vibration during turning

[jack620]

This evening I started work on my slitting saw arbor. The material is 1 1/2" 1020 BMS from Lockbar in Moorabbin. I'm attempting to turn one end down to 16mm. My setup is shown below.

No matter what I do I'm getting terrible vibration during turning. The tool is on centre. I've tried turning with and without the live centre. The chuck jaws are tight. I've tried low speed back gear and various speeds up to 960RPM. I've varied the DoC and feed but I always get a horrible vibration. I varies from a rattle at low RPM to a high pitched whine/howl at high RPM. Lube helps a bit, although the insert manufacturer says lube isn't required on MS. The insert is a Lamina Technologies TNMG 160408 NN.

I used exactly the same setup to face the end of the piece and it cut as smooth a silk.

Appreciate any advice.

Chris

[Jekyll and Hyde]

This evening I started work on my slitting saw arbor. The material is 1 1/2" 1020 BMS from Lockbar in Moorabbin. I'm attempting to turn one end down to 16mm. My setup is shown below.

No matter what I do I'm getting terrible vibration during turning. The tool is on centre. I've tried turning with and without the live centre. The chuck jaws are tight. I've tried low speed back gear and various speeds up to 960RPM. I've varied the DoC and feed but I always get a horrible vibration. I varies from a rattle at low RPM to a high pitched whine/howl at high RPM. Lube helps a bit, although the insert manufacturer says lube isn't required on MS. The insert is a Lamina Technologies TNMG 160408 NN.

I used exactly the same setup to face the end of the piece and it cut as smooth a silk.

Appreciate any advice.

Chris

That's a pretty large nose radius on that insert, have you got any smaller? I've only recently bought some 08 radius inserts, but I'm yet to try them out, so I don't know what they do... I've found 04 seem to work pretty well on a lot of stuff, and 02 work quite well for some of those horrible steels that seem to tear rather than cut...

Looks like your machine might be a bit smaller than my AL335 too, so maybe it's just not rigid enough to power on through with that nose radius?

*Edit* - Also, have you had a good look at the edge of the insert you're currently using to make sure the edge/tip hasn't been knocked off or chipped? Maybe turn it around to use a fresh corner and see if that helps...

[shedhappens]

Clearance angle at the front ?

Maybe that tip needs to go in a tool holder that is angled down slightly to the front ?

[jack620]

J&H,
No, 08 is the only radius insert I have. I'm going to get some smaller radius inserts this week. My lathe is a 3/4HP Hercus 260. Not sure how it compares to a AL335 size wise. I have used these inserts on my lathe before with very impressive results. I mean huge curls of dark blue swarf flying in all directions- pretty scary actually. So I reckon my machine has the cajones to to do the job. You mentioned horrible steels and I'm beginning to think I might have a piece myself. I have bought 1020 from Lockbar before and it cut like butter.

I'll try rotating the insert to a new edge.

Shed,
The photo doesn't show it, but the insert is angled down. I bought the holder and tips together.

[Ueee]

Have you tried another tool? You have a diamond too holder don't you?
If you have chatter, which the surface finish doesn't look like you do, but if you do, you will need to take a decent cut off to remove all traces of the chatter otherwise the tool will just follow it.

[jack620]

Thanks Ewan. I'll try the DTH tomorrow. The piece I'm turning is from the end of the bar. It was pretty seriously out-of-round when I started. Maybe you're right and I'm just following the chatter from the previous cut.

[morrisman]

Hi It could be a loose or worn bearing in the headstock spindle ? Mike

[jack620]

I guess it could be a bearing Mike, but I've never had vibration like this before (except occasionally when parting off).

[Steamwhisperer]

Hi Chris,
have you tried feeding by hand so you can change the feed rate and see if you can find one that stops the chatter. You may have to try a heavy feed rate. Otherwise you could try and rotate the tool so the cutting edge is less than 90 degrees and see how that goes.
Just thinkin'

Phil

[nearnexus]

It sounds like you may be better off just slightly below centre as that is a negative rake tip and you have no front relief.

