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VFD with Pratt Whitney 10” engine lathe to bypass broken back gear?

GPW1013

Plastic
Joined
Oct 14, 2020
Hello. First post here but I’ve read around quite a bit. I Picked up this 1920s? Pratt Whitney 10” engine lathe and looking to get into metal working. It has all the threading gears and came with some tooling and both a 3 and 4 tooth chuck etc... Already ordered a couple books to learn more.
My question is regarding setting up a motor. I have single phase 120 and 240 power in my shop and a 3 phase 3hp GE induction motor (1725 rpm). Planning to buy a foreign 5hp VFD to run it in my shop.
My problem is: the back (bull) gear has a few broken teeth and therefore is probably not useable unless fixed..?
Would I be able to run the lathe without the back gear via direct drive from motor to the spindle and rely on the VFD for speed control?
Or should I plan on running an idle shaft with say a 3” v belt from motor to 10 or 12” v belt pulley on idle shaft with a smaller flat belt pulley also on idle shaft to the spindle? Basically use pulleys to slow the rpm?
Or.. should I run direct drive from motor to spindle and attempt to repair the broken teeth on the bull gear so I can use the it if needed.
Probably just doing hobby stuff won’t really run if for more than an hour at a time most likely.

Thanks in advance.



9499A679-5200-4BDB-9F6B-555D7F9593E0.jpgB0CF25E2-4BEA-4E4A-8C2E-ECD351F27186.jpg9D8CB998-0546-4EAA-B4FA-2B8ECF481B1F.jpg
 
It depends on how slow it’ll spin in it’s lowest non-back gear speed.
Take that rpm and divide by five.
If that results in an rpm of 50 or less, it’s workable for threading.
If your lathe spins 250 rpm at lowest now with a vfd at 60 hz... it will give you 50 rpm or so at 12 hz. You still get ok torque for threading at 12 hz, the motor shouldn’t be slowed enough to feel “coggy” at that speed, and it’ll still cool itself a bit if you’re not running that slow for a long time.
I ran a little Grizzly lathe with broken back gears just this way, worked fine.
 
Thanks Johnoder. What method would you use to fix three adjacent teeth? Drill and file pins? Or is there another method??
 
drilling and threading in a row of pins and filing them to shape tends to be the easiest durable solution without having to braze metal on, and you can find lathes with teeth fixed that way in use for decades after the repair without any problems. Based on what I see, it's the method I'd probably choose to fix that lathe.
 
Thanks I’ll look into that some more. I’m sure there are threads detailing the process. Any tips?
 
Thanks Johnoder. What method would you use to fix three adjacent teeth? Drill and file pins? Or is there another method??


Indeed- and it turns out to be "self gaging" as to how much to file off the pins

put in pins, reassemble (if disassembled) and turn back gears past the repair - and of course at first it stops dead at unprofiled pins - and progresses from there

Then - when it turns past repair almost effortlessly, its time to run and continue the filing until the KNOCK goes away

Might even want to involve a little marking medium such as high spot blue
 
Curious where you are in Idaho? I suspect not close to where I'm at but I have made a number of gear repairs in the past. I'm in south central Idaho.

Sent from my SM-N976U using Tapatalk
 
It depends on how slow it’ll spin in it’s lowest non-back gear speed.
Take that rpm and divide by five.
If that results in an rpm of 50 or less, it’s workable for threading.
If your lathe spins 250 rpm at lowest now with a vfd at 60 hz... it will give you 50 rpm or so at 12 hz. You still get ok torque for threading at 12 hz, the motor shouldn’t be slowed enough to feel “coggy” at that speed, and it’ll still cool itself a bit if you’re not running that slow for a long time.
I ran a little Grizzly lathe with broken back gears just this way, worked fine.

I'm wondering if your motor is Single Phase like the OP's motor. I'm thinking the centrifugal switch on a Single Phase motor will engage the start winding when RPM's are that low.
 
