Grinding chip breakers on HSS toolbits
Posted: Thu Jan 11, 2007 3:33 am
I had offered to talk about HSS tool grinding, and did so with hesitation. My reservations were well founded. It's interesting how a person can do things on a routine basis, with success, and be at a loss to explain how or why, but that's where I find myself. It's nothing short of shocking to be without words, particularly when I rarely have a clue that it's time to shut up.
Grinding toolbits, for me, has been routine for so long that I have no memory of not being able to grind them. The skills involved were accumulated in practice, over a period of time, with the tools put to use in a production environment. By applying the tools under demanding circumstances, tool design was determined and performance evaluated on the machine. From this, I'd like you to understand that there is no substitute for experience. You can not learn the things that make one a good tool grinder from reading, you must experience them, and see the end result in order to understand the effects of subtle changes in tool design that spell success, or failure.
Functional turning tools are ground to a mental picture, with relief angles ground to fit the requirements of the task at hand. While these angles are important, they are not carved in stone, and vary with circumstances. It becomes a matter of course to grind in such a fashion, so no guides, aside from profile gages, are necessary. That's not to diminish the importance of relief angles, but they are flexible enough that they are not a serious concern unless one finds himself in a production environment, where a maximum performance over a long period of time is a requirement.
Often times, an alteration of an angle, particularly a rake angle, will allow for better performance for the short haul, often long enough to complete a rather unpleasant task. Turning the corners off a stainless plate cut from a square is a good example. Excessive positive rake, coupled with a very light feed, using a plunge turning technique changes the job from one of misery to one with few, if any, problems. It's easy enough to achieve functional angles without gauging, so the eye should be trained accordingly.
Having said that, I still need to say enough to help those that struggle. Which ones, and for what reasons, remains a mystery, so I'll start the ball rolling and try to answer questions if there are any. Best thing for me to do at this point is to talk about basic machining concepts, and what influences performance. The principles remain pretty much the same as you move from one type tool to another, taking into account that working in bores requires a serious rethink of front relief angles, which are influenced by bore size.
Tools that are ground with simple angles perform reasonably well, but often with higher cutting pressure than might be desired, and they rarely have any features that manipulate the chip. Surface finish and production, under those circumstances, can suffer. Chip control is often just as important as the cut itself----and must be dealt with accordingly. Faster operations are one of the benefits, as is safety through the elimination of stringy chips.
The mechanism by which this occurs is a ground chip breaker. Such an addition to your tool opens the door to greater rake angles, lower cutting pressure, and chip control Chip breakers can be ground with sufficient positive rake to seriously improve cutting characteristics and still have excellent edge strength, due in part to reduced heat at the point of cut, a result of positive rake. An offset to the positive rake is accomplished by grinding a cylindrical breaker, in which the chip reverses direction quickly after leaving the cut, more or less counteracting the effects of positive rake (hogging). The radius of the chip breaker, in conjuntion with the proper amount of front relief, tends to control hogging and chatter. Properly ground, the breaker will roll the chip, yielding either C's, 9's, or an endless coil. The coil isn't as desirable, but from the standpoint of safety, is much better than strings, and is often much easier to dodge than hot C's or 9's. There are times when a coil becomes the chip of choice, such as when boring a hole that has minimal bar clearance and it's desirable for the bore to not accumulate chips.
Metals, for the most part, respond to being "peeled" (shorn via positive rake) much better than they do to being displaced by brute force (negative rake). For those that use HSS, not carbide, the advantages to using reduced rake are few. It is not an option when machining steel, and offers no advantages when machining aluminum, so it's pretty much restricted to use in soft cast iron and free machining copper alloys, in both instances to control hogging, with no real advantages in machining aside from a prolonged edge when machining cast iron. In those examples, it is generally accepted that rake be reduced to 0, with no real need to go negative. Chip breakers, by sharp contrast, can be applied under almost all circumstances with huge benefits, except for machining free machining copper alloys, and, once again, soft cast iron.
