VBMT Insert Moving in Pocket (Swiss Application)

Hi Everyone,

---------- BACKGROUND INFO -----------------
We have an application where we turn 41L40 from .562" down to .200" in one pass (swiss) with a VBMT221 (.016 rad X 35 degree.) Most of the time it's fine and we'll get about 1,000 pcs per insert but from time to time the insert will shift in the pocket. This can happen right upon putting a new insert in, 500 pcs into a run, or anywhere in-between...the turned OD's get small by .001-.002 and the part gets long by .001-.002. When this happens I know what I am going to see when I take the holder out to inspect the insert....the (RH) insert essentially rotated CW a tick or two about the screw. (The attached picture shows this exaggerated for clarity-sake. The red circles indicate where the side of the insert contacts the holder after it shifts.) The insert doesn't have to rotate very much before its relief no longer protects the holder and then it - the holder - starts to rub on the material, and the cutoff then rapids into a spot that's supposed to be turned back but it's not so it blows up the cutoff and maybe its holder, blah blah blah.

Anyways, the Kennametal guy says the inserts should NEVER move in the pocket. Even if it IS just "too heavy of a cut" for that wimpy little VBMT it should still break the insert before it moves in the holder.

------------ SO HERE'S MY QUESTION --------------

Does anyone know what would cause an insert to move like that?

------------- MORE BACKGROUND INFO --------------

I have made sure the holder is clean, the insert is seated right and the screw it torqued down nice and tight, this has happened on new holders and old holders. I recently just turned the feed down from .002/rev to .001/rev and that didn't help so I don't think it's just load. At one point we were using VCMT inserts in the VBMT holder and I thought, "AH HA!" that's it! But alas it is still happening with the VBMT insert/holder.

------------- BONUS QUESTION ---------------

I know how a stronger insert geometry, like a CCMT (80 degree) over a VBMT (35 degree), makes the nose radius area of the insert stronger but does it make the rest of the insert any stronger somehow? Like, are they more stable in the holder or anything?


THANKS, AS ALWAYS, FOR ANY INPUT!!

My first thought is that a lot of the pocket's side contact area has been removed by the corner relief cut. What's left doesn't give much resistance to the insert twisting. V inserts being what they are, there is a lot of moment arm available to twist the insert, by the screw deforming, or the remaining pocket side area getting "smushed in" a little.

See if there is a V #2 toolholder that has less, or more optimally arranged, corner relief.

C insert would be much more stable in the pocket, as the twisting moment arm is much shorter, and the corner relief can be less pronounced than your V holder.

Regards.

Mike

Have you tried a VBGT insert?
Your insert might not be a true flat and straight 35. The M leaves you a lot of manufacturing tolerance to play with when making the insert.
This is a bigger problem with Vs than other shapes.
Due to the sharp vee these "climb" out of the pocket much faster than any other shape with any shift.
How thick of a shim can you put between the insert and its wall at the opening where you show the large gap?
It could simply be a worn holder. Again, due to the geometry these holders don't last long.

Generally I recommend you never use a "V" unless you absolutely have to.
Even if all new and a ground insert with everything perfect the tip height repeatability is terrible.
A +/-.001 variation in insert IC ("G" tolerance) is a +/-.0033 variation in tip height. Now add 15 minutes in the pocket angle, 5-10 minutes in the insert sides........and you quickly get unhappy customers.

IMO your Kennametal guy is full of poop up to or over his .....
There is never a perfect insert/pocket fit. It always moves when loaded and a swiss deep does this in spades.
Sometimes the hold down method preloads the right direction for use, sometimes not. Sometimes there is no right direction for general use cncs.

To help with your problem here and in all shapes pockets were normally made 15 minutes "open" in the back. This assures that the twist is on the front of the walls not the back corner which reduces the "wandering" error at the tip.

Some of this may not make sense but maybe trust me that your crew will hate making both holders and inserts in this geometry if you have good measuring equipment.

If you can fit a "C" or "S" shape into this cut run away from the "V" as fast as you can.
It is an insert that should only be used as a last resort. It's not an edge strength problem and I'd put that way, way down my list of worries even at 0.200 deep.

Just my two cents worth from the still learning everyday clubhouse and hoping Jasley73 and Exkenna spot this and have some input from their real world in this stuff.

Bob -.... ...

I'm in the lose the VBMT camp, I know they're considered past it, but back in the day, triangular inserts were good for very deep DOC's - plenty of edge length and well supported.

IMHO try a different holder from a different source, i agree with the above also though, its a intrinsically less than ideal shape. Any chance you can switch it out for a VNMG holder with the top clamp, like this the pin pulls it into the pocket and the top clamp really locks it down far better. I know your space limited, but when ever i encounter these kinda issues, going to the top clamped style holders is the best option i have found for both V and W inserts

You might want to consider switching to sandvik's coroturn TR system. It was designed specifically to address this problem.

