Need help with lathe finish

[kenickie22]

Hello. I have a Jet 13 x 40 geared head lathe with 3 phase 230V multi-speed motor that I'm running on from single phase power. I'm currently using an Anderson Converters 3HP/15A rotary phase converter. I can run the lathe in the slower speed range (2HP/8.1A), but the converter doesn't have enough oomph to start the motor at the faster speed range (3.2HP/13.2A).

My issue is poor surface finish when turning & threading (in the slower speed range; 45 - 900 RPM). I haven't always had this issue, and am hoping someone can help me troubleshoot it. I've checked what I believe to be the usual mechanical suspects (gibs are tight, tools are sharp (free hand ground HSS, diamond tool-holder, carbide), tools are on center or slightly below, compound rest is locked, chuck is tight, gears are tightly meshed, etc. I've also tried a few different cutting oils.

I suspect that I may have a motor issue. One of the terminals for the power coming in to the lathe appears to be burnt (black wire on bottom).

Examples of poor turned and threaded finish in 416 stainless.

Any suggestions on how to determine if the motor is the issue, or other suggestions? Thanks!

[whateg0]

Is this something that came on suddenly? Or did it gradually happen and get worse over time? Does this happen with all materials? Are some materials worse?

Do you have any better pictures? Those look awfully grainy and I don't think it's just the way it looks on my phone.

Dave

[kenickie22]

It seems to have gotten worse over time. I have had similar results in steel, but haven't tried any other materials. Hopefully the resolution on these photos will be better. Thanks for your reply.
https://uploads.tapatalk-cdn.com/202005 ... 1bd4b0.jpg
https://uploads.tapatalk-cdn.com/202005 ... 836996.jpg

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[Patio]

That screw that is black, is loose! It will create a high resistance connection and an imbalance of your motor electrical system. You need to pull it apart and clean the wire, the contact pads and maybe replace the screw. That should help a lot.

[armscor 1]

Your motor is single phasing, hopefully repairing the terminal will bring it back to full power.
Check the motor tail insulation, sometimes they melt and cause a ground fault.

[kenickie22]

Thank you both. I'm going to try and find a replacement terminal today.

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[John Hasler]

That screw that is black, is loose! It will create a high resistance connection and an imbalance of your motor electrical system. You need to pull it apart and clean the wire, the contact pads and maybe replace the screw. That should help a lot.

Also apply anti-oxidant grease. It would be best to install proper ring or spade terminals on all four wires.

[kenickie22]

That screw that is black, is loose! It will create a high resistance connection and an imbalance of your motor electrical system. You need to pull it apart and clean the wire, the contact pads and maybe replace the screw. That should help a lot.

Also apply anti-oxidant grease. It would be best to install proper ring or spade terminals on all four wires.

Thanks for the suggestion John. Will do.

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[Bentworker]

I worked on one of those Jet lathes with a two speed motor a few years ago for an acquaintance. On most of those Jet machines there is a wiring diagram glued to the inside of one of the panels. You should turn the power to the machine off, and start removing covers. If you find the schematic take a picture and post it here. My guess is that since the machine works in low speed the burnt power input terminal is not the problem, but a result of a different problem. There has to be something that is causing single phasing when you try to run it in high speed. Something else I will say is that you should look at the schematic of the lathe and make sure the generated leg from your phase converter is not being used to power the control transformer. You want the control transformer to be connected to the non generated legs, as they are much more stable upon inrush current if the motor starting. I have seen it where the control transformer is connected to the generated leg and the inrush of the motor drops the voltage enough that the control contactors and relays chatter.

My guess is that you have a bad switch or contactor that is open on one of the legs to the motor when trying to run it in high speed- causing single phasing- and a whole lot of current draw which heated up your power input terminal.

Dig into it a little further and see what you find.

[kenickie22]

Bentworker, I have this screenshot from my manual. I'll check under the electrical panels when I get home tonight. Thanks.

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[Bentworker]

I’m curious what the condition is of that switch labeled SA1

Also looks like your control transformer is connected between terminals S&T. I’d double check to make sure your generated leg is not on one of those terminals.

[John Hasler]

That's my #1 suspect as well.