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2_many_hobbies
02-09-2009, 10:39 PM
I have a Taig Micro Lathe II that I originally bought with the Marathon Motor and it ran great but as in any one sitting I would turn 2" to 1/4" 6061-T6 and then mill 2"x0.5" T6 to mild steel I was getting tired of constantly changing belt positions for every different bit and operation.

My friend bought a bunch of golf cart motors for his go-cart tinkering and sent me one for $50. It seems like a hefty motor but as their is only a sticker giving the voltage and current ratings I have no idea If it is any good. I will explain my setup and maths and you guys can advise me.

Ok I made a shaft sleeve so I can use the stock Taig pulley set and have the dc motor temporary attached to the lathe with the belt the same tightness as with the AC motor (tight but still springy) and I built a PWM controller and have the motor fed from an old PC power supply that delivers 12V @ 15A and has current protection so when I stall the motor I need to wait for the psu caps to drain before I can start it again.

My comparison math:
Marathon Motor: 120v 1/4hp 1725rpm
746watts per HP so its a 185Watt motor.
DC Motor: 12v 15a 5000rpm
12v times 15a its a max 180Watt Motor.
Figured its pretty close so it should preform just as well.

Well I have discovered that at anything below 1500rpm i cant even use a 3/32 cutoff tool to part my work as the motor stalls with even the lightest pressure and I can stop the workpiece from rotating by hand. So im currently using the pullys to run the motor as fast as possible and gear it down which makes the whole DC conversion quite pointless....

So my question is what are the ratings of DC the motors do you guys run? and what kind of controllers do you use? A imagine if I had a 15a dual h-bridge I could feed higher current at lower voltages and create more torque but a 15A h-bridge controller is not cheep by any means.

Also my question is since I use the lathe to mill as well I would like to setup the best pulley ratio for my setup and am not sure what speed range I should be looking for. At 5000rpm with a 1/4in 4flute endmill im scared the world is going to end and turning anything in my 4jaw chuck over 2000rpm scares the **** outta me too. What range should I be looking for? (I rarely ever turn stock less than 0.25in the last thing I cut smaller was a screw shorter)

Thanks!

2_many_hobbies
02-10-2009, 03:23 AM
Oh I have another question. Does anyone know where I can buy polyurethane v-belts like the gates ones that come stock with the taig? I need a few 7in ones for some nema17 steppers but have no idea where to get them.

Jeff-Birt
02-10-2009, 09:28 AM
A 1/4 HP motor = ~186 Watts. The big problem is the RPM range difference between the Taig motor and you DC motor. Your DC motor is spinning 3 times as fast so you need to gear it down 3:1 to get into the same torque/rpm range as the original. The other unknown issue is your PWM controller and it's efficiency.

As for the belts, someone on the Taig Owners Club site was asking about the Gates belts. I was able to find them on Gates website, and as I recall they had a list of distributors too. I don't recall the URL but pop over to the TOC and look for the thread I mentioned.

2_many_hobbies
02-10-2009, 09:09 PM
Hey, thanks for the reply. My dc motor on the two highest ratios of the taig pulleys gives me 2300/1300rpm but I still have no power, the parting tool on a 1" piece of T6 at 1300rpm will always stall the motor. Maybe something is up with my power supply, im gonna hafta try to dig up an old tranny to tap.

My PWM controller is a 55v/30a Mosfet with a snubber diode across the motor so its about as simple and efficient as im gonna get.

I found your link on TOC to the gates catalog and my it is not the nicest one to paruse when your looking for specific lengths and it just gives a wack of part numbers. I remember their is a somewhat similar website that sells sprockets and such that has categories on the left and it was kinda yellow or something. They had a good selection but I cant find the bookmark.

Im specifically looking for a site that carries 2-5mm v-belts in a 4-12in range, as well as toothed timing belts for sprockets for the main drive of a few projects. Currently I make my own sprockets but if I can get 1-2in ones for under $15 its cheaper than a few hours of my time.

Jeff-Birt
02-11-2009, 10:52 AM
For small timing pulleys and belts check out, Small Parts: http://www.smallparts.com/b/16310181?searchRank=salesrank. They have a wide range of such products. Another valuable resource is Stock Drive Components: http://www.sdp-si.com/, they have a lot of neat stuff too.


Hey, thanks for the reply. My dc motor on the two highest ratios of the taig pulleys gives me 2300/1300rpm but I still have no power, the parting tool on a 1" piece of T6 at 1300rpm will always stall the motor.

