Thread: Mars Lathe DC motor conversion.

Nice going Ewan.

Glad it all worked out.

I'm nearing completion as well.

Be interesting to see how my gearing works out, as it's set for 60% motor speed, which according to that link I posted is about where the best power/torque level is.

Good to see the controller is a success.

Cheers

Rob

Congratulations Ewan,

Good to see it working and looks very promising!


Regards
Ray

Well I got my DC conversion up and running today, and I can only say that my fears about lack of power from the 1.5 HP DC motor in comparison to my original 3/4 HP motor are completely groundless.

Yes they are real horses and not DC ponies. The little motor is awesome.

There's no sign of commutator sparking or cogging, and it works perfectly - no issues what so ever.

It's unreal when doing a heavy cut to simply turn up the speed slightly and watch the controller compensate and drive the motor harder.

You can see the toolpost flex as you turn up the heat. Not recommended procedure, but you can see what the setup is capable of.

Watching the revs as the cutter enters the job on a heavy cut at say 500 rpm, the revs drop by about 20 rpm, come back up by about 10, and stay there.

My only issue is some vibration from my drive belts, which has always been present and I have to address, but overall it's certainly an impressive modification.

I have three pulley speed options with the current setup.

0 - 2000 rpm
0 - 1650 rpm
0 - 575 rpm

Maximum motor speed is set at about 80%

When I get it all tidied up I will post some photos.

Cheers

Rob

Well done Rob.
It is a completely different feel to an ac motor isn't it. I will elaborate on some things later when I am at home and not on my phone, but what position is the ir pot set at, and how is the speed regulation at low speeds, I found it varies more at lower speeds like in the video.

Thanks for all the kind words guys, and the help. It's ok Phil, just think of it as magic, then your head should remain intact! LOL.

Originally Posted by Ueee

It is a completely different feel to an ac motor isn't it. I will elaborate on some things later when I am at home and not on my phone, but what position is the ir pot set at, and how is the speed regulation at low speeds, I found it varies more at lower speeds like in the video.

Yes, it's quite different in the way the torque comes in.

I left all the controller settings at the recommended factory levels as out of the box, except for the maximum speed which I set at about 80%.

It all seems pretty smooth with very little lag when altering speed and/or load while actually turning.

At very low motor speed there is slight cogging without load, but no speed variation as such. OK with a load on it though.

That super low ratio I have available will be great for threading and big heavy jobs.

Also using the 1650 mid ratio I timed the lathe stopping time and it was virtually the same as with the AC motor - about 1 second from 500 rpm.

It's interesting to vary the speed and watch the cutting finish change - very nice being able to just dial in the finish

I also got the red lathe tacho working - set the multiplier to 0.0167 (what figure did you use Ewan?). It looks and works good.

My $12 hand held LCD laser tacho has been an absolute god send for this job to check ratios and dial in the lathe tacho. They are fantastic/excellent units for the money.

Still a couple of small jobs to do to tidy up, but it's all usable again and definitely a success. Been a lot of work though, just making everything fit and look neat.

Worked out pretty cheap overall - less than $100 for the complete package.

Cheers

Rob

G,Day Fellas,
To each his own, but the thought of being able to turn the speed down to zero with the pot frightens the bejeezus out of me, can you imagine bumping the control knob accidentally in the middle of a chuck or tooling change ?, I set my minimum speed to 30 RPM and I know when I hit the stop button it's going to stay stopped until I hit start again.
Regards,
Martin

Originally Posted by toolman49

G,Day Fellas,
To each his own, but the thought of being able to turn the speed down to zero with the pot frightens the bejeezus out of me, can you imagine bumping the control knob accidentally in the middle of a chuck or tooling change ?, I set my minimum speed to 30 RPM and I know when I hit the stop button it's going to stay stopped until I hit start again.
Regards,
Martin

I still have a normal on/off switch as well. The pot switch is there for threading, so i can slow down coming up to a shoulder and turn the lathe off without having to take my hand off the knob.

I currently have a 3-1 and 3-2 reduction to the countershaft and 1-1 and 2-1 to the spindle. I think i really need a 2-1 and a 4 or 5-1 to the countershaft. The bottom end speed is not gutsy enough at 6-1, 8 or 10-1 would be better. Changing belts is fairly easy with both belts loosening with the over-center lever. My lathe still takes a good 5 sec to slow down from 500rpm, so i can't wait to take delivery of this braking resistor and hook it up.

How big a cut does it take to flex your toolpost Rob?

Originally Posted by toolman49

G,Day Fellas,

To each his own, but the thought of being able to turn the speed down to zero with the pot frightens the bejeezus out of me, can you imagine bumping the control knob accidentally in the middle of a chuck or tooling change ?, I set my minimum speed to 30 RPM and I know when I hit the stop button it's going to stay stopped until I hit start again.
Regards,
Martin

Hi Martin,

That's not an issue as you still stop the system in the same way with the master cutout switch. Well, I do anyway.

