Help chatter while machining a shaft (CNC lathe).

[1NoobMachinist1]

Hi everyone as the title says, currently I am faced with a challenge and I am not sure how to remedy it. I am currently experiencing some chatter while machining EN24T 110mm diameter shaft that's 660mm long. it us chucked into a hydraulic chuck with cut soft jaws to fit with a tailstock and rotating center. I am using a kyocera DNMG insert with a 0.4mm (CA525) radius I've attempted to use the recommended values after doing the calcs (RPM @ 478, F120, 1mm DOC (2mm overall)) here's the part that stumps me near the end roughly 200mm before reaching the chuck I get a smooth clean finish how ever the other 400mm+ length closer to the tailstock experiences some chatter? What can I attempt to do to reduce this chatter? I will add I've attempted to cut from the back (chuck to tailstock) with a 0.25mm DOC (0.5mm overall) to try and get rid of the chatter marks I'd say roughly 2/3rd of the length comes out smooth but then all of a sudden I start experiencing chatter again. I am pretty new to it and sucks big time to have to machine EN24T (I've machined 304 SS before (16mm shaft) and EN19T (65mm shafts) which after a long battle I got to come out smooth) I attempted to follow the same procedure as I do with the EN19T however I am practically terrified to run the machine at 700RPM with such a big and long piece chucked in.

 

[???]

Standard practice with chatter is to drop your speed and up your feed.

 

[Mechanola]

You didn’t mention the holder. What are the angles of the insert to the work? Geometry is important.

 

[rjwalker1973]

Does your machine have SSV Spindle speed variation?

 

[1NoobMachinist1]

Standard practice with chatter is to drop your speed and up your feed.

I did try this to "clean" up the chatter that was present from the previous cut. but the it results in a vinyl disc finish which is a no go once I come to finish sizes.

 

[1NoobMachinist1]

You didn’t mention the holder. What are the angles of the insert to the work? Geometry is important.

Apologies the holder is a Kyocera DDJNR2525M-1504, also I can't tell you what the angles are as I have no idea, the tool holder is placed within a 6 post turret flush to the face so if I had to assume the angle would be 93 deg? also that is the insert I am using DNMG150608PS (DNMG442PS)

 

[1NoobMachinist1]

Does your machine have SSV Spindle speed variation?

I wouldn't even know where to look for this option is this a g-code perhaps? the controller is a GSK988TA (similar to a fanuc) if that helps anything

 

[guythatbrews]

DNMG150608PS (DNMG442PS)

In post 1 you spec a 0.4mm tnr but the tool above is 0.8mm tnr. You need to get a 0.4 tnr insert and that should solve the problem.

 

[1NoobMachinist1]

In post 1 you spec a 0.4mm tnr but the tool above is 0.8mm tnr. You need to get a 0.4 tnr insert and that should solve the problem.

Hi apologies I didn't even noticed that I was showing the 0.8mm I googled quickly to just get a picture of it, I am using a 0.4mm insert that looks like that.

On a side note I've resorted to putting the fixed steady in place for the one side (I don't have much "meat" to play with to risk it) to get my finish size to 99.97mm it came out pretty clean and smooth, I will attempt different methods on the other side as I have to machine down to 69.98mm, the reason I posed this question is due to the fact that I am attempting to find the sweet spot for machining such shafts without the use of a running steady and fixed steady respectively (I've managed to do this with 65mm EN19T shafts that are 550mm sticking out from the chuck. So I thought it should be possible to do so here with this part as well

 

[guythatbrews]

There is a ton of difference in live centers. The center you are using may not be much help.

Going to a 0.2mm TNR is a big help also.

 

[Mtndew]

How much tailstock pressure are you using?

 

[rjwalker1973]

I wouldn't even know where to look for this option is this a g-code perhaps? the controller is a GSK988TA (similar to a fanuc) if that helps anything

From what I see your control does not. Yes it is a M-code that will vary your spindle speed during operation to break harmonics.

 

[bigjon61]

Reduce your speed by half and report back.

 

[charlie gary]

I don't see the machine you are using mentioned anywhere, but that's a pretty big piece of hard steel. If you're getting good results close to the chuck but bad results farther away, it appears your tailstock (or the whole machine) is too flimsy for the amount of cutting pressure being applied.
If it were me, I would go to an even smaller radius on the finish tool (.2mm), a light depth of cut (.2 to .25mm) and whatever feed rate required to meet your finish requirements.

 

[Ox]

I'd also be looking at the tailstock / pressure / live center for issues.


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I am Ox and I approve this here post!

 

[1NoobMachinist1]

There is a ton of difference in live centers. The center you are using may not be much help.

Going to a 0.2mm TNR is a big help also.

the current tailstock I am using back end is a MT5x4, (Material being cut EN19T). Also the only other insert I have with a 0.2mm radius are the TNMG160402(not sure if its PS or GP will have to look when I am back in the workshop)

 

[1NoobMachinist1]

Reduce your speed by half and report back.

Will attempt this tomorrow morning and do so.

 

[1NoobMachinist1]

I don't see the machine you are using mentioned anywhere, but that's a pretty big piece of hard steel. If you're getting good results close to the chuck but bad results farther away, it appears your tailstock (or the whole machine) is too flimsy for the amount of cutting pressure being applied.
If it were me, I would go to an even smaller radius on the finish tool (.2mm), a light depth of cut (.2 to .25mm) and whatever feed rate required to meet your finish requirements.

I will say it is my first time machining EN24T hardest material I've ever machined was 304ss rods and EN19T.
I cant' really tell you what the machine is as its most likely just a rebrand from a Chinese machine I've attached images with the specs given and how the machine looks except it doesn't have a toolpost instead its fitted with a 6 toolholder turret (first cnc machine I bought that I could afford brand new as the used machines market and financing in South Africa was a ridiculous), tailstock is a manual one I clamp it pretty tight and push the live center pretty hard in until the handle doesn't want to rotate.

Also I will attempt to use a 0.2 radius insert although its in a TNMG shape.

 

[1NoobMachinist1]

I'd also be looking at the tailstock / pressure / live center for issues.


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I am Ox and I approve this here post!

I'll attempt to clamp even tighter down tomorrow when I setup for the other side. I'll also reduce the tailstock stickout as little as possible as the turret has this huge table that kind of forces me to have the tailstock shaft stick out almost completely. the setup is pretty similar to this for the EN24T (pictured is EN19T) had a lot of issues getting this to also gie me good cuts without chatter.

 

[Mtndew]

EN24T is just 4340 here in the U.S. Nothing special just a little harder to machine.
Run it about 70% as fast as you would a mild steel and you should be golden.

Go to a smaller corner radius on your insert to help with the chatter and adjust your depth of cut and feed rate accordingly.

Removing existing chatter from a shaft can be almost impossible sometimes. If the O.D. isn't finished, use a file at a slow RPM, sandpaper, then resume cutting.

Cutting left to right with a normal holder will only make it worse because you'll have more of the insert in the cut.