But what speed is the motor turning? Perhaps the motor is not up to snuff?

2_many_hobbies
02-11-2009, 08:48 PM
Stock Drive Components was the website I mentioned, I really like their layout with the tabs on the left and an actual picture of the category your in. Thanks for the help.

About the motor when I have it geared the spindle is running at 1300rpm and the motor is at ~5000rpm. I figured at full voltage when its just turning the piece its at 3A and figured it would just draw more current to keep up the speed but instead it just slows down until the motor stalls and the motor current maxes around 9A and then over currents when it stalls. I just cant imagine how these could move a cart with at least 150lbs in it without either a much better power source than mine or an expensive controller that manages its current as well as voltage.

escott76
02-11-2009, 09:33 PM
Stock Drive Components was the website I mentioned, I really like their layout with the tabs on the left and an actual picture of the category your in. Thanks for the help.

About the motor when I have it geared the spindle is running at 1300rpm and the motor is at ~5000rpm. I figured at full voltage when its just turning the piece its at 3A and figured it would just draw more current to keep up the speed but instead it just slows down until the motor stalls and the motor current maxes around 9A and then over currents when it stalls. I just cant imagine how these could move a cart with at least 150lbs in it without either a much better power source than mine or an expensive controller that manages its current as well as voltage.
Without reasonable specs on the motor it's going to be difficult to determine much. Couple things that will effect the operation of this motor in a golf cart. The biggest one would be it's gearing. Gearing is used to match the speed of the load to the maximum efficiency of the motor. The other is the power source, which is usually an SLA, which can source a large amount of current. It may well be run at higher voltage in the cart.
The motor doesn't draw current to keep up RPM, it draws current to make torque. The the motor is acting like both generator and a motor at the same time. At no load generated current comes close to supplied current, minus the motors frictional etc losses so you have your low, no load run current. As load is applied the RPM drops, which causes the generator side to generate less "back" current, and the net current into the motor rises. DC motors will have the most torque at just before stall, and you'll want to be operating away from that. Halfway between no-load and stall isn't bad. If you had a rated load RPM for the motor you could use that to figure out where you wanted to be and gear the motor accordingly.
You don't mention details about the controller you are using. It's switching speed and resistance specs will effect the entire system. Certain ones are better than others.

Jeff-Birt
02-12-2009, 05:34 PM
You don't mention details about the controller you are using. It's switching speed and resistance specs will effect the entire system. Certain ones are better than others.

That is a very valid point. Try and measure the voltage drop across your controller, and the drop across the motor and then measure your power supply both at load and freewheeling. That will give you an idea of how much power your loosing in the controller and how your loading down the power supply.

2_many_hobbies
02-13-2009, 08:38 PM
The motor is bulk surplus and the only bit of sticker on it says 12v, 15A, 5000rpm. no load current is 3.15A, no load voltage is 11.98V (directly attached to psu), motor resistance is 1.8ohms with a multimeter, 3.8ohms calculated, source voltage is 12.43V (old pc psu, 12v/17a rail), voltage across the motor with the controller attached is 11.86v

Controller is an IRFP260N 200v, 46A N-Channel Mosfet controlled with an LM324 powered off a separate rail (only draws 3ma anyways)

Its about as efficient as one could hope for in a controller but the motor has the same pathetic torque characteristics without the controller.

Al_The_Man
02-13-2009, 09:12 PM
That appears to be an extremely small motor for a Golf cart unless geared down appreciably,
If they are permanent magnet motor and used surplus, it is possible they could have been repeatedly driven into high overcurrent and demagnetized the PM field.
Al.

2_many_hobbies
02-13-2009, 11:10 PM
They where new surplus. To help me compare what are the ratings of the dc motors you guys use?

Al_The_Man
02-14-2009, 12:39 PM
With a DC motor controller for a spindle, you need some way of monitoring the reduction in speed due to load, this is done in non-feedback types of controller by monitoring current, and then increase the output voltage accordingly.
The best way is by means of Tachometer or other feed back method.
Any load on a DC motor the rpm will drop and if not remedied, rpm will drop with load.
Al.

2_many_hobbies
02-14-2009, 01:45 PM
I was figuring it would need a much more complicated controller. I am looking into making my own discreet mosfet h-bridge with current sensing for a side project but since my lathe will soon have an encoder on the headstock (for indexing) I will easily be able to write something up in my microcontroler to handle being loaded better.

I hope.