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Black_Moons
08-23-2009, 12:30 AM
Hi, Today im wondering about lathe bit angles.
Now, correct me if im wrong, but I understand that (given the angles shown at http://wpcontent.answers.com/wikipedia/commons/thumb/0/0e/Tool_Bit_Geometry.JPG/300px-Tool_Bit_Geometry.JPG )

Side clearance helps control hoging by letting the side rub if it trys to hog, but can prevent heavy feeds in aluminum by the same action (Similar to how relief clearance on a circular saw teeth sets how fast you can easily cut)

Front clearance just prevents rubbing, especialy if you accidently where a mil or two above center. Doesnt seem very critcal at all (other then has to be at least a few degrees, and excessive angle reduces cutting edge support) I guess it could also help control drawing in the part.

Back rake controls how 'sharp' the tool is. But trys to draw the work into the bit?

Side rake controls how sharp the tool is, but promotes hogging because it trys to bend the bit into the cut.

But what im trying to understand is what mix of side and back rake to use and when to use more side rake then back rake, or back rake then side rake.

I can kinda understand back rake doesnt cause feed rate hogging, but would'nt it be able to deflect (by drawing in) small diamiter parts into the cutter?

Could'nt that more easily setup a chatter that would damage finish then side rake? (or less rake all togethor)

Also, would'nt side rake chatter (the HSS bit itself chattering insted of the workpeice?) cause less surface finish deformation because its only going left/right insted of changeing the diamiter of the work?

Also, How does that translate to fly cutter bits? As your now cutting with the tip of the bit insted of the side, side and back rake are effectively fliped, but the rigidity of your bit is still very high for back rake (Can't strech a HSS bit) but low for side rake.

Im wondering if maybe back rake verus side rake might depend on 'part' rigidity for lathe bits. (use less back and more side rake when dealing with small diamiter softer parts being turned?)

Also, Why does brass/bronze require 0 rake? I know 'because it hogs' But what causes it to hog? Like why when cutting it does the bit hog in yet in other metals its fine?

Jim Shaper
08-23-2009, 12:42 AM
In school we had a really good video about the mechanics of the cut while turning. You're not actually "cutting" when you turn something. You're shearing the material off on a plane just forward of the cutting edge.

I think once you understand that, it's easier to see why and how the different edge profiles work.

This is also how you end up with negative rake tooling in higher powered machines, not to mention that it actually works at all.

Paul Alciatore
08-23-2009, 03:52 AM
You are thinking about it way too much. Get some HSS tools that are already ground and cut some metal. Start with a mild steel and make chips. Keep your eyes open and OBSERVE and remember. Make notes if necessary.

Then try other metals with the same tools. See where they work and where they do not. Now, you know where you need to change the geometry. Don't make changes just to be making them, do it for a reason. Like, because you are not getting the results you want from the tools you have. If you must change the tool geometry, start with the rake angle and leave the clearance alone.

As for the "hogging in" thing, I have seen very little of it. Perhaps I avoid the metals, like brass, that are known for it. Perhaps I take more conservative cuts. It is not because I have tight machines as my lathe is older than I am and has a lot of backlash. Brass is not the only material that is known for this. Many plastics are a lot worse. Soft aluminum can do it. For brass, I use a zero rake angle. Softer materials allow the tool to get drawn in. It probably has something to do with the way the material reacts in that zone just ahead of the tip of the tool. Harder materials do not allow this to happen as easily.

I have a set of drills set aside for brass. As I use them, I grind them to zero rake. If I am turning brass, I use a tool with no or little rake. I do this because experience has shown it is necessary. And it works.

Clearance is ALWAYS needed. It allows the tool to actually cut into the work. Without clearance, the tool will just ride along the surface withour penetrating. One block of metal just sliding across another. That is a design for a bearing, not a cutter. But you don't want too much or the tool will be weak and tend to crack.