The sort of symptons you describe are typical of lack of front relief.

Try it.

Normally negative rake tips are used in holders with a slight downward angle to give the relief required.

Rob

[morrisman]

It may be that your cross slide or top slide gibs are slightly loose . Mike

[Clubman7]

Try wrapping some emery cloth around the bar then clamp in the chuck.
takes up any gaps in the jaws.
Elsewise, check to make sure the insert is correctly seated in the holder.
Reduce overhang of the holder and it has a clean surface to clamp to.
Reduce extension of the tail stock spindle.
Make sure gibs are adjusted up.
If you still have it after all that, then your headstock bearings are worn.
good luck.

[cba_melbourne]

Looks like a Hercus lathe? For such lightweight lathes, you should only use tools with positive rake. They take less power to remove a given volume of metal. Less power means less chatter. The insert you are using looks like zero or negative rake, such tools are only good on rigid and heavy industrial lathes: they do not cut a chip, they shear a chip and that takes a lot more power and causes much stronger cutting forges. And inserts for zero or negative rake very often have on purpose a chamfered or radiussed cutting edge (not nose radius, I mean a rounded blunt cutting edge, they do it to make inserts last longer on heavy industrial lathes).

Chatter can also be caused by loose spindle bearing preload (in case your lathe is fitted with roller bearings). A loose cross slide gib can also cause chatter. If your lathe has variable speed, you may try to slightly change rpm up or down to avoid the speed regions with the worst resonance. If you do not have variable speed, you may consider it: one grat advantage is exactly the ability to avoid resonances on lightweight lathes.

[jack620]

Gents,
thank you all for the feedback. The first thing I did this morning was rotate the insert to a new edge (thanks J&H). This went part of the way to solving the problem. The vibration reduced and the swarf was silver coloured whereas it had been dark blue or brown. I feel like a goose for not trying this first up, but I had only used this tool once previously and only on mild steel, so I figured the tip was still sharp. :doh:

However I still had some vibration, so I switched to back-gear 170RPM. I then upped the feed rate in increments until it was almost at max (thanks Phil). By taking a 0.5mm max DoC at a high feed rate I was able to finish the job without vibration. While the source of the vibration was obviously the tool on the workpiece, most of the noise was caused by the hinged side door vibrating on the headstock. A bit of double-sided tape at the contact point knocked that source of irritation on the head. The workpiece is now being held in a 16mm ER40 collet and I have been able to up the RPM again without any dramas.

I will take the advice of a few of you and check the cross-slide gib strips.

Chris,
Thanks for the info on negative rake tools. In future I'll only use this tool when I really need to. I'll finish this job with the DTH. Variable speed is on the wish list. I lost count of how many times I changed the belt settings trying to sort out this problem.

I am going to cut and paste all your tips here into a word document and keep it handy in the workshop for the next time I run into similar problems. :2tsup:

Cheers,
Chris

[nadroj]

Try HSS.

Jordan

[Clubman7]

At 170 RPM you might as well use HSS as there wont be any benefit using Carbide insert tool at that diameter.
At 250m/min material should been running at 2000RPM+.
Replacing worn edge helped, but i think there is still an underlying problen to your lathe.
Sometimes these things are sent to try us.

[wbleeker]

Try HSS.

Jordan

Nailed in one line, speed and horsepower as well as machine/bearing rigidity are what you need for negative rake tools.
Where do you live Nadroj,we might be neighbours?
Will

[nadroj]

Carbide tips look to have some shape to them at the cutting edge, that changes the angle at the cutting point.
Could be even negative? Even though the flat base of the insert is at a positive angle.
Not an expert on this, just an observation.
I do have a few carbide tipped tools, but rarely use them due to the versatility of HSS.

Jordan
Penrose

[cba_melbourne]

> Carbide tips look to have some shape to them at the cutting edge, that changes the angle at the cutting point.

