Cutoff Tool

[John Hasler]

Having never used a hobbyist type lathe I have no experience here.

That being said, I can not see any advantage to parting from the back unless one has run out of room for another tool on the cross slide or cycle time is a priority.

I suppose that lifting the cross slide off of the ways may impart a bit of chatter relief, this again I do not know, interesting concept however.

Parting from the front the toolpost bends forward, causing the tool to dive into the cut. Cutting from the back the toolpost tends to rock back, pulling the tool out of the cut. Might not be better, but at least it's a different set of problems.

 

[Wreck™Wreck]

Thank you.

I sometimes run LH tools from the back when I do not have the correct tool for the job, if making 50 or more parts the chance of running the spindle the wrong direction is almost assured at some point, carbide insert tooling does not like this (-:

 

[Profkanz]

Upon reading the Kyocera parting tool literature that I posted above a few things pique my interest. I have never read the booklets that come with the tools that I use because I am lazy and they are not my tools.

They advise setting the tool above the center line , I suspect that this is to account for a certain amount of tool movement even if you are running the coolest Swiss lathe.

Setting the cutoff tool slightly (.005-.010) above center and ensuring that there is plenty of end relief makes for better chipflow and less end back pressure in the cut.

 

[mikey]

Count me in with the folks who want to know why a certain method or tool works better or worse than another. Before I make a special rear parting holder and blade it would be important to understand the critical design features that will make it better than what I already have.
The 'whys' that I have learned so far about using a lathe properly have been fascinating and satisfying.

Toz, I can't tell you that parting from the rear is better for you; just better for me and apparently many others. As to why it works, there are many theories as mentioned but nobody has proven anything to date as far as I know. My opinion on the advantages are:

• The cutting forces lift the tip out of the cut if the cut falters; dig ins are rare, as is chatter.
  
• It improves the apparent rigidity of the lathe when parting, however it does it.
• Cutting fluids get to the tip more readily instead being lost with the chips, thereby reducing friction and cutting temperatures.
• Most rear mounted tools are bolted to the cross slide itself instead of being perched on top of a tool post. This greatly improves rigidity whether parting from the front or rear.
• It works good! It allow me to part at very high speeds, which makes parting smooth and easy because I don't worry about chatter or dig ins. Because of the design of my tool I can go from pulling the tool out of the drawer to cutting in under 10 seconds - I do this all the time. I haven't chattered or dug in in over a decade. Is it any wonder that I think this is a viable option?

I usually try to stay away from discussions on parting other than to offer things that may help. The reason is that the discussion about rear mounted tools always evolves into a "how does it work?" kind of thing or "what proof do you have?" I know how I think it works and have proven it to my personal satisfaction. I don't expect anyone else to buy it; I can only hope folks can keep an open mind about it.

 

[Tozguy]

Toz, I can't tell you that parting from the rear is better for you; just better for me and apparently many others. As to why it works, there are many theories as mentioned but nobody has proven anything to date as far as I know. My opinion on the advantages are:

• The cutting forces lift the tip out of the cut if the cut falters; dig ins are rare, as is chatter.
  
• It improves the apparent rigidity of the lathe when parting, however it does it.
• Cutting fluids get to the tip more readily instead being lost with the chips, thereby reducing friction and cutting temperatures.
• Most rear mounted tools are bolted to the cross slide itself instead of being perched on top of a tool post. This greatly improves rigidity whether parting from the front or rear.
• It works good! It allow me to part at very high speeds, which makes parting smooth and easy because I don't worry about chatter or dig ins. Because of the design of my tool I can go from pulling the tool out of the drawer to cutting in under 10 seconds - I do this all the time. I haven't chattered or dug in in over a decade. Is it any wonder that I think this is a viable option?

I usually try to stay away from discussions on parting other than to offer things that may help. The reason is that the discussion about rear mounted tools always evolves into a "how does it work?" kind of thing or "what proof do you have?" I know how I think it works and have proven it to my personal satisfaction. I don't expect anyone else to buy it; I can only hope folks can keep an open mind about it.

Mikey, thanks for sharing your thoughts with us. Please know that I am not asking for proof of what you say. I believe every word you wrote and don't question the validity of your statements. My questions only relate to my relative ignorance and my need to understand the 'why' of it. My approach to this hobby is not to just get the job done but to learn as much as possible about a lathe. Sorry if I sound skeptical sometimes because its not the case.

 

[atlas ten]

I like to think simple when using lathe. Kinda like the quote "each action has an equal and opposite reaction".
Jack

Sent from my SGH-I337M using Tapatalk

 

[mikey]

Mikey, thanks for sharing your thoughts with us. Please know that I am not asking for proof of what you say. I believe every word you wrote and don't question the validity of your statements. My questions only relate to my relative ignorance and my need to understand the 'why' of it. My approach to this hobby is not to just get the job done but to learn as much as possible about a lathe. Sorry if I sound skeptical sometimes because its not the case.

Okay, I just didn't want to leave the subject hanging. Unfortunately, this is one of those "why's" for which there are a lot of "I think's" but not a lot of "I know's". I will say that back in the old chatter days I was ready and willing to try anything, whether or not it made sense to me at the time. This was one of those times when I'm glad I did.

 

[Tozguy]

So far the discussion has helped me to see the limitations of my current parting tools and motivate me to try something with a better concept. So I thank you all.

 

[Woodemp]

Hi Mark. I've used this type of cut-off tool with good success. But, on large rigid lathe's. I learned the tool needs to be grounded first. I would increase the front clearance to between 10 to 15 deg.
Grind the front cutting edge back about the same amount from left to right.
Grind a chip breaker with extreme back rake.
Hand feed. Feel the machine. Use the rate of feed determined by the chip. If you don't give it enough feed it will rub and wear the tool and then hook.

Also, depending on the size of the workpeice, I would set it slightly below center. A 4" diameter I would set it about 010" below center.

Good Luck

 

[Wreck™Wreck]

Not a parting operation but similar, plunge cut a V-Groove, 104° included X .308" depth, I would not have plunged it but didn't have a suitable tool otherwise.
Ground a brazed 90° bit on a surface grinder, the parts are 3.307" OD 304SS and they ran very quietly without chatter with an excellent finish until the 7th of 10 when I had to sharpen the tool. 45RPM's, .001 IPR feed rate with a 2 second dwell .025" from the bottom. I did use a live center.

The second groove has the same angles but a wide flat bottom, ground another brazed tool and profiled this feature, the chips were a NIGHTMARE with this tool. 530 RPM's, .025" DOC, .008 IPR feed rate roughing, .008" DOC, .004" IPR feed for finishing. The bore has several diameters and 1 internal groove.

Done with a Bridgeport 15 X 48 CNC chucker lathe.

All in all some very tedious parts.