Fly cutters are no different. Only the orientation of the tool and work is changed. Keep in mind that they cut on the OUTER edge as the tool/spindle is moved across the work (or the work moves under the spindle). So assuming your fly cutter is in a vertical mill, the "front clearance" is on the outside of the circle the cutter makes and the "top rake" or "back rake" is on the almost radial fact of the tool.

Also remember that the fly cuter is actually cutting INSIDE a circle, like a boring bar, so the clearance angle must be large enough to prevent rubbing on this round surface it has just cut - a bit larger actually. Depending on the actual diameter of the fly cutter, a secondary clearance facet may need to be ground at a greater angle than the primary to prevent this.

Black_Moons
08-25-2009, 01:25 AM
Im still wondering about Side rake vs Back rack. Iv been grinding bits, I don't want to grind bits till the cows come home if someone can inform me the diffrence beween the two. (Effectively, I know what 'angles' they are)

Also, when dulling a drill for brass use, do you grind off the tip of the cutting edge (reduce/eliminate clearance aka relief), or do you grind along the flutes to actualy remove the rake?

Paul Alciatore
08-25-2009, 02:54 AM
Im still wondering about Side rake vs Back rack. Iv been grinding bits, I don't want to grind bits till the cows come home if someone can inform me the diffrence beween the two. (Effectively, I know what 'angles' they are)

Also, when dulling a drill for brass use, do you grind off the tip of the cutting edge (reduce/eliminate clearance aka relief), or do you grind along the flutes to actualy remove the rake?

Back rake is generally along the long axis of the tool. Side rake is generaly perpendicular to back rake. There are probably tools where this definition does not make sense and you would need to go by some other guidlines, but I think this is the basic idea and it should apply to most lathe tools. A fly cutter would be more difficult as many have the tool bit set at some odd angle so the directions may need to be in relation to the work.

When altering a bit for brass you grind on/in the flute to produce a flat that is parallel to the axis of the drill. The tip is left alone as the direction of penetration is toward the tip and you MUST have clearance there. If you don't believe this, I can show you some import bits that don't have the proper clearance angle on the tip and they do not cut.

Don't get hung up on the differences between "side", "front", and "back". When grinding a tool, consider how it cuts. The rake angle should be perpendicular to the cutting edge. If there are more than one edge that will be doing the cutting, then a compromize will be needed, perhaps 45 degrees to both. Any cutting edge MUST have clearance below it. If the edge is expected to cut, then grind clearance below it - side, front, back even; if it cuts, it MUST get clearance.

In addition, any edge that will be essentally parallel to the side of a cut as the cutting progresses, should also be relieved to some extent to prevent rubbing. So, a grooving tool may have a wider tip than the area behind it. Saws are either hollow ground to provide this clearance behind/below ther teeth, or the teeth have a "set" to provide it. And the front of a lathe tool that is used for turning down the diameter and which will travel across the newly cut surface, will be ground at an angle away from that cut surface so it does not rub.

It is not front, side, or back that determines how the edge is ground, it is what that edge is expected to do in use. It is the direction that the tool will be fed in while cutting.

Black_Moons
08-25-2009, 04:24 AM
Ok I believe you on the clearance :) I did'nt think that grinding off the clearance was proper, its just it was hard to figure out what some of those web page instructions where doing by the discriptions they gave.. and if they even knew what they where doing. Not all webpages are correct.

Im still wondering about the rake angles themselfs and back vs side.. just seems to me a subject there exists very little information other then charts we are supposed to follow. I like knowing the *why* behind the charts, Even if its usless to know and I still follow the charts blindly, it usally allows me to clue in on other things, like another piece of the puzzel that is the universe.

Ed P
08-25-2009, 07:54 AM
For all the information that you could possibly want on the subject
of sharpening cutting tools for the lathe and miller:
"Design and Use of Cutting Tools" by Leo J. St.Clair
It is now out of print but your local library should be able to get a
copy through inter-library loan. It will answer *all* of your questions,
I guarantee.

Ed P

davidfe
08-25-2009, 10:29 AM
For all the information that you could possibly want on the subject
of sharpening cutting tools for the lathe and miller:
"Design and Use of Cutting Tools" by Leo J. St.Clair
It is now out of print but your local library should be able to get a
copy through inter-library loan. It will answer *all* of your questions,
I guarantee.

