1". I would likely bore to within .001 and polish the rest of the way to sneak up on it and ensure a good smooth finish.
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- Dec 20, 2012
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I left a lot about solid bars out of the Primer to cut the sheer volume of that thing down. Perhaps going into it a bit more here might be helpful.
Solid bars are subject to the same cutting forces as an inserted bar is. What differs is that a solid bar has the geometry ground into it, while an insert can have a plethora of different surface topographies. Still, side and end relief, back and side rake and the presence or absence of a nose radius does make a difference in how the tool produces and handles cutting forces. The key advantage we have with solid bars is that we can alter the radial and axial rake of the tool by changing the position of the tool in the tool holder and by the way we angle the bar in the bore. Let me try to explain.
If you watch a solid bar cut a bore, you will see that most of the stock reduction is done by the end cutting edge; the finish is produced primarily by the side cutting edge. While this may sound weird, the same thing happens with a turning tool; the side does the stock reduction and the edge produces the finish. Tangential cutting forces increase with depth of cut because more of the end and side cutting edges are engaged in the cut. Because side rake and back rake are fixed and their effect is theoretically fixed, depth of cut is a MAJOR determinant of the amount of tangential cutting forces the tool produces.
Now here is the key advantage of solid bars that I alluded to above. By rotating the tool in the tool holder so that the tip of the tool points upwards a few degrees, you have effectively increased side and back rake and this can have a major impact on reducing the amount of tangential force produced. You can simultaneously turn the tip of the tool towards the wall of the bore a few degrees and effectively reduce radial cutting forces as well. It doesn't take much of either of these rotational maneuvers to make a huge difference in how well and how accurately a solid bar can cut. Now when you take a bigger depth of cut, forces are lower. You will also find that by altering the angle of your tool as above, finishes are also improved.
While the nose radius does have an impact on finishes, this becomes less important if you can understand how to use your edges effectively. Keep in mind that the larger the nose radius, the greater the radial deflection it produces. This is not a big deal if your tolerances are in the half-thou range but if you are trying to hit something in the low tenths then yes, it does make a difference. This is why I suggested to you that a sharp nose would be better. With practice, it is possible for you to cut well into the low tenths with Micro 100 solid carbide bars and produce a very fine finish while doing so.
It does not matter which kind of bar you use, solid or inserted carbide, the boring bar holder is a critical tooling component. I would rather see you use a really good boring bar holder and a cheap HSS boring bar than a junk holder and a Micro 100 bar. It is that important.
Whatever bar you choose, learn to hone it and then practice until you can cut what you intend. Emery cloth is not a substitute for skill.
Solid bars are subject to the same cutting forces as an inserted bar is. What differs is that a solid bar has the geometry ground into it, while an insert can have a plethora of different surface topographies. Still, side and end relief, back and side rake and the presence or absence of a nose radius does make a difference in how the tool produces and handles cutting forces. The key advantage we have with solid bars is that we can alter the radial and axial rake of the tool by changing the position of the tool in the tool holder and by the way we angle the bar in the bore. Let me try to explain.
If you watch a solid bar cut a bore, you will see that most of the stock reduction is done by the end cutting edge; the finish is produced primarily by the side cutting edge. While this may sound weird, the same thing happens with a turning tool; the side does the stock reduction and the edge produces the finish. Tangential cutting forces increase with depth of cut because more of the end and side cutting edges are engaged in the cut. Because side rake and back rake are fixed and their effect is theoretically fixed, depth of cut is a MAJOR determinant of the amount of tangential cutting forces the tool produces.
Now here is the key advantage of solid bars that I alluded to above. By rotating the tool in the tool holder so that the tip of the tool points upwards a few degrees, you have effectively increased side and back rake and this can have a major impact on reducing the amount of tangential force produced. You can simultaneously turn the tip of the tool towards the wall of the bore a few degrees and effectively reduce radial cutting forces as well. It doesn't take much of either of these rotational maneuvers to make a huge difference in how well and how accurately a solid bar can cut. Now when you take a bigger depth of cut, forces are lower. You will also find that by altering the angle of your tool as above, finishes are also improved.
While the nose radius does have an impact on finishes, this becomes less important if you can understand how to use your edges effectively. Keep in mind that the larger the nose radius, the greater the radial deflection it produces. This is not a big deal if your tolerances are in the half-thou range but if you are trying to hit something in the low tenths then yes, it does make a difference. This is why I suggested to you that a sharp nose would be better. With practice, it is possible for you to cut well into the low tenths with Micro 100 solid carbide bars and produce a very fine finish while doing so.
It does not matter which kind of bar you use, solid or inserted carbide, the boring bar holder is a critical tooling component. I would rather see you use a really good boring bar holder and a cheap HSS boring bar than a junk holder and a Micro 100 bar. It is that important.
Whatever bar you choose, learn to hone it and then practice until you can cut what you intend. Emery cloth is not a substitute for skill.
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Another great write up Mike ! 
Totally true . You can peel off a tenth with a dead sharp bar .
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I meant to refer you to this thread where I posted an article on boring bar geometry from Micro 100. Hope you find it useful.
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Oh yeah ! The 20mm grenades for the OICW gun . Fun project and I sure miss that cnc lathe . 
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