Exploded a GTN3 holder

Parlo said:
It seems you don't see the correlation between MMR & tool pressure, so regretfully I will resist looking for simpler examples to explain this.
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Okay, I knew there had to be some engineer-type guy out there who could explain things better. How about this:

If the machine tool’s spindle speed remains constant as the parting tool moves to the center of the workpiece, the cutting speed will gradually decrease until it reaches zero at the center. A decrease in cutting speed is disadvantageous for the tool and can severely stress the cutting edge. As the edge approaches the center, pressure increases as the tool is fed at the decreasing cutting speed. In CNC lathes, the control increases the spindle speed as the tool moves toward the center to maintain the same cutting speed. Eventually, however, the machine cannot go any faster and the cutting speed decreases as the tool gets close to the center of the workpiece.

Simple enough for you?
 
Aukai said:
I think as you go to the center at the same advancing speed, the rotating speed slows down, and is making a deeper cut. That's how I'm thinking it's working.
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A good way is to imagine what is happening, consider a pointed the tool scribing a scroll on the face of the part as it feeds accross the face. The distance between the lines will remain constant. This distance is the feed per revolution. What happens as the diameter decreases is that the speed of the chip accross the top of the tool will decrease. The chip thickness ( feed per rev ) is unchanged throughout just less material is removed per revolution as the diameter decreases.
 
Another source, different material tested, with similar results. Also explained as the reason carbide inserts give better finishes and longer tool life at the appropriate speeds than they do at lower speeds. Low speeds are better for sharp HSS tools.




It's a long read, .pdf download.

www.researchgate.net

(PDF) Cutting Parameter Optimization when Machining Different Materials

PDF | Flank wear occurring on the cutting edge increases as machining time goes on. This results in an increase in the cutting force and the surface... | Find, read and cite all the research you need on ResearchGate
www.researchgate.net www.researchgate.net


A quote from the article @mikey posted, which supports the information in both of the articles I posted:

"As the edge approaches the center, pressure increases as the tool is fed at the decreasing cutting speed."
 
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Parlo said:
It takes the same thickness of chip at all diameters. It removes smaller and smaller amounts of material per revolution as the diameter decreases, lowering the tool pressure as it goes.
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As the tool moves to the center it is seeing slower and slower surface speeds for the same depth of cut, the tool cuts less and less efficiently and pressure required to complete the cut goes up.

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You seemed convinced, and I don't agree with your physics, so we'll have to just let it go :)
 
Parlo said:
The metal removal rate and therefore the load on the tool decreases as the diameter reduces when using a constant rpm and constant feed per rev. If the feed was excessive it would show at the beginning of the cut where the forces are greatest. I suspect the blade was too weak for the operation and a deeper blade and possibly wider would have been more suitable
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Sincerely, this is about as far from my experience as possible. Please, if you are uncertain about parting operations, read and believe Mikey’s approach, which he has shared widely for a long time…it works.
 
Aukai said:
You seemed convinced, and I don't agree with your physics, so we'll have to just let it go :)
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Can you please elaborate or demonstrate where or how the physics are flawed. No Googling :).
 
Parlo said:
This article has a constant metal removal rate & does not consider the ever reducing metal removal rate in parting off.
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No, it shows that slower surface speeds produce higher cutting forces for an equal depth of cut. Which is exactly what happens as you part off a piece at a constant spindle speed, as the diameter decreases the cutting force will increase of the depth of cut remains the same. Sounds like exactly what happened to the parting tool in the original post.

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