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Do tools stay on center when changing inserts?

arkyengineer

Plastic
Joined
Oct 10, 2022
I'm still pretty green when it comes to machining, I have a small machine shop and we have been in business for a year. I got a brand new HAAS ST-15 here a while back and I have been really happy with it. There are definitely better machines out there, but its the biggest and nicest machine I have owned up to this point and also my first CNC lathe. I'm having a problem and I want to know if its a common one, is there something wrong, or maybe a quick fix? Or is this something that is a non-issue for 99.9% of people and because of the demands on this job its just going to always be a pain in the butt. This is a 2-axis lathe, no fancy Y-axis (or this would be a non-issue) but my finishing tool is leaving a small nub and on the parts I'm running, its unacceptable because its a cosmetic face. Now the material is 304 SS and I went through the process a few weeks ago of getting the tool shimmed to hit perfectly on center. I was getting really nice parts but its been about a month since I ran these parts and I have run steel, aluminum, stainless, and plastic parts through since I last ran these, constantly changing inserts along the way. Now I'm back to these parts and it looks like I never adjusted it to begin with. Now, for pretty much every other part I have run, the finish has been perfectly fine, but like I said, these parts NEED a 100% flat surface where there is zero nub. I will supply some photos but its hard to see in photos and like I said before, for 99.9% of machining jobs, tool is perfectly on center, its just this one job where its critical that the surface is 100% flat and you can see or feel any nub.

I have a feeling its going to be one of those things where the tool and machine is fine and its just a pain in the butt type of work that I will have to set up each time I run it. Or maybe, if I'm lucky, I just have to use a brand new insert every time I run the job and hope its on?
 

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plastikdreams

Diamond
Joined
May 31, 2011
Location
upstate nj
Might want to work on the rest of that "cosmetic" surface while you are at it.

If you are putting the same insert in there should be no change in center, as long as the pocket is clean.
 

Goff

Aluminum
Joined
Dec 16, 2007
Location
CA, USA
Most inserts have a positive or negative angle in the tool holder. Whenever the nose radius, insert thickness, or the hone on the edge the insert changes, the nose position with regard to part centerline will change, and be either above or below center, and leave a nub.
 

wmpy

Hot Rolled
Joined
Dec 16, 2011
Assuming you are using a negative insert like a CNMG, the tolerance on the thickness of that insert is +/- .005".

It's not uncommon for a tool that used to be cutting to center to need some additional shimming after being changed.
 

johnmontrose

Aluminum
Joined
Sep 25, 2007
Assuming you are using a negative insert like a CNMG, the tolerance on the thickness of that insert is +/- .005".

Just to amplify the point above, see the insert codes here:


The third letter is the tolerance.

The other thing to bear in mind is that as you near the centre of the part, the cutting speed reduces to zero.

Maybe do some investigation and measure the tip height each time you change an insert to see how it varies.
 

Sam I

Plastic
Joined
Nov 6, 2020
In practice I have not personally found the tolerance of the insert to be an issue. One thing to consider is your programmed X finish position. It should be below centre to compensate for the tool nose radius. If you've got a .4 nose rad you should be feeding to at least X-0.8. You could try facing the part with the hand wheel below centreline and see if that gets rid of the pip. If so then that could be your problem.

Another thing could be if the machine has had a bump since you shimmed the tool. Again, I've not personally found the need to shim the tools in my ST-20. They all cut to centre no problem as is. If the machine has had a bump at some point then it could be that it's off the machines centreline and therefore needs the shims to compensate.
 

michiganbuck

Diamond
Joined
Jun 28, 2012
Location
Mt Clemens, Michigan 48035
It used to be that inserts from the same run number would be near dead on the same size. yes, that was Carbloy, Valentine Iscar, VR Wesson, and the like big outfits.
But yes it would b good to micrometer the first insert, and the shim as needed,

Knowing this and trusting a carbide source a big shop can often go from a high-cost insert to a lower-cost insert by adjusting the machine parameters when one run number goes to the next run number.
 
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Fancuku

Cast Iron
Joined
Dec 7, 2018
The tool is not on center. That's all there is to it.
And that finish is not exactly cosmetically pleasing.
 

arkyengineer

Plastic
Joined
Oct 10, 2022
To everyone who said the surface is not exactly cosmetically pleasing, that is true. I have gotten cleaner looking faces off it, but in fairness that is a really close up photo that doesn't really flatter the part. Instead of saying cosmetic, I probably should have phrased it as uniform. Needs to have a uniform finish and the nub stands out when looking it over.

In practice I have not personally found the tolerance of the insert to be an issue. One thing to consider is your programmed X finish position. It should be below centre to compensate for the tool nose radius. If you've got a .4 nose rad you should be feeding to at least X-0.8. You could try facing the part with the hand wheel below centreline and see if that gets rid of the pip. If so then that could be your problem.

Another thing could be if the machine has had a bump since you shimmed the tool. Again, I've not personally found the need to shim the tools in my ST-20. They all cut to centre no problem as is. If the machine has had a bump at some point then it could be that it's off the machines centreline and therefore needs the shims to compensate.

Sam I: I am going below centerline by about 0.03" any less and point where the radius of the tool goes straight doesn't reach the center and you have a pointy nub.

Here is another question, there is a small spacer under the insert. Can you swap that out for different sizes and shim it that way? Or do you have to remove the whole tool holder and shim it there?
 

706jim

Stainless
Joined
Jun 14, 2006
Location
Thunder Bay Canada
I don't know if this comment is relevant here but when machining on an engine lathe where tool height is easily adjustable, I used to point out to students that a sharp nub indicated a tool below center while a smooth nub indicated that the tool was above center. On an engine lathe the carriage would just get pushed away from the work when the tool was too high unlike a CNC.
 

