Grinding parting tools
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pkastagehand
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Thanks for all the input. Lots of info there and I didn't realize there were so many variations.
So, Tom, as regards your double beveled top profile do you just grind the front relief and you're good to go or do you do as others suggest and grind a back rake to get a flat cutting edge?
If you don't grind off that double top bevel do you think I could get away with not grinding my single top bevel? In some ways it seems like a way to have it remove the tit from the parted off piece if the flat side is set at center. But it may also mean I have some slight side thrust that may cause problems and the beveled side would be below center which may also change the dynamics.
Paul
So, Tom, as regards your double beveled top profile do you just grind the front relief and you're good to go or do you do as others suggest and grind a back rake to get a flat cutting edge?
If you don't grind off that double top bevel do you think I could get away with not grinding my single top bevel? In some ways it seems like a way to have it remove the tit from the parted off piece if the flat side is set at center. But it may also mean I have some slight side thrust that may cause problems and the beveled side would be below center which may also change the dynamics.
Paul
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Paul, double or single bevel, the blade is not shaped as a 'cutting edge' as bought, they are just HSS blanks.
If you consider it - presenting this to the work as is presents a tool with negative side rake on both sides with the double and an unbalanced profile with the single. I'm not saying it doesn't work (I can say I've never tried it) but I think you will find it will cut much better with a slight amount of top rake ground on and a lot less inward cutting force required too.
Without wishing to labour a point heres that broken tool from the previous pics as reground
If you are not convinced then try grinding just a short piece back - say 2-3mm and see how the cutting improves. If you're still not convinced then all you need do is grind it back.
BTW you do not have to grind the side faces as shown above. This was done simply to reduce the width of the tool for my personal situation but the top angle is about right for most save brass where it is better to have zero top rake.
As John says - many do find parting off a real stumbling block compared to the rest of the turning operations - giving yourself a head start with sharp tooling will be a big help.
Regards - Ramon
If you consider it - presenting this to the work as is presents a tool with negative side rake on both sides with the double and an unbalanced profile with the single. I'm not saying it doesn't work (I can say I've never tried it) but I think you will find it will cut much better with a slight amount of top rake ground on and a lot less inward cutting force required too.
Without wishing to labour a point heres that broken tool from the previous pics as reground
If you are not convinced then try grinding just a short piece back - say 2-3mm and see how the cutting improves. If you're still not convinced then all you need do is grind it back.
BTW you do not have to grind the side faces as shown above. This was done simply to reduce the width of the tool for my personal situation but the top angle is about right for most save brass where it is better to have zero top rake.
As John says - many do find parting off a real stumbling block compared to the rest of the turning operations - giving yourself a head start with sharp tooling will be a big help.
Regards - Ramon
Paul,
No - I just leave the double-bevel top of the blade as it comes. I know that what Ramon says (about them just being blanks which need grinding) is correct, but I didn't bother and it works for me (I have an ancient Myford Super 7).
I rarely have parting problems (but tend to only work on stock less than an inch diameter).
Regards Tom
No - I just leave the double-bevel top of the blade as it comes. I know that what Ramon says (about them just being blanks which need grinding) is correct, but I didn't bother and it works for me (I have an ancient Myford Super 7).
I rarely have parting problems (but tend to only work on stock less than an inch diameter).
Regards Tom
pkastagehand
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Thanks everybody for the input.
Ramon, what you're saying is what I've always done in the past. I just hated that I end up with something less than full blade with with that top rake and so needing to grind off and start over (or grind some side relief farther back on the tool) for a longer reach. I suppose I could do like many of you and have several parting tools for different purposes/materials/depths of cut, etc. Just hate spending money if I don't need to.
Tom,
I have tried parting CRS with an unground top with that funny (Armstrong) profile and it worked but I wasn't pleased with it overall. Might work OK in other materials, especially brass?
I will possibly buy a couple more tools and have some different shapes eventually but for now will probably just go back to putting the top rake on it.
Paul
Ramon, what you're saying is what I've always done in the past. I just hated that I end up with something less than full blade with with that top rake and so needing to grind off and start over (or grind some side relief farther back on the tool) for a longer reach. I suppose I could do like many of you and have several parting tools for different purposes/materials/depths of cut, etc. Just hate spending money if I don't need to.
Tom,
I have tried parting CRS with an unground top with that funny (Armstrong) profile and it worked but I wasn't pleased with it overall. Might work OK in other materials, especially brass?
I will possibly buy a couple more tools and have some different shapes eventually but for now will probably just go back to putting the top rake on it.
Paul
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