4
Sat 4th Dec 2010

A naughty/nice detector. Press the button on the Santa model and he randomly raises either the Naughty or Nice flag. Easy! I've tried this before though. It turns out that randomness is not as easy as it looks.

A friend of mine tells a story of how he went on a team building visit to Whitby. They were split into groups and given a variety of problem solving tasks to complete. One task was to find the height of the television transmitter mask that loomed over the nearby cliff tops. Many of the groups rushed off to the local shops to buy protractors and measuring devices using clever mathematics and observation to calculate the height of the tower. In those days before the internet my friend's group rang the BBC and asked. They came away with the easiest to find  and most accurate answer.

That story has always stuck with me. It came to mind when I was making the ratchet model. I'd used a complex system to apply friction to the main wheel. Regis pointed out a much simpler, more elegant solution.

So it is with that in mind that I head with trepidation towards the creation of random movement. Am I missing something obvious? Is there an easy way of achieving the same ends? We'll see.

My starting point is a spinning rotor. Spin the rotor and let friction stop it in a random position. Just like a little tiny wheel of fortune. Where she stops, nobody knows!

Hopefully I'll be able to kick the wheel into action using a toothed bar pulling up past the square rotor shaft.

I'm planning to fit the rotor into a holder (above, left) where it will be free to spin.

This module will then be fitted inside a holder and be free to move up and down. As it moves up it will be moved passed a toothed bar (not shown here) to make it spin. This should be where the randomness comes in. When the rotor is pushed back down it'll settle into a fixed position. I'll cut alternate holes in the rotor (outlined below) and depending on which hole is facing downwards either one arm or the other will raise. Naughty or nice? Only Santa knows for sure.

Here's a first draft rotor. Too wide, and the shaft too fat but you can see where the alternate holes will go. I'll have another go after I've had a coffee.

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5
Fri 26th Nov 2010
£2.50
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The completed model.
This model lets you see how cranks work and could be the starting point for your own designs.

Print out the parts onto thin card (230 gsm / 230 micron). Score along the dotted lines, don't cut the holes out yet. Carefully cut out the parts.

Fold the push rod ends in half and glue them together to make double thickness card. When the glue has completely dried cut out the hole in the middle with a sharp knife...

...then cut out the part with a pair of scissors.

Repeat with the second push rod end.

Glue the box strengthener into place over the box side so that it is accurately line up. This is to make the crank hole double thickness.

The completed box side.

Once the glue is completely dried carefully cut out the hole with a sharp knife.

Fold round and glue down the lower flap on the box side to make a triangular tube section. Use a square box to make sure that the part is at right angles. (I used Northern Exposure season four DVD box set but season three would probably work just as well.)

Fold over and glue down the top flaps as shown above then pop it up in a square section tube.

Repeat the process with second box side.

Fold the five sections of the crank in square sections and glue them down.

Fold up the first two sections and glue them into position. Don't do the other ones yet.

Fold round and glue up the push rod. Glue the push rod ends to the push rod.

Thread the push rod onto the centre of the crank. Fold up and glue the rest of the crank making sure to keep the angles accurate.

Fold up and glue together the moving platform. Glue the top and bottom into place.

Glue the push rod to the tab on the moving platform.

Glue the base to one of the box sides lining up the creases in the base with the front and back of the box side.

Fit the crank through the hole in the box side.

Offer up the second box side and thread the crank into place. Note that the moving platform fits between the two rails on the box sides but mustn't be glued!

Glue the second box side to the base.

Fold the end into place as shown, glue down the tabs in the places arrowed.

Fold round and glue down the triangular section on the box end. This will give rigidity to the completed box. Glue the box end to the tab on the base. (Arrowed)

Fold up and glue down the box end.

Fold round and glue the the two square sections of the handle.

Fold one section into the other and glue.

Roll round and glue down the long tab.

Complete the model by gluing the handle to the crank. Once the glue is dry, turn the handle and the platform moves back and forth. This mechanism could have all sorts of uses in your own automata design from moving a tortoise head in and out of its shell to driving the arms of an exercising bear!

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0
Mon 15th Nov 2010

From an idea by Regis in the comments in the last post. The original version of the ratchet model used a pair of friction loops to stop the tooth wheel rotating when it wasn't supposed to. Why not use a second pawl suggests Regis. It turned out to be a both simple and elegant solution. I cut away the friction loops with a sharp knife and removed them. The second pawl was then just a simple fold of card glued to the box. It worked perfectly. I'll redo the full instructions with new parts shortly, meanwhile, if you want to give it a try, the extra parts are free to download at the end of this post. On the original model the friction loops helped keep the main wheel centred. To replace this function I've added a couple of washers. (Not shown in this picture.) The replacement axle just adds a couple of lines to show where the washer need to be lined up. Thanks Regis. Nice idea!

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7
Sat 13th Nov 2010
£2.50
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The completed ratchet model. Turn the handle and the main wheel advances on tooth at a time.

