Construction

Point Motors: Method

Printed Circuit Board:

I've started with the PCB because all other dimensions flow from this component.

The first task is to design your board, mine is shown here:

The spacing of the tracks is one eighth inch to match the edge connector. The edge connector is pushed onto the right hand side. Other connectors may need different spacing.

The two way, double pole switch that controls the point works as a reversing switch feeding the first 5 tracks of the board, as follows.

Working from the top down:

A common return, input from one side of the two way switch, that connects one side of the supply directly to the motor.
The input from the other side of the two way switch feeding the limit switches on the next three tracks.
For the slider contact, pick up at the left hand side of the pcb, enables the slider to be moved to the right.
For the slider contact, pick up at the right hand end of the pcb, enables the slider to be moved to the left.
For the slider contact, and is the output to the other side of the motor.
For the output of the two way switch, activated when the slider is at the left hand end of the board, by the contact on track 7
For the output of the two way switch, activated when the slider is at the right hand end of the board.
Input to the two way switch, by the contact on the slider.
Tracks 10 -12 , second two way switch.
Tracks 13 - 15 , third two way switch.

The slider connects together, by way of the contacts:-

tracks 3, 4 and 5,
tracks 7 and 8,
tracks 10 and 11,
tracks 13 and 14.

The slider: a) the board

Mounting (not to scale)

Copper side (not to scale)

Three variations of the slider

The slider is 1.9 in wide, and about 1 in long. I cut mine all by hand, including the separation tracks. A jig of some kind would make the work easier, and more consistent, although the dimensions are not critical, except that the width should be consistent if a lot of motors are required. It just helps to assemble them all more easily.

The soldering of the pick up wires needs reasonable accuracy, again a jig helps.

Reasonable care should be taken drilling for the post, so that the fixing screw is clear of the pickups, and doesn't cause a short circuit. The post *should* be a 'sloppy' fit as it drives the slider, so that it doesn't have to be lined up with great accuracy. If the motor spindle, screw thread and post are out of alignment, there should be enough flexibility to work easily.

The slider: b) the post

The post is a short length of 3/16 in brass, about 3/8 in long, with a7BA thread across one end, and a small treaded hole down the other, for a fixing screw. (I used th North American size 2-56). A jig is essential at this point.

Jig

I had a friend make up a small jig from 3/4in square steel bar. A 3/16in hole is drilled down the middle, to take the brass rod. A 1/16in hole is drilled down 3/32in from one end to intersect the 3/16in hole. The grub screw was added by my friend to hold the rod when drilling. Once the end hole was drilled across the rod, a thread was cut to take the 7BA screw thread. It was then cut to length, put back into the jig to be held as the end hold was drilled and tapped for the small screw. You may need to alter some of these measurements to suit your materials.

The slider: c) the screw thread

The screw thread is about 1.5 in long. The critical factor is to make it from steel, to run in the brass post without binding. Brass in brass is not recommended.

The screw thread needs a secure, flexible fixing between itself and the motor spindle. I used plastic 'tube' created by stripping out the wire from some telephone cable. Use whatever is at hand.

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Last Modified: 1st December 2003