This is a 2 part opinion question that could qualify to be posted in 3 or 4 of the forum categories.
As some of you know, I recently acquired a Logan 6560 variable speed lathe. I also opted for a VFD to power the 3 phase motor.
Since this will allow me to control spindle RPM both mechanically and electrically, does anyone have knowledge or an opinion for the base frequency set-point for the VFD ?
My gut says it should be the USA native 60 Hz, but thought I'd ask what others with VFD experience thought.
As I pondered the possibilities this presented, I had another thought. I've been a big proponent of the Touch DRO App. Lately, many of the updates have dealt with added functionality for use with lathes.
Since becoming aware that CNC lathes adjust RPM to maintain optimum feeds & speeds as material is removed, it made me wonder if this could be incorporated into the App that would allow it to adjust the output frequency of a VFD to optimize turning speeds for surface finish and time.
It probably isn't worth the effort, but I'm considering recommending a possible addition to the App that would include an output control function for VFD's to control RPM based on an option to input the cutter type (HSS, carbide, etc), material being cut, saddle feed rate, and cross-feed DRO position for depth of cut.
What do you think ? Do you see any advantages to this ?
I've been mulling over your question and thought I'd share my experience{s} with vfd's and rpm's. The one almost universal issue I have when a vfd is added is folks very rarely consider the heat that gets accumulated in the motor. usually because the speed is set so low that the attached fan cannot generate enough airflow to perform the needed work. And I am familiar with fractional to motors of 800 HP that were setup perfectly and never had an issue until someone decided to slow them down to solve some other problem. Relative to this conversation? Maybe, maybe not... just sharing :smiley_bignews:
I've expressed the same concern based on videos I've seen, but Shawn and others state never having any issue with it.
That said, I've seen stated that the newer motors have a rating designation expressly for variable frequency use.
As hobbyists, we seldom push our machines to their potential which helps alleviate some of the overheating potential.
Well...
There's several caveats to your ideas.
Motor speed - yeah, slow can create heat and reduce cooling, but you're also not going to be too keen on running the motor at 25% speed just for the amperage issue - you can overload the VFD, nevermind the motor.
40-80hz is a good healthy range for an induction motor. If you want a more wide spread range you need an inverter duty motor. Likely has a built in cooling fan running at 100% no matter motor speed.
You'll still find yourself running through gears/mechanical speed change, but we're talking if you're going from a 6" OD work piece to a .500". Just for the torque.
If you could figure out a way to connect touch dro to the VFD somehow, you'd be a genius. That would be incredible.
Yes, the dro's have multiple speed input capabilities (mV, resistance, etc)
But, you may also find as you engage the cross slide lateral feed to face off, it's real easy to adjust the speed dial as it closes in on center. Same thing with turning diameters.
Get yourself a project box and setup some push buttons and a potentiometer for speed and bolt it in a real convenient spot.
Also, eBay a speed/rpm readout with the proxy switch. They're real nice too.
Also, some dro's have feed speed on the display. So you can really dial in.
There are a few variables and differences between lathes that doesn't make for a "One size fits all" situation. Things like different SFM requirements for different metal and for different cutters (HSS/Carbide) under that.
For my situation, the Logan has an infinitely variable speed ajust with a 2 speed gear box. 40-220 RPM in low gear and 250-1,400 in high.
As for interfacing it with Touch DRO, that's beyond my ability, but I think it wouldn't be too difficult for someone with the coding skills.
A VFD has Aux ports for things like an On/Off switch, FWD/Rev switch, and for connecting a potentiometer for frequency/speed control.
Touch DRO controllers are set up to have a Tach input and up to 3 or 4 scale inputs.
For calculating SFM all you need is RPM (from Tach), material diameter (from X-axis scale, 2X Radius), and material type (this would require a change to the firmware to allow selecting the material type and have a coded chart/library of speed & feed ranges).
The processor could do all the math and output a signal via a DPOT (digital potentiometer) that output a signal to the VFD Aux port that would increase/decrease RPM to maintain SFM within spec.
I think a person with the coding skills could do this with an Arduino independent of Touch DRO. An audio alarm and SFM display could be added to alert the operator if going outside the optimum SFM range.
To me, it sounds like a neat idea, but not sure it would be worth the time and effort for someone to build the controller and write the algorithm to make it work.
I don't think knowing the SFM is a requirement. I think the speed change could be linear.
You set the speed yourself, and the DRO only vary the speed on a linear scale based on where the cross slide was when you set the speed. And the cross slide crank handle becomes the POT, essentially. Hell, that in itself wouldn't be terribly difficult to rig up. Is it possible to have two variable resistors run in series somehow? the pot on the cross slide handle would have to be a multi-turn or infinite.
I find to keep the VFD at around 60 hertz and adjust the speed mechanically on my 14" Rockwell lathe. I've actually tripped off the VFD running it too low and kicked in the clutch.
With my VFD hooked up to my Bridgeport, I'm doing everything wrong! I typically set the vfd at 15-20 Hz. And I have the vfd direct wired to the switch on the mill. I do forward-reverse all the time. I have never seen an error code pop up. I only wired the vfd that way because the manual didn't say anything different? But now on the net everybody says that's a no no. As for motor temp. I have a magnetic thermometer stuck on the side of the original Bridgeport motor. In my Hobby Shop environment, I have never seen any alarming temperature. Maybe 5° above ambient is the highest I have seen. And its been about 20 years now.
Quote from: chips and more on May 20, 2024, 09:35 AMWith my VFD hooked up to my Bridgeport, I'm doing everything wrong! I typically set the vfd at 15-20 Hz. And I have the vfd direct wired to the switch on the mill. I do forward-reverse all the time. I have never seen an error code pop up. I only wired the vfd that way because the manual didn't say anything different? But now on the net everybody says that's a no no. As for motor temp. I have a magnetic thermometer stuck on the side of the original Bridgeport motor. In my Hobby Shop environment, I have never seen any alarming temperature. Maybe 5° above ambient is the highest I have seen. And its been about 20 years now.
Well that's just nutty!
Quote from: chips and more on May 20, 2024, 09:35 AMAnd I have the vfd direct wired to the switch on the mill.
Huh, wverything I've read says to wire the VFD directly to the motor, no switch in between, My guess was it had to do with soft starts and/or potential to overload/damage the VFD from the current surge on start-up.