Exploded a GTN3 holder

Navy Chief said:
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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So you believe that metal removal rate has no effect on tool forces. The graph shows that it does with the varying feedrates ( increased feedrate shows an increased tool load ).
The tool in the example broke " very quickly " which I read as near the outside diameter where the cutting speed was near ideal.
 
Parlo said:
So you believe that metal removal rate has no effect on tool forces. The graph shows that it does with the varying feedrates ( increased feedrate shows an increased tool load ).
The tool in the example broke " very quickly " which I read as near the outside diameter where the cutting speed was near ideal.
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As I read it the cut was silent after some initial chatter and was going well with the auto feed engaged, this would imply that it was not right at the outside diameter.

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Actually, if you read post #9, the cut was about halfway through when he began to have issues. This is right around the time that SFM is beginning to drop and tool pressure starts to increase.

Parlo, please show us your formulas for MMR so we can all attempt to see where you're coming from.
 
From the article Mikey posted. Specifically, the article is talking about parting:

"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."

Author​

Scott Lewis
Scott Lewis is with Sandvik Coromant Co.



So, if the spindle speed and the feedrate are constant, then it stands to reason that the material removal rate and surface speed are reduced as well, as the tool plunges deeper.
Yet still, it says that In that scenario, the pressure would increase as the tool moves toward center.
 
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How is this even an argument?

When you part by “hand”, you have to increase the feed rate in X the closer you get to the center of the part.

A player doesn’t need math or physics. The feel let’s you know. The sketchiest time of the operation is when you get closest to zero radially.


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mikey said:
Actually, if you read post #9, the cut was about halfway through when he began to have issues. This is right around the time that SFM is beginning to drop and tool pressure starts to increase.

Parlo, please show us your formulas for MMR so we can all attempt to see where you're coming from.
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Let's address the initial post. Parting 316 stainless steel without coolant breaks tools. Parting 316 stainless steel without coolant and with a dull insert previously used without coolant, always breaks tools. This is the overriding cause for the tool breakage in this example. I'm sure we can all compile a list of other minor contributing factors.

Let's consider the relationship between SFM and MMR.
I won't start scouring the internet for articles to reinforce my point, I will use the graph kindly posted by Ken226 in post #32.
The graph shows a gradual increase in tool pressure as the SFM drops. I get that.
The graph also shows a decrease in tool pressure as the feedrate or MMR reduces. Agreed?
The graph appears to indicate a larger decrease in tool pressure due to the lower MMR than the increase in tool pressure resulting from lower SFM.

I appreciate that the rates may not be linear and may reverse at tiny SFM but generally speaking the relationship remains the same. The graph refers to a constant MMR so does not account for facing and parting operations. It also does not take into account the increasing weakness of the ever decreasing diameter right near the centre which eventually won't have enough strength to impart a lot of pressure on the more rigid tool.

The question is:
If the tool pressure from both variables - MMR & SFM were averaged at various diameters and plotted on a graph, would the tool pressure rise or fall as the diameter decreased?
I believe it will fall until the diameter get's too small to be able to impart any significant pressure on the tool. This is just a considerd opinion, I believe the reduction in MMR has a greater effect on tool load than the reduction in SFM. I remember conducting lathe tool trials 40+ years ago with what I recall as a lathe tool dynamometer, I wish I had access to the facilities now to discover exactly what does happen when facing or parting off.
 
Parlo said:
Let's address the initial post. Parting 316 stainless steel without coolant breaks tools. Parting 316 stainless steel without coolant and with a dull insert previously used without coolant, always breaks tools. This is the overriding cause for the tool breakage in this example. I'm sure we can all compile a list of other minor contributing factors.
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Aukai used cutting oil from the get go. Not sure if the insert was new but coolant was used throughout so no, I don't agree that this was the cause of tool breakage in this instance. As I have stated multiple times, I believe it was due to the cutting conditions that developed as the cut was made. It was not the blade. It was not the insert. I have parted 303, 304 and 316 on numerous occasions with a used GTN-3 insert without any issues. I have parted SS even more often with HSS and I know those blades were often not freshly sharpened and I had no issues parting with them, either. The difference between Aukai and me doing it was that I controlled the feed manually, that's all. No problems, no chatter, no crashes, no stalling.

I will leave further discussion to you and others. I wouldn't want you to injure yourself trying to simplify things for me or have to scour the net for articles to reinforce your point. But I do congratulate you, Parlo, for being the single person to have made my Ignore list.


 
mikey said:
Aukai used cutting oil from the get go. Not sure if the insert was new but coolant was used throughout so no, I don't agree that this was the cause of tool breakage in this instance. As I have stated multiple times, I believe it was due to the cutting conditions that developed as the cut was made. It was not the blade. It was not the insert. I have parted 303, 304 and 316 on numerous occasions with a used GTN-3 insert without any issues. I have parted SS even more often with HSS and I know those blades were often not freshly sharpened and I had no issues parting with them, either. The difference between Aukai and me doing it was that I controlled the feed manually, that's all. No problems, no chatter, no crashes, no stalling.

I will leave further discussion to you and others. I wouldn't want you to injure yourself trying to simplify things for me or have to scour the net for articles to reinforce your point. But I do congratulate you, Parlo, for being the single person to have made my Ignore list.
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There is a significant difference between cutting oil and coolant, especially in this instance.

Cutting oil was manually applied, not flood coolant as I suggested.
The insert had cut one previous part without flood coolant. The cutting conditions did develop / deteriorate as the cut was made, due to the lack of coolant.
I expect any tool to still cut past half way at lower than 50% of the SFM.
You seem rather vexed if your opinion is challenged. A forum is for all to exchange views, not a competition.
 
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