The scotch yoke is a simple, effective mechanism for converting rotary motion (the handle round the back) into reciprocating motion (the yoke and slider at the front). This project allows you to make your own working mechanism and learn how it works first hand. Members can download the parts for free, non-members for £2.50

Find out more about the scotch yoke mechanism here and here

Print the four parts sheets onto thin card. Score along the dotted and dashed lines and cut out the holes before carefully cutting out the parts.

Fold round and glue up the axle and the pin. Fit them in place in their respective holes in the wheel piece.

Fold the wheel piece in half and glue it together to make double thickness card. Make sure it stays flat as it dries. One the glue is dry carefully cut around the circle.

Glue the pin tab to the wheel 1mm above the wheel.

Roll the tab round tight and glue it down to make a circular pin.

Glue the two box sides to the box body as shown above.

Fit the box inner into place so that it runs across the centre of the box on either side of the hole.

Fold the box round and glue it together.

Make up the two box ends, thread the bushes through the holes in the box sides and glue them into place.

Thread the wheel through the box. Fix it into place using the washer. It must be free to rotate.

Fit the yoke vertical to one of the two shafts. Use the grey area for alignment. Repeat the process with the other half of the yoke.

Glue the two ends to the yoke vertical…

…and complete the yoke.

Fit one of the box sides into place on the end of the box. Line up the tab edge with the edge of the box to make sure that it is straight and square.

Thread the yoke into place as shown.

Fit the other end of the box into place making sure that it is straight and square.

Assemble the handle in three steps. Fold the two halves round and glue them to make two square section tubes. Fold one tube into the other and glue it down. Roll the long tab round and glue it down.

Complete the project by gluing the handle into place.

Turn the handle and the yoke/slider will move back and forth! Scotch yoke!