my lathe needs more torque.

[MCRIPPPer]

i have the 7x12 mini lathe equipped with the brushless dc motor and belt drive. it would be nice to get some more torque, and less rpm. i have never turned up the dial much past half way. has anyone replaced the pulleys with a different ratio? right now it looks to be something around 2:1 or 3:1 but i have not counted the teeth on them. ideally my mini lathe would have a synchronous ac motor and belt cvt!

 

[pete]

To get what your looking for you won't achieve it by using any electronic speed control invented yet. One way or another you will lose varying amounts of torque. Probably removing the motor from from it's current position, building another mount for it to be bench mounted, then adding extra pulleys and shafts to give you the speed reduction and torque increase will be the most simple way to go. It could also be done using gearing to maybe come up with a more compact system.

A much larger hp motor would allow dialing the speed down while still giving you the torque your looking for using electronic speed control. But again it probably won't fit in the stock location, so your again going to need to bench mount it. It's really too bad that all lathes don't come with a back gear system.

But to be honest you probably still need at least most of the higher rpm range the lathe came with for a lot of logical reasons. I'd be a bit careful about losing that high rpm capability.

Pete

 

[Wizard69]

I have to agree with Pete, you need a speed reduction via another set of pulleys.


You might be able to get some improvement by changing the ratio between the stock motor and spindle. However most belt technologies can't handle much more than a 10:1 ratio and 10:1 in most cases is really pushing it. Plus you need room for the pulleys as realistically you can only go so small with the pinion. You can look into new pulleys to give you a better ratio but I don't think you have much room to play here. Also as Pete indicated you don't want to loose your top end speed.

In any event what makes you think torque is the problem? Further is the drive amplifier set up correctly? I'm not familiar with this specific drive but most drive have setup options which if not set correctly can lead to the motor being soft and unresponsive to load.

 

[MCRIPPPer]

i have the classic little 7x 12 lathe that you get from littlemachine shop, micro mark, harbor freight, and other places. it has a brushless motor that is controlled by a motor driver circuit board that converts ac line power to dc and then drives the motor through mosfets. the motor simply drives a belt directly to the spindle. the motor speed is controlled by a potentiometer knob. the problem is the motor does not have that great of HP when it is spinning 100rpm. my thinking is if a could "gear" it down somehow, the motor could spin a little faster and be in a better power range. i want to make something that lets me select a lower ratio or go back to the higher ratio easily.

 

[Tin Falcon]

i have the classic little 7x 12 lathe that you get from littlemachine shop, micro mark, harbor freight,

Sounds like you are mixing apples and oranges I am confused . Littlemachineshop.com and Micromark sell the hi Torque version of this machine that has a 500 watt motor HF,Grizzly and others sell a similar version with a 350 watt motor. only 70% of the power.
Tin

 

[MCRIPPPer]

i have the 500w one.

 

[Tin Falcon]

You could always add a treadmill motor more POWER.
Tin

 

[chucketn]

MCRIPPPer ,
Your the first I've heard say the 500W brushless motor was not powerfull enough. I have the older 350W brushed motor on my MicroMark 7x14 and its powerful enough for me. But it does have the 2 speed head stock.
Now my X2 mill is another story. I did put a 1.5 hp treadmill motor on it and love it. I replaced the origional motor and controller because the controller blew a mosfet. Ordered replacements, but haven't installed them yet. I like the treadmill motor and its MC-60 controller and will probably leave it there.

Chuck

 

[MCRIPPPer]

thats funny my 500w sx2 mill is plenty powerful for me but my lathe seems like it could use a bit more torque for things larger than about 1.5 inches dia. for smaller things it has plenty of power. lately i have made some parts about 2 inches diameter and it is not hard to accidentally stall it.
now that i fixed the gibs and adjusted everything the rigidity is greater than the horsepower.

 

[Wizard69]

thats funny my 500w sx2 mill is plenty powerful for me but my lathe seems like it could use a bit more torque for things larger than about 1.5 inches dia. for smaller things it has plenty of power. lately i have made some parts about 2 inches diameter and it is not hard to accidentally stall it.
now that i fixed the gibs and adjusted everything the rigidity is greater than the horsepower.

