# Parting off blade shape.



## Parlo (Jul 21, 2022)

Does anyone believe that a parting tool only cuts on the leading face.
There is a lot of discussion on this subject at the moment.
One opinion believes that _only_ the face of the tool cuts when plunging in.
The other opinion is that the face _and_ edges cut, even if relieved.


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## rabler (Jul 21, 2022)

Joe Pie (youtube) did a short video where he made a HSS trapezoidal grooving tool, essentially a wide parting blade, wider at the front and narrowing significantly, and plunged it into a piece of brass.  I believe it was in answer to this discussion.


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## mmcmdl (Jul 21, 2022)

Wouldn't see how the relieved sides would cut ? If they cut , they aren't relieved !  You could part with a form tool of course such as a pulley groove tool in which the sides would cut . If you're talking about a regular parting groove tool , be it HSS or carbide insert type , the face cuts .


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## Cadillac (Jul 21, 2022)

I think it’s easy to see where it’s cutting look at the chip. The face and top are what make the cutting surface. Reliefing the top I find helps the cut.


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## SLK001 (Jul 21, 2022)

You should cut only on the face.  If you're cutting with the edge, then your tool isn't perfectly perpendicular with the work and you are going to break your tool.


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## Winegrower (Jul 22, 2022)

Parlo said:


> The other opinion is that the face _and_ edges cut, even if relieved.


Perhaps some advocate of this theory would explain why he/she holds this opinion?


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## Parlo (Jul 22, 2022)

Winegrower said:


> Perhaps some advocate of this theory would explain why he/she holds this opinion?


One opinion is that the sides of the chip do not need to be sheared and are detached from the part by tearing. So only the face cuts.

The other theory is that looking down on the top of the blade, the chip has a shear area which is the width of the blade x the feed per rev. This gives the chip a u shaped perimeter of 3 sides that are all need to be cut, even with a trapezoidal ( dovetail ) shaped tool.


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## cathead (Jul 22, 2022)

Sometimes I will add a small groove down the middle of the top of the cutter to force the chips to cave in some resulting in
better chip removal.  Another thing that is handy is to sharpen the cutter at a slight angle on the face to produce a nice clean
cut on the part being cut off.


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## Winegrower (Jul 22, 2022)

I saw the Joe Pie video before I started this thread, so of course I will just leave this question for the machining philosophers.


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## Parlo (Jul 22, 2022)

Winegrower said:


> I saw the Joe Pie video before I started this thread, so of course I will just leave this question for the machining philosophers.


I understand:
There is a palpable reluctance to stand up and be counted on this one. It really is lesson 101 in cutting tools.
For example:
Is it possible to cut a slice of pizza with one cut? - No.. - two sides of the triangle need to be cut to remove the slice from the disc.
Is it possible to cut a triangular thread with one side of the tool? No.. once again the perimeter needs to be gradually cut to remove the chips.

A proposed theoretical one dimensional line ( leading face of the tool ) will never shear the sides of a groove, how are they cut?


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## T Bredehoft (Jul 22, 2022)

Parlo said:


> leading face of the tool ) will never shear the sides of a groove,



You  don't suppose the ends of the leading edge cut to the ends, do you.  Well, they do, whether you call it cutting on the side or the face.


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## Parlo (Jul 22, 2022)

T Bredehoft said:


> You  don't suppose the ends of the leading edge cut to the ends, do you.  Well, they do, whether you call it cutting on the side or the face.


Don't forget, the chip has a thickness equal to the feed per rev. This creates a 3 sided shear requirement which is the blade width plus the feed per rev per side. So yes, the blade shears at the front and also has to shear at the sides.
Are you saying that there must be cutting around the corners of the leading face to detach the swarf from the sides?


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## Shotgun (Jul 22, 2022)

Parlo said:


> For example:
> Is it possible to cut a slice of pizza with one cut? - No.. - two sides of the triangle need to be cut to remove the slice from the disc.


Never seen big guys like myself eat pizza, have you?  One slice.  Fold over.  Nice triangle that doesn't drop the good parts.


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## Mitch Alsup (Jul 22, 2022)

Parlo said:


> Does anyone believe that a parting tool only cuts on the leading face.



If the parting tool is normal (90º) to the work, then only the face cuts--the sides are present to hold the face from vibrating.



