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Models for grinding HSS Lathe Tools
- Thread starter mikey
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- Dec 20, 2012
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bfk, thank you for your kind words. I really appreciate the sentiments from you and all the other guys.
May I suggest you change from 1/4" tool bits to 3/8"? I think you will find that the Sherline lathe responds very well to a stiffer bit, and you will see this when you take fine finishing cuts; your accuracy goes way up.
The maximum depth of cut a tool will produce is great but quite honestly, I don't often cut that deep. I know the tool can handle it but why waste material? What really matters to me is how accurately I can turn with that tool. If you keep the nose radius down to about 1/64" or so, the square tool will size accurately enough to come in dead on size most of the time. The cool thing is that when you have a half-thou to take off, it will take that much off, too. Try interpolating your depth of cut to 0.00025 and, if the tool is sharp, it will take that off the diameter.
Cutting force reduction isn't just about how deep you can go; it is also very much about how that reduction enables very fine cuts. This is why I keep saying that for a small lathe, HSS is often the better option - accuracy!
Anyway, good for you, bfk! Keep grinding, experimenting and enjoying your lathe.
Mike
May I suggest you change from 1/4" tool bits to 3/8"? I think you will find that the Sherline lathe responds very well to a stiffer bit, and you will see this when you take fine finishing cuts; your accuracy goes way up.
The maximum depth of cut a tool will produce is great but quite honestly, I don't often cut that deep. I know the tool can handle it but why waste material? What really matters to me is how accurately I can turn with that tool. If you keep the nose radius down to about 1/64" or so, the square tool will size accurately enough to come in dead on size most of the time. The cool thing is that when you have a half-thou to take off, it will take that much off, too. Try interpolating your depth of cut to 0.00025 and, if the tool is sharp, it will take that off the diameter.
Cutting force reduction isn't just about how deep you can go; it is also very much about how that reduction enables very fine cuts. This is why I keep saying that for a small lathe, HSS is often the better option - accuracy!
Anyway, good for you, bfk! Keep grinding, experimenting and enjoying your lathe.
Mike
- Joined
- Dec 20, 2012
- Messages
- 9,408
I was corresponding with one of the guys tonight and mentioned how to hone a tool. I am reproducing that advice here. Please don't be insulted by this; it is simply my way and I thought you might find it useful:
Remember that it is easier to hone a tool at the end of the day than it is to let it get dull and have to regrind it.
- Use an extra-fine diamond stone and water to lube it. Be sure the stone is clean; wash with Ajax or similar first and get it clean and flat.
- If you can, submerge the stone in water and then place it on a solid surface.
- Bring the side cutting flat into contact with the stone and make sure it sits dead flat, no rocking. Gently stroke towards you. Going back and forth will make you rock the tool and you will round the flat. Just pull and apply gentle even pressure. You are not trying to grind the surface; you are simply honing it.
- Do the same to the end cutting face. Be careful not to tip the tool; just keep it flat and pull lightly. EDIT: I should tell you that your goal is to raise a burr at the top of the face you're honing. You will remove this burr in the next step.
- Finally, hone the top of the tool. You MUST keep this flat. It usually only takes a few strokes per face to hone (EDIT: to remove the burr) the tool.
- Do not touch the nose radius unless it is getting too small. If you need to recut it, go easy and try to keep the radius even from top to bottom and side to side.
Remember that it is easier to hone a tool at the end of the day than it is to let it get dull and have to regrind it.
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- Dec 12, 2012
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- 109
Thanks guys. I've ground a few tools in the past using the SB "How to Run a Lathe" for guidance, but I never put that much time or effort into the grinder setup. I also used to just guess at the angles. I didn't get a chance to grind HSS last night but am excited to do so.
Mike, thanks for the honing tips. I've always dragged the tools across the abrasive so that a burr would be left on the dull end, i.e. the sharp edge leading the drag across the stone, if that makes sense.. No rhyme or reason for this, just what I thought was right. Should I drag the opposite way? Or does it matter?
Barry
Mike, thanks for the honing tips. I've always dragged the tools across the abrasive so that a burr would be left on the dull end, i.e. the sharp edge leading the drag across the stone, if that makes sense.. No rhyme or reason for this, just what I thought was right. Should I drag the opposite way? Or does it matter?
Barry
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I try to raise a burr on the top of the tool, Barry. That way, when I hone the top surface I take that burr off, leaving a sharp edge behind. After shaping the tool on the belt sander, I hone really carefully with the coarse stone. I am looking to get the coarse grind marks off without changing the angles of the face, creating facets or grinding down an edge. When I go to the fine stone, all I want to do is remove the marks from the coarse stone. Same with the extra-fine stone, but here I am making sure that I have zero light reflecting off the cutting edges. Once I get that, I stone the nose radius on and hone the top surface again and the tool is ready for use. It normally takes me far longer to hone a tool than to shape a tool.
A tool sharpened this way will shave hair, cut meat and will definitely cut metal. If I shaped the tool with the right angles, it will stay sharp for a long time. As I work, I keep an eye on the edges. If I see reflected light (which I rarely ever do), I stop and hone it with a few strokes; takes all of 15 seconds to do. Most guys don't pay much attention to their edges but once you see how a sharp tool cuts you will begin to. When you're trying to come in on size on a critical part take the time to sight your edges because a dull edge will deflect and this affects your accuracy.
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In support of the Junior shooting program Mike donated a couple of Rex AAA cutters he made, one knife and the standard turning to cuts 416R SS specifically. Following Mike's instruction on angle of attack of the turning tool, made turning 20 inch barrels a lot easier, played with the angle till i got the results. Pointing the cutter towards the chuck when roughing and the other way finishing is the key, which I never practiced before with indeaxble carbide bits.
