Solid Bench Top Mill Build (warning large photos)

[RCaffin]

I can't even stall it by holding onto the 12mm shaft at the lowest rpm the controller can manage

That's why I LIKE brushed DC motors, especially industrial ones with huge heat capacities.

To explain: imagine an 180 V 2 A DC motor: 360 W, right? Rated at 3,000 RPM. But this motor probably has a DC resistance of 2 ohms or less.
Now, spin this motor at 3,000 RPM and the back emf will be about 180 V, and the current draw might be 0.1 A. So there it is spinning away happily on a 18 watts of power.
Now slow it down by 10%. What happens?
The back emf drops by 10%: it is now 162 V. But the supply is 180 V. So now we have 18 V across 2 ohms: the motor current goes from 0.1 A to 9 A. The torque goes up by a factor of 90x.
That's GRUNT! And what is happening in that gold motor.

My spindle is rated at 180 V and ~360 W on a 24/7 duty cycle. So the nominal current at full load is 2 A, as above.
He-he-he - when drilling the current sometimes goes up to 4 A or even 6 A. Do I care? Nope. The torque goes WAY up. I buy GOOD drillbits.
It did get to 8 A once when I was drilling titanium, but I dropped the feed rate a bit that time. Well, let's be reasonable.

Try getting that with a stepper!

cheers
Roger

[LeeWay]

Hi Feist.....would it be possible to stack a second motor on top of this one, both mounted in a tubular housing to get the same RPM but twice the torque.....maybe even three.

The length would not be too intrusive as opposed to a bigger HP motor that tends to get bigger in diam as the HP rises.
Ian.

I would not try that without some sort of clutch unless you can sync them together. It may actually work if you happen to get them paired identically. The weak one though would be dragging the stronger one down and I would think they would both wear out prematurely.

[RCaffin]

I doubt that two Brushed DC motors in series would do any harm to each other. They will load-share quite happily, provided that they are the same motors. All that will happen is that one motor will draw slightly more power than the other one. I can't see any reason why that would increase the wear on either motor though.

Now, if you try to do that with other sorts of motors, even if identical, there could (maybe) be problems. You would have to get the phasing exactly right.

Unlike transistors, motors are very tolerant things. Fortunately.

Cheers
Roger

[G59]

I doubt that two Brushed DC motors in series would do any harm to each other.

That little motor he's using is a brushless unit(BLDC). Totally different animal.

I too love the big DC motors. Max torque starts at 1 rpm. But I just hated replacing brushes every year or so.

[RCaffin]

As I said, with something like a brushless DC motor you would have to get the phasing right. Do so and I see no problems.

Dunno why you might have to replace the brushes every year. I did have to replace the brushes on my LandCruiser generator a few years ago - but that was after 20 years of serious use.

Cheers
Roger

[handlewanker]

Hi....... with a brushless DC motor the controller only sees one motor......if you couple two DC brushless motors together in line you then would need to have the poles and magnets exactly lined up to work with the output pulses from the single controller.........with two seperate controllers, one for each motor, you wouldn't have that problem, but that's really grasping at straws, but it would work perfectly if you needed more HP in a small packet.

BTW.......there's a very good reason why the high speed spindles are water cooled.......I suppose if you filled a bucket with ice blocks and then blew air from it directly into the motor in question it would put out the HP as indicated without blowing up or melting something.

With a brushed motor if you doubled the length of the commutator you'd extend the working life enormously.
Ian.

[G59]

Dunno why you might have to replace the brushes every year.

I exagerated here. I actually only replaced them once in 10 yrs and that was only because the original drive let go and I went with an SCR drive. That was a mistake and I went through a set of brushes in 6 months. I since replaced the SCR with an AMC drive and it works so much better and quieter than the original.

[Eldon_Joh]

I exagerated here. I actually only replaced them once in 10 yrs and that was only because the original drive let go and I went with an SCR drive. That was a mistake and I went through a set of brushes in 6 months. I since replaced the SCR with an AMC drive and it works so much better and quieter than the original.

interesting.

i have an scr drive on my lathe, driving a 3/4th hp 90vdc baldor motor. it was very loud, the 120hz hum was loud enough you could feel the table vibrate.
so i added an inductor. its 1.4 ohms of 19 or 20 gauge wire, the choke weighs 2.8 kilograms, it was manufactured to be the inductor/reactor for a 500/400/300 watt, 220v arc lamp.

anyhow, after adding the inductor the motor is dead silent now, almost can't hear it running.

[G59]

Yes with the SCR drive, I could also feel the vibrations through the large castings of the mill and it had a deafeningly loud hum throughout the operating range. The spindle would spool up much slower then the original drive and stopping was slower.
Lesson learned I guess. I didn't know about the Inductor/Filter back then. But, the AMC drive pulled through with minimal changes plus I gained back the braking resistor and had full control of it under Mach3.

Today, I couldn't fathom the idea of SCR anything. PWM is the way to go for DC motors unless you want it directly connected to power.

[G59]

double post

[RCaffin]

I agree about the SCR drives. Very noisy, and rough. I replaced mine with a switched MOSFET one.

But the comment about putting a large inductor in series is a VERY good one. Thank you.
(It also works to limit radiated noise from a MOSFET drive - pretty crucial in fact.)

Cheers
Roger

[DICKEYBIRD]

Any updates Feisty? I just bumped into this thread yesterday & was transfixed by the build. Fantastic job! Did you finish it or did you have to put it on hold due to work & life in general? Love to see a video of it murdering hunks of steel!

I'm building a fixed-gantry router/mill to cut aluminum & am gathering stuff & working on the design right now. I'm intrigued by your moving-rail Z-axis design. I guess I missed the discussion about this feature but are you happy with how it turned out? I'm considering a blatant plagiarization of it if I may.