Please take the following with a grain of salt because I don't have one of these lathes. However I have plenty of experience with small motors.
1.
The first thing I'd do is look for a manual for the motor controller. Ideally the manual will have tuning info for the motor. If so hopefully it can point you to tuning for low end performance.

2.
Tuning may or may not be a problem but it is one of the dirt things I verify at work if motor or servo problems crop up. Frankly I'm not even sure this Amp can be tuned. The problem is the Amp can only compensate so much for the lack of low end torque. You may want to talk to the company you bought the lathe from. The thing that I'm wondering about is why the slow RPM anyways. 100 RPM just seems slow.

3.
Given that the 100 RPM is required for what you are doing and the motor and driver are right your only alternative is to gear down with the current motor. As you note get your motor speed back up to the point where you have real power. Power is what you are looking for.

4.
The 500 watt motor really isn't that bad, I'd be resistant to putting a new motor in place. This is a small lathe and as such it can only really handle so much in the way of a drive motor. You may get your torque with a treadmill motor at low speed but what happens if you are running full speed and something crashes or jams. Yeah I know that isn't suppose to happen. If you do go the DC tread motor route realize that the torque curve on a DC brush motor is basically a ramp from zero torque to full torque. The maximum power comes at the half way point. There is a web site out there someplace that explains this, the point is you still need good RPM to provide real power to the cut.

5.
Carbide can allow for higher surface rates which would mean more RPM and thus power. In other words dial up the RPM and learn to deal with hot chips.

 

[MCRIPPPer]

i dont think there is anything wrong with my motor, but it is just the nature of the beast to not have much power when spinning slow. i figure most nice lathes have a gearbox with multiple speeds for a reason. i mainly want a lower gear for the occasional large diameter cut where i have to slow it down. i often slow it down alot for larger drills as well as threading.

the brushless motor version has a governor type speed control that gives it more juice when it slows down, but when i first start the cut, it almost completely stops due to the load before it realizes it and speeds up again, which is annoying. i bet it would do this alot less if the motor could spin faster. the speed regulation is definitely much better at higher speeds. another problem is that at slow speeds, you can hear each phase change as the motor goes around and it is not as smooth as higher speed.

i do not want to swap the motor because i think it has enough mousepower to do everyhting the lathe is capable of if it is in its power band.

 

[DJP]

Sharper tools and smaller cuts help my small lathe get heavy jobs done. My lathe and mill are not meant for production operation so I just take more time.

I agree with the suggestion to consider carbide tooling and higher speeds. It does work.

But if all that fails get yourself a bigger lathe. You have the justification for a business case that I would support. Bigger is better.

 

[MCRIPPPer]

maybe my problem is i use a 20 horse one meter between center lathe at work lol.




i use carbide on my lathe and i can take a pretty good size cut with it without chatter. it actually cuts better when i take a larger cut, but the motor doesnt like it!


maybe i will build a jack shaft thing kind of like what people did to the old 350w ones that has two sets of pulleys. one with a low gear and one with the factory ratio.

might be worth it just to replace the oddball crappy stock belt. it is non standard as far as i can tell and is not of the best quality as far as mesh with the pulleys.

 

[tombstone]

perhaps you could increase the inertia of the spindle at low rpm by attaching a heavy weight to the end of the spindle like a heavy piece of steel turned up so it is balanced and start the cut when spindle gets to correct rpm
Ray

 

[Wizard69]

i dont think there is anything wrong with my motor, but it is just the nature of the beast to not have much power when spinning slow.

This is very true.

i figure most nice lathes have a gearbox with multiple speeds for a reason. i mainly want a lower gear for the occasional large diameter cut where i have to slow it down. i often slow it down alot for larger drills as well as threading.

You might want to search the net to see if other owners have come up with a solution. The classic solution would be a jack shaft to give you a speed reduction

the brushless motor version has a governor type speed control that gives it more juice when it slows down, but when i first start the cut, it almost completely stops due to the load before it realizes it and speeds up again, which is annoying.

This is why I think you need to look into drive tuning. Either that or you have a troublesome drive. At some point you drives ability to control RPM at low speed will evaporate. However the fact that the drive tries to compensate but rather slowly indicates to me a tuning issue. At best we could get the input of owners with the same drive mechanism installed in their lathes to see if they experience similar behavior.

i bet it would do this alot less if the motor could spin faster. the speed regulation is definitely much better at higher speeds. another problem is that at slow speeds, you can hear each phase change as the motor goes around and it is not as smooth as higher speed.

