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Parting off chatter problem with one speed

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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 :borg:

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. :scratch: :lol:

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.

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