Models for grinding HSS Lathe Tools

[mikey]

The local metal supply has added a bunch of machines making a corner into a sort of showroom for tools. Mostly Jet brand, I saw this 2x42 belt / 8" disc grinder. It is a bit pricey but if it would be good for grinding bits I could start watching for a second hand one, finding a used one can't be any more of a long shot than finding the old Craftsman like you have.

http://www.jettools.com/us/en/p/j-41002-2-x-42-bench-belt-disc-sander/577004

You might PM @Aukai - he owns one and can give you first hand info on it.

 

[mikey]

Sorry this took so long, Guys. I wrote and re-wrote this multiple times, trying to simplify it. I hope this works.

Have you ever thought about what a good lathe tool does? I have. To me, a good tool should do its job without calling attention to itself. If you need it to rough, it roughs well without any annoying chatter. If you need a fine finish it should give it to you simply by angling it toward the tailstock. If you need accuracy, you should be able to dial in what you need and the tool should cut it. This is what all good lathe tools should do, in whichever material you happen to be working with, but they must be designed to do it. You can’t just grind up any old shape and expect it to work well.

Not to overstate the obvious but no tool does it all. Every material group we work with has its own characteristics and if you want your tool to perform well then it has to be designed to accommodate those characteristics. Designing such a tool takes some thought and work but if you do it well then you will have a tool that is all but invisible – it will do what you need it to do without you having to struggle to achieve it.

I know that some of you have ground the Square Tool and have hopefully found it useful. I designed it and I’m proud of it but I know it for what it is – a compromise that works with most stuff but is not great at any one thing; it is an “okay” tool. If it is good enough for your needs then stop here and be happy but you should know that a material-specific tool ground for high performance will easily kick a Square Tool’s butt!

So just what is a high performance, material-specific “good tool”? It is a tool with a tip geometry designed to minimize cutting forces and cutting temperatures while accommodating the characteristics of a specific material group. Note that cutting forces and temps comes first, at least in my book, because these tools are intended for use on smaller lathes, maybe 12” and below. I do not need a bigger lathe; I just want mine to cut like one.

Now, as for the material-specific part, each material group has certain characteristics that we need to deal with and we need to know what they are in order to design a tool to accommodate them. I’m not referring to the chemical composition of the material, although that does matter. I am more concerned about other factors:

• Machinability refers to the ease with which a material can be cut. It is a comparative rating using 1212 mild steel as a standard. Here is an easy to use list: http://www.carbidedepot.com/formulas-machinability.htm. We need know how “hard” the material is to cut because this influences how durable the cutting edge must be. It also influences your choice of blank; it might be better to go with cobalt instead of M2 for tool steels, high carbon steels and stainless steels.
• Work Hardening –Work hardening occurs when the cutting tool transfers the heat from the cutting operation to the work piece instead of the chips and is aggravated by materials that have low thermal conductivity and retain the heat in the body of the work; stainless steels are an example of such a material. Stainless Steels and many of the higher carbon heat treatable steels will work harden. Sharp tools, rapid chip evacuation and a savvy lathe operator are required for these materials.
• Heat sensitivity – some materials, especially plastics, are very heat sensitive. If the work gets too hot it tends to melt and that can affect finishes and dimensional accuracy. The tool must cut freely and remove heat as quickly as possible. Aluminum-specific tools work pretty well for most plastics.
• Variations within the material group. For example, the 300 series of Austenitic Stainless Steels include 303, 304, 316, 316L, etc. All of them will work harden, especially 304 and 316 – they get hard if you look at them sideways. 303, on the other hand, contains Sulfur, which makes it much easier to machine. It will work harden, too, but you can cut it easier so avoiding high temperatures in the cut is simpler. Stainless Steels are not particularly hard in the annealed state in which we usually buy it; it gets hard if we don’t cut it the right way or use sharp tools.

My point is that in every material group there will usually be different alloys. Your job is to know about distinguishing factors in the group that will affect your ability to cut it and design around it.