The fact that you think you hear the phase switching happening is very interesting. Way back in the day I worked on old PTI drives that output square waves to the motors and I don't recall actually hearing the phase switch at low speed. So I'm not sure if this is something due to the motor / drive combination or something faulty.

i do not want to swap the motor because i think it has enough mousepower to do everyhting the lathe is capable of if it is in its power band.

Your first thing to consider is if a new set of pulleys can be installed to allow for a more favorable ratio. The problem here is that I don't think you have a lot of room to work with. So the second option would be a jack shaft. A jack shaft is a better choice because it gives you more options.

As for a motor swap, the deal there is that you go to a bigger motor to get far better torque at the low speed end. That can mean excessive power at the high end for the machines mechanics which is something you should always consider. Even then sometimes it makes sense to have at least two ratios between the spindle and the motor.
---------
In any event before doing anything other than maybe looking at tuning, I'd seriously consider finding different ways to machine your metals. As suggested Carbide can allow for higher spindle speeds and thus surface speeds as can some high speed steels.

Of course sometimes you don't want high speeds due to wanting to be able to keep an eye on what is happening or are running unbalanced parts. This puts you back to a jack shaft solution. Actually I mention jack shaft but you could look for a bargain on a gear box or even make a gearbox. Belts are far cheaper though

 

[DJP]

The motor may not be getting a pure DC voltage from the speed controller. On a mini-mill that had a bad speed controller I wired directly into the motor to apply DC from an external source. Try running the motor on a 12 V car battery or maybe two in series to get the speed that you want.

A DC motor should have lots of torque even at low rpm.

One more idea for your consideration.

 

[mu38&Bg#]

another problem is that at slow speeds, you can hear each phase change as the motor goes around and it is not as smooth as higher speed.

I don't have any experience with this machine. If the motor is not running smoothly you may want to check if the three phase wires (assuming 3 phase brushless DC motor) are connected and that the coils are not open. It would run on two phases if the control uses hall or other rotor position sensors.

Greg

 

[MCRIPPPer]

my motor is brushless so i cant run it right off a car battery, and besides, the motor needs something like 90v to work right. the mini mill you had used a universal motor, which will run off ac, and dc.

i know my motor is wired right. it does not run rough as if it had a dead spot like if a phase was missing, but you can clearly hear when it switches onto the next magnet and the motor torque changes. im talking around 100 rpm here, so the motor is spinning very slow! i can not hear the actual electrons tho lol.

this thread is turning into a diagnosis of non existent problems lol. the problem is my motor is low horsepower and even lower horsepower when it spinning slow, so i wanted to know if others have done anything about it via gearbox or belt.

i want to be able to turn things slow for my 4 jaw and other things that are large diameter, or off center.

 

[velocette]

Hi MCRIPPer
Wizard69 has covered the alternatives very well for modifications or additions.
I can understand that that you are somewhat frustrated with moving from a large and powerful lathe to a small benchtop lathe having done the same excercise.
"Poly Vee Belts" and pulleys are easy to fabricate and fit and fairly compact.
Gear it down if you need more torque at low speed use stepped "Poly Vee Pulleys" to give a selection of ratios.

Eric

 

[Wizard69]

my motor is brushless so i cant run it right off a car battery, and besides, the motor needs something like 90v to work right. the mini mill you had used a universal motor, which will run off ac, and dc.

i know my motor is wired right. it does not run rough as if it had a dead spot like if a phase was missing, but you can clearly hear when it switches onto the next magnet and the motor torque changes. im talking around 100 rpm here, so the motor is spinning very slow! i can not hear the actual electrons tho lol.

this thread is turning into a diagnosis of non existent problems lol. the problem is my motor is low horsepower and even lower horsepower when it spinning slow, so i wanted to know if others have done anything about it via gearbox or belt.

There are so many ways to do this that ultimately you have to pick one and run with it.

i want to be able to turn things slow for my 4 jaw and other things that are large diameter, or off center.

The only way to get a torque increase with what you have, is to gear the motor down. That of course means buying mechanical parts. A real alternative is to buy a larger motor that provides more torque at the low end and a more limited amount of mechanical parts. Which avenue is best for you is unknown.