> There is a lot of discussion on this subject at the moment.
> One opinion believes that _only_ the face of the tool cuts when plunging in.
> The other opinion is that the face _and_ edges cut, even if relieved.



If the tool is properly setup in the tool holder, only the face cuts.
If the tool is not set normal, all sorts of strange effects may be encountered.


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## Parlo (Jul 22, 2022)

Mitch Alsup said:


> If the parting tool is normal (90º) to the work, then only the face cuts--the sides are present to hold the face from vibrating.
> 
> 
> 
> ...


Just to clarify. The parting tool is a dovetail shape. Square at the face and relieved sides.
The chip cutting perimeter, looking down on the tool is the same shape as the tool. It stays the same throughout the plunge.
The face cuts the bottom of the groove and the sides of the tool cut the swarf from the sides of the groove .
The swarf thickness is the feedrate per rev, which is the same as the depth of cut per rev, which is also the same amount the sides need to cut per rev.
What part of the tool detaches the chip from the sides of the groove if only the front cuts?


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## Shotgun (Jul 22, 2022)

How much top rake does the tool have?
If you're only feeding a couple thou per rev, then the leading edge could be slicing under the material and pulling it off the sides instead of cutting.  That'll leave a nasty finish, but isn't that what we see from most parting operations.


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## Parlo (Jul 22, 2022)

No top rake, completely flat to keep thing simple and not introduce any more variables..

I agree; if the sides of the tool are not cutting & the material is 'pulled off ' in parting and grooving operations, a poor finish is probable.
If a good finish is acheived on the walls of the groove, would you agree that the sides of the tool are cutting?
'


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## Jim F (Jul 22, 2022)

As long as I get through, without breaking the blade, I really don't care........


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## Shotgun (Jul 23, 2022)

Parlo said:


> No top rake, completely flat to keep thing simple and not introduce any more variables..
> 
> I agree; if the sides of the tool are not cutting & the material is 'pulled off ' in parting and grooving operations, a poor finish is probable.
> If a good finish is acheived on the walls of the groove, would you agree that the sides of the tool are cutting?
> '


I'll agree, as long as the beer holds out.  But this very much reminds me of a serious debate about the number of angels that will fit on the head of a pin.


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## Cadillac (Jul 23, 2022)

Parting blade cuts on the face in normal operations. When cutting straight in you have no side pressures. You swarf will be as thick as the infeed till it breaks off. 
 When looking at the parting blade what gets dull. The face top edge not the sides, and I e seen some carbide parting inserts with side relief no way for sides to touch.


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## Parlo (Jul 23, 2022)

Cadillac said:


> Parting blade cuts on the face in normal operations. When cutting straight in you have no side pressures. You swarf will be as thick as the infeed till it breaks off.
> When looking at the parting blade what gets dull. The face top edge not the sides, and I e seen some carbide parting inserts with side relief no way for sides to touch.


Good point, the wear on the top of the blade is a true indication of where the the cutting is done. No visible wear is seen on relieved sides as they don't touch the sides of a wider groove.
Wear can be seen on the corners though, this is from the depth of cut which is the feed per rev. This indicates clearly that the cutting is also performed on the sides, but only on a small distance from the cutting face = to the feed per rev.


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## mmcmdl (Jul 23, 2022)

Parlo said:


> Wear can be seen on the corners though, this is from the depth of cut which is the feed per rev. This indicates clearly that the cutting is also performed on the sides, but only on a small distance from the cutting face = to the feed per rev.


I'm not following you on your thinking here . 



Parlo said:


> Good point, the wear on the top of the blade is a true indication of where the the cutting is done. No visible wear is seen on relieved sides as they don't touch the sides of a wider groove.


This is your answer , you said it yourself . Of course you'll see wear on the corners , they are part of the face which is doing the cutting !


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## Parlo (Jul 23, 2022)

mmcmdl said:


> I'm not following you on your thinking here .
> 
> 
> This is your answer , you said it yourself . Of course you'll see wear on the corners , they are part of the face which is doing the cutting !


Very true. A corner has two sides, one is the face and the other is the side.


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## mmcmdl (Jul 24, 2022)

On ignore .