The more you know about your materials, the easier it is to design the tool to accommodate them. Do your homework, guys.

Designing a tool

If you’ve read through this thread this far, you have been exposed to what each tool angle does. We are going to use that knowledge to design our tools. To assist us, I’ve summarized them below:

• Relief angles – reduces cutting forces by improving penetration and improves finishes. It also weakens support under the cutting edge and reduces edge life if taken to extremes.
• Side Rake – the key effects produced by increases in side rake are the reduction of cutting forces and cutting temperatures without impacting on support under the cutting edge. It does this by increasing (narrows) the included angle at the cutting edge to enhance penetration, thins the chip, increases chip flow rate, and extends edge life.
• Back Rake – reduces cutting forces and cutting temperatures and controls where cutting forces are focused. As back rake increases, the cutting forces shift closer to the tip.
• Nose Radius – the bigger the radius, the greater the deflection. Deflection is minimized when the depth of cut exceeds the nose radius. Therefore, small nose radii are good radii.

Next, let’s look at a typical angle grinding table. The information in these tables is derived from manufacturing, machining and experiential data collected over many decades and is an industry “standard”.



Note the following:

• All the angles change to suit the material type the tool is being ground for. Distinctions are not made for different alloys within a material class; sometimes this matters, sometimes it doesn’t.
• The side relief angles are pretty constant; this is where the concept of “clearance is clearance and enough is enough” comes from … Hmmm.
• Side relief is greater than end relief. That smaller end relief angle adds mass behind the cutting tip for strength but it also concentrates heat so while the tip is stronger, it cuts hotter. This is why I recommend making the end relief the same as the side relief; you don’t need the mass in a non-production setting and you certainly don’t need more heat in the cut.
• Side rake angles vary widely depending on the material class. Note that the side rake for machine steels (higher carbon steels) and stainless steels are higher. This is because greater amounts of side rake clears chips faster and that reduces cutting temperatures, which reduces work hardening. This tells you that the table actually does account for at least some of the known properties of the materials.
• Back rake also varies widely but recommends higher amounts of back rake for aluminum and copper. These softer materials are also “sticky” when cut and the higher back rake shears these materials more effectively. Now you see that the table actually understands tip geometry; it just isn’t blatant about it.
• One last one. Did you notice that brass and bronze have a side rake range that is slightly positive to slightly negative? Most of us just use Zero rake, and I do it because I’m lazy. I tested tools on brass and bronze with various amounts of side rake and found that brass finishes better with slight positive rake and bronze finishes better (less tearing) with slight negative rake but only when the relief angles are at conventional angles. When the relief angles are increased to about 15°, I saw little improvement in the finishes with changes to side rake in these materials, which is good because as I said, I’m lazy. So, when I make these tools, I use a 15° side and end relief and just hone the top flat – one less surface to grind. It is also interesting to note that a positive rake Square Tool will cut these materials to a very fine finish; there goes the “neutral rake” and the “tool will dig in” argument. Try it and see.

Okay, so now we know that the table sort of does account for the characteristics of each material class and while it is subtle, understanding how the tool angles work allows you to see the nuances contained therein. This knowledge and 5 bucks will get you a Venti Hazelnut Mocha at your local Starbucks!

There is a reason for dragging you through the table discussion. The table gives you two really valuable things: it gives you baseline angle data to begin your angle changes from, and it tells you which materials can potentially work harden (higher carbon steels and stainless steels).

Now you know what the tool angles do and you have some baseline tool angles to consider. Next you need to consider the characteristics of the material you’re going to work with. I’m not going to list the characteristics of every material class for you. You need to research this yourself but we will design a tool for Stainless Steel as an example of the design process.

NB: To possibly save you some work, I’ve already given the angles for my mild steel, brass and aluminum cutting tools in post #36 of this thread if that is of interest to you. You never know; they might just work well on your lathe.

