You spin me right round

[jjgoertz]

I'd like to know how or why you picked that motor? It looks like the electric motors used in some RC Airplanes.

I wanted a motor that already had hall effect sensors installed, so that cuts out most of the market of brushless motors, and then I was looking for something with a low kV (rpm per volt) so that the 1024 speed steps the controller has as much functional resolution as possible, IE I want the motor at the upper end of it's rpm range on my input voltage so that each step is a smaller percentage of the total speed.

This motor is way bigger than what I would really need, but it's easier to find low kV motors in larger sizes. It's nominally a 48v motor with a free running speed of 9,030rpm, so that's a kV of 190 rpm. Then I'm running the motor on 12v, with the 12 inch to 6mm gear ratio that means I'm at 74% of the max speed. That will go down a little bit when I make the convex pulley.

[Æ]

My new motor came in this week. It's a Maxon 283869, has hall effect sensors installed from the factory, and also doesn't exhibit any cogging when you spin it by hand.

I'd like to know how or why you picked that motor? It looks like the electric motors used in some RC Airplanes.

[Æ]

Yep, concave for round belts and convex for flat belts.

The motor should be fine in terms of heat dissipation. My watt meter shows less than 1/10th of a watt of input power to the controller.

That will be one of the biggest obstacles for me, if and when I ever get back to my DIY turntable. I'm using a 24 pole hysteresis synchronous capacitor run motor (300 RPM) and I will have to lathe the motor spindle pulley very precisely in order to obtain 33.3 RPM exactly at the platter.

[Millstonemike]

Do not make the motor spindle pulley with a concave cut. I know it might seem counter intuitive,....

Counter intuitive - that's were experience trumps theory. Thank you.

[jjgoertz]

Do not make the motor spindle pulley with a concave cut. I know it might seem counter intuitive, but convex is how they most often do it, certainly so on turntables I'm familiar with.

Yep, concave for round belts and convex for flat belts.

The motor should be fine in terms of heat dissipation. My watt meter shows less than 1/10th of a watt of input power to the controller.

[Æ]

T's beautiful. One of a kind art meets engineering meets precision implementation.

Trying to help ... some observations ... that you've likely already thought about ...

Might the motor need more heat dissipation (dunno know)? Increased exposure to air - possible at the bottom of the relief in the plinth through to the underside.

The belt seems to ride down to the top of the walnut motor mount; extra friction, ugly wear line? I could only think of: (a) a roller to keep it higher; or (b) machine the motor spindle with a concave curve running top to bottom - 360^o around - to keep the belt vertically centered on the exposed spindle (I have no idea if I said that right).

Do not make the motor spindle pulley with a concave cut. I know it might seem counter intuitive, but convex is how they most often do it, certainly so on turntables I'm familiar with.

[Millstonemike]

My new motor came in this week.

T's beautiful. One of a kind art meets engineering meets precision implementation.

Trying to help ... some observations ... that you've likely already thought about ...

Might the motor need more heat dissipation (dunno know)? Increased exposure to air - possible at the bottom of the relief in the plinth through to the underside.

The belt seems to ride down to the top of the walnut motor mount; extra friction, ugly wear line? I could only think of: (a) a roller to keep it higher; or (b) machine the motor spindle with a concave curve running top to bottom - 360^o around - to keep the belt vertically centered on the exposed spindle (I have no idea if I said that right).

[jjgoertz]

My new motor came in this week. It's a Maxon 283869, has hall effect sensors installed from the factory, and also doesn't exhibit any cogging when you spin it by hand.



Made a motor mount from 3/4" walnut and 1/2" baltic birch, with a relief cut in the plinth for the motor to hang down into, and an additional hole to route the wires to the bottom.





The motor mount is attached from the bottom to hide the fasteners.



And she's playing. Still using a regular rubber band for the drive, need to find something with a more consistent thickness, and preferably in black.

The band is riding directly on the motor shaft right now, which works fine but to tune the belt tracking for that I'm playing with the tension on the four screws mounting the motor. In the future I'll make a small crowned pulley.

The final thing I need to do some work on is the spindle to get it more concentric. I have a bit of audible wow right now. My plan is to bore the end of the shaft the bearings ride on in a lathe, and press a longer spindle into that. Then the platter will have a clearance hole for the spindle so I will have a concentric center no matter what the wood does.

[jjgoertz]

Knocked together a little rack for the records last night. I'll have to build a little table for this thing too, I'm thinking something along the lines of a 30x48 kotatsu, nice and cozy when winter rolls around.

[jjgoertz]

My new controller showed up this week, a Maxon DEC 50/5.



I had been under the impression that this thing would work in open loop mode without hall-effect sensors on the motor, but that was wrong. I got some little A3144 hall effect sensors to install in my motor, using the gaps in the stator to get an even 120 degree spacing. They were initially super glued into position and then I came back and potted them with epoxy and some chopped fiberglass.





The motor does run, but the controller seems to be unhappy with it. I'm getting over current alarms if I try to go below about 75% of full speed, despite my watt meter telling me that I'm drawing 0.0 amps, and the controller being rated for a continuous 5 amps. My best guess is that the controller isn't compatible with the high winding resistance of this motor.

I will be switching over to a Maxon EC-max 283869 motor that I got a pretty good deal on used.



I also did some experimenting with my bearing system this week. I tried out some needle bearings in place of my teflon bushings, but those ended up being much too loud with only a minimal reduction in friction. I ended up splitting the bottom teflon bushing in half to make the axis alignment a little less critical with the way I floated the bottom bushing into position to level the platter.



The magnet bearing is the only thing I did this week that actually went as planned. One of them is bolted to the threaded hole in the bottom of the platter's shaft, and the other is attached to a little bridge hanging from the bottom of the plinth. The only tricky part is getting the gap set correctly so the platter floats in between it's two end stops.





With the decision to change motors I will also be using a much smaller drive pulley than I had originally planned, about 3/8", and because of that I will be switching to a flat drive belt. To keep the belt on the platter I put a nice crown on it's edge. This also got the platter centered more accurately to the shaft.

[jjgoertz]

I wonder how much force it takes to move the tonearm? How far does the cantilever deflect before the tonearm actually moves? How much lateral force is required to move the tonearm? After it is all set up and accomplished, you might want to play a test record and check that you are getting equal performance from both channels of the pickup/cartridge.
I really don't know very much about linear tonearms, but all the commercial versions that I've seen have some sort of driving mechanism to help the tonearm more laterally.

It's actually quite difficult to get a measurement of how much force it takes to roll the carriage. The best measurement I got was from holding my digital scale on it's side, and then elevating one side of the rail until the carriage rolled down hill into the scale. With that method I get about 0.3 grams.

[filmslayer]

Exceptionally nice photographs too! It's really a treat to see so many of them.

couldn't agree more ...

[Æ]

in the 2.5 years i've been here this is probably the coolest thread i've seen . good work

Exceptionally nice photographs too! It's really a treat to see so many of them.

[filmslayer]

in the 2.5 years i've been here this is probably the coolest thread i've seen . good work

[jjgoertz]

Took Daryl's suggestion and twisted up the wire pairs. The wire hanger thing was a little top heavy so I swapped out the brass tube for a smaller and lighter tube, and then filled the base block with some more lead shot.

Finally I made a little queueing arm. It's some 90 thou music wire covered in heat shrink for that nice soft touch.