It is however the weirdest thing after being so used to the lathe making all it's usual noises and 50 Hz vibrations from the motor to now have an almost perfectly smooth motor and different noises.

It's hard to believe it's spinning the chuck so fast as there's no noise - very deceiving.

Also the motor responds totally differently as you apply load to it.

The AC motor under full load would let out that 50 Hz growl letting you know it wasn't happy and was about to stall, but with DC that just doesn't happen. There's no noise or vibration and it just runs slightly erratically as the extreme cutting load varies.

Also you can put a huge cut into a piece of steel and the motor doesn't give any feed back whatsoever - the old AC unit would be snarling and growling, but not with DC.

It's a bit of a learning curve.

Cheers

Rob

Originally Posted by Ueee

How big a cut does it take to flex your toolpost Rob?

They all flex Ewan.

It's a matter of how much load you put on them and the cutter you use

Today I was digging carbide in deep enough that it was rolling the leading edge of the cut upwards like a can opener and that DC motor just treated it like nothing was happening.

Awesome power from such a little motor.

I rechecked the max speed setting and it is 67% which is dead in line with recommendations.

One other thing with this setup is that having independent PC fans for cooling you can have them running when the motor is off and cool it down quickly while you do other things. The original impellor fan that was fitted was pretty weak, so the PC fans are an improvement anyway. Plus they are independent of motor RPM and thats a big plus.

Cheers

Rob

I agree Rob, it doesn't matter what size cut you are taking there will always be some flex....no matter how tiny. I was just wondering how big of a cut your machine can handle with the DC motor? I started on a plug gauge for making backplates today and i noticed some belt flap too, its really bad at certain revs, i guess it just resonates at the right speed.

I received my braking resistors today......waste of $7 that was. At 650 ohm they barely do a thing, maybe cut the 1000rpm to stop time down by a second (so maybe 9 sec instead of 10). So i took the risk and simply hooked that great big choke in as a 1 ohm resistor......like magic the lathe stops in maybe 2 secs from 1000rpm, the "resistor" is converting just on 10amps to heat. At 50rpm there is pretty well no difference, it takes a 1/4 of a turn to stop either way, but the thing with the resistor is that the higher the revs the higher the voltage so the more powerful the braking. At 1k there is a small flash of blue from the brushes, nothing like you get on a brushed AC motor that is braked though. I think i will wind a 2 or 3 ohm resister myself and go from there...although the choke is doing a good job, i'm a little worried about the relay....

Hi Ewan,


I haven't really measured the cut, but yesterday I was turning down some welded steel plate to make a support for the intermediate pulley spindle and the cuttings were comming off as long blue hot spirals maybe 1 mm thick (using carbide).

That was using the low pulley ratio(max spindle speed of 575 rpm) and the spindle speed was dropping about 100 rpm in that situation at full speed with the potentiometer. So 2 mm is quite possible.

This little lathe has always had good power and cutting ability, the only limit being eventual motor belt slip, but now with a bigger pulley on the motor it's never going to slip, so either the DC motor stalls or something breaks

You still need a low pulley ratio with this setup for really heavy, deep cutting, slow jobs (combined).

When I get time I am going to make a two step motor pulley to cut the low speed some more - the original 3/4 hp went down to 120, so the DC should be at about 240 to equal it, given the HP difference. So I will probably halve it from 575 as it is.

Is it the flat or V belt that's flapping ?

My V belts are not dead smooth, only because both have a bulge at the butt join (different brands and good quality) which is a bit annoying.

Overall everything is a lot smoother than before.

Where this system shines is when finishing off, you simply wind up the speed (eg. facing off is classic example) and the result is brilliant.

Cheers

Rob

You guys are great salesmen for DC motor conversions! I like what I'm hearing.

Simon

Originally Posted by simonl

You guys are great salesmen for DC motor conversions! I like what I'm hearing.

Simon

He He you might be right.

I said from the beginning I was bit worried that the DC horses would be ponies after reading some comments about this - but that's not the case in my conversion.

Then the 2 HP motor failed to perform and I was a bit disappointed and had to use the second fiddle 1.5 HP I had secreted away, but once again it surprised me, as it has much more power than my old 3/4 HP AC motor and the torque just keeps piling on as the revs drop.

So yes it is a worthwhile conversion although it's been a lot of work and a very slow process nutting things out and trying to keep it all neat and tidy.

I had a look this morning and I can make up a centre for the original pulley and fit that - which will give me all my original ratios and more lower ratios.

I'm surprised it turned out as well as it has, as I didn't really have high hopes for a successful outcome. It all seemed too cheap to be effective

Cheers all

Rob

Originally Posted by nearnexus

I'm surprised it turned out as well as it has, as I didn't really have high hopes for a successful outcome. It all seemed too cheap to be effective

Cheers all

Rob

I love it when a plan comes together!