Designing a tool for Stainless Steel

Material considerations:

Stainless steels are produced in a large variety of alloys. Perhaps the commonest group of SS is the Austenitic group, the 300 series: the most commonly used SS is probably 316, followed by 304. The easiest alloy to machine is the Sulfur-bearing 303; this is a good choice for hobby guys to use.

The 300 series is not really that hard in the annealed state. If you look at the machinability chart you will see that 303 has a rating of 78%, while 304 is about 45% compared to 1212 mild steel at 100%. So, 303 is only slightly harder to cut than mild steel and 304 is about twice as hard to cut. The cutting speed for the Austenitic group is about 120 SFM for HSS tooling so again, not that hard to cut. It can actually be a little “gummy” and will tend to produce stringy chips.

Again, the one characteristic we need to be aware of is the tendency for SS to work harden. Basically, if the work heats up too much at the point of cut the surface will form a hard layer that can be very difficult to cut, much less take a finish cut. You have to get under that layer to cut it if it forms so we need to avoid work hardening when possible. To minimize work hardening, use a good tool and avoid dwelling in the cut. We can discuss how to cut SS in more detail if there is interest.

SS Tool Design

Now we know that we need to focus on reducing cutting forces to produce a tool that will move through our SS with relative ease while also getting the chips out of the cut fast to avoid work hardening. We also know we intend to alter the standard tool angles in the table to do this. The question is how do we change them and by how much?

The answer is based on my experimental data. I apologize but this is all I have to go on; there is no published data beyond this recommendation as far as I know. My method is to add 25-40% to the standard baseline angles and base the specific amount on your priorities. For example, if the material is soft then you can increase the relief angles by 40% without worrying about edge life but if the material is hard, like tool steel, then you may want to leave the relief angles at baseline and try reducing cutting forces with your rake angles. It’s a judgement call that will later be tested in a cut.

Sorry guys, I know this sounds nebulous. The best advice is to start low, say 25%, then increase it until you either get the result you want or things get worse. Over time, you will learn the judgement you need to adjust things with confidence. You will also find that this 25-40% range works rather well. Going above 40% usually doesn’t provide much benefit; it tends to lead to premature wear, especially in harder materials. There are also times when going past 40% makes sense; I do this if the tool keeps cutting better and better without going dull too fast. Usually this is on soft stuff like plastics.

Keep in mind that when we change an angle to produce a desired advantage, it will produce that advantage. But if we change another angle the effect is additive; each angle change augments the effect of other angle changes. In other words, no tool angle works in isolation. If you change one, it affects them all. This is a good thing, by the way, and it is good to know that a change here and a change there adds up to a lot so you do not need to go hog wild with your angle changes. Go slow, be conservative, and base further changes after you assess how well the tool cuts. After you have some experience with this tool angle change thing I think you will be very surprised to see how even a degree or two can produce BIG results.

Okay, now we can get to designing our tool

Tool angles from the table: Side relief: 10°, End relief 8°, Side rake 15-20° and Back rake 8°.

What these angles tell you is that a conventional tool attempts to deal with the work hardening tendency of SS by using the rake angles – smart. The remainder of the angles are rather ordinary – not so smart.

• Relief angle considerations: Since SS is not that hard we can add a bit more relief angle to reduce cutting forces without worrying too much about affecting edge life. We need not go overboard with this because the effect of our rake angles will be additive to whatever we do with the relief angles. For this tool, a 40% increase will do, so we can make the side and end relief 14° but 15° will work better with minimal impact on strength.
• Side rake: side rake is the key angle here because it reduces both cutting forces and cutting temperatures in a single stroke. If we use 40% of the standard 20° we would have 28°, which is just about right.
• Back rake: standard amounts of back rake will put the cutting forces at the side cutting edge, which is fine, but if we increase back rake it will assist in chip thinning, reduce cutting forces and will also accelerate chip flow to reduce cutting temperatures even more. It will also put the cutting forces closer to the tip, which is where a smaller lathe will apply power during a roughing cut. Let’s add a full 40% of the standard 8° to give 12° of back rake.

