PM 1236 VFD Conversion

[mrbreezeet1]

Do you guys think that "the pounding of dynamic braking" hurts a non gear driven lathe.
When you say "dynamic braking" are you talking about a deceleration stop of say 3 seconds, or are you talking about one with a breaking resistor added to the system?
Take for example, My Oliver 159 wood lathe, I just put a VFD on. It uses a reeves drive.
And I may put one on my Logan 10" lathe, it is just v belt to the flat wheel on the jack shaft, and flat belt from the jack shaft to the spindle. It does have back gears.
Thanks,
Tony

Metric threading certainly seems like a good use for dynamic braking. Still though, adding VFDs to manual lathes is (in certain respects) still in it's infancy. By my accounting, it wasn't that long ago that VFDs were becoming common-place and when they first came out, there was a wave of poeple burning up their old motors because they weren't designed for running at speeds other than 50 or 60 Hz. Now, let's extend that logic just a bit more and ask ourselves if our 1950's design lathes (yes, our modern lathes are pattern variations of mid-1950's technology) were really made to take the pounding of dynamic braking. Don't get me wrong... I don't have all the answers and am figuring this out as I go but, my intuition tells me to do some thinking before proceeding.

When big gears turn small gears and vise-versa, the torque applied and stress on the teeth must be considered. That's just one of the reasons why transmissions have stepped gears in the first place... -So you don't knock the teeth off all the small gears. If you were going down the road in your 6 speed manual transmission at 65 MPH, pressed-in the clutch, shifted to 1st gear and dumped the clutch... Good Bye Car! So what's the difference between that and dynamic braking? -Nothing.

Somone out there probably knows the answers to these things but, often times, people come back and say "I've been doing it that way for years so, it's OK"... -Sorry, that kind of answer doesn't cut it for me. It's a valuable data point but not a good answer.

Ray

 

[Ray C]

Tony,

My understanding (and I could be misguided) is that dynamic braking involves a resistor AND the VFD supplies a reverse voltage causing the motor to stop within fractions of a second. Anyhow, I don't think a belt will suffer the possible consequences that a gear would although it certainly would place more stress on it -how much, I don't know but, belts are more forgiving when it comes to transmitting torque.

Ray

Do you guys think that "the pounding of dynamic braking" hurts a non gear driven lathe.
When you say "dynamic braking" are you talking about a deceleration stop of say 3 seconds, or are you talking about one with a breaking resistor added to the system?
Take for example, My Oliver 159 wood lathe, I just put a VFD on. It uses a reeves drive.
And I may put one on my Logan 10" lathe, it is just v belt to the flat wheel on the jack shaft, and flat belt from the jack shaft to the spindle. It does have back gears.
Thanks,
Tony

 

[mrbreezeet1]

I see, Ok Thanks,

Tony,

My understanding (and I could be misguided) is that dynamic braking involves a resistor AND the VFD supplies a reverse voltage causing the motor to stop within fractions of a second. Anyhow, I don't think a belt will suffer the possible consequences that a gear would although it certainly would place more stress on it -how much, I don't know but, belts are more forgiving when it comes to transmitting torque.

Ray

 

[Ray C]

I've got a little project going on (actually, 5 of the same thing) that is finally putting the VFD to good use. VFD's are certainly optional equipment but, it's paying off on this 2.5" piece of 316 SS. This piece is ornamental and must be perfect. It was very nice to set a base speed with the gearbox and tweak the R's down to make a perfect cut. Don't know if this picture shows it well but, the face cut on that piece was done with auto-crossfeed and the VFD was used to gradually increase the R's as the diameter decreased. The finish is perfect, no rings, no ripples, no gear ghosts...

In addition to 3 PH motors showing less vibration artifacts, fine speed control is why VFD's kick-butt!



Ray

 

[darkzero]

Very nice finish Ray!

 

[Ray C]

Very nice finish Ray!

Thanks, Will. And I maintained the finish on the 1st pass all the way to the last one. With VFD, you set the lathe to the diameter instead of cutting until you find the sweet-spot diameter given the closest RPM as per table look-up. Did the bulk removal in several cuts, saving the last 15 thou pass for the finish. I want everything on this project within 0.0001 of spec. I took the temperature at the end, made the expansion calculations, dialed-in and took the cut. 2 hours after it cooled, it was dead-on. Immediately after making the cut, while still hot, it was 0.0006 over.

Ray

 

[Ray C]

... And the VFD really helps with threading too. This was cut with HSS and I was able to tweak the RPMs fast enough to cut a nice thread and still have good control of the halfnut lever. The finish inside the threads came out very smooth. I do a fair amount of work on SS shafts and prior to having the VFD, there was a always a sacrifice in thread quality leaning toward slow RPMs to not crash the bit. I have 9 more of these to make and I'll try some carbide cutters later on.