Parting off chatter problem with one speed

[PekkaNF]

Everybody has had one time or another few parting off problems, but they are normaly pretty easily solved.

I have been lately looking go new paring off, threading and grooving tools. One friend dropped me a polyvee pulley form tool he brough me a brought me some MGMN 200 inserts and MGEHR shanks to try them.

Worked like a charm, ecept belt on my lathe was slipping....chinese lathe, bad belts and pulleys, stepped up he belt on next step pn pulley. Normal turning was fine, but parting off anything more than 30 mm OD aluminium was a pain. Constant really bad chatter. On 30 mm OD bright steen and on 20 mm tempering steel.

Thought it was the setup we had been earlier playng with. Disassembled the toolpost mount, cleaned all mounting surfaces, tightened the gibs and cleaned the chuck. Hlafan a hout later we cleaned the tool holders, cleaned ad remounted the inserts, tried different toolshanks and inserts and still were getting the chatter.

Hour later chatter we descided to try a lower speed and the chatter was gone. Then we tried faster speed and the chatter was gone

Chatter returned when the "chatter" speed was tried. On close examination it looked like chatter was beltdrive induced....belts and pulleys seem to vibrate a lot at that setting and maybe that excites the vibration?

I have been very tempted some time to modernisize the two stage V-belt drive with a capacitor start small AC motor sytem into two step polyvee belt drive + threaphase motor + VFD.

What do you think?

Pekka

[sparky961]

I was about to chime in with tips for parting off, but you kinda lost me here. :P

It would seem like you ended off with a question about the drive system rather than parting off.  Maybe you're just thinking out loud.  I do that a lot.

[sparky961]

One thing to consider:

Do you need more than one parting speed?  If this is for your own hobby use, what's the rush?  Pick the fastest speed that works reliably and be content with that.

[PekkaNF]

Uhh...Spent a whole evening in the garage, thinked that part of the problem got identified to crappy belt drive and that is possible to fix. Then slept three hours, woke up, could not sleep and vomited everything here...I think I lost of the plot myself.

You are right one parting speed would be workable, but there is a nagging feeling that this could be improved and if not dealt now I'll suffer later. As a intermediate step I'll try to locate new belts and check the sheaves.

The thing is that my lightweight chinese lathe has move value to me than economical value if sold, so I might as well modify it somewhat and hopefully even improve. Parting off is one pretty clear indication when something is just passable or not good enough at all. Therefore I have been trying to expand the envelope systematically find out what works and what does not.

I'm pretty confident that part of the problem is the original belt system and the capacitor run motor is another limitation. Drive system is one part that can be build and tested separately. Other issue is cross slide does not show very good contact when blued and spotted, but that is another separate issue.

I am tempted to build very rudimentary rear toolpost for parting off (but normal one, not inverted one). Any Ideas for a quick change design that the whole post would come out clean and would index back without much waffing about?

Pekka

[sparky961]

If making machine modifications and improvements is something you enjoy, then by all means do so.  But don't get caught fooling yourself into thinking that you can make that machine inherently better than it is.  They are what they are.  Useful? yes, but with many limitations.  I've come to accept this myself but if I ever find myself with an excess of money and space, I'll be ditching the import crap for quality heavy iron in a heartbeat.  Until then I try to appreciate the imports for what they are.

[gerritv]

My experience with a King KC1022ML is that the cross slide moves unless you over tighten the gib screws, then back the off a 1/8th turn. The way I check this is to pull up on the tool post while placing a finger across the cross slide/carriage joint. If I feel any movement then I over tighten the gib again etc.
Lubrication and feed in speed also impact chatter. I have learned to feed faster if chatter develops.
Variable spindle speed is my next step, looking for a treadmill to butcher.

Gerrit

[PekkaNF]

Sparky....you are right on this one. No matter what I do to that lathe it's still small chinese lathe with all of it's limitations. However, half decent second hand machines are rare here. I don't have space for 3-5 metre lathes - those are cheap here.

Think it would be whole lot more nice to use with 0,75 kW three phase motor with VFD and good belt drive. I checked it this evening. It's not only belts, the pulleys are out of round too and the drive system is way too weak. And anyway, I like re-building and/or improving machines.

Gerit, I can conform all that. Today I was parting off happily with no problem at all. It was that one speed that was giving all the resonance and chatter.

Pekka

[PekkaNF]

MGEHR1616-2 & MGMN200-G vs. SGTHR 20-2 & GTN2.

GTN wins.

Pekka

[sparky961]

So, which of the two shown side-by-side is the winner?  I've never been great with all of the number/letter designations for holders.  Which criteria are you using to determine your winner?

In your last picture, just looking at the chips below (small short curled chips lower left, which may or may not be from the active parting cut), it looks like you're getting a lot of heat.  It would help to get some soluble oil coolant in there via a squirt bottle or more fancy delivery system, or straight cutting oil - which will do the same job with probably a lot more nasty smoke. ;)

What feed/speed are you using?  Or are you feeding by hand?

[PekkaNF]

GTN is in the second picture on the left. One ended insert, friction seating no clampping screw to hold it.

It winns because no chatter and cuts cleanly. Probably much of the reason the inser holder that has less overhang. Also difficult material chip was formed better on the Iscar GTN inser than on the two sided cheap chinese one. Although I must say that the one chinese I got was surpricingly well made and the tool holder is about 1/4 of the price  paid for iscar. Inserts are 10 times cheaper tha iscar. I got one box of the chinese iserts that are next to uselees: they cut bright drawn shaft, but chip formation is bad and they break often. These on the picture were much much better. Still no markings on the box but these insers actually work.

I have pictures of the two sided grooving system (MGEHR1616-2 & MGMN200-G ) at the end. Last picture of the cheap but surpricingly good insert after the I cut two cuts on MOC, four cuts on drill rold (silver steel) and whole evening of general parting off (maybe dozen of cut outs) on structural steel. If I can find aluminium inser for it I would dedicate that holder for aluminium parting off. However, on more difficult materials and deeper cuts than parting of maybe 35 mm dia I will be using GTN2 system.

The material that gives most trouble is pretty hard tempering steel, known here as 34CrNiMo6 to me, these are other names for same stuff (or close) 4340, 817M40, En24 the one I got is tempered to pretty high hardness. Produces very long swarf and seems to cut best when run hot. Cip formation takes a whole lot from chip breaker.

Pekka

[PekkaNF]

It is the pulley....tried new belts and one speed is terrible.

I never have been too happy with the powertrain of this lathe, alternative to completely new construction is to fix two pulleys...i guess I could take them out and take to friend to true them. Or use my lathe and make completely new set of pulleys for polyvee.

Pekka