Gina

I have implemented this method of Z drive using C-Beam Double Gantry Plates.  With the overall height of the drive (except for motor) being just under 500mm the Z range is 300mm.

With the heating problems with the 400mm square print area I have decided to abandon this size of print bed and go for the 300mm x 300mm as used in the previous Titan printer.  In view of this and the inaccurate plywood panels currently used in the box and having found a source of cut-to-size hardwood plywood, I have decided to make a new box slightly smaller in footprint but slightly higher.  As a result of all this the build volume will be around 300mm cube (just under 1ft cube).

Improved the Z axis by replacing the gear drive with timing belt and pulleys.

This printer is not quite good enough for some precision parts I'm trying to print so I'm looking into possible improvements again.

Have the clock running after a fashion but it's irregular and doesn't keep going for very long.  A new escape wheel, anchor and adjustable escapement have helped but I still don't think the escape wheel is accurate enough.

Going train and escapement.  The moving end of the adjustable suspension lever will have an M6 threaded rod, bearing and thumb wheel to adjust the height.

Designed a lever system to adjust the height of the anchor so that I can set the separation of anchor from escape wheel.  I've replaced the original round arbour on the anchor with a square section to avoid the movement I was getting before.  This was intended to allow for getting the clock "in beat" but it was too coarse and also tended to move in use.  I'll think about that adjustment later.  One possibility is bending the crutch as is often done with pendulum clocks to get them "in beat".  With printed plastic this involves heat to soften the plastic.  Alternatively I could replace the plastic with solid wire.

Here are screenshot of the CAD models.  Adjustable suspension, anchor and crutch.

Made up a new pendulum with 12mm hardwood dowelling, 3D printed coupling to the suspension spring and a new 3D printed bob.  This should help but there are two adjustments which could do with making variable - the spacing of the anchor from the escape wheel and to get the escapement "in beat" without tilting the whole clock.  (In beat means getting the "tick-tock" equally timed.)

Clock mechanism in case.  The pendulum rod is a steel rod from an ancient wall clock but seems to be too flexible.  I may try a dowel rod instead though the air friction will be much higher.

Made a few tweaks and assembled the mechanism again from chain drive to escapement and pendulum crutch and been testing.  Making progress.  The escapement waggles nicely with a small pull on the chain.  Next job will be to add the pendulum and mount the clock into the case to give the pendulum room to swing.  Then connect a weight to the chain and see if the clock will run and if so how much weight it needs.  I have a variable weight in the form of a one litre screw cap bottle to which I can add water as required.

I had thought of having one winding motor in the middle driving both the going train and the striking train but if the motor is to go behind the back clock plate it can't be in the middle because the pendulum occupies that place.  I could have the motor on the right driving the main winding sprocket and that could go on to drive the strike winding sprocket.  To go behind the back plate it would have to be a NEMA14 as a NEMA17 would be too big.  I shall need to confirm that the NEMA14 is powerful enough to drive the clock. 

Whilst it would save costs a bit to use the one motor to wind both the clock and the striking system, I could save much if not all the reduction gearing by having a separate motor for the striking part.  With a separate motor it could run it much less often and save gearing or even make it match the striking sequence with suitable Arduino code.  Anyway, I can look into that later, as long as I leave room.

Going train with escapement.

The striking mechanism will also have automatic winding but this case is quite different.  In a period of 12 hours the clock strikes 1+2+3+4+ --- +10+11+12 = 78times.  If the motor winding is constant (easier to implement) the weight will move up and down as the rotation rate of the chain wheel is not only intermittent but varies a lot over the 12 hour period.  The total motion will be 78 units in 12 hours but 12 of those are in the last hour and only one in the first hour, 23 in the last two hours etc.  I think the easiest way of catering for this is allow for the full 78 units of weight drop in the space inside the case.  This allows a considerable safety margin, maybe unnecessarily but is a starting point.

The upshot of the above is that 78 revolutions of the strike wheel would result in in the chain moving twice the weight hight range in the case.

Calculating...

1. Height of case less height of weight plus pulley/sprocket is around 1.5m
2. Each chain link pair is 20mm
3. Using the same design of drive sprocket as the main clock, with 18 teeth, one revolution is 18 x 20 = 360mm
4. Total chain length for full weight range is twice weight range = 3m
5. Number of revolutions of striking drive sprocket is therefore 3000/360 = 8.33
6. Allowing for the full striking sequence within the case height means 78 revs of the strike wheel corresponds to 8.33 revs of the motor sprocket
7. Gear ratio is 78/8.33 = 9 which conveniently works out as two pairs of gear with 3:1 ratio.

Now for the motor drive gearing...

