Objectives of the counter:
- An electrical pulse should advance the units digit.
- A different electrical pulse should advance the tens digit.
- Another different electrical pulse should advance the hundreds digit.
- Advancing from 9 to 0 should also advance the next reel.
- A mechanical movement should allow the reels to be reset to 0.
At first I tried using Electromagnets to advance the reels. The way I tried was via a Ratchet Wheel which has twenty teeth and a 1:2 ratio to the reel. So each one tooth movement of the ratchet wheel will move the reel by a tenth of a revolution.
This did not work well because the Meccano Electromagnets are not powerful enough.
Instead all the reels are on a friction drive from a motor and the reels try to revolve at about 60rpm. They are held in position by escapement mechanisms. There is a 1:5 ratio to the escapements from the reels. There are two diagonally opposite stops on the escapement. Half a turn of the escapement moves the reel by one tenth. The Electromagnet pulls a rod disengaging the escapement on one side, but engaging it on the other so the reel doesn't continuously turn if the Electromagnet is on for more than a short pulse.
To make the 9 to 0 transition work, the reel pushes a rod which acts as if the next Electromagnet had been pulsed. This means no Electromagnet is needed for the thousands reel.
The reset to 0 is achieved by two steps.
Firstly stops are moved to each reel stopping them just shortly after the 9 position.
Secondly the escapements are disengaged allowing the reels to turn freely.
Once the two operations are reversed the reels make their final movement to 0.
It didn't work too well at the April 2013 meeting due to me not having enough parts, which were bought on the day.
After the April 2013 meeting I made the following improvements:
- The detents have been redesigned, using Pallet Pins as the stops instead of long Grub Screws.
- The escapements have been redesigned. They used to be directly underneath the detent. They are now to one side.
- The friction drives have been moved to a separate rod away from the reels. This results in less wear on the compression springs, and means the reels once resting are completely stationary.
I seems to be working quite well now, but still needs more adjustment.
A big problem with the chassis is torsion. Just connecting a number of Angle Girders at right angles to each other does not work.
Instead I used a 'compound Angle Girder'. Two Angle Girders are connected to each other by a number of 3 Hole Couplings. A 1 1/8" Bolt passes through a hole in the girder, the coupling is added and then the other girder put on the end. Three of these compound girders are equally spaced along the base, going from front to back. Extra Angle Girders are placed at right angles.
The playfield is inclined at about 6°. There was a lot of trail and error in attempting to achieve this angle. The actual support for the playfield which is just a 24½" Angle Girder each side is easy, but the outer frame needs careful positioning of Flexible and Strip Plates.
Each leg is Angle Girders forming a 2½" x 2½" support. These are screwed into the frame using four Bolts. This is too unstable so 5½" Angle Girders are also connected from the legs at an angle to the frame. For transportation purposes the legs are removed.
This is made out of MDF 6mm thick and seems quite sturdy. It needs to be cut with care as the dust is hazardous. Tools used are a drill, long arm fret saw, sand paper, and a large saw.
I did at first consider using Narrow Strips all the way around the playfield but standard Strips are good enough. Nine hole and seven hole Curved Strips come in useful because of their radii. I don't usually but non-Binns Road parts but have made an exception for the pin table. The strips are typically fitted in using ¾" Bolts or 1 1/8" Bolts. Heavy duty rubber bands are used to allow the ball to bounce off. 2½" rubber bands are used for lane separators.
The kick-out part is just a Pawl with Boss. The motor needs to operate forward to kick the ball out, and then reverse to get the Pawl back to its resting position. There are three Bush Wheels between the motor and the drive. The Bush Wheels have Bolts in them so they can freewheel for almost one revolution. This allows the motor to pick up speed before operating the kick-out.
The flippers are a Crank with a 2" Narrow Girder connected to them. Some rubber bands are placed over the Narrow Girder to reduce noise. The flipper mechanism is fairly simple. A motor continuously runs, and Electromagnets operated by the flipper buttons move a Rod slightly to engage a friction clutch. This moves the respective flipper up very quickly. The flippers are sprung to allow them to move to their resting position once the Electromagnet has switched off.
Why not just use an Electromagnet to move the flipper up? Meccano Electromagnets are not powerful enough. Five could be used in parallel maybe, but that would use 3½ amps and power supplies are pricey.
There are seven lanes on the pinball machine. Three at the top and two either side of the flippers. The ball rolls over the switch, which is a 1½" Corner Bracket. This makes an electrical contact to advance the score. The ball is not very heavy so the switch needs a very light trigger. It is balanced to about 2 grams. The switch which is a 1" Wiper Arm is placed ½" away from the pivot. The Corner Bracket is 1½" away from the pivot. This means the up force from the Wiper Arm is reduced three fold.
Arcing is a big problem here, as the contacts arc they rapidly loose conduction. There were two solutions to this.
- Instead of directly operating the Electromagnet at 0.7A, operate it through a Meccano Relay at 0.02A. This considerably reduces the arcing.
- Add a 'snubber' circuit in parallel with the switch. This is a capacitor and a resistor in series. When the circuit is open the capacitor charges. When the circuit closes, the capacitor discharges, meaning there is a rapid change of potential difference over the contacts, but not immediately 12V. When the circuit opens again there is a reverse operation in effect meaning there is no immediate potential difference between the contacts. According to a formula the capacitance needed is 47pf and the resistance 15 ohms. This is for a 700 ohm Meccano relay.
The pinball machine will have a x10 mode on some sections. For this there needs to be a timer mechanism to activate it and deactivate it. The timer mechanism is simply a motor running through a gear reduction to an Axle Rod which rotates at about 0.6 rpm. When the timer expires the rotating Axle Rod switches off the motor and stops it turning. To start the timer an Electromagnet pulls one of the Axle Rods and disengages the gear on it. Another Axle Rod further down the chain is weighted to one side and rotates the Axle Rods to restart the timer.
The drop targets consist of three in a row. When the ball hits a target it drops, scoring points. The targets are raised on a ledge, when they are pushed back they come off the ledge and drop.
A difficult part was to get the dropping of a target to provide a sufficient intermittent pulse to the digit counter. This is achieved by a slow moving armature that the drop target pushes. It is a counterbalance but the balance is fine, so it moves slowly.
The raising of the targets is achieved by a cam operated by a motor. There is another armature above the targets and once they have all dropped the armature lowers completing a circuit to the motor.
There are two minor problems with the design. If two targets drop at the same time only one score is registered. When the targets a raised they score again, but this could be seen as a feature.
SELMEC 2013 Exhibition
Remarkably successful. It worked for 5 hours with only 15 minutes downtime. The top score of 690 was made by a youngster. There was always somebody waiting to play. I did not mange to take any photos on the day but hope to obtain some made by friends. The down time was caused by a 50-tooth Gear Wheel which was free running on a shaft, but it started to crawl in one direction causing the flipper to raise even though it was not being used. The solution was to replace the Gear Wheel.