Chip breakers are sensitive to speed, depth of cut and feed rate. Therefore it's nearly impossible to describe one that suits all purposes. Again, the basic principle remains a constant, so all one need do is make subtle changes to the breaker to improve performance when it does not perform to expectations. Likewise, a change of feed rate, spindle speed or depth of cut often will yield good results with an existing grind.
A narrow, deep breaker will usually yield poor performance, although it might serve for finishing cuts. Armed with that idea, one would widen a breaker that did not permit free flowing of the chip. It is also desirable for the chip to have the ability to flow freely, with a subtle change of direction such that there are no corners to trap the chip and prevent flow. A gentle radius is the most preferred design, although other configurations can work. By choosing a radius, positive rake can be introduced to the cutter without building any traps, with minimal effect on heat transfer. .
Please take note: The information I'm providing is pretty much useless for rocker tool post use. Between the included positive rake of the post, and the angle necessary to keep the post away from the cut, the geometry is totally different. Tools I recommend are intended to be mounted in a holder without rake, and at right angles, or very near right angles, to the turn. Both of those features become impossible with a rocker post. The details on chip breakers can, indeed, be applied to tools ground for use in a rocker tool post, but the extreme angle differences must be taken into account. In general, poorer performance can be expected as compared to holding cutting tools in a different post.
In order to facilitate the presence of a chip breaker, tool design is usually slightly different form conventional grinds. Where the included angle of the top of a right hand turning tool might have been in the vicinity of 60° for conventional grinds, an angle of 80° is more desirable, assuming it isn't in the way of the use of a center. In such a case, the tool is generally relieved to clear the center, leaving the bulk of the material that isn't in the way. By providing the greater angle, there is room for the full chip breaker to be ground without sacrificing much, if any, of the rear portion. That keeps the chip breaking properly, but plays no role in how the cut behaves otherwise. The bulkier tool is somewhat better at handling heat, so that is a side benefit.
For starters, lets consider the tool, below. It's a boring tool, one that is very useful in removing stock rapidly. The cutting edge is ground at a slightly less than 90° to the shank, so that the tool is able to bore to depth and face. For through bores, the tool can be mounted at a slight angle, so the cutting edge has a minor amount of lead. This design works exceedingly well in both circumstances, so there is no need for two different tools for boring. The cutting edge in this case is the end of the bar, which I will reference as the front. The relief angle will work as low as 5°, and up to 8°, and is affected by feed rate. You'll notice as you alter the angle, the tool will have more or less tendency to hog, and often chatter. You can use that feature to help control the performance of the bar.
Side clearance is in keeping the requirement for the tool of choice to enter the hole for which it's intended. There is no point to giving specific guidelines, for each hole has it's own demands. Just make sure that the heel of the tool, with the cutting ege in contact with the bore, has clearance. 5° or more beyond the radius of the part is very acceptable.
Pay particular attention if the tool is very close to bore size, so the back bottom edge of the bar doesn't drag in the bore. Remember, none of the angles are critical, so long as the tool is kept within reasonable boundaries, it will function very well. Avoid extreme clearance angles to help control hogging and to provide the maximum edge strength possible. If, by chance, you find the tool does not cut freely, inspect the tool for signs of dragging in the bore------and check the chip breaker to insure that the chip being generated is flowing freely. Chip stack up will quickly snap a tool if left unchecked. Chip welding alone can be the source of much grief. Use a tool of this design with lubrication. Brush application is adequate, just don't run a chip breaker dry. If, when machining aluminum, you find it has a tendency to streak off on the breaker, it's always a good idea to spin a small cylindrical stone in a chuck and apply the breaker to the stone to polish the surface. The smoother the better. Be careful to not round the cutting edge should that be necessary.
The chip breaker is ground freehand, using a standard grinding wheel with a radius dressed on the corner. Alternately, an old wheel can have a radius hand dressed and used exclusively for such grinding. That eliminates rounding the corner of your wheel, prolonging the useful life. The radius on the wheel can be smaller than the one you desire, but it is not as easy to get a uniform grind.
