Acceptable TIR of motor shaft?

[.LMS.]

After watching the troubleshooting adventures of @TX COWDOC in this thread (https://www.hobby-machinist.com/threads/facets-in-my-16-tpi-threads-need-some-help.89625/), I decided to try the "glass of water on the headstock" test on my 3phase 1236T and saw some concerning vibrations at various RPMs. I have been noticing some issues with some cuts and seeing them get better and worse with some minor RPM changes via the VFD.

I thought I'd take all the advice on that thread and others about eliminating vibrations. I plan on replacing the motor pulley and getting a notched belt (already replaced the stock belt), but I thought I'd start looking at the motor first and work my way towards the chuck.

I measured the TIR of the motor shaft at 0.0015 and have no idea if that's acceptable or not. Can anyone advise?

 

[mksj]

You will always get some nodes of vibration when using a VFD, you are hitting the resonant frequencies where the amplitude peaks. VFD's can be programmed with skip frquencies to avoid those RPM's. On bigger motors/systems these vibration nodes can tear equipment apart. The 1236T and 1340GT are relatively light lathes, have a narrow footprint and a tall headstock. These all contribute to some degree of vibration at different RPMs. The stock belts tend to be pretty poor, seem to through rubber all over the place and also will take a set when sitting which results in vibration. A good cogged belt like the like the Gates Tri-Power BX or similar I found was a significant improvement on my PM 1340GT. Proper belt tension and motor/pulley alignment is also important, do not over tighten the belt, I used the weight of the motor with some light finger pressure pushing down on it and then locked the motor mount.I have seen increased vibration from too tight belts. If you are running a VFD then use the larger motor pulley to the larger headstock pulley (I.e. flip the motor pulley) use a BX27 belt and you should get full spindle RPM range with the VFD (something like 30-90 Hz).

As far as motor runout, I would expect a bit less but 0.0015" but it is not significant given pulley runout and belt variations. I would not replace the motor based on that factor, and there are very few motors that will fit the 1236/1340GT models and have 2 Hp. The stock motors I found worked well, I did replace mine with a Baldor IDNM vector motor because it was being sold at a ridiculously low price. You need to significantly change the pulley ratio with a smaller motor pulley and run the motor to a much higher Hz then stock.

The issues TX COWDOC had were due to other issues which effected the rigidity of the tooling, when these were corrected the threading issue went away. He is also using a single phase motor, which in some cases can worsen the vibration issue and cause surface finish issues. It is somewhat idiosyncratic as to when it pops its head up.

This is someones else's 1340GT with the motor pulley flipped with a Gates Belt.

I had a vibration node on my PM1340GT at around 725-750 RPM, bit otherwise was quite stable.

 

[.LMS.]

Mark;

Thanks for your usual well thought out response. Just wondering - did you place your motor pulley or keep the stock one?

 

[Winegrower]

You can download a free vibration app for iPhone/iPad that does 3 axis modal analysis. If you like quantified amplitudes and frequencies, this is terrific.

 

[.LMS.]

I googled "vibration app iphone" and got some very interesting results. (Think the women's magazine Cosmopolitan).

 

[COMachinist]

when I tuned my 12x36t for a better finish on threads and turned surfaces I found this brace loose and flopping around. I changed the belt and aligned the pulleys better and made a. Vibration dampener for that brace and it did help some. I still running the factory motor until it dies which should not be long, on these Chinesium motor then I ’ll do the 3ph/VFD conversation. For now I can live with the less than the perfect finish. With max rpms and the right feed rate, I’m get this finish on 4140. The threads are M24-1.0 using cheep Shars carbide.