SB9A restore and retrofit 3 phase/VFD
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    Default SB9A restore and retrofit 3 phase/VFD

    Hello, All.

    This is my first post with a call for help on belts and pulleys.

    I struggled for years making a Chinese mini-lathe meet my needs and finally 1 month ago acquired a 1956 9A. The significant event here is a conversion to a 3 phase Baldor and KBMA VFD. The 45 lb. countershaft is gone. I have made a motor mount from 3/16" steel plate that is sandwiched between the left base and the ways. Photos to follow.

    Because sellers want a fortune for the V-belt cone spindle pulleys, I am forced to try and make a flat serpentine belt work. And BTW, I am targeting a 1:2 or 1:1.5 reduction ratio on the 1750 rpm motor, using the largest rightmost flat spindle pulley.

    So, if I had a second lathe, I would make a 2" in diameter crowned pulley, 5/8" bore with 3/16" keyway, but I don't. Every thing is available on the internet but this.

    Any advise on this pressing matter?

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    Sound like you are wanting to eliminate the countershaft entirely??

    BTDT - it basically sucked...much better plan it to use the existing 3 step cone,and design a new compact drive to give you 6 speed "ranges"(including back gear)t

    If understand your proposal correctly you will have squat for low end power.

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    Quote Originally Posted by JimBuchanan View Post

    Any advise on this pressing matter?
    Keep a watchful eye on your local Craigslist for a really scruffy "cousin" of your lathe that does have the countershaft, buy the second lathe at a modest price because of its shoddy or in other ways incomplete condition, keep/ use the countershaft, and then sell the rest for parts so that you can be one of those folks getting silly amounts for SB9 parts to then help pay for what you put into buying the scruffy lathe?

    Or, make yourself a hardwood pulley, using a holesaw to rough it out and then giving it a crown with a file whille spinning it in a drillpress or even an electric drill held in a vise, then use the wood pulley, even though it may be crude, to get your lathe going, and then once the lathe is going, make yourself some better aluminum or hardwood pulleys in a cone set, so that you will really have the option of speeds.

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    I hope you didn't throw away the "45 lb countershaft!" That's a major mistake and trying to directly drive the spindle on that lathe is going to result in a whole lot of trouble with speed of the spindle and torque required from the motor. This will not end well. Put the countershaft back in place and then drive that with the 3 phase motor and VFD.

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    "Any advise on this pressing matter?" It's unanimous. "Don't do it!"

    Maybe that's the reason I can't find more info on VFD motor conversions replacing the Countershaft. Nobody knows how. In a world of 1 phase motors 100 years ago, the countershaft was the only way to provide motor reduction speeds from 1200 rpm down. Yes, I will admit a mechanical advantage in the torque at slower speeds, but that can be compensated for with the 1:5 reduction of the back gears and running a 1 hp at no less than 30hz. (1/2 speed)

    I'm glad to continue this experiment for your reading pleasure. I have a robust motor mount that is 1/3 of the volume the c/s takes up. I'm targeting a motor pulley of 2.25" diameter. Maybe I will end up with a V-belt cone pulley for the extra grip of the V-belt.

    Full speed ahead, as they say.

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    It *can* be done...but would take a very expensive motor(like 8 pole TENV) IMO....but if you can pull it off more power to you!

    Pics and specifics?

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    It's not that people don't know how to do it, it's that it doesn't make sense. A 9" South Bend is fundamentally a low speed machine with many, many turning operations run in the 100 - 200 RPM range. You're going to have a difficult time dealing with that speed range with a direct drive between motor and spindle. Yeah, theoretically you could come up with a 12 pole 3 phase motor that might get you into the ball park but why do it when the simple solution is right there. And if you think you're going to be able to use the back gears as a speed reduction device to get down to the normal turning speed, you may very well get to experience lubrication failure in between the spindle and cone pulley. Further, threading is done at speeds around 50 RPM and I don't see how you're going to get there.

    3 phase motors (4 or 6 pole) can "clock" down to 100 rpm but they are not stable at those speeds and maintaining speed stability under load will be difficult.

    South Bend's are "old fashioned" cast iron machinery and they work best if they're left to just be that.

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    As the analysts say on all the corporate conference calls, can you provide a little more color on the following comments:

    BTW, I thought I was pretty clear on eliminating the countershaft.