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## Parlo (Jul 24, 2022)

Here is a link that describes how the formation of the spiral grooves either side of the groove are greated by the sides of a relieved tool.
The same pattern can be replicated by facing with the same relieved tool, a coarse feed will show the spiral better than a fine feed.
The section of the spiral will be the same as the shape of the tool corner giving a saw tooth finish.
I think this helps to describe how the side of the tool shears away the chip from the sides by leaving the tool corner shape on the part.
The advancing tool won't knock off the peaks formed by previous revolutions as it is smaller due to the relief angle.









						parting tool.jpg
					






					drive.google.com


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## Parlo (Jul 25, 2022)

Thanks for all the posts.
It seems from this small straw poll that:
4 - posts say on the front only.
5- likes for front only.
Just one like for sides and face.

Unfortunately the diagram and my descriptions haven't convinced this small cohort.
Before I demonstrate my expectations on video, is there anyone who has seen the image willing debunk my conclusions.

Cutting material away from the parent stock with tools that generally need relief to cut, is at the heart of most of the posts on the forum. Please post why you agree the trapezoidal tool cuts on the sides, or if you disagree, the reasons why not.


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## OCJohn (Jul 25, 2022)




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## Parlo (Jul 25, 2022)

OCJohn said:


>


Thanks John for posting the video.

The demonstration only shows that a fine feed produces a fine finish. The initial plunge cutting of the groove was at such a very slow hand feedrate it could not show the saw tooth ( phonograph ) section grooves on the sides, as the overlaps were so close together.

When the face was cut under power feed it produced a much coarser finish and somehow actually managed to break the sharp corner and even remove the scratch where the tool was retracted, both off camera.

Apparently the video was produced to disprove the now deleted comments from Bill Shiff that categorically proved that a relieved parting tool will cut on 3 sides and produce a sawtooth finish on the walls of the groove. To disprove his conclusive comments the sides were " removed " actually just relieved to be precise in an attempt to prove cutting was only on the face.

Realizing the comments were too descriptive and overwhelmingly correct, all the supporting comments were deleted and then....

The video description was changed to infer that Bill's comments suggested that a dovetail tool could not produce a groove at all with a tool of this shape. *Not,* as Bill confirmed, that the tool would produce a groove but with a saw tooth spiral on each side, which it simply does. The video attempts to hide the sawtooth grooves by handfeeding the tool as slow as possible to make them less visible.

Thereby suggesting Bill's conclusions were wrong and the apparent victory assured. Which is all that matters?


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## FOMOGO (Jul 25, 2022)

I've had great luck parting on the old Atlas 12" lately with a standard HSS blade. Sharp and straight seem to be the important components. That and a little practice. Lower rpm also seems to help on machines that aren't supper rigid. I touch it up on the grinder after every few operations. Mike


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## jaek (Jul 25, 2022)

Parlo said:


> Does anyone believe that a parting tool only cuts on the leading face.
> There is a lot of discussion on this subject at the moment.
> One opinion believes that _only_ the face of the tool cuts when plunging in.
> The other opinion is that the face _and_ edges cut, even if relieved.



What kind of nonsense is this? The parting tool cuts on the top, not the front or the sides.


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## Susquatch (Jul 25, 2022)

jaek said:


> What kind of nonsense is this? The parting tool cuts on the top, not the front or the sides.



Ding ding ding!!!  I think we have a winner! The top is what does the cutting. The top cuts from the front edge of the top back to the depth of cut determined by the feed rate. All that the front and sides do is support the top.


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## jaek (Jul 25, 2022)

Susquatch said:


> The top cuts from the *front edge of the top* back to the depth of cut


Clear language makes it so much easier to be understood, eh?


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## Parlo (Jul 26, 2022)

Susquatch said:


> Ding ding ding!!!  I think we have a winner! The top is what does the cutting. The top cuts from the front edge of the top back to the depth of cut determined by the feed rate. All that the front and sides do is support the top.


Thanks for the simpler explanation confirming the cutting perimeter.

Because the cutting edges have two faces, horizontal and vertical, the cutting perimeter seemed easier to describe using the vertical faces i.e. the front face and the side faces.

In hindsight, I should have just posted the drawing earlier as it clearly shows the cutting perimeter without needing a written description.


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## jaek (Jul 26, 2022)

This ASME document names the various parts of a lathe cutting tool. You might want to read it.