I also suggest that tools be ground without the use of a rest, which I will address in a different post. Lets see if this has helped anyone, or if it has raised any questions that may need to be addressed. The next tool will likely be a right hand turning tool. Once you have seen a couple designs, you should be able to envision a tool of almost any design, and grind accordingly.
Grinding toolbits, for me, has been routine for so long that I have no memory of not being able to grind them. The skills involved were accumulated in practice, over a period of time, with the tools put to use in a production environment. By applying the tools under demanding circumstances, tool design was determined and performance evaluated on the machine. From this, I'd like you to understand that there is no substitute for experience. You can not learn the things that make one a good tool grinder from reading, you must experience them, and see the end result in order to understand the effects of subtle changes in tool design that spell success, or failure.
Functional turning tools are ground to a mental picture, with relief angles ground to fit the requirements of the task at hand. While these angles are important, they are not carved in stone, and vary with circumstances. It becomes a matter of course to grind in such a fashion, so no guides, aside from profile gages, are necessary. That's not to diminish the importance of relief angles, but they are flexible enough that they are not a serious concern unless one finds himself in a production environment, where a maximum performance over a long period of time is a requirement.
Often times, an alteration of an angle, particularly a rake angle, will allow for better performance for the short haul, often long enough to complete a rather unpleasant task. Turning the corners off a stainless plate cut from a square is a good example. Excessive positive rake, coupled with a very light feed, using a plunge turning technique changes the job from one of misery to one with few, if any, problems. It's easy enough to achieve functional angles without gauging, so the eye should be trained accordingly.
Having said that, I still need to say enough to help those that struggle. Which ones, and for what reasons, remains a mystery, so I'll start the ball rolling and try to answer questions if there are any. Best thing for me to do at this point is to talk about basic machining concepts, and what influences performance. The principles remain pretty much the same as you move from one type tool to another, taking into account that working in bores requires a serious rethink of front relief angles, which are influenced by bore size.
Tools that are ground with simple angles perform reasonably well, but often with higher cutting pressure than might be desired, and they rarely have any features that manipulate the chip. Surface finish and production, under those circumstances, can suffer. Chip control is often just as important as the cut itself----and must be dealt with accordingly. Faster operations are one of the benefits, as is safety through the elimination of stringy chips.
The mechanism by which this occurs is a ground chip breaker. Such an addition to your tool opens the door to greater rake angles, lower cutting pressure, and chip control Chip breakers can be ground with sufficient positive rake to seriously improve cutting characteristics and still have excellent edge strength, due in part to reduced heat at the point of cut, a result of positive rake. An offset to the positive rake is accomplished by grinding a cylindrical breaker, in which the chip reverses direction quickly after leaving the cut, more or less counteracting the effects of positive rake (hogging). The radius of the chip breaker, in conjuntion with the proper amount of front relief, tends to control hogging and chatter. Properly ground, the breaker will roll the chip, yielding either C's, 9's, or an endless coil. The coil isn't as desirable, but from the standpoint of safety, is much better than strings, and is often much easier to dodge than hot C's or 9's. There are times when a coil becomes the chip of choice, such as when boring a hole that has minimal bar clearance and it's desirable for the bore to not accumulate chips.
Metals, for the most part, respond to being "peeled" (shorn via positive rake) much better than they do to being displaced by brute force (negative rake). For those that use HSS, not carbide, the advantages to using reduced rake are few. It is not an option when machining steel, and offers no advantages when machining aluminum, so it's pretty much restricted to use in soft cast iron and free machining copper alloys, in both instances to control hogging, with no real advantages in machining aside from a prolonged edge when machining cast iron. In those examples, it is generally accepted that rake be reduced to 0, with no real need to go negative. Chip breakers, by sharp contrast, can be applied under almost all circumstances with huge benefits, except for machining free machining copper alloys, and, once again, soft cast iron.
Chip breakers are sensitive to speed, depth of cut and feed rate. Therefore it's nearly impossible to describe one that suits all purposes. Again, the basic principle remains a constant, so all one need do is make subtle changes to the breaker to improve performance when it does not perform to expectations. Likewise, a change of feed rate, spindle speed or depth of cut often will yield good results with an existing grind.
