Where to find upside down parting holder?

A conventional lathe tool of any type in conventional orientation with the work rotating in the normal direction will place downward force on the carriage, a force diagram makes it clear -

LatheCuttingForces.jpg
A parting tool will not be subject to axial feed force but the rest is is relevant, pertinent and accurate.
 
I have always wondered why someone would want to part it upside down. The force indeed is to lift the carriage, rather than push into the bed.
 
I'm sorry dickey, but that's incorrect. I have background in physics and I'm 100% sure about this.

With the tool holder on the front of the cross-slide, the blade upside down, and the spindle rotating clock wise, the cutting action will try and lift the carriage in the front. This is why the first bullet point on the eccentric page is as follows.

https://www.eccentricengineering.com.au/index.php?option=com_content&view=article&id=31&Itemid=45
1: The inverted blade design significantly reduces the chance of a jam up and blade breakage.
Click to expand...
When the cutting forces get to high, the carriage will lift and the tool eases out of the cut. Even if you have your carriage locked it will still lift, just not as much, as usually the lock is on the right side of the carriage.

When the tool is right side up, the cutting forces push the tool and the carriage down onto the bed. when the cutting forces are to high, or the set-up isn't rigid enough, the parting tool flexes down because that's the only way to relive the force. This usually leads to the tool breaking. It either breaks because the force is to high, or because the work rides up on the tool and breaks it. Excessive backlash in the cross-slide can exacerbate the issue.


The Aloris #7R style holders are designed to be mounted on the back of the cross slide with the spindle turning counter clockwise like it normally does. This to tries to lift the back of the carriage, but it can't as the back of the carriage have gibs that run against the underside of the bed and prevent the carriage from lifting. Even that wasn't exactly what they where designed for.

On a manual machine they would most commonly be used on a turret lathe like in this video.
 
ddickey said:
It is hard to visualize but running the blade upside down in reverse will direct forced downward. Eccentric makes one specifically for screw on chucks.
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I think you have missed the point that screw on chucks will be unscrewed by a tool used as you describe above ;-)
 
Seems to make sense and that is what I thought. If you're 100% certain I'll go with it. That video of the turret lathe has the blade mounted upside down so exactly the same forces.
My lathe parts very well with this set up.
 
magicniner said:
I think you have missed the point that screw on chucks will be unscrewed by a tool used as you describe above ;-)
Click to expand...
you're correct. I guess you could only use a rear parting blade held upside down with a screw on chucks.
 
Dan_S said:
I'm sorry dickey, but that's incorrect. I have background in physics and I'm 100% sure about this.

With the tool holder on the front of the cross-slide, the blade upside down, and the spindle rotating clock wise, the cutting action will try and lift the carriage in the front. This is why the first bullet point on the eccentric page is as follows.
Click to expand...

I'm not as sure of that as you are, Dan. Here is why. Please tell me if I'm off base.

The way I see it, the parting blade is a lever and tool post mounting screw is a fulcrum. If you try to lift the carriage/cross slide/compound/tool post assembly by lifting on the business end of the parting blade, the whole assembly will try to turn around the fulcrum, dipping in the front and rising in the back. Rotating the spindle/work piece clockwise against an upside down parting tool in a front tool post does just that. So, it seems to me that the upside down blade and CW spindle rotation combine to push the front of the carriage down, not lift it up. Am I wrong?

Tom
 
higgite said:
I'm not as sure of that as you are, Dan. Here is why. Please tell me if I'm off base.

The way I see it, the parting blade is a lever and tool post mounting screw is a fulcrum. If you try to lift the carriage/cross slide/compound/tool post assembly by lifting on the business end of the parting blade, the whole assembly will try to turn around the fulcrum, dipping in the front and rising in the back. Rotating the spindle/work piece clockwise against an upside down parting tool in a front tool post does just that. So, it seems to me that the upside down blade and CW spindle rotation combine to push the front of the carriage down, not lift it up. Am I wrong?

Tom
Click to expand...

You call the tool post mounting screw a fulcrum in a lever system which it isn't, the fulcrum is not part of the moving assembly, it is the point around which the moving assembly moves freely.
The rest goes generally downhill from there ;-)
 
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