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## Parlo (Jul 26, 2022)

jaek said:


> This ASME document names the various parts of a lathe cutting tool. You might want to read it.


Thankyou for the link, it really helps to show how more than one cutting edge is required.
The document has an excellent description to explain cutting principles using the nomenclature of major and minor cutting edges.
Using the nomenclature from the document, the major edge of the trapezoidal tool in this discussion is the front and the minor edges are the sides.
Figure 4 on page 11 demonstrates the effect of the minor cutting edge(s) on the surface finish, which in this case are the sides of the groove.


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## Susquatch (Jul 26, 2022)

jaek said:


> This ASME document names the various parts of a lathe cutting tool. You might want to read it.



Sometimes it's good to review such things. 

After doing so, I think it might have been more productive to have discussed cutting edges instead of faces. That might have more quickly narrowed the discussion. But even then, the feed rate determines the size of the bite and that still involves more of the top face than just the top front edge.


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## Susquatch (Jul 26, 2022)

Parlo said:


> Using the nomenclature from the document, the major edge of the trapezoidal tool in this discussion is the front and the minor edges are the sides.



Yes. I contemplated this major minor comparison too. But I think the feed rate gets lost in the discussion and that is what decides the size of the minor edge. 

What gets lost in doing that is the rectangularity of the cut itself. It really isn't just an edge. Edges are lines. But the cut has length and depth which is an area or surface not a line. In fact, it is this surface that dictates both a major and minor cutting edge. 

Moreover, a detailed stress analysis of a cut would show a distribution of forces both in the material and in the cutting tool that go beyond the cut surface itself.


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## Parlo (Jul 26, 2022)

Susquatch said:


> Yes. I contemplated this major minor comparison too. But I think the feed rate gets lost in the discussion and that is what decides the size of the minor edge.
> 
> What gets lost in doing that is the rectangularity of the cut itself. It really isn't just an edge. Edges are lines. But the cut has length and depth which is an area or surface not a line. In fact, it is this surface that dictates both a major and minor cutting edge.
> 
> Moreover, a detailed stress analysis of a cut would show a distribution of forces both in the material and in the cutting tool that go beyond the cut surface itself.


I hoped that the diagram I posted would show the area on the top of the tool which is the same as the chip section.

Here is the link - https://drive.google.com/file/d/1QyfOBTufVaYmN_7zdSYRsrYRQBbSRcW5/view?usp=sharing


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## Susquatch (Jul 26, 2022)

Parlo said:


> I hoped that the diagram I posted would show the area on the top of the tool which is the same as the chip section.
> 
> Here is the link - https://drive.google.com/file/d/1QyfOBTufVaYmN_7zdSYRsrYRQBbSRcW5/view?usp=sharing



Your drawing shows the dark rectangular cutting surface on the top of the tool perfectly.

Too bad there is no way on here to show a live 3D video of the cutting stresses in the tool and in the part as the tool cuts the part. That is what is missing in this whole discussion.

Does anyone on here have access to an engineering workstation powerful enough to run CAE software that can simulate parting off and create a short video?

Ideally, the video would show several cross sections of the part and the tool at various places and demonstrate the stress gradients of the parting process.


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## WobblyHand (Jul 26, 2022)

Susquatch said:


> Your drawing shows the dark rectangular cutting surface on the top of the tool perfectly.
> 
> Too bad there is no way on here to show a live 3D video of the cutting stresses in the tool and in the part as the tool cuts the part. That is what is missing in this whole discussion.
> 
> ...


Would an IR camera would show the heating of the cutting edges?  Might be useful to confirm the simulation?


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## Parlo (Jul 26, 2022)

Susquatch said:


> Your drawing shows the dark rectangular cutting surface on the top of the tool perfectly.
> 
> Too bad there is no way on here to show a live 3D video of the cutting stresses in the tool and in the part as the tool cuts the part. That is what is missing in this whole discussion.
> 
> ...


I am going to film a trapezoidal section tool facing off a part at a high feed to show the saw tooth steps formed. This should confirm that the tool cuts on the minor edges and leaves it's form as a spiral groove.


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## Susquatch (Jul 26, 2022)

WobblyHand said:


> Would an IR camera would show the heating of the cutting edges?  Might be useful to confirm the simulation?