So, if we hold to the recommended 25-40% increases over standard table angles we could grind our tool with 15° relief angles for both side and end, 28° of side rake and 12° of back rake. Our tool will have adequate support under the cutting edges and pretty large amounts of side and back rake to get the chip out of the cut quickly to reduce cutting temperatures and cutting forces will be reduced significantly. Note that these angles are a guess on my part but based on past experience, it’s a pretty good guess. If I were a new guy, I would start with a 25% increase for each angle and work my way up until the tool cut really well. There is no magic number or formula.

I would use a 1/64” nose radius on this tool to minimize deflection. The reasoning is that too large a nose radius will tend to deflect with smaller depths of cut and if it deflects it won’t cut and if it doesn’t cut then it builds heat and if it builds heat then it work hardens and then we can’t take a finishing cut. Whew!

And that is how you design a tool for Stainless Steel – Toldja’, not that hard!

How to test your tool design

When I design a tool that I’ve never made before, I grind one blank to the conventional angles in an angle table and put my modified angles on another blank or on the end opposite the conventional one. I keep careful records on depth of cut, speed and feed, then push the conventional tool until it chatters. Then I do sizing cuts and then finishing cuts. Once I know how that conventional tool works and I am sure of its limits, I test the modified tool.

Usually, I will grind the modified tool with conventional angles except for one single angle. Typically, this will be a modified relief angle. Then I push that tool hard and record the results. I often start the modified angle at about 25-30% and increase the amount if the edge holds up well. If I can take a cut deep enough to chatter and the edge remains razor sharp then I consider that optimized. Sometimes I will go beyond 40% if the edge is holding up well. If you later notice that your side edge is dulling in use, go back to your notes and reduce the relief angles on the next tool.

Once I get the relief angle optimized, I move on to modifying side rake alone, then modify back rake. What I am looking for here is to see how much deeper I can cut before the tool chatters or the lathe bogs down. Do not be surprised if your modified side and back rake allow you to double the depth of cut over a conventional tool. I also watch to see if the chips clear faster and better and also if the chips thin as expected; they should. I consider rake angles optimized when I can at least double the depth of cut over that of a conventional tool. If the tool is for a material that work hardens, then it must be able to exceed the depth of cut that causes the conventional tool to chatter at the very least; your tool should easily make that same cut without chatter and without work hardening. Typically, your cutting edges should be as sharp as they were at the start of the cut.

How do you assess whether the tool reduces work hardening? You take a decent depth of cut and keep the tool cutting continuously. Then you take a fine cut with a relatively slow feed, maybe 0.005” deep, and the tool should shave off a chip with a fine finish. If it does this, that work surface is not hard and your tool angles worked. You can also test the work for hardness with a hardness file and compare it to an un-cut piece.

I always start with a very small nose radius, one I can just see with the naked eye, then increase it until it gets to about 1/32” so I can see how the tool cuts with that large a radius. To me, 1/32” is a big radius and I try hard to keep it smaller if I can. For soft materials like Aluminum, brass or plastics a larger nose radius is okay but for steels, I usually go with 1/64”.

Once you feel comfortable that you’ve optimized your tool angles and you’ve actually done some practice pieces and know that the tool works well then grind a keeper from a blank you trust and you’re done. It may take me several months to get to this point but I know that my keeper tool is going to be a “good tool”.

As I said, this is not hard to do. It requires work and thought but you will end up with a tool that is unique, and you will know exactly why it is ground the way it is. Moreover, if something goes wrong with it down the road, you will know exactly how to address it.

Trust me; one day you will rough out a work piece, then size it quickly and then take a micro-cut to come in on size. You’ll notice that the finish is really fine and the lathe and tool handled everything without effort. Then you’ll realize that you didn’t even stop to think about the tool once; you just made your cuts and the tool cut. On that day, you’ll know that you designed and ground a really good tool.

