Monday, 13 April 2009

Unexpected find

While looking through my collection of salvaged stepper motors I found a couple of NEMA17s. This one came out of the hard drive in the first PC that I bought, an 80286 AT clone for about £1200 in the 1980's.

All the subsequent hard drives I have owned had voice coil head servos, but this one, which was a full height, 51/4", 20MB MFM drive, was built more like a floppy drive with a stepper motor to move the heads.

The motor had a plastic wheel with an endstop on it preventing it making more than one revolution. On removing it I was surprised to find that it was also a resonance damping device.

It seems to consist of a brass flywheel isolated from the shaft by a ball bearing, but coupled to it with a viscous fluid, probably some type of oil. I think it behaves like an electrical snubber, which is a resistor and a capacitor in series use to dampen voltage transients. I think this will have an analogous effect on velocity transients.

I found a similar motor in a 51/4 floppy drive, but that was uni-polar whereas this one is bi-polar, and it did not have the damper. It looks like they were pushing the performance of steppers as far as they could before moving to voice coil servos.

I don't know if it still works, it is more than 20 years old and I damaged it a bit removing it from the shaft as it was glued on. I don't think I will need it when driving a high friction, low inertial load like an extruder drive.


This may be an evolutionary dead end, with the move to stepper motors and pinch wheels, but I wanted to try a couple of things that have been on my "to try" list for a long time.

The main issue that I have had with the pump part of the original extruder is that the bearings wear out fairly quickly. Both the half bearings themselves and the lands on the shaft. One problem is that being only half bearings, any lubrication soon gets carried away by the plastic.

The best lifetime I have had is with stainless steel bearings and a stainless steel shaft. The downside of a stainless steel shaft is that you cannot solder a nut on to provide the drive. I have found two ways round this:-
  1. Use a hex head bolt. For some reason stainless steel bolts never seem to have thread all the way to the top. Since the thread needs to be sharpened with a die anyway, it can be extended at the same time. It is hard work tapping stainless steel though. You need a split die, set to its biggest diameter to start with, and you need cutting compound. The hex head allows you to get a good grip to stop it turning and the original thread makes it easy to start off square.
  2. Drill through the nut and shaft and insert a pin. If, like me, you break lots of drills then broken drill shafts make perfect pins. I now buy drill bits in packs of five or ten!
I replaced the two half bearings with three ball bearings. At the top is an M5 bearing to take the axial thrust. At the bottom I use two M4 bearings as rollers to take the radial load.

The downside of this arrangement is that you still need to turn a land on the bottom of the shaft. It could probably be done with a file and drill though. It actually works without removing the thread, but I expect it might wear away the rollers.

This design works but there are a few things I would change if I built another: -
I made it compatible with the existing filament guide to avoid having to reconfigure my machine for HDPE. Ideally the screw holes at the bottom end need to move out to allow longer bolts to hold the rollers and the size needs increasing from M3 as the threads strip eventually.

I left clearance to allow the top bearing to be inserted from below, but left no access to the nuts. Consequently it was very difficult to assemble and I had to make undersized nuts.

I used the smallest outside diameter bearings I could find for the given inside diameter. That was a mistake because it is hard not to foul the outer part of the bearing with a washer as the moving part is so small. Star washers seem to just grip the inner and provide enough standoff to clear the outer. I used counter sunk heads to clear the outer face of the rollers. I expect larger diameter bearings use bigger balls, so perhaps have higher ratings.
All easy things to put right with a design iteration.

Another thing I have been meaning to try is the GM17 gearmotor. I have had some for a long time, but without a second shaft, adding a shaft encoder is not trivial, as it is with the GM3. Solarbotics now sell a cheap magnetic encoder that fits inside the casing, making it a more attractive proposition.

To fit the motor in place of the GM3 a new mounting bracket and a shorter version of the shaft coupler is needed.

Here is the completed pump: -

And here it is built up into an extruder: -

I am waiting for the magnetic encoder to come from Canada so I tested it open loop with a couple of bench power supplies.

The GM17 is a bit quieter than the GM3, but not that much when heavily loaded. It extrudes at a similar rate, but the speed seems to vary a lot with load, so it would be useless without closed loop control. It seems to labour and get quite hot at 12V, so I don't imagine its life would be a lot better than GM3. It overruns a lot when the power is disconnected, so it would need a full H-bridge and reverse thrust to get decent stopping.

I still have lots of things to try: stepper drive, a roller instead of the filament guide, an offset screw drive to avoid the rollers.

Saturday, 11 April 2009

Nutty tip

If you want to use a nut, but find there is not enough room for it, here is an easy bodge that I have used a few times: -

Just take a nut one size below, drill it out and tap it to the size you wanted. This is very easy to do because the outer thread size of the smaller nut is about the same as the tap drill size of the bigger one, so you only really have to drill the thread away.

The nut on the left is a proper M5 nut, the one on the right is an M4 nut tapped to M5. Obviously it will have a lower maximum load but it can get you out of a hole.