I was thinking something like that earlier near the beginning of this post. My thoughts at the time were that it's entirely possible that part heating and tooling heating might expand the sides of the parting groove enough to precipitate some side cutting. Then it hit me that the whole premise of the discussion was wrong. It isn't the front or the sides, it's the top rectangle and it's edges that does the cutting. 

@jaek 's comment about nonsense crystallized it all for me. It's funny how we get locked into nonsense so easily when we ignore the obvious.


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## Susquatch (Jul 26, 2022)

Parlo said:


> I am going to film a trapezoidal section tool facing off a part at a high feed to show the saw tooth steps formed. This should confirm that the tool cuts on the minor edges and leaves it's form as a spiral groove.



Keep in mind that these spiral marks only show the end result, not the process itself. It could well lead to a misleading conclusion about what is really happening. I think high-speed video that can be slowed down might be more telling because it might show the compression of the material as it buckles and fails ahead of the tool.


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## Parlo (Jul 26, 2022)

I'll make sure I have a rigid setup and hope to feed at least 0.2"/rev. The stock will probably be Delrin or pvc about 4" diameter. If the large relief angle on the tool is reproduced on the face of the part, I will be convinced that the sides of a dovetail shape tool cut & not just the face.
I expect to see a sawtooth profile on the spiral which would be replicated both sides if machining a groove.


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## Parlo (Jul 27, 2022)

Here are some images of a tool & how its profile is transferred to the workpiece despite being acutely relieved.
And a short video of cutting the groove.


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## jaek (Jul 27, 2022)

It is indeed possible to create a very large surface finish with a high secondary cutting angle, tiny nose radius, and a high feed rate.

Is there some larger point being made here?


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## Parlo (Jul 27, 2022)

jaek said:


> It is indeed possible to create a very large surface finish with a high secondary cutting angle, tiny nose radius, and a high feed rate.
> 
> Is there some larger point being made here?


Several YouTube comments from retired engineer of 55 years, Bill Shiff were deleted because he proved that a trapezoidal ( triangular ) parting tool cut around its perimeter and not, as stated just on the primary edge. With respect for Bill, this is the best evidence I could supply to dispel the myth and to respect his wisdom.


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## jaek (Jul 27, 2022)

Every trapezoidal cutoff tool I've seen has a top cutting face with parallel side edges. This page has some drawings.

Someone could hypothetically grind one like you did, which would result in less rubbing but a worse surface finish. Doesn't seem like a common choice.

Why should people care which edges a cutoff tool cuts on?


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## Parlo (Jul 28, 2022)

jaek said:


> Every trapezoidal cutoff tool I've seen has a top cutting face with parallel side edges. This page has some drawings.
> 
> Someone could hypothetically grind one like you did, which would result in less rubbing but a worse surface finish. Doesn't seem like a common choice.
> 
> Why should people care which edges a cutoff tool cuts on?


The trapezoidal shape in my demonstration is looking from above, your link refers to the shape from the front of the tool.

Parting inserts are relieved to reduce rubbing and improve surface finish.

What's wrong with demonstrating how the perimeter of a tool cuts, it's fundamental in all machining.
The video shows a simulated magnified view of the process for the benefit of those who haven't had the opportunity to have seen a demonstration before.
Lots of the previous posts believed that only the front of the tool did the cutting, so it has helped to dispel that belief.


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## jaek (Jul 28, 2022)

If parting inserts are relieved and they cut on the sides why doesn’t that make the surface finish worse? How can they cut a square groove without square sides?


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## Larry$ (Jul 28, 2022)

Looking at an insert they almost all have some radius on the corners. The radius reduces the area that is in essence torn off. The point where the metal fails. Is that torn area then burnished to create the beautiful finish we see? I use HSS parting blades that I either eyeball sharpen, lazy method, or more precisely grind on the tool grinder. Even though right off the grinder these have "perfectly square corners", so the sides would not be cutting and only a torn chip would be produced the thickness of the feed, almost instantly a corner radius is worn on to the tool. This is because an absolute sharp corner has little material supporting it. So we now have a form tool with a radius  corner with progressive shear cutting until the metal fails and some tearing occurs. The wiping action of the progressively thinner cut is therefore going to burnish the surface a little bit in the torn area. This produces a flowing of the metal much like using a sharpening steel on a knife. SWAG System! 