Ed P

Ed,

Here is the free down load.

http://www.archive.org/details/toolschucksfixtu00dowdrich

Nice book.

Willy
08-25-2009, 02:56 PM
Thanks for the link Davidfe.
It's always nice to get a free textbook on machining related topics.

Me likes books a lot.:D

Black_Moons
08-25-2009, 07:38 PM
While that book looks extensive, its listed as
'Tools, chucks and fixtures; a comprehensive and detailed treatise covering the design and use of cutting tools and holding devices employed in turning and boring operations in modern manufacturing plants for obtaining accuracy and increasing production (1920)'
by Dowd, Albert Atkins, 1872-

You would'nt happen to have a link to the '"Design and Use of Cutting Tools" by Leo J. St.Clair' book as well and posted that one by mistake would you?

davidfe
08-25-2009, 08:21 PM
While that book looks extensive, its listed as
'Tools, chucks and fixtures; a comprehensive and detailed treatise covering the design and use of cutting tools and holding devices employed in turning and boring operations in modern manufacturing plants for obtaining accuracy and increasing production (1920)'
by Dowd, Albert Atkins, 1872-

You would'nt happen to have a link to the '"Design and Use of Cutting Tools" by Leo J. St.Clair' book as well and posted that one by mistake would you?

BM,

opps....didn't read closely enough.

Looked on www.scroogle.org (http://www.scroogle.org) and found it available for inter-library loan.

Don't know where you are. Near me there were about a dozen
available.

HTH

J. Randall
08-26-2009, 12:12 AM
Black Moons, I have thought about the theory of back and side rake on the top of a ground tool quite a lot. The only thing that makes any sense to me is this, Picture cutting right to left on a bar in the lathe, and a pretty substantial cut. The back rake is going to benefit your cut the most when you first plunge in, and then when you start your longitudinal cut down the bar you get the benefit of the side rake. Having said that I have ground some with side rake only and they cut pretty good. The benefit of side rake only is you can continue to sharpen the bit as it wears without reshimming to get your center height right. Does that make any sense?
James

Ed P
08-26-2009, 11:46 AM
BM,


Looked on scroogle.com and found it available for inter-library loan.

HTH

What am I missing? When I go to scroogle.com it's porn site!

Ed P

Ed P
08-26-2009, 11:49 AM
Black Moons, I have thought about the theory of back and side rake on the top of a ground tool quite a lot. The only thing that makes any sense to me is this, Picture cutting right to left on a bar in the lathe, and a pretty substantial cut. The back rake is going to benefit your cut the most when you first plunge in, and then when you start your longitudinal cut down the bar you get the benefit of the side rake. Having said that I have ground some with side rake only and they cut pretty good. The benefit of side rake only is you can continue to sharpen the bit as it wears without reshimming to get your center height right. Does that make any sense?
James

I think you may find the relationship between side and back rake is
more about controlling the movement of the chip, where do you want
it to go?

Ed P

Black_Moons
08-26-2009, 12:21 PM
Oh, I want the (scalding hot) chips to jump right onto the feed control handle of course. Doesnt everybody?

Carld
08-26-2009, 06:13 PM
Moons, I very seldom use back rake. Your much better off with the top of the tool flat and on the center line of the work in the lathe. When you have back rake the leading edge tip of the tool is on center line BUT, the rest of the leading edge cutter is below center line at an increasing angle. For soft metals like aluminum, brass etc it works fine that way and is actually best.

As others have said, your analyzing the cutting angles to much. Read the Machinery Handbook as to the best angles to use on various materials and do it that way until you have experience to make changes that may work better for you. There is a whole section on tool angles for different materials so use it.

davidfe
08-26-2009, 07:50 PM
What am I missing? When I go to scroogle.com it's porn site!

Ed P

Ed P.

It is www.scroogle.org (http://www.scroogle.org)

so sorry!