End Notes:

Guys, there is something about a 15° relief angle that just works. I don’t know why this is; it just works. I’ve done thousands of test cuts in all sorts of materials and somehow, that 15° relief angle is almost always where I end up. It reduces cutting forces nicely and stays sharp over time. I also tested relief angles for threading tools and 15° produced burr-free threads and held up in just about every material I’ve used it on, including tool steels. I tested angles above and below 15° for threading tools but this angle cuts really well and holds up the best. Whenever I need to grind a new tool for some material I haven’t cut before, I will start low and dial in the relief angles but somehow I end up with 15° as the best setting … dunno’.

I know this is a lot to absorb but while tedious in words, it is not difficult. It may look that way but it really isn’t. It is the only way I know of to grind a tool that will work on your specific lathe with you in the driver’s seat and I encourage you to try it.

I hope this helps you. If you have questions, I am here.



Mike

 

[TTD]

Awesome post (as usual), Mikey.

Being relatively new at grinding cutting tools (at least properly, anyways), I’ve still got a while to go yet before all of this becomes “second-nature”. Not knowing any better when I first started out a couple of years ago, I used to think that as long as only the tip of tool was touching the work & nowhere else was rubbing, that I was good to go. In fact, NONE of my tools back then even had any back rake…just “a bit” of side rake & side/end relief.

Now, thanks to this thread, I can see the errors of my ways & I’m starting to actually visualize in my mind what’s happening at the tool tip while cutting. Started paying more attention to tool geometry instead of the “that’ll-be-good-enough” mindset & the difference has been (no surprise) night & day.

I, and more importantly my wee lathe, can’t thank you enough Mike for taking the time to explain all of this so even dummies like me can understand it. It is immensely appreciated, my friend! Hands down, the best tutorial out there for beginners learning to grind their own tools.

 

[mikey]

Thanks, Todd. What a nice way to start my day!

 

[ttabbal]

Thanks again @mikey! I'm going to try your design method out on some 304 SS, just because I have some.

A couple questions I've bumped into that might be helpful for everyone to see answers to...

1) I see why turning the tool toward the tailstock helps with finish, but what about when you are turning to a shoulder on the left? Would it be better to make a tool for this? Perhaps by increasing the angle on the left, decreasing on the right to mimic the tool post angle change?

2) Stringy chips. I've had some of this on 6061 in particular. You can break them by stopping the cut, but is there more we can do to help them break on their own? I know chip-breakers can become a whole new topic, but perhaps a couple ideas for one material would be appropriate?

3) Tip radius. I made some using the belt grinder, but they aren't that consistent. Tend to be larger at the tip, and thin out toward the bottom. It works, as the top is where the cutting happens, but I would like to be more consistent to make measuring easier. The diamond cards are a bit slow to cut this. Would a light touch with a file be a good idea? Perhaps some sandpaper on a flat surface?

 

[ttabbal]

I'm putting this in a different post to try to keep things organized.. I need to turn some cast iron for my collet chuck. It doesn't seem to have a hard surface, which I read can be a problem. Sadly, I don't have scraps to work with, so testing will be a little limited. I was leaning toward just taking a light cut for testing, as I only need to remove about 0.1".

Tool angles from the table: Side relief: 10°, End relief 8°, Side rake 12° and Back rake 5°.

This puts them in range of the 15° square tool, with a lower back rake. I read it tends to cut a bit dusty, and can be a bit abrasive. So I'll cover the ways and use a vacuum to keep dust down. I was leaning toward doing 15/15 on the side/end. This is to a shoulder on the left, so the previous post's question about that will come into play here. I'm leaning toward going to 15 on the side rake, 8-10 on back rake. 15 mostly so I can be lazy and not adjust the table angle. Increasing the back rake to move cutting forces toward the tip, as like most here, I have a small lathe.

My lazy side wants to toss a 15° square tool in there and give it a go. I suspect it would cut reasonably well. Perhaps not ideal, but it IS a one-off. I don't anticipate turning much cast iron. Of course, now that I've said that, I'll need to do a ton of it.