Which ever way the handle is turned, the wheel always advances in the same direction. Weird!

A working paper mechanism for you to download, print out and make. Members can download the parts for free at the end of this post, non-members can download the parts for a modest fee. Print out the parts onto thin card. (230 micron / 230 gsm) Score along the dotted lines and cut out the solid black lines. Don't cut out the circular holes yet.

Fold the wheel pieces in half and glue them together to make double thickness card. Make sure that you don't get glue on the tabs.

Carefully cut out the wheels.

Set them aside for the glue to dry.

Fold the pawl bearings in half and glue them together to make double thickness card. Let the glue dry completely then cut out the hole with a sharp knife. Complete the piece by cutting round the outside.

Fold up the box inner as shown above left, note that the creases are valley folds. Flip the piece over and glue the box outer to it to make a double thickness of card. Line up the parts as accurately as possible.

Cut out the two holes. Glue the friction loop holders to the grey areas on the inside of the box.

The friction loops add a little resistance to the wheel to that it is not turned as the pawl moves across the wheels surface.

Fold a coin holder strip round two 20mm diameter coins (pennies) and glue it tightly into place.

Fold up and glue together the friction loop, glue the coin pack into place. The coins add weight pulling the friction loop onto the axle and so increasing the friction.

Slip the friction loop into the friction loop holder. It should be free to move. Repeat on the second side of the box.

Fold round and glue together the wheel inner.

Glue the wheel inner to one side of the wheel carefully lining up the tabs with the grey areas on the wheel.

Fit the second wheel into place on the other side of the wheel inner rotating it so that the tabs on the wheel centre are running in the same direction.

Fold round and glue together the axle. Thread the axle through the wheel centre and line it up with the grey lines on the axle then glue it into place to complete the wheel.

Fold round and glue together the pawl as shown above. Glue the pawl bearings onto the pawl using the grey areas for alignment.

Fold a coin holder round a single 20mm coin (penny) and glue it into place in the end of the pawl as shown.

Assemble the three crank pieces as shown above. The diagonal piece adds strength and rigidity.

Glue one of the short crank pieces to the longer piece using the grey areas for alignment.

Thread the crank through the bearings and glue the third crank piece into place again using the grey areas for alignment. The crank should be freed to turn in the bearings.

Glue the base to one of the sides being careful to line up the edges and corners accurately.

Thread the axle through friction loop and into the larger hole in the box side. It is important that the wheel is the correct way round. Make sure that the flats of the top teeth are facing toward the pawl, use the picture to help.

Thread the crank through the second, smaller hole in the box side.

Thread the second box side over the crank and axle and glue it into place on the box base, again being careful that the parts are lined up accurately.

Glue the box ends into place on the box base. (Box side not shown for clarity)

Fold over and glue in the box ends to the inside of the base. Try twisting the box a little before you glue in the ends the again after the ends are glued into place. Notice how much more rigid they make the box!

Assemble the handle in three steps. Fold round and glue the two square sections. Fold one section into the other and glue. Roll round and glue down the long tab.

Complete the model by gluing on the handle.

That's it! Once the glue is dry, turn the handle. the main wheel will advance one tooth at a time always in the same direction no matter which way the handle is turned. Use this model to find out about mechanism and how they work or as a starting point for your own models.

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0
Thu 11th Nov 2010

Having put the model together at the end of yesterday I really wasn't happy with the way the paper leaf springs worked. They were stiff had a limited range of movement. Back to good old gravity then. To add a bit of friction to the main wheel shaft I have a loop of card curved over the top of the shaft. At the bottom of the shaft are a couple of coins. The whole assembly slips into a tube so that its not flapping around.

I've stuck with the pawl being at the bottom of the wheel. With it at the bottom its movement is easily restricted, if it was over the top of the wheel it would be free to lift right off and over. I'd have to add a cover to keep the pawl in place.

So there it is complete. It works great! Turn the handle either direction and the main wheel advance one tooth at a time, always in the same direction which is weird. I need to add some box sections to the box to stop it twisting then that'll be it done. Excellent.

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0
Fri 5th Nov 2010

First draft of the crank mechanism, looks promising but has a few problems. The box needed to be taller and longer. The shaft was too long and the moving platform didn't quite fit. One of the many advantages of designing on the computer is that you can easily make a few changes and reprint. So that's what I did...

Spot on!

Notice the triangular section on the front of the box. The box itself if very open and so easy to twist. The triangular section adds stiffness.

The end result works great. The moving platform could be the starting point for all sorts of different models. For some reason, when I look at it I see a person using a vacuum cleaner. Feet on the end of the box, hoover on the sliding platform. Hmm.

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1
Thu 4th Nov 2010

Continuing my ongoing project to mechanify everything into paper, I've been working on a crank based model. I spent longer than usual with the sketchbook just trying to work out how I'm going to lay out this mechanism. I've finally settled on a crank that drives a platform back and forth. First step, the crank.