    "BTDT - it basically sucked" but what did you do and why did it suck?

    "...much better plan it to use the existing 3 step cone,and design a new compact drive to give you 6 speed ranges(including back gear)" Good idea.

    "squat for low end power" I will address this at a later time.

    Be assured I have a saved search on my local Craigslist for a really scruffy SB9 "cousin"

    I even found the makeshift hardwood pulley a good idea, but I would need to make a keyway on the steel pulley. I currently don't have a arbor press or 3/16" broach.

    "That's a major mistake and trying to directly drive the spindle on that lathe is going to result in a whole lot of trouble with speed of the spindle and torque required from the motor. This will not end well." But, please elaborate.

    "Put the countershaft back in place and then drive that with the 3 phase motor and VFD." If you use a c/shaft, you don't need a VFD. If you use a VFD, you don't need a c/shaft. Sorry, that's the way I see it, today.

    "...but would take a very expensive motor(like 8 pole TENV) IMO" I agree a 8 pole motor would be expensive, and I was told to stay away from a TENV motor due to no ventilation.

    "It's not that people don't know how to do it, it's that it doesn't make sense. ...100 - 200 RPM range... You're going to have a difficult time dealing with that speed range with a direct drive between motor and spindle. ...And if you think you're going to be able to use the back gears as a speed reduction device to get down to the normal turning speed, you may very well get to experience lubrication failure in between the spindle and cone pulley.

    South Bend's are "old fashioned" cast iron machinery and they work best if they're left to just be that."

    Very good points. Thank you, for them all. I've been sensitive to the slower speeds from the beginning. We will see if these issues can be overcome.

    If budget were no object, I'd have the most affordable lathe with power crossfeed years ago, and that would be the Precision Matthews 10x22V. In California, I'd have to pay $200 sales tax and $200 shipping for about $3,000. I am on a budget and have less than $1,000 in this power crossfeed, american made lathe, save for the motor drive pulley and spindle pulley cone dilemma.

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    pay for shipping both ways and you can borrow my 3/16" keyway broach and 5/8" bore broach plug :P or send it to me and I'll broach it for you and get it back to ya! Or I can just make you the pulley you're asking about.

    Your idea, your rules. Why tell you something is a bad idea when you can figure out whether it is or isn't for yourself. I'm just offering the means to proceed forward! For science!

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    Who said to stay away from TENV???? A high quality invrter rated unit is made for extremes, you ain't gonna hurt one running a lathe....TEFC is fine also but you have to be mindful of running one below nameplate speed since the fan will also slow...a rule of thumb I used for TEFC was 45-90 hz for general purpose motors.

    I cant type much at a time but for some good advice, you should post in the trAnsformers and vfd section....like I was saying it can be done and those guys can help you if you are serious.

    BTW- poly v (serp) is better than V belt for power transmission IME.
    Last edited by iwananew10K; 05-05-2019 at 01:35 AM.

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    no follow up??

    post up what you have, lets have a think tank.

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    I'm getting a lot of information to digest.

    First, is a photo of the new motor infrastructure as suggested in the first post. There is a right foot version to re-gain straightness. The added benefit is a closing of the foot cavity to chips filling up over the years. There is a round leg with leveling screw to help support the weight of the motor. While it is yet to be seen if the weight of the motor provides enough force to keep the flat belt/V-belt taunt, there will be a turnbuckle type arm to stabilize the system. So, the vertical, horizontal and motor torsion forces should be controlled.

    There is a square tube mount with a hole ready for a pedestal for the VFD. Note that this bench is in the house laundry room where I used a Chinese mini-lathe for years for my camera lens conversion business, and space is a minor reason to eliminate the SB c/shaft. It won't fit. No comment on the quality of welds.

    Point taken on the TENV inverter rated motors. I may have got miss-information or miss-understood. That could be a future upgrade to get/regain torque at low motor rpm's. The 3 phase general purpose Baldor was very cheap.

    The spreadsheet indicates using the largest headset pulley only and 2" motor pulley for highest gear reduction. The VFD cycles range between 1/2x and 1 1/2x 60 cycles is indicated. There is a "gap" between 250 rpm and 400 rpm where there could be less torque.