What do you think Professor Mike? Am I heading the right direction?

 

[DHarris]

GREAT write-up Mikey!

Ok, I've taken this latest chapter and placed it at the end of the text book (we are now at 37 pages). I've attached it to this post for Mikey to review and let me know if there is a better "order" for the flow of the document. Let me know what changes to order of topics you would make, and I'll make them - format the document better (page breaks & text flow around photos / diagrams etc) and post a final version to this thread.

All input welcome

p.s. and I'll make a table of contents in the front from the highlighted titles.

edit: file is in an older version of MS word - so everyone should be able to open it up.

 

[mikey]

A couple questions I've bumped into that might be helpful for everyone to see answers to...

1) I see why turning the tool toward the tailstock helps with finish, but what about when you are turning to a shoulder on the left? Would it be better to make a tool for this? Perhaps by increasing the angle on the left, decreasing on the right to mimic the tool post angle change?

If you use a general purpose shape, like our Square Tool shape, then it will cut into a shoulder with no problem. When doing so, you are taking light cuts with the nose radius and end cutting edge. The tool angle is the same as you would use for a facing cut. Usually you will cut with the end edge until you hit the shoulder and then face out.

2) Stringy chips. I've had some of this on 6061 in particular. You can break them by stopping the cut, but is there more we can do to help them break on their own? I know chip-breakers can become a whole new topic, but perhaps a couple ideas for one material would be appropriate?

Certain materials, most notably aluminum and stainless steel, will string. This is especially true when we take light cuts. It has to do with the higher ductility of the material - thin chips don't break off; they string. You can break them by taking heavier cuts at higher feed rates or you can pause momentarily in the cut. Unfortunately, light cuts will string and that is just the way it is. If you use a carbide insert with a chipbreaker it can help but you have to take a heavy enough cut for the chipbreaker to work so what do you do when you have take light cuts? You string, plus you will have issues with accuracy when taking finishing passes. There is no perfect answer, sorry.

3) Tip radius. I made some using the belt grinder, but they aren't that consistent. Tend to be larger at the tip, and thin out toward the bottom. It works, as the top is where the cutting happens, but I would like to be more consistent to make measuring easier. The diamond cards are a bit slow to cut this. Would a light touch with a file be a good idea? Perhaps some sandpaper on a flat surface?

For me, the fastest way to cut an accurate nose radius is with a coarse diamond stone. I cut a flat with it using firm pressure, then switch to a fine or extra-fine stone to round and blend the edges. It takes me maybe 60-90 seconds from start to finish. I can do it with a fine grit belt if I'm careful but I prefer the diamond stones - far more likely to give me an accurate radius. It would be unlikely to have a file cut hardened HSS but a sandpaper might work - see you in about a year or so. All kidding aside, use the diamond stones.

 

[mikey]

I'm putting this in a different post to try to keep things organized.. I need to turn some cast iron for my collet chuck. It doesn't seem to have a hard surface, which I read can be a problem. Sadly, I don't have scraps to work with, so testing will be a little limited. I was leaning toward just taking a light cut for testing, as I only need to remove about 0.1".

Tool angles from the table: Side relief: 10°, End relief 8°, Side rake 12° and Back rake 5°.

This puts them in range of the 15° square tool, with a lower back rake. I read it tends to cut a bit dusty, and can be a bit abrasive. So I'll cover the ways and use a vacuum to keep dust down. I was leaning toward doing 15/15 on the side/end. This is to a shoulder on the left, so the previous post's question about that will come into play here. I'm leaning toward going to 15 on the side rake, 8-10 on back rake. 15 mostly so I can be lazy and not adjust the table angle. Increasing the back rake to move cutting forces toward the tip, as like most here, I have a small lathe.

My lazy side wants to toss a 15° square tool in there and give it a go. I suspect it would cut reasonably well. Perhaps not ideal, but it IS a one-off. I don't anticipate turning much cast iron. Of course, now that I've said that, I'll need to do a ton of it.