Sam I

Plastic
Joined
Nov 6, 2020
Sam I: I am going below centerline by about 0.03" any less and point where the radius of the tool goes straight doesn't reach the center and you have a pointy nub.

Here is another question, there is a small spacer under the insert. Can you swap that out for different sizes and shim it that way? Or do you have to remove the whole tool holder and shim it there?
To my knowledge those are just insert seats and are standard but it could be worth a call to your tooling provider.

Has the machine had a bump though? It could be an indication of a more serious problem. If the machine axis are off then it won't cut straight and facing cuts will be concave or convex depending on the out of alignment condition.
 

arkyengineer

Plastic
Joined
Oct 10, 2022
To my knowledge those are just insert seats and are standard but it could be worth a call to your tooling provider.

Has the machine had a bump though? It could be an indication of a more serious problem. If the machine axis are off then it won't cut straight and facing cuts will be concave or convex depending on the out of alignment condition.
To the best of my knowledge no. There are only 2 of us here at the shop and we are both on the hook for the loan so a major bump would have been addressed if either of us had one. Plus everything else seems to be good. For example, our rougher doesn't really make a nub as big at the finisher. The face finish isn't as good, but there is never a nub. Our rougher is in slot 1 and our finisher is in slot 3. Slot 1 is the "master" slot on this machine and all other positions are based off it, so maybe swapping those is what I need to do? If so, that's odd cause all the tools should be good regardless of the slot but I think I remember a tech who I talked to mentioned something about having the more critical tools in lower position pockets...

And to clarify something that I think I explained poorly earlier. My tool is going past centerline when facing by about 0.025" to account for the radius of the tool. This material is 304SS and the finishes I get on other materials is much nicer, I have yet to get a really perfect cosmetic finish on this, but that is the goal. Uniform finish is more important that a cosmetically flat finish cause these get laser marked and if the surface isn't uniform that the marking will look off and the nub in the center of the part stands out like a sore thumb after laser marking. I'm using all HAAS tooling and inserts, I have started to use other brands of inserts and while more expensive, I have seen MUCH better surface finishes, especially in Aluminum. Sumitomo aluminum inserts have been amazing. I imagine there is a better insert for better finishes in 304SS that I'm not using. That I can fix by buying different inserts for this job, the nub is the biggest problem and I'm having a hard time understanding the cause or if it is just an inevitability. I would hate to spend money on a more expensive tool holder only to have the same problem. If I can understand why it happens, then I can make an informed choice about how to deal with it.
 

michiganbuck

Diamond
Joined
Jun 28, 2012
Location
Mt Clemens, Michigan 48035
To my knowledge those are just insert seats and are standard but it could be worth a call to your tooling provider.

Has the machine had a bump though? It could be an indication of a more serious problem. If the machine axis are off then it won't cut straight and facing cuts will be concave or convex depending on the out of alignment condition.
it is Ok to add a shim under and insert, If a hole in the shim is needed one can drill a hole with using a Dremel (hand-held grinder) and a point-mounted wheel.
Some parts can be end, nub removal rubbed on a plate with a slip of abrasive paper/Auto wet paper is good/OK.
 

CarbideBob

Diamond
Joined
Jan 14, 2007
Location
Flushing/Flint, Michigan
Inserts from the same maker?
Things that effect it:

The nominal size for a #3 thick insert is .1875 +/- what ever tolerance group it is.
Shim seats are +/-.005 also but most are made to +/-.001. Holders are normally made +/-.003 on this "H" dimension to a master insert.
If you mic the center of a CNMG-432 it will likely be .194 to 0.199 thick. This is because the center of a molded chipbreaker insert is tall than the actual cutting edge.
This is so that when grinding the top and bottom you do not touch the molded edge. Ideally the cutting edge comes out .1875 from the base of the insert.
Some people make the insert .1875 in the center so the cutting edge is .003 below that.

A typical negative rake holder has 5 degrees built in so the cutting edge rises .002 thou from insert tip to the radius tangent point in this case.
High shear chipbreakers often dip down from the tool tip so the opposite effect on centerline.

Many inserts have a honed or radiused cutting edge.
You cut where this radius blends with the side of the insert so this drops the tools actual shear point .002 to .005 down the side of the insert.
T landed tools will always have a lower cutting edge.

Add all this up and you can easily have a +/-.015 difference in tool height from what should be nominal even inside the same brand name.

Making an add on shim from a piece of shim stock is fine and done often for your problem.
You can also just put shim stock under the holder itself.
I have had customers request custom ground shim seat kits in various sizes with the sizes etched on them but this is expensive and overkill IMO.

Bob
 
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guythatbrews

Hot Rolled
Joined
Dec 14, 2017
Location
MO, USA
You can also just put shim stock under the holder itself.

Bob
This is what I do if needed. I face past center bit more than the TNR, like .040 for a #2. Helps and doesn't seem to hurt the insert. Once it's set don't seem to have issue when the insert is changed.

If this is a constant problem for you dedicate a turret position for this and set and forget. Some lathes have an eccentric tool block locating pin you can rotate to bring the tool on center.
 

michiganbuck

Diamond
Joined
Jun 28, 2012
Location
Mt Clemens, Michigan 48035
The free cutting and rigidity of the part can have an effect on a nub, a thin part that deflects away with cutting forces, poor sharpness or rake-attitude so the forces change at the end of travel affect the position or rigidity of the part may have a nub effect. Also consed SFPM from 3" to zero so sharpness may and rake along wit SFPM be a factor.

The part finish looks like some compaction /pushing back may be happening.
 
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