-
Welcome back Guest! Did you know you can mentor other members here at H-M? If not, please check out our Relaunch of Hobby Machinist Mentoring Program!
Triangle vs Diamond inserts
- Thread starter nnam
- Start date
- Joined
- Dec 20, 2012
- Messages
- 9,422
@nnam, my last post was off the top of my head. I just re-read it and it is all true but the wording might be "above your head" if you don't yet understand inserted cutting tools. I apologize; I didn't intend that. Let me try to be a bit more useful, and in doing so I am going to assume you are not running a CNC lathe.
Negative rake tooling requires a big, powerful, rigid machine. These tools do not require speed as much as they require rigidity and power. A negative rake tool like a Trigon negative rake tool used in a big lathe, say 20", will put a beautiful finish on a part while also taking what would be a monster cut on a hobby lathe. Put that same tool in a 10-12" lathe and the tool will not cut nearly as well. It will cut but your depths of cut will be limited and your finish will suffer some. Not only that, your accuracy will suffer if you cannot take a big enough cut to bury the nose radius. This is why most of us are trying to steer you away from negative rake tooling. I don't know what kind of lathe you have so take this with a grain of salt, okay?
The vast majority of hobby guys will be better served with positive rake tools. The term "positive rake" can be confusing because the insert holder or the insert can have positive rake. For the tool holder, a positive rake means the insert is angled up a bit. For an insert, the cutting edge and the slope of the top of the insert behind the edge can slope up, meaning it has positive rake. All of these inserts will have a relief angle ground under the cutting edge; this is characteristic of positive rake tools. Note that you can have a neutral rake tool/insert holder and use positive rake inserts in it; this will give you a net positive rake cutting tool.
Positive rake tools cut with lower cutting forces but rake is only a small part of the cutting forces the tool generates. Your cutting conditions (depth of cut, speed and feed) also impact on the forces the tool generates, as does the nose radius. The nose radius is perhaps the most important factor to consider when choosing inserts and the tools that will hold those inserts. If you are using a light lathe to take small cuts on common (not hard) materials then you want a positive rake tool and insert with a small nose radius, get it?
Note that the class of insert you use in a given tool holder, like a CCMT insert for an SCLCR tool holder, will have different edge geometries depending on which operation you are doing. Roughing inserts will usually have a wider land between the cutting edge and the raked part of the insert; medium cutting inserts will have a slightly narrower one, while finishing inserts usually have almost no land behind the cutting edge; the raked part is right behind the cutting edge. Of the three types of CCMT inserts, the finishing insert will produce the lowest cutting forces. If you also use the smallest nose radius you can find in a finishing insert then you will be able to take smaller cuts on a smaller, less rigid and less powerful lathe.
I hope this sort of helps to visualize what we've all been saying here. If I had to recommend a single tool holder for general use on a hobby class lathe, it would be an SCLCR tool holder that uses CCMT and CCGT inserts. Both of these inserts are positive rake insert. I would focus on finishing inserts that can rough, take medium cuts and finish well for you. I would also choose the smallest nose radius you can find for that tool. Note also that bigger tools are not always better because small nose radii are not found on bigger inserts. If accuracy is important then pay close attention to the size of the nose radius; the bigger it is, the more it will deflect. The most accurate cuts will use a depth of cut at least equal to the nose radius so sometimes smaller is better, especially on a small lathe.
Negative rake tooling requires a big, powerful, rigid machine. These tools do not require speed as much as they require rigidity and power. A negative rake tool like a Trigon negative rake tool used in a big lathe, say 20", will put a beautiful finish on a part while also taking what would be a monster cut on a hobby lathe. Put that same tool in a 10-12" lathe and the tool will not cut nearly as well. It will cut but your depths of cut will be limited and your finish will suffer some. Not only that, your accuracy will suffer if you cannot take a big enough cut to bury the nose radius. This is why most of us are trying to steer you away from negative rake tooling. I don't know what kind of lathe you have so take this with a grain of salt, okay?
The vast majority of hobby guys will be better served with positive rake tools. The term "positive rake" can be confusing because the insert holder or the insert can have positive rake. For the tool holder, a positive rake means the insert is angled up a bit. For an insert, the cutting edge and the slope of the top of the insert behind the edge can slope up, meaning it has positive rake. All of these inserts will have a relief angle ground under the cutting edge; this is characteristic of positive rake tools. Note that you can have a neutral rake tool/insert holder and use positive rake inserts in it; this will give you a net positive rake cutting tool.
Positive rake tools cut with lower cutting forces but rake is only a small part of the cutting forces the tool generates. Your cutting conditions (depth of cut, speed and feed) also impact on the forces the tool generates, as does the nose radius. The nose radius is perhaps the most important factor to consider when choosing inserts and the tools that will hold those inserts. If you are using a light lathe to take small cuts on common (not hard) materials then you want a positive rake tool and insert with a small nose radius, get it?
Note that the class of insert you use in a given tool holder, like a CCMT insert for an SCLCR tool holder, will have different edge geometries depending on which operation you are doing. Roughing inserts will usually have a wider land between the cutting edge and the raked part of the insert; medium cutting inserts will have a slightly narrower one, while finishing inserts usually have almost no land behind the cutting edge; the raked part is right behind the cutting edge. Of the three types of CCMT inserts, the finishing insert will produce the lowest cutting forces. If you also use the smallest nose radius you can find in a finishing insert then you will be able to take smaller cuts on a smaller, less rigid and less powerful lathe.
I hope this sort of helps to visualize what we've all been saying here. If I had to recommend a single tool holder for general use on a hobby class lathe, it would be an SCLCR tool holder that uses CCMT and CCGT inserts. Both of these inserts are positive rake insert. I would focus on finishing inserts that can rough, take medium cuts and finish well for you. I would also choose the smallest nose radius you can find for that tool. Note also that bigger tools are not always better because small nose radii are not found on bigger inserts. If accuracy is important then pay close attention to the size of the nose radius; the bigger it is, the more it will deflect. The most accurate cuts will use a depth of cut at least equal to the nose radius so sometimes smaller is better, especially on a small lathe.
Last edited:
Thank you all for taking time explaining the in and out of inserts. It turns out that the shape does not matter as much as the others.
I was lucky not fully aware of this but bought the recommended CCMT inserts and holders.
They have a 0.4mm nose radius. I just ordered a set of 10 inserts CCGT with 0.2 mm nose radius for finishing cut and aluminum cuts.
My lathe is 15x30 Leblond regal. It has 3hp I think, but some are equipped with 5hp.
I was lucky not fully aware of this but bought the recommended CCMT inserts and holders.
They have a 0.4mm nose radius. I just ordered a set of 10 inserts CCGT with 0.2 mm nose radius for finishing cut and aluminum cuts.
My lathe is 15x30 Leblond regal. It has 3hp I think, but some are equipped with 5hp.
- Joined
- Mar 26, 2018
- Messages
- 2,724
Awesome! Nice lathe. That would be at the bottom end of being able to actually use negative rake tools. Your lathe is bigger than most on this site.
My lathe is a 1000lb 12x36 with 1.5 HP. Just a bit too small for a CNMG432
My lathe is a 1000lb 12x36 with 1.5 HP. Just a bit too small for a CNMG432
- Joined
- Nov 24, 2018
- Messages
- 239
Would like to meet up with you. Also live in Tucson. In the process of renovating a Bridgeport Milling Machine. Also have a 13-40 WWII Southbend lathe. Hope to hear from you soon. Thanks. Rolly