What do you think Professor Mike? Am I heading the right direction?

As usual, the answer is more complicated that just tool angles. Cast iron comes in a variety of alloys and this matters - a lot. If your plate is grey cast iron then it machines readily. Ductile cast iron has higher strength and is a bit harder to cut. There are several other alloys that I haven't machined before so I cannot advise you there. The problem for me has been consistency in the material, or lack thereof. Chinese CI is not usually of high quality and can have soft and really hard spots throughout the casting. Unfortunately, it is the most readily available and the cheapest. European CI is usually pretty consistent. USA CI is also usually pretty good to very good.

I normally use a HSS tool to cut CI but I have also used carbide inserts with TIN coating and they worked well. I prefer HSS because I can keep the tool sharp. I also use cobalt HSS, not M2. The machinability rating for CI varies with the alloy but in general, it is about 45-50%. Bear in mind that this material is abrasive so in addition to being harder, it tends to grind down edge. That is why I tend to use cobalt. It is also why I tend to be more conservative with my relief angles.

I cannot find my CI tool to measure the angles but my records show that it had 12 degree relief angles to contend with the hardness and abrasion. When turning CI with a cobalt tool, I tend to go slower (about 60SFM) so cutting temps are not that big an issue but cutting forces can be huge so I did increase side rake to 15 degrees. Back rake was 10 degrees on my tool. My records show that I was concerned about a built up edge but I didn't go into more detail (and I can't remember that far back). Regardless, the tool cut backplates well.

I will need to grind another CI tool soon because I have some backplates to do, too. I will very likely put some thought into it but I will probably end up close to these numbers. Sorry to be of so little help. I do agree that a Square Tool will probably work but the only way to tell is to try it. If the edges wear too fast, switch to cobalt.

 

[ttabbal]

If you use a general purpose shape, like our Square Tool shape, then it will cut into a shoulder with no problem. When doing so, you are taking light cuts with the nose radius and end cutting edge. The tool angle is the same as you would use for a facing cut. Usually you will cut with the end edge until you hit the shoulder and then face out.

Ah, hold it like a facing cut was the bit I missed. Thanks! I kept visualizing trying to come into a shoulder with the tool post angled toward the tailstock, so the shoulder might well hit the tool or the tool post before the cutting edge got there. I knew that couldn't be what you were saying to do.

Certain materials, most notably aluminum and stainless steel, will string. This is especially true when we take light cuts. It has to do with the higher ductility of the material - thin chips don't break off; they string. You can break them by taking heavier cuts at higher feed rates or you can pause momentarily in the cut. Unfortunately, light cuts will string and that is just the way it is. If you use a carbide insert with a chipbreaker it can help but you have to take a heavy enough cut for the chipbreaker to work so what do you do when you have take light cuts? You string, plus you will have issues with accuracy when taking finishing passes. There is no perfect answer, sorry.

I suspected that might be the answer, but I wanted to make sure I wasn't missing anything obvious. Strings aren't the end of the world, just have to be a little more careful around the machine when they are present. And try not to let them get into the cut, messing up my finish. I see why 12L14 is so popular. I can cut that with near anything and get nice easy chips.

For me, the fastest way to cut an accurate nose radius is with a coarse diamond stone. I cut a flat with it using firm pressure, then switch to a fine or extra-fine stone to round and blend the edges. It takes me maybe 60-90 seconds from start to finish. I can do it with a fine grit belt if I'm careful but I prefer the diamond stones - far more likely to give me an accurate radius. It would be unlikely to have a file cut hardened HSS but a sandpaper might work - see you in about a year or so. All kidding aside, use the diamond stones.

I'm blaming lack of coffee for the idea of using a file on HSS. I forget how hard these things are, the belts just cut them so quickly.

Push harder with the coarse diamond, got it. I tend to be a little too gentle on the honing pressure.