What you see could be just a chip breaker. Chip breakers have little to do with the cutting geometry itself.

> Could be even negative? Even though the flat base of the insert is at a positive angle.

Inserts for negative rake can often be used from both sides (from a triangular insert you get 6 usable tips, 3 on top and 3 on bottom). If you look at such insets from the side, they are rectangular. Such inserts ONLY work in negative rake toolholders, that present the insert with the tip sloping down towards the workpiece. Without this downwards sloping, there would be no clearance angle below the cutting edge. Then the tool only rubs and chatters.

Negative rake inserts are sold very cheaply, it is the simplest to make inserts, and it is the oldest inserts there are, since some 60 years ago it was not yet possible to make intricate inserts like nowdays. The problem is when people buy such cheap negative inserts, maybe with the view to flip it over and use twice the number of cutting edges, and then install these inserts into toolholders designed to take positive rake inserts. Without the downward sloping seat. The result can be complete frustration with insert tooling and a return to HSS.

For lightweight lathes with low HP motors, positive rake insert tooling works as good or better than HSS. For example CCMT or TCMT inserts are excellent for roughing and finishing steel etc. For Aluminium and plastics and even wood CCGT-ALU or TCMT-alu with 20 to 30 degrees positive rake work wonder, these are as sharp as a freshly ground and carefully honed HSS. I also uuse these for fine finishing steel, they are so sharp that you can take off cuts of just a few microns.

Sharpness matters a lot on a manual lathe. The rule is, that you cannot take a depth of cut that is less than the radius of the cutting edge itself (not the tip radis, that is another thing, I mean the radius of the cutting edge itself as seen under a microscope). That is why it is not possible with a carbide insert designed for roughing, to take off a very fine chip on a manual lathe, like necessary to turn something like a ball bearing seat that requires an accuracy of better than 0.001mm. If you try, the insert just rubs and leaves behind an ugly irregular finish. On a cnc lathe the same insert can produce a nice finish and hold accurate dimension, because you program it to take not less than 0.1mm for the finishing cut, and the machine is rigid enough to do that with accuracy and without the need to take control measurements as you approach final dimension. On a light lathe you need to approach final dimension slowly and take measurements, and to do this you do need a very sharp tool, either HSS or a very positive rake polished type insert like a CCGT-Alu.

Sharpness also matters to reduce or eliminate chatter. The less sharp an insert, the greater the cutting forces, the more chatter. But also, the less sharp an insert the longer it lasts on a powerful heavy lathe, and believe it or not, also the better the surface finish - but only on a very rigid heavy lathe. Negative rake inserts very often have on purpose chamfers or radiusses on the cutting edge to make it blunt.

Undortunately the literature from the many insert makers is directed to where 99% of their customers are - powerful heavy industrial CNC lathes. Their recommendations do rarely apply to smaller manual lathes. Believe me, if your lathe is less than say 250kg and 2HP and less than 3000rpm top spindle speed, only sharp and positive rake inserts will work well for you - either that or good old HSS.

- Uncoated inserts are always sharper than the same insert coated. There is little benefit in coating ouside industrial use.
- Either use flood coolant, or no coolant at all. Using drip or brush on coolant thermally shocks the carbide. The cutting edge of such shocked insert under a microscope looks full or microcracks, which greatly shorten the insert life.
- If the lathe allows it, try to use inserts at about 3 times the surface speed you use for HSS. Take as deep a cut as reasonably possible. Inserts hate too shallow cuts. When roughing, chips should be very hot and blue, carbide just loves this condition. When finishing to a very precise diameter and best possible surface finish either use special polished supersharp high top rake inserts, or use HSS.

[jack620]

Great info Chris. I'm going to track down some positive rake inserts and a new holder. Where do you get your -ALU inserts from?

[GSRocket]

cba... That is a 5 star essay.
I shall name the essay ... The Idiots Guide to Carbide Tooling.
Well done and thankyou.:wub: