Trunnion clamped into the jig and ready for bending

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6th June 2010 Meccano-built Trunnion bender

Trunnions stripped and ready for bending alongside finshed parts

A couple of weeks ago the latest edition of the International Meccanoman Journal landed on the doormat. Amongst its pages was a suggestion from Philip Webb Drawing regarding bending trunnions to create a very useful looking part. This started a discussion on the Spanner II list debating how to achieve these bends. For those, like me, who do not have access to workshop facilities I have made a simple Meccano-built jig that will allow anyone with a hammer and a hard flat surface to make neat folded trunnions.

Build it…


Two views of the finished trunnion bending jig

After a couple of aborted attempts at making a Meccano bending break type machine I decided that there was a much simpler solution. The trunnions need to be bent so there is a clear ½ inch between internal faces. This means bending it around a former. The trunnion also needs to be held in place while the bending is carried out. To make this simple jig, select 14 of P/N 2, 5½ inch  standard strips (11 hole), and bolt them together through holes 1,2, 10 & 11. Use a couple of drifts or rods to ensure they are all aligned. Then bolt two fishplates, through their slotted holes, through holes 3 & 9 of the strips using pivot bolts. This will help maintain good alignment. A stack of 4 P/N 5, 2½ inch standard strips (5 hole), are bolted through one of the fishplates and secured with a couple of washers and a locking nut. This should be tight but allow the stack of stripes to be swung clear for insertion and removal of the trunnion. A long bolt and a threaded coupling are used to lock the jig up and clamp the trunnion in place. Finally two 2 inch long rods have collars fitted and are used as alignment pins.

The jig ready for action

How it works…

Making the inital bend with thumb power!

Clamp a trunnion in place using the alignment rods passed through the top and bottom holes of the trunnion and through holes 5 & 8 of the jig. Clamp it up tight. As the fishplates are slightly thinner than the trunnions, there is plenty of clamping force applied. Bend the trunnion over the former with your thumbs to get it started and then use a hammer to get it the rest of the way.

Gentle hammering will make the bend sharp

Turn the jig over and repeat the exercise to fold up the other side. Finally place the jig on a flat solid surface and gently tap the corners square.

Tap the corners square

The finished part

I was suprised how accurately this little jig produces nice neat little parts. You will need to repaint after the bending so you might as well strip them first and bend them up in their stripped state. You can then decide how to finish them, if at all. If you leave them as bare metal some form of rust inhibitor will be required. I have just given these ones a spray of WD40 and polished them dry with a rag for now but I think I will be repainting them eventually - or even zinc plate them...


And another...

At the time of writing I had not realised that Philip Webb had already made a similar jig and mentioned it on the Spanner II list. Here is his explanation of how he made it and how to fold the trunnions:

Trunnion forming block

Make up a thick bar of 5½ inch strips, bolting them together at their ends with long bolts or threaded rods. This should be at least ½ inch thick. PW JigThe Trunnion is secured by bolting through into the two Threaded Cranks mounted beow the
With your hands on the bar, push the free edges of the trunnion slowly and firmly down against a hard wooden surface to begin the bend on each side. Finish off to a good right angle by tapping the corner with a small hammer. You will obviously have to repaint the edges!

Philip Webb

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