The knife tool is excellent in cutting or finishing the tenon shoulder. I needed to adjust the headspace on one barrel 0.0005, decided to use the knife tool, touch off the tenon, monitoring the tool post with 0.0005 indicator, moved the carriage and cut. And it indeed shaved off 0.0005 on the tenon shoulder.
On Mike's suggestion I will grind my own and duplicate and save his for models to copy. The first one I will duplicate is the knife tool specifically for cutting crown
Mike, thank you.
The knife tool is excellent in cutting or finishing the tenon shoulder. I needed to adjust the headspace on one barrel 0.0005, decided to use the knife tool, touch off the tenon, monitoring the tool post with 0.0005 indicator, moved the carriage and cut. And it indeed shaved off 0.0005 on the tenon shoulder.
On Mike's suggestion I will grind my own and duplicate and save his for models to copy. The first one I will duplicate is the knife tool specifically for cutting crown
Mike, thank you.
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- Dec 20, 2012
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Thank you, Bamban. With your consent, can I post the angles for the tools so others can duplicate them for working 416R? It will be a good exercise to show how we chose the angles we chose and thanks to you we know we have a working tool that others can duplicate for their work. What do you think?
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Okay, Bamban and I discussed his work before deciding on what tool geometry to use. He is turning 416R stainless steel rifle barrels. (I have not turned 416R before.) It comes prehardened to somewhere between Rc 24 to 36 so maybe semi-hard. Like all stainless steels, 416R will work harden so we needed a tool that would cut with lower forces to keep the tool from bogging down in the cut and producing heat. We also needed to clear the chips out fast to keep cutting temps low. Most of the work is general turning and facing but rather than just make a turning tool, we chose to make a general purpose tool and a knife tool.
HSS can cut 416 but a tungsten cobalt tool would handle the heat and abrasion better. We chose a Rex AAA bit because it has 5% cobalt but 18% Tungsten, the highest tungsten content of any bit made by Crucible. This would allow the tool to handle high heat and abrasion.
Here are the specs we chose:
The side rake is 7 degrees more than the angle table calls for. The goal is to greatly improve chip clearance while also reducing cutting forces. Given that the relief angles are increased and side rake is also boosted, the risk of reducing edge life is a real thing so I encouraged Bamban not to hog with this tool.
Back rake is increased by only a few degrees. This is intended to keep the cutting forces focused juuust to the side of the nose radius. It would have been nice to increase back rake more but then we would need a larger nose radius to finish well and this would lead to deflection, less cutting action and more heat production so I opted for more side rake and kept back rake reasonable. Focusing the forces here will enable the tool to rough well and face very accurately.
We chose a general purpose shape and a typical knife tool shape but used the angles above for both tools. The nose radius on the turning tool was held at about 1/64", while the knife tool radius is smaller than that. The reasons for these radii are to reduce radial forces to improve accuracy first; finish is a secondary consideration. If a better finish is required, the nose radius can be increased.
This is an example of how we can take a general purpose turning tool shape and alter its geometry to accommodate the peculiarities of the material. I will admit that since I haven't turned this material myself, the angles chosen were a guess ... but it was an educated guess. We chose a tool blank with the desired properties so it should hold up under frequent use; I think we chose well. Here are the tools right off the belt sander:
My thanks to Bamban for allowing me to share the specs on his tools. I wanted to show you how a tool blank was selected and how the angles were chosen. Bamban already told you how he altered lead angle to both rough and finish, which is helpful.
Now we have a tool that is proven to work with 416R. If you have a tool geometry you would like to share, please feel free.
HSS can cut 416 but a tungsten cobalt tool would handle the heat and abrasion better. We chose a Rex AAA bit because it has 5% cobalt but 18% Tungsten, the highest tungsten content of any bit made by Crucible. This would allow the tool to handle high heat and abrasion.
Here are the specs we chose:
- Side and end relief = 15 degrees
- Side rake = 25 degrees
- Back rake = 10 degrees
The side rake is 7 degrees more than the angle table calls for. The goal is to greatly improve chip clearance while also reducing cutting forces. Given that the relief angles are increased and side rake is also boosted, the risk of reducing edge life is a real thing so I encouraged Bamban not to hog with this tool.
Back rake is increased by only a few degrees. This is intended to keep the cutting forces focused juuust to the side of the nose radius. It would have been nice to increase back rake more but then we would need a larger nose radius to finish well and this would lead to deflection, less cutting action and more heat production so I opted for more side rake and kept back rake reasonable. Focusing the forces here will enable the tool to rough well and face very accurately.
We chose a general purpose shape and a typical knife tool shape but used the angles above for both tools. The nose radius on the turning tool was held at about 1/64", while the knife tool radius is smaller than that. The reasons for these radii are to reduce radial forces to improve accuracy first; finish is a secondary consideration. If a better finish is required, the nose radius can be increased.
This is an example of how we can take a general purpose turning tool shape and alter its geometry to accommodate the peculiarities of the material. I will admit that since I haven't turned this material myself, the angles chosen were a guess ... but it was an educated guess. We chose a tool blank with the desired properties so it should hold up under frequent use; I think we chose well. Here are the tools right off the belt sander:
My thanks to Bamban for allowing me to share the specs on his tools. I wanted to show you how a tool blank was selected and how the angles were chosen. Bamban already told you how he altered lead angle to both rough and finish, which is helpful.
Now we have a tool that is proven to work with 416R. If you have a tool geometry you would like to share, please feel free.
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