    Workable until an inverter rated motor is installed
    sb-speeds.jpg
    _dsc2259.jpg

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    I'm getting a lot of information to digest.

    First, is a photo of the new motor infrastructure as suggested in the first post. There is a right foot version to re-gain straightness. The added benefit is a closing of the foot cavity to chips filling up over the years. There is a round leg with leveling screw to help support the weight of the motor. While it is yet to be seen if the weight of the motor provides enough force to keep the flat belt/V-belt taunt, there will be a turnbuckle type arm to stabilize the system. So, the vertical, horizontal and motor torsion forces should be controlled.

    There is a square tube mount with a hole ready for a pedestal for the VFD. Note that this bench is in the house laundry room where I used a Chinese mini-lathe for years for my camera lens conversion business, and space is a minor reason to eliminate the SB c/shaft. It won't fit. No comment on the quality of welds.

    Point taken on the TENV inverter rated motors. I may have got miss-information or miss-understood. That could be a future upgrade to get/regain torque at low motor rpm's. The 3 phase general purpose Baldor was very cheap.

    The spreadsheet indicates using the largest headset pulley only and 2" motor pulley for highest gear reduction. The VFD cycles range between 1/2x and 1 1/2x 60 cycles is indicated. There is a "gap" between 250 rpm and 400 rpm where there could be less torque.

    Workable until an inverter rated motor is installed

    capture55.jpg
    _dsc2259.jpg

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    What model is your Baldor?


    as much as I hate to say your mount is way too flimsy....and the sandwiching is a bad idea...a very bad idea....make the drive a stand alone unit isolated from the lathe and you will be a zillion times better off...really I agree with Doberman here, until you do some research you should just use the original...with a poly v belt you can use a j440 belt which is much shorter than original so it won't take up as much space.

    FWIW- this is the type of motor that gets you near your goal...but not quite unless you sacrifice something..even with something like this your best option would be to run between 30-120hz with things sized so 30hz gets you near 50rpm in back gear.

    You do not want to exceed much over 200 rpm in back gear.

    Baldor 3/4 HP Motor, 3Ph, 208-230/460V, 1150 RPM, 56H Frame, 60Hz, 35K850Q175G1 | eBay

    By all means, experiment but I suspect you will come to the same conclusion we all have.

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    Alright now for my suggestion that no one asked for! My goal is to stay inline with OP's views as tightly as possible. Basically, the only modification this would require is to drill and tap 6 holes in the step pulley, these are blind and will have no impact on the pulley or it's oil passages. Fabricate a v-belt "sheath" that would fit over the middle step of the step pulley, not a difficult part to fabricate, ~2-3 hours maybe. And yeah, pretty much just look at the pics, it's pretty obvious about how I would go about it. Also to remedy the low RPM torque issues (and someone correct me if my understanding of Vector drives is incorrect) would be to just simply use a Vector drive instead of a VFD. The information available leads me to believe that they're specifically designed to be a VFD with High torque at zero speed and also a lot more stable in its speed ranges. Again someone please correct me if that's wrong.

    Attachment 255936Attachment 255937Attachment 255939Attachment 255938Attachment 255940

    this was all done just for fun mostly. This is basically the simplest way I can imagine up to do what OP wants, it's not perfect and requires altercations to the original design, but I struggle to see it working "efficiently" any other way.

    - Inventor Professional 2018, DM if you want the files. All dimensions used are accurate to a South Bend 9A. (yes I took the time to model a true involute gear)
    - higher quality images here: HD pulley porn
    Last edited by naru; 05-06-2019 at 04:07 AM. Reason: fixed images

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    Quote Originally Posted by naru View Post
    Also to remedy the low RPM torque issues (and someone correct me if my understanding of Vector drives is incorrect) would be to just simply use a Vector drive instead of a VFD.
    But you need the back gear to get the 40 foot pounds of torque needed to drill a 1" diameter hole, at 40 rpm, the slowest back gear speed you can get from the 1/3rd hp motor those lathe shipped with. and that's all the torque the 5/8" wide cast iron 16 dp gear can deliver safely before reaching fatigue limits, so you can stall the lathe without breaking the teeth in the lowest speed with the stock motor.

    your double V belt can deliver far more torque than 5/8" wide cast iron gear teeth can, if you tension the belts properly. this is something you would need to adjust on the fly unless you're far less worried than i am about wearing out the bearings.


