Lathe tool inserts

[gard]

I just looked at "How to run a Lathe" I have revision 56 from 1966, it states to set the bit 5 degrees above center. Like a lot of text books there is not an explanation of why or what it effects.. I guess this must be based on some experiments and experience at the time using the south bend lathes and HSS bits. They do mention other cutting bit materials but nothing about carbide insert tools. I do not know how common they were in 1966. The book was originally copyrighted in 1914, no idea how much it was edited in later revisions. Here is a photo of that page, I have a well worn hard copy but pretty sure I have seen soft copies on the internet someplace. I made the little gauge shown and use it often when grinding bits. I looked around on the internet and could not any great info on the internet so I am afraid I do not have a good answer for Jeff.

 

[Provincial]

If the bit is set above the centerline of the spindle, there are two issues.

First, the actual depth of cut will be slightly less than what is dialed in, because the tip of the tool bit is moving at an angle to the surface of the cut. Hard to explain, but it is a geometry thing. The movement of bit is not perpendicular to the diameter of the work.

Second, if there is any flex in the supporting parts of the machine from the cutting forces, it lowers the tip of the tool bit, which is now taking a deeper cut because of the factor mentioned first. If the bit is on centerline and things flex, it will relieve the forces, because it is taking a shallower cut. I can see where either setting could introduce or amplify chatter, but the above center cut is the more likely source.

 

[MikeInOr]

I recently saw someone say the following:

Raise the tool height...carbide cuts better above center.

Is this good advice?

I recently saw a youtube video where an engineer I respect said the exact same thing... BUT context is everything. In the video he was specifically referring to taking a boring cut inside of the hollow in the piece he was working on. His advice was "just a touch above centerline can help with chatter". His advice was directed at and only relevant to inside boring cuts though. For normal outside cuts the advice would have made no sense at all!

 

[gard]

The HSS bit is sharpened to a much sharper included angle at the edge. SB recommended 61 degrees for mild steel vs most carbide I have seen that is closer to 80 or 90 degrees. This sharpness is what I think makes HSS cut better with the less horsepower and lower cutting forces that I use on the old lathes. This sharpness allows relief on the front and side not possible with carbide and allows the HSS bit to be set slightly above center but why would you? Sometimes it seems to work better for me. I can only assume someone did some experiments at some time in the past and came up with 5 degree above center number for HSS in mild steel. The HSS can be ground to the achieve the same angles relative to the work in quick change tool holders so you can give it a try sometime.

First, the actual depth of cut will be slightly less than what is dialed in, because the tip of the tool bit is moving at an angle to the surface of the cut. Hard to explain, but it is a geometry thing. The movement of bit is not perpendicular to the diameter of the work.

This is correct, If I did the math right, a 5 degree higher bit with a depth of cut set to 1 mil will actually remove 0.996 mils. I don't care. Perhaps if I had DRO or CNC and dialed in 1" inch of cut, I would have 0.004" error this could be a problem. The errors will be much more when cutting tapers.

Second, if there is any flex in the supporting parts of the machine from the cutting forces, it lowers the tip of the tool bit, which is now taking a deeper cut because of the factor mentioned first. If the bit is on centerline and things flex, it will relieve the forces, because it is taking a shallower cut. I can see where either setting could introduce or amplify chatter, but the above center cut is the more likely source.

This could very well also be correct but as a thought experiment how about if the bit starts just slightly above center and during the cut it deflects to just slightly below center, where the depth of cut is the same. Less chatter? As the height changes, the cutting angles also change which may have a larger effect on cutting force. When I see chatter on my lathe it appears to be more about deflection of the workpiece than deflection of the tool bit. Deflection of workpiece may be due to head stock bearings, lathe chuck and part stick out from the chuck.

 

[gard]

I posted this question on a different forum and got a little more insight.

from gustafson
"To make sure it is not below center. If it is below center, when you lower feed or stop feed it will move up and make a groove in the part. When it is above center, the force of the cut pull the tool down into the larger diameter of the part, and when you stop feeding, the lowering of the cutting force allows the tool to move up to a smaller diameter, thus no groove"

from PCD,
"In books from the late 1800's far more extreme angles above center are recommended. The justification being that this causes some of the cutting force to load the crossfeed screw.
Why that was an important consideration with old lathes I do not know. Maybe they used a lot more rake and had problems with the tool getting sucked into the work.
But it does prevent cutting accurate diameters with a DRO or the crossfeed dial and also requires constant fiddling with height which are pretty significant disadvantages."

I thought it was Interesting information to pass on. Next time I am in a library with old books I may do a little more research.