CoroTurn TR

Most of the literature shows D shape inserts, but the range also includes V.

Drop a shim between the insert and wall if you can.

Finegrain said:

V inserts being what they are, there is a lot of moment arm available to twist the insert

C insert would be much more stable in the pocket, as the twisting moment arm is much shorter, and the corner relief can be less pronounced than your V holder.

Regards.

Mike

Click to expand...

That makes sense! A RH 80 degree would be almost fully/directly supported by the holder against Z cutting forces, instead of how the 35 degree is kind of "hanging out in space." Thanks!

CarbideBob said:

Have you tried a VBGT insert?
Your insert might not be a true flat and straight 35. The M leaves you a lot of manufacturing tolerance to play with when making the insert.
This is a bigger problem with Vs than other shapes.
Due to the sharp vee these "climb" out of the pocket much faster than any other shape with any shift.
How thick of a shim can you put between the insert and its wall at the opening where you show the large gap?
It could simply be a worn holder. Again, due to the geometry these holders don't last long.

Generally I recommend you never use a "V" unless you absolutely have to.
Even if all new and a ground insert with everything perfect the tip height repeatability is terrible.
A +/-.001 variation in insert IC ("G" tolerance) is a +/-.0033 variation in tip height. Now add 15 minutes in the pocket angle, 5-10 minutes in the insert sides........and you quickly get unhappy customers.

IMO your Kennametal guy is full of poop up to or over his .....
There is never a perfect insert/pocket fit. It always moves when loaded and a swiss deep does this in spades.
Sometimes the hold down method preloads the right direction for use, sometimes not. Sometimes there is no right direction for general use cncs.

To help with your problem here and in all shapes pockets were normally made 15 minutes "open" in the back. This assures that the twist is on the front of the walls not the back corner which reduces the "wandering" error at the tip.

Some of this may not make sense but maybe trust me that your crew will hate making both holders and inserts in this geometry if you have good measuring equipment.

If you can fit a "C" or "S" shape into this cut run away from the "V" as fast as you can.
It is an insert that should only be used as a last resort. It's not an edge strength problem and I'd put that way, way down my list of worries even at 0.200 deep.

Just my two cents worth from the still learning everyday clubhouse and hoping Jasley73 and Exkenna spot this and have some input from their real world in this stuff.

Bob -.... ...

Click to expand...

Wow! Thanks for the reply, Bob! I don't want to sound like I am throwing my K guy under the bus - he DID say (just like you) that the same mfg tolerancing can cause amplified problems in those holders when compared to others and that he DOES see more problems in those (V) than any others. He also claimed something like when he NEEDED a good V holder he made sure it was coming from Germany...I don't know if that's true...it's just what he said.

He also said to put a .001" shim between the wall and insert, tighten it down, and see if the shim comes out. I haven't done that yet and I'm not going to because for the reasons you all are telling me, along with my own experience of ruining parts and holders, I am going to start making the part the other way around that will allow me to use a C. It MIGHT take a couple seconds longer per part but, like TOM said in another post, I believe the overall increase in reliability (meaning less time chasing down errant 35 degree insert positions) will more than outweigh the couple seconds/part it MIGHT cost me.

Thanks, again, for your help!

adama said:

IMHO try a different holder from a different source, i agree with the above also though, its a intrinsically less than ideal shape. Any chance you can switch it out for a VNMG holder with the top clamp, like this the pin pulls it into the pocket and the top clamp really locks it down far better. I know your space limited, but when ever i encounter these kinda issues, going to the top clamped style holders is the best option i have found for both V and W inserts

Click to expand...

Thanks, Adam! There might be room in that teensie weensie cross slide for a top clamp. I'll try that if doing the part the other way around with the C insert doesn't work for some reason!

gregormarwick said:

You might want to consider switching to sandvik's coroturn TR system. It was designed specifically to address this problem.

CoroTurn TR

Most of the literature shows D shape inserts, but the range also includes V.

Click to expand...

Sure enough - D and V! That may be the ticket if I can't flip the part around the way I'd like to. Thanks!

plastikdreams said:

Drop a shim between the insert and wall if you can.

Click to expand...

I will if I have to run this again before I can try any of the other proposed routes. I bet I'd get at least a .002" in there. When I put the insert in I always look at the assembly from the bottom to make sure there is an even band of the bottom of the insert overhanging the seat of the holder....kind of like a solar eclipse? There's maybe a .010" overhang all the way around. After I run into trouble it's clearly scootched over to more like .020" on one edge and .000" on the other.

adama said:

IMHO try a different holder from a different source, i agree with the above also though, its a intrinsically less than ideal shape. Any chance you can switch it out for a VNMG holder with the top clamp, like this the pin pulls it into the pocket and the top clamp really locks it down far better. I know your space limited, but when ever i encounter these kinda issues, going to the top clamped style holders is the best option i have found for both V and W inserts

Click to expand...