A narrow, deep breaker will usually yield poor performance, although it might serve for finishing cuts. Armed with that idea, one would widen a breaker that did not permit free flowing of the chip. It is also desirable for the chip to have the ability to flow freely, with a subtle change of direction such that there are no corners to trap the chip and prevent flow. A gentle radius is the most preferred design, although other configurations can work. By choosing a radius, positive rake can be introduced to the cutter without building any traps, with minimal effect on heat transfer. .
Please take note: The information I'm providing is pretty much useless for rocker tool post use. Between the included positive rake of the post, and the angle necessary to keep the post away from the cut, the geometry is totally different. Tools I recommend are intended to be mounted in a holder without rake, and at right angles, or very near right angles, to the turn. Both of those features become impossible with a rocker post. The details on chip breakers can, indeed, be applied to tools ground for use in a rocker tool post, but the extreme angle differences must be taken into account. In general, poorer performance can be expected as compared to holding cutting tools in a different post.
In order to facilitate the presence of a chip breaker, tool design is usually slightly different form conventional grinds. Where the included angle of the top of a right hand turning tool might have been in the vicinity of 60° for conventional grinds, an angle of 80° is more desirable, assuming it isn't in the way of the use of a center. In such a case, the tool is generally relieved to clear the center, leaving the bulk of the material that isn't in the way. By providing the greater angle, there is room for the full chip breaker to be ground without sacrificing much, if any, of the rear portion. That keeps the chip breaking properly, but plays no role in how the cut behaves otherwise. The bulkier tool is somewhat better at handling heat, so that is a side benefit.
For starters, lets consider the tool, below. It's a boring tool, one that is very useful in removing stock rapidly. The cutting edge is ground at a slightly less than 90° to the shank, so that the tool is able to bore to depth and face. For through bores, the tool can be mounted at a slight angle, so the cutting edge has a minor amount of lead. This design works exceedingly well in both circumstances, so there is no need for two different tools for boring. The cutting edge in this case is the end of the bar, which I will reference as the front. The relief angle will work as low as 5°, and up to 8°, and is affected by feed rate. You'll notice as you alter the angle, the tool will have more or less tendency to hog, and often chatter. You can use that feature to help control the performance of the bar.
Side clearance is in keeping the requirement for the tool of choice to enter the hole for which it's intended. There is no point to giving specific guidelines, for each hole has it's own demands. Just make sure that the heel of the tool, with the cutting ege in contact with the bore, has clearance. 5° or more beyond the radius of the part is very acceptable.
Pay particular attention if the tool is very close to bore size, so the back bottom edge of the bar doesn't drag in the bore. Remember, none of the angles are critical, so long as the tool is kept within reasonable boundaries, it will function very well. Avoid extreme clearance angles to help control hogging and to provide the maximum edge strength possible. If, by chance, you find the tool does not cut freely, inspect the tool for signs of dragging in the bore------and check the chip breaker to insure that the chip being generated is flowing freely. Chip stack up will quickly snap a tool if left unchecked. Chip welding alone can be the source of much grief. Use a tool of this design with lubrication. Brush application is adequate, just don't run a chip breaker dry. If, when machining aluminum, you find it has a tendency to streak off on the breaker, it's always a good idea to spin a small cylindrical stone in a chuck and apply the breaker to the stone to polish the surface. The smoother the better. Be careful to not round the cutting edge should that be necessary.
The chip breaker is ground freehand, using a standard grinding wheel with a radius dressed on the corner. Alternately, an old wheel can have a radius hand dressed and used exclusively for such grinding. That eliminates rounding the corner of your wheel, prolonging the useful life. The radius on the wheel can be smaller than the one you desire, but it is not as easy to get a uniform grind.
I also suggest that tools be ground without the use of a rest, which I will address in a different post. Lets see if this has helped anyone, or if it has raised any questions that may need to be addressed. The next tool will likely be a right hand turning tool. Once you have seen a couple designs, you should be able to envision a tool of almost any design, and grind accordingly.