All this is merely a bit of mental gymnastics. If I manage to get an acceptable cut I'm happy and go on to try and limit the next mistake learning opportunity.


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## Parlo (Jul 28, 2022)

jaek said:


> If parting inserts are relieved and they cut on the sides why doesn’t that make the surface finish worse? How can they cut a square groove without square sides?


Why would cutting on the sides ( around the corner radii ) make the finish worse? It's common to pop a radius on a corner to improve surface finish. It only cuts the sides one feed/rev deep. Most cutting tools are relieved i.e. have clearance following the cutting face to prevent rubbing.

If you see the earlier video and this image https://drive.google.com/file/d/1QyfOBTufVaYmN_7zdSYRsrYRQBbSRcW5/view?usp=sharing it will explain how a groove is cut with square sides. The quality of the finish on the sides, as always, depends on the feedrate. This determines the overlap between the side grooves and ultimately the surface finish which is measured by the height of the peaks.

" how can they cut a square groove without square sides " takes the thread back to the original posts.


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## Parlo (Jul 28, 2022)

Larry$ said:


> Looking at an insert they almost all have some radius on the corners. The radius reduces the area that is in essence torn off. The point where the metal fails. Is that torn area then burnished to create the beautiful finish we see? I use HSS parting blades that I either eyeball sharpen, lazy method, or more precisely grind on the tool grinder. Even though right off the grinder these have "perfectly square corners", so the sides would not be cutting and only a torn chip would be produced the thickness of the feed, almost instantly a corner radius is worn on to the tool. This is because an absolute sharp corner has little material supporting it. So we now have a form tool with a radius  corner with progressive shear cutting until the metal fails and some tearing occurs. The wiping action of the progressively thinner cut is therefore going to burnish the surface a little bit in the torn area. This produces a flowing of the metal much like using a sharpening steel on a knife. SWAG System!
> 
> All this is merely a bit of mental gymnastics. If I manage to get an acceptable cut I'm happy and go on to try and limit the next mistake learning opportunity.



_'The radius reduces the area that is in essence torn off.'_
The material is sheared around the radius, producing the wall finish. There is no burnishing after the shearing, as the tool width becomes less than the slot width as the tool progresses deeper.

_'so the sides would not be cutting and only a torn chip would be produced the thickness of the feed, almost instantly a corner radius is worn on to the tool.'_
The corners wear because the material is shearing the feed depth around the corners. This feed / rev depth is the length of shear on the sides of the tool ( approximately equal to the wear radius ).


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## Shotgun (Jul 28, 2022)

I realize that times are rough, but are we expecting this horse to become edible if we beat it enough?


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## Parlo (Jul 28, 2022)

If a post has errors, should I leave it or attempt to further explain the concept?
Perhaps this video will convince the doubters that a parting tool needs to cut on the sides and can't just cut at the front.


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## Jim F (Jul 28, 2022)

Shotgun said:


> I realize that times are rough, but are we expecting this horse to become edible if we beat it enough?


I was just thinking the same thing..........


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## Susquatch (Jul 30, 2022)

Parlo said:


> Several YouTube comments from retired engineer of 55 years, Bill Shiff were deleted because he proved that a trapezoidal ( triangular ) parting tool cut around its perimeter and not, as stated just on the primary edge. With respect for Bill, this is the best evidence I could supply to dispel the myth and to respect his wisdom.



Jezz, and here I thought this thread was looking for opinions on what edges of a parting tool do the cutting. It seems obvious to me that it is three edges or as a wise user pointed out earlier, the front top surface.

I didn't know that we were trying to validate the opinions of a wise old engineer. But as a slightly less old and much less wise old man I agree with his view completely.


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## Parlo (Aug 1, 2022)

Susquatch said:


> Jezz, and here I thought this thread was looking for opinions on what edges of a parting tool do the cutting. It seems obvious to me that it is three edges or as a wise user pointed out earlier, the front top surface.
> 
> I didn't know that we were trying to validate the opinions of a wise old engineer. But as a slightly less old and much less wise old man I agree with his view completely.


Thanks mate, you were a tough crowd but I got there in the end lol. Now onto YouTube to debunk the video.

A surprising result arose after shooting the coarse feed video, the part stayed the same length! I was measuring over the remaining peaks.


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