How is a V-negative setup going to behave at 0.182 DOC? You may not have enough clearance below the cutting edge that far back from the tip on that small diameter material. At best, you'll knock up a giant burr (unless you taper out of the cut) and get high thrust.

Nerdlinger said:

I will if I have to run this again before I can try any of the other proposed routes. I bet I'd get at least a .002" in there.

Click to expand...

Doesn't the retainer screw push the insert into the pocket? I thought single-screw toolholders had the screw slightly off-center so it jams the insert firmly into the pocket when tightened.

Possible that the screw is now bent, or the threads are janked, such that this no longer happens ...

Regards.

Mike

rklopp said:

How is a V-negative setup going to behave at 0.182 DOC? You may not have enough clearance below the cutting edge that far back from the tip on that small diameter material. At best, you'll knock up a giant burr (unless you taper out of the cut) and get high thrust.

Click to expand...

All depends on what you get in there insert wise, get something like this Korrolly HA chip breaker

VNMG-HA Carbide Turning Inserts (PC811 Grade) - Korloy

And you can do pretty dang well, i use them a lot and there very free cutting, more so than some so called positive chip breakers.

Either im being dumb or i just don't get why there would not be enough clearance. Spouse it all depends just how crazy he is going feed wise but presuming as he is removing so much and taking it straight to finished i doubt he is feeding all that hard relatively speaking?
.

Finegrain said:

Doesn't the retainer screw push the insert into the pocket? I thought single-screw toolholders had the screw slightly off-center so it jams the insert firmly into the pocket when tightened.

Possible that the screw is now bent, or the threads are janked, such that this no longer happens ...

Regards.

Mike

Click to expand...

NO it does...I meant AFTER it tips. I can't get anything in there when it's freshly inserted.

adama said:

Spouse it all depends just how crazy he is going feed wise but presuming as he is removing so much and taking it straight to finished i doubt he is feeding all that hard relatively speaking?
.

Click to expand...

Only between .001-.002 ipr so there’s plenty of clearance to make that cut before the material would rub on the holder.

Nerdlinger said:

...the turned OD's get small by .001-.002 and the part gets long by .001-.002. When this happens I know what I am going to see when I take the holder out to inspect the insert...

Click to expand...

I'm extremely suspicious that an insert moving in the pocket, would create ONLY .002" max change in the diameter & length. If the insert really were moving, I'd expect about 10x that amount in change.


I actually had to work through an insert-moving-in-the-pocket issue in a past life in the shop. We had production parts where we used WNMG inserts/holders in finishing.


If you were using the insert (any insert, any shape) in a strictly turning scenario - meaning NO facing, only turning - then the risk of an insert moving in the pocket is very low, since the insert is always loaded in the same direction.

If however, you are doing turning & facing, or even worse - turning & back-turning, then the risk goes up a lot, as you're loading the insert in both directions.

In our case with the W inserts, it was because we used the same tool turn diameters, and then face also. My memory is pretty fuzzy on specifics, but I believe we saw changes of something like .2mm on lengths, and .3-.4 on diameters when this became an issue.

So to hear that you're only seeing changes of .002" max - I'm suspect of that, especially considering the V-shape's length/leverage over the clamping system makes it especially vulnerable...



So, corrective actions then...?

The easiest thing to try, would be to take CarbideBob's advise & try a VBGT insert. See if that helps.
Next, move to a "D", or even "C" insert.
I would like to throw in a recommendation too, for something in Walter's WSM01 grade. This is a very well-suited grade/coating combo, that provides especially low cutting forces, in large part to it's super thin & slick coating.

If the problem persists, I'd start looking for mis-alignemnts in the machine, or wear in the guide-bushing, or leveling, or something else mechanical/geometric that's causing your variations.

Jashley73 said:

I'm extremely suspicious that an insert moving in the pocket, would create ONLY .002" max change in the diameter & length. If the insert really were moving, I'd expect about 10x that amount in change.

Click to expand...

Not my take but that is why we have insert guys here so we can disagree and argue about application we can not taste, smell and feel in real life.
The shift is often microns. Dual direction loading is not a normally a swiss problem.
You are a bit deep in the cut for this size screw and holder.
It has a large flank but you you should not use all of it.
Going to other shapes to get that flank you bump the IC and screw size.
Bob

At first I thought that I was on the same page with Adama, but I realized not.

He mentioned a top clamp and I thought that he meant VPGR style. That will pull back tight into the pocket. You may need to persuade it back out of the pocket actually.... I have used them for years with good results. I have gone down to the other styles for boring bars as min bore precludes, but for external - I go with the VPGR's. (in a profiling app)

Now - whether you have room for this style in your Swiss ... ???






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