Finishing passes with 1/2" carbide ball end mills in steel

I'm trying to run toolpaths in some high carbon stainless steels to make knife bevels, hardness is about 255BHN. I'm getting my speeds/feeds from Gwizard calc, running a Tormach 770 for my mill, which tops out at 3250 RPM. My surface finish has been less than ideal, and I'm wondering where I'm going wrong.

I'm reducing a piece of stock that's only 3mm thick down to 1mm at where the cutting edge will be, I've not been doing any roughing passes. Because the knife bevel needs to have as smooth a surface finish as possible, I have been using a 1/2 inch wide ball end mill.

How would you approach this problem? How deep would you attempt to cut during a pass?

Put the knife in a vise so you're using the side of the tool rather than the end.

Hi Joe - The ball mill will not give you a good finish as I imagine you are using the "end" which has a very small non cutting surface in the middle. Either use a straight mill tool and turn the blade up as Gerry suggests (to cut on the side) or use a small end mill for your roughing (cut on the end). This will give you a good cut and finish.. Then I imagine you draw file to finish? Peter

To discuss - The centreline of a blade is "straight" in its plane so this means if the blade is supported at its intended half angle the cut surface is planar? (No, after thought) So a very sharp blade is 15degs so support at 7.5degs or a typical sharp blade is 20degs so support at 10degs. Is this what you are doing? As the blade edge is curved I think the only way to do it is to hold it at the half angle as then the cut angle remains the same to the blade irrespective of the shape of the blade & you use the side of a milling tool. Or you can incrementally machine it if the blade is vertical but then it will have steps in it to be removed. Does this make sense? Are you trying to achieve a "hollow" bevel?

Thought this through and its tricky. If you get a 10deg draft cutter you can cut it on the tool side and have the knife vertical. Then the tool path has to follow the "vertical" blade shape or belly. Or you can tilt the machine head but does the Tormach do this? Peter

https://conicalendmills.com/cutting-...pered-end-mill

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Originally Posted by peteeng

Hi Joe - The ball mill will not give you a good finish as I imagine you are using the "end" which has a very small non cutting surface in the middle. Either use a straight mill tool and turn the blade up as Gerry suggests (to cut on the side) or use a small end mill for your roughing (cut on the end). This will give you a good cut and finish.. Then I imagine you draw file to finish? Peter

To discuss - The centreline of a blade is "straight" in its plane so this means if the blade is supported at its intended half angle the cut surface is planar? (No, after thought) So a very sharp blade is 15degs so support at 7.5degs or a typical sharp blade is 20degs so support at 10degs. Is this what you are doing? As the blade edge is curved I think the only way to do it is to hold it at the half angle as then the cut angle remains the same to the blade irrespective of the shape of the blade & you use the side of a milling tool. Or you can incrementally machine it if the blade is vertical but then it will have steps in it to be removed. Does this make sense? Are you trying to achieve a "hollow" bevel?

Thought this through and its tricky. If you get a 10deg draft cutter you can cut it on the tool side and have the knife vertical. Then the tool path has to follow the "vertical" blade shape or belly. Or you can tilt the machine head but does the Tormach do this? Peter

https://conicalendmills.com/cutting-...pered-end-mill

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Thanks for the in depth response! I actually intend to leave the machined finish on the blade if I can get it smooth enough.

Unfortunately my Tormach doesn't have the ability to tilt it's head, otherwise I could use my current fixture and machine the bevel using the side of an end mill as you and Gerry suggested. Since this isn't a one-off project I've made a fixture plate to hold four pieces of stock flat; that is, knife blade lying on its side, and using mounting holes is held against the surface of the plate. I actually don't mind having SOME variance in the blade bevel, having some evidence of cusps left behind is fine. I have seen other knife makers use a similar setup, and so I'd like to make it work if possible. Turning the fixture on its side and using the side of a cutter would be something I would explore if I can't manage to machine the blade on the horizontal.

I actually don't want a hollow grind for this particular project, so that kind of simplifies things in that regard.

I think you're quite right about running with the bottom of the ball end mill, as although the fluting does extend down to the bottom of the cutter, it ends in sharp corners around the center, and the fluting isn't as deep there... that coincides with how the surface finish has come out so far.

Two possibilities have occurred to me, though. Do you think it would work well to have a ball nose end mill run along the profile of the bevel, from the edge to the spine / flat side of the blade? Or do you think that running the cutter upwards from the edge to the spine directly, stepping over and repeating from low Z to high? Basically with those two methods I'd be attempting to cut the steel using as much of the side of the cutter as possible, instead of the bottom as I have been. Alternatively, the same approach using a regular end mill with a larger corner radius?

Hi Joe - Doing the blade on the flat is the easiest set-up, which means it will have to be a round tool and use a small stepover. I think it's time to talk to a tooling supplier to find the correct tool for the job. A toolmaker will know the answer as this is what a plastic injection mould would have to do in tool steel. Speeds and feeds will be important to get the finish you want. Peter

Quote:

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Originally Posted by peteeng

Hi Joe - Doing the blade on the flat is the easiest set-up, which means it will have to be a round tool and use a small stepover. I think it's time to talk to a tooling supplier to find the correct tool for the job. A toolmaker will know the answer as this is what a plastic injection mould would have to do in tool steel. Speeds and feeds will be important to get the finish you want. Peter

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That's not a bad idea, I'll talk to a local supplier and see what they recommend for the job. Thanks again for the insight, you've been very helpful.

Hi Joe - Have to ask, "what would Titan do? BOOM. Peter try Kennametal tools.