VFD lathe retro

[itsmoked]

I've got a lathe that has an extremely complex gear box that has failed in multiple ways. A 2-1/2HP motor was the original prime mover. We have a 7-1/2HP VFD motor which is even plumbed with an integral 3ph blower. We'd like to replace the 2-1/2 and the transmission with this motor and a VFD - belt driven. Any suggestions or concerns you guys can think of?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

What turning speeds are you looking for? If your machine's back gear is part of the knackered transmission then anything large will become more awkward. You can obviously run carbide tooling which will allow a higher cutting speed, if the cut is continuous and the material will tolerate it. You really want back gear if you're turning something with an intermittent cut like a casting.

Are your power feeds and leadscrew driven from the transmission? Even if they're separate you need to be able to keep the speed down a bit for screwcutting on a manual machine.

Does the transmission drive a lube oil pump for the main spindle? There's usually a sightglass on the head somewhere if there is a pump.

There are a lot of folks more knowledgeable than me over at Practical Machinist, a real mix of enthusiasts / home workshop owners and professionals. You might get a proper answer if you know the machine type. And of course there's our very own forum281 full of experts.


Oh, and please tell me you haven't broken a Monarch or CVA - I would love one of them for my home shop!

 

[Compositepro]

Machine tools are often considered "constant horsepower loads". Speeds need to be adjusted to the diameter of the work or tooling, but as speed goes down torque requirements go up. This can only be done with a mechanical transmission.

 

[jraef]

To clarify Compositpro's statement, when you use gears to change speed, you exchange speed for torque: output speed goes down, output torque goes up. But when you change speed with a VFD, the torque remains the same. Now in this case you are increasing the motor size 3 fold, so that may end up making up some of that, but it will depend on the ratio of the lowest speed in your back gear. For example if it's a Monarch 10EE, I believe the lowest speed is a 5:1 ratio, so you will end up with less torque on the work shaft using a 7.5HP VFD driven motor.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[itsmoked]

Scotty; I believe it's a Monarch. It's the "only lathe with speed control in the direction lever". Their is an oil sight glass in the housing above the spindle on the front. I think the thread cutting is tied to the head.


Compositepro, Jeff; Yeah I realize the torque issue. That's why I'm asking you guys. There is a final gear ratio that we're banking on helping us here. Unfortunately as-is it's very limited because the sheave we go to is next to the drive tail so it can only get so large.. (about 4.5") It's less than 2:1 though so I've warned the guy that we should put in an intermediate shaft for more reduction. Essentially being geared way low and then taking advantage of the motor's 6kRPM name tag would be a better way to go. I'm trying to figure out what the minimum reduction would have to be to get us in the ball park with the original drive now that our motor is 300% of the original. It should be at least 1/3 the reduction of the original gear reduction correct?



Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Yes, I think so. Generally these motors were at least 20% maybe 25% over sized because that's the way things were done back in those days, so you might be able to get away with slightly less, but for sure 3:1 should do it.

By the way, for some reason when I read your original post I didn't even read the name. Had I known it was you Keith, I wouldn't have bothered explaining the torque issue... sorry.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[Compositepro]

There is no question that you can "get away with it". A mechanical transmission simply allows you to get as full horsepower out of a motor at various speeds of the chuck. With a VFD you will have to limit your depth of cut, if you exceed the allowable torque on the motor. There is always something setting limits. Usually it is the stiffness or strength of the cutting tool or the support structure, or it is the torque or horsepower of the motor.

 

[itsmoked]

Jeff you help so many ^$#@^8 people every day I certainly don't fault your missing a name occasionally.

Comp; I'm told this lathe is "all about accuracy and smooth function NOT hogging out 1/2" cuts of steel". So I don't think loss of some torque is going to be an issue. As you say, something will always limit you.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[MikeHalloran]

Can you program the VFD to limit the torque? That would be nice.

It would even be nicer to have it turn on a lamp when the torque gets near the limit, so the operator will know to back off the depth of cut a little, or sharpen the tool.

The other warning would of course be smoke from the belt; you want to set the torque limit a little under that.


Mike Halloran
Pembroke Pines, FL, USA

 

[jraef]

Smoke detector above the belt perhaps?

Most higher function drives will have the ability to indicate a torque warning, but limiting torque sometimes involves allowing it to override the speed command, so be careful about that.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[itsmoked]

Scotty; It's a Gaziano.

Looks like the second one down:

http://fac-staff.seattleu.edu/fshih/web/14_misc.asp

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Nice looking machine. Visually not dissimilar to the smaller British CVA:

http://www.budgetmachinery.co.uk/newsite/data/bm/lathes/cvalathe.htm.

The CVA would be my first choice for a smaller machine to sit alongside my old Holbrook which is on here somewhere:

http://www.practicalmachinist.com/vb/antique-machinery-history/holbrook-lathe-164407/index2.html