1. If I use the same design of motor sprocket with 8 teeth, that's 8x20 = 160mm of chain.
2. Since the chain would move 3m in the 12 hours driving the strike mechanism then this amount needs winding up in that time.
3. 3m of chain with 160mm per rev of the motor sprocket means 3000/160 turns of the sprocket in 12 hours ie. 3000/(160x12) = 1000/(160x4)
4. In one second the sprocket wants to turn 1000/640x3600) revolutions ie. one rev in 640x3600/1000 = 64x36 = 2304 secs
5. Motor turns once in 200s so we want a gear ratio overall of 64x36/200 = 8x36/25 boiled down to the lowest common factor
6. That's either a lot of gears or includes a ratchet drive.

Decided to simply go for a 1:4 gear ratio rather than 4:1 on the chain drive to centre wheel gear pair.  The means just 1/16th the weight required but running 16x faster.  This is no problem with the stepper motor drive, in fact it's easier.

Calculating...

1. The centre wheel turns once per hour so the drive sprocket 4 times an hour or 15m per revolution.
2. Chain drive sprocket has 18 teeth and motor sprocket 8 teeth.
3. Motor sprocket wants to turn 18/8 times in 15m or 18x4/8 = 9 times an hour ie. 9/60 = 3/20rpm or 20/3 mins/rev which is 20x60/3 = 400 secs/rev
4. I expect to use a NEMA17 or maybe NEMA14 stepper motor with 200 steps/rev.
5. If I were to use 1 step/sec motor speed the motor shaft would rotate at 200 secs/rev
6. I plan to drive the motor sprocket with a pair of spur gears from the stepper motor so the ratio would be 2:1 - very convenient. 

The gears seem to be alright now but there is still too much friction.  It would need several kilos weight to drive the clock even without the escapement.  I think I need to reduce the gear ratio between chain drive sprocket and centre wheel.

The centre wheel and the intermediate wheel were both at fault and I'm reprinting both.

Going train with escapement assembled into the two acrylic plates.  Unfortunately it's binding somewhere and won't run.  Have to take it apart again and reassemble in stages until I find the problem.  This is the clock mechanism out of the case.  The pink part takes the drive chain.

Printed the crutch and tried to assemble it all but found the anchor wouldn't fit so redesigned it and just finished printing it.

Now printing the brackets which hold the bearings for the anchor shaft and pendulum suspension.

Tests with that escapement seem favourable so I'm going ahead with the construction.

Here is a photo of the gears of the "going train" and escapement laid out on the table.  I'm trying a larger escape wheel and anchor to reduce the need for precision 3D printing.

Printed anchor fine now printing escape wheel (2nd attempt) with 0.3mm nozzle and 0.2mm layers.

Started to look at this project again.  First thing is to get the escape mechanism working - maybe with a test rig.  I have make improvements to my Mini 3D printer giving better results so I might just succeed this time.  Pretty much everything will be printed in PLA.

Plan of stages :-

1. Get test escapement working.
2. Take a new pair of acrylic sheets for the front and back plates, drill holes and add ball bearings.
3. Construct and add the "going train".
4. Add the gears that drive the hands and the hands themselves.
5. Add motor-winder for the going train.
6. Add mechanism to automatically adjust effective pendulum length to regulate the clock.
7. Add the chiming mechanism.
8. Add motor-winder for chiming train.

I'm having second thoughts about the moon dial as I already have a moon dial clock in this room.  I would rather concentrate on the case and getting the whole clock looking presentable.

Edited to add extra sections I'd missed out.

Been having lots of problems with the Z drive - very noisy and tending to stall one side or the other even at quite a low speed - so decided to dismantle the Z drives and see if I could find the problem.  Tested the threaded rods for straightness and they are fine.  One problem is that the printed Z carriages have warped and the wheels are loose on the Z rails.  So I shall need new Z carriages anyway.  From what I can discover and discussions, I think the main problem may be the accuracy of alignment of the threaded rods with the nuts on the carriages.  To make alignment easier and save myself a lot of work I think I may add more cash than effort and order some ready made parts.

I would want two sets - one each side.  There are other useful parts too such as top and bottom plates that hold the rods precisely in ball bearings but aluminium parts are quite expensive at £10.50 each (so 4 off would be £41) and I think 3D printed ones would be good enough.  This is a system designed for precision in CNC machines and with the taller gantry plates just the job for the Z axis.  Firstly though, the smaller and cheaper gantry plate.

C-Beam Gantry Plate  £11.00

The double gantry plate is longer that the standard one which is only 60mm between wheel centres and would give better resistance to twisting.

C-Beam Double Gantry Plate  £16.50

C-Beam Linear Rail – Cut To Size  £13.62

I'm already using NEMA23 stepper motors for the Z drive so it all fits.

I added some bracing to the frame and now this printer is working fine so I shan't be messing about with the Z drive.  It ain't broke so I'm not fixing it!  I did have to fix broken piezo Z probe wires though - replaced the devilishly fine wires with a pair from a ribbon cable.  Working again.

Been checking the NASA Images web site and found they are generally not copyright and I can show them in my talk.

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