    40 foot pounds at 1500 rpm is 11 hp. so, you're going to need a pretty creative solution if you want to be able to get that wide range of torque and rpm from a cheap motor and drive if you want to do away with any mechanical methods of exchanging rpm for torque. so certainly you can make a v belt conversion for a direct drive. you can replace the cone pulley with something you carved from a block of wood if you really needed to. but it doesn't replace the fact that you need a cone pulley to get a wide range of rpm.



    for what its worth i chucked up a 56 pound object on my lathe, the center of mass was about 2 feet from the headstock, so it was applying on the order of 150 pounds force straight down at the headstock and about 100 pounds upwards on the rear of the spindle bearing. i was surprised to see that only perhaps 40 rpm was needed to maintain a rather low friction oil film. so, you may be ok with tensioning the spindle belt enough to drill say a 1" hole and doing away with the back gear completely, but i don't know how much extra heat you're going to get from the spindle at 1500 rpm with that much extra load on it.

    anyhow regarding vector drives about the most torque you can get out of an induction motor is about twice or three times its nameplate hp/torque. that torque comes at a steep expense, current is about proportional to torque and heat in the windings follows amps squared. so, yes practically for a few momemts a good vector or vfd drive can deliver a substantial amount of torque if the drive can dump the amps into the motor and if its programmed correctly and if the motor doesn't overheat and burn up.

    but you can't get 40 foot pounds of torque from a 1 hp motor and 1 hp vfd, directly driving a spindle intended to hit 1200 rpm. you have to either change the belts or the gear ratio.

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    I've done the C/shaft removal trick with a milling machine (P&W number three horizontal) to very good effect. Comments:

    Upsize the hp on the motor as they are constant torque below nameplate speed, you lose hp as you go down in speed.
    Get the largest motor you can run with a 120 volt single phase VFD which I suspect is one hp.

    Get an 1800 rpm motor.

    Get a VFD that is sensorless vector capable. Be sure to autotune the motor when in place in the machine. Hitachi makes
    a nice drive but the Teco in my milling machine works fine.

    The countershaft and associated belt does rob hp so you gain that way. Probably the main source of slip will be
    the flat belt so let that be an automotive serpentine belt rather than leather.

    Sensorless vector motor will deliver power down around 10 rpm, no problem. If you are planning a lot of sub 60 cycle work
    consider an aux. cooling fan on the motor. For hobby use probably not needed.

    For this machine do NOT try to stop the spindle by hand, even at 10 Hz:


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    You replaced a 45# casting/shaft arrangement with some sheet metal and a door hinge (bolted between the bed and leg, no less) then expect us to believe that your superior engineering skills will lead to a new way to power a 70 year old machine?

    Okie dokie.


    A constructive edit- you'll never deliver any torque through that mount, it will twist like a belly dancer and then the belt will slip. If you're stuck with sheet metal weldments (which can be very stout) at the very minimum it needs boxed and cross braced. Disturbing the mount between the bed and leg is not a good idea, in fact I don't think I've ever seen it done. Like mentioned above, isolate the motor mount to your table and apply belt tension from the headstock to the mount, properly arranged this will mitigate your twisting.

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    Quote Originally Posted by iwananew10K View Post
    What model is your Baldor?

    FWIW- this is the type of motor that gets you near your goal...but not quite unless you sacrifice something..even with something like this your best option would be to run between 30-120hz with things sized so 30hz gets you near 50rpm in back gear.

    You do not want to exceed much over 200 rpm in back gear.

    Baldor 3/4 HP Motor, 3Ph, 208-230/460V, 1150 RPM, 56H Frame, 60Hz, 35K850Q175G1 | eBay

    By all means, experiment but I suspect you will come to the same conclusion we all have.
    Yes, I am experimenting. It remains to be seen if I can overcome all issues. I think I solved the motor issue by purchasing the motor you suggested. It is a 6 pole inverter rated unit that gets me in the ballpark for low cycle torque.
    Thank you, for the recommendation and your other constructive criticisms.

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    I didn't mean for you to buy it, but was meant as an example of the type of motor that was capable..that said I think if you plan out everything and make the necessary changes it can work for you.


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