<snip>
3 phase 3hp GE induction motor (1725 rpm). Planning to buy a foreign 5hp VFD to run it in my shop.
<snip>
Do NOT buy a cheap VFD unless it is of the sensorless vector drive operation. You can't get low Hz operation that does not cog the motor shaft which is transmitted to the spindle. And it will stall out at low RPMs.
My 10x20 bench top is a V-belt driven lathe, and is left in the middle belt setting at all times. I can tap or die cut threads (up to 1/2x13) at low spindle RPM (<10) without changing this setting or engaging the back gear and no stall. That was not true with the scalar drive I had before (Teco FM100). Also for 3HP you do not need to oversize the VFD drive when used on single phase input.
I also single point thread with low RPM without backgear. And as the motor is 4 pole, I enable max Hz to be 120. This gives a wide speed range. I don't know how well this will work on a flat belt driven machine.
 
Thanks Johnoder. What method would you use to fix three adjacent teeth? Drill and file pins? Or is there another method??

1) mill a slot at the root of each broken teeth an silver solder a steel strip in place. File or mill the new 'teeth' to profile.
2) Or, the pin and file method, this repair was done when dirt was new:

SF_gear01.jpg


SF_gear02.jpg


SF_gear03.jpg
 
It will work perfectly fine with a normal VFD of any type including chinese cheap ones.

The VFD won´t matter, and in practice the motor wont matter (95%).

Slowest speeds for threading with high torque are needed when threading large D parts and/or deep cuts.
It is very unlikely You will ever thread 10" D steel blanks on a 10" lathe.
Typical work is 1/2 x D or == 5" at most for a 10" lathe.

But if You do need to thread or cut the larger workpieces, it will still work just fine.

HP=HP
.
As long as the VFD and motor are similar, and the lathe is upto it, Your work will proceed just fine.

That said ..
for some work backgear is highly desirable in manual lathes.
Mostly for very large workpieces where You want lower SFM in cutting speed.

I set up mine with a proper industrial AC brushless servo motor.
30x better than any VFD or similar.
But expensive, with HTD 8/30 belt drive, pulleys, and mounts made.
 
Ok. Thanks for the advice. Does type of metal of the pins matter much? Planning to tap 10-32 holes and basically use 10-32 threaded bolts with thread lock as my pins to file. Curious if any old bolts will work
 
Many prefer using brass pins/screws for this type of repairs: strong enough and more likely to work themselves to the correct profile than causing odd wear on the mating teeth (true only if kept free of any grit).
Others prefer to cover the row of undersized/partially shaped pins with JB Weld or similar compounds, cover with a layer of stretch-wrap film and shape the tooth profile by rotating against the mating gear (will need still a light filing after curing to re-establish proper clearance of the rebuilt teeth).

Paolo
 
It will work perfectly fine with a normal VFD of any type including chinese cheap ones.

The VFD won´t matter, and in practice the motor wont matter (95%).

So the 'ebay special' 3hp VFD and my GE tri clad induction 3hp motor would work just fine? If I can go with the cheaper VFD no problem that would leave some more money on the table for tooling etc...
Then again I dont want to spend the money on the cheaper one and have it not work well or blow up on me..
 
So the 'ebay special' 3hp VFD and my GE tri clad induction 3hp motor would work just fine? If I can go with the cheaper VFD no problem that would leave some more money on the table for tooling etc...
Then again I dont want to spend the money on the cheaper one and have it not work well or blow up on me..

I'd step up to 5.

Has anyone priced a new gear via Boston Gear or something? Probably not cheap, but that looks like a pretty nice lathe.
 
I'd step up to 5.

Has anyone priced a new gear via Boston Gear or something? Probably not cheap, but that looks like a pretty nice lathe.

I bought a bunch of Boston gears from your Amazon.com of all places! 1/4 the price of buying them direct. They don't have all of them but check the one you need there first.
 
E7B265C7-E01B-4F73-AD5E-B22FC74EE757.jpgI repaired the bull gear and got it meshing nicely. Will see how it holds up. Went with steel in the center tooth for a little more strength and brass for the others. If it doesn’t I’ll have to go another route. Would probably be worth having a new one made in the long run but this should get me started. Hoping to order a VFD this week, probably a teco 3hp.
 








 
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