I've made the crank from a single piece, sort of like the handles that I use for my models but with a few more corners!

With the push rod in place its just a case of folding it up and gluing.

Next, the box. Oh and the push rod will need re-doing because it is too short.

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3
Mon 1st Nov 2010
£2.50
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As part of my on-going project to create working paper versions of all the mechanisms in the world I present to you, the Quick Return Mechanism! Members can download the model for free at the end of this post, non-members can download the parts for a small fee.

Print out the parts onto thin card (230 micron / 230 gsm) Dotted lines are valley folds, dashed lines are hill folds, solid lines show where to cut. Grey areas show where to glue.

The completed quick return mechanism.

Fold round and glue together the drive shaft.

Glue the drive shaft to the drive wheel using the grey areas and the black triangles for alignment.

Glue the two stiffeners to the wheel and the drive shaft as shown.

Glue one of the washers to the drive shaft using the grey areas and the grey line for alignment.

Assemble the box as shown. Make sure you are as accurate as possible.

Glue the box to the base and glue up the sides. Notice that the folds on the base are valley folds.

Fold round and glue together the bar. Notice that there is a tab free on the triangular end of the bar.

Assemble the pin and glue it into the slider. Fold the slider round and glue it together.

Thread the drive shaft through the holes in the box. Glue on the second washer to hold it into place. Make sure that the drive shaft is free to rotate.

Thread the slider onto the rod.

Fit the pin into the hole in the drive wheel. Glue the tab on the end of the rod to the grey area on the base.

Make the handle in three steps. Fold the two parts into square section tubes. Fold the two parts one inside the other and glue. Roll round the long tab and glue it down.

Glue the handle to the drive shaft to complete the model.

That's it! Turn the handle and the main bar moves slowly to the right then whips back quickly to the left. Find out more about the quick return mechanism in the mechanisms section.

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0
Tue 26th Oct 2010
£2.50
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Print out the parts onto thin card. (230 microns / 230 gsm) Start by scoring the dotted lines and cut out the holes. Both these jobs are easier on whole sheets. Once complete, carefully cut out all the parts apart from the gear.

The completed work gear model. Turn the handle twenty four times and the main gear turns once.

To make the gear, fold it in half and glue it together to make a double thickness piece. Make sure that the tabs are not glued down. Once the glue is dry cut round the outside of the gear...

...then cut out the teeth.

The completed gear.

Fold round and glue together the worm shaft.

Starting from the handle end (with the two large grey squares) glue the first of the worm parts to the shaft using the red line for alignment.

Apply a small blob of glue to the second worm part at the point shown, apply some glue to the tabs. Glue the worm part to the shaft as shown so that it overlaps the first worm part.

Repeat this process until the worm is complete. Glue the two washers to the shaft.

Glue together the gear shaft and glue on one washer on the back of the shaft.

Glue together the two parts of the box. Make sure it is all square and accurate.

Glue the base onto the box.

Glue together the two worm bearings.

Thread the gear shaft through the holes in the box. Fit the remaining washer.

Glue the gear into place on the gear shaft using the grey areas and grey line for alignment. The gear must be free to turn.

Assemble the handle in three steps. Fold the parts round to make two square section tubes. Fold the tubes one into another and glue. Roll round and glue down the long tab to complete the handle.

Glue one of the bearings into place as shown above, use the grey areas of alignment and make sure that the edges of the bearing are lined up with the edges of the box.

Fit the worm into place then glue on the second bearing, again making sure that parts are lined up accurately.

Complete the model by gluing on the handle. That's it, a 24:1 reduction gear. Make it for some hands on experimentation or as the starting point for your own models!

Part of the Paper Mechanisms Multi-Pack - Buy and save!

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5
Sun 24th Oct 2010

I'm working on a worm gear for my next model. The plan is that I'll use it to make a simple mechanism and as the heart of a model. In a worm gear, for each complete turn of the worm advances the gear wheel by one tooth. This makes worm gears perfect for dramatically gearing down movement.

I've tried making them before but with only limited success. I had originally opened up a few washers and glued them to a drive shaft in a spiral. The problem I was that I ended up with a kink in the card like in the picture above. Looking at the parts it looked like the centre of the worm section where it glued to the shaft shouldn't be a square at all.

Time to crack open the maths. I had a spiral line (red on the shaft above) going round the 10mm shaft advancing 2.5mm across each face. Pythagoras - he say 10.3mm.

Working out the distance across the diagonal of the tube is another application of Pythagoras. The result this time is root 225 or exactly 15mm.

So, each corner of the centre of the worm part is a isosceles triangle with base 15mm, and sides 10.3mm. To make the parts I used the red construction lines to create the black line.

...and here is the resulting part, complete with orange square so you can see the difference.

The result works well with no kinks! Members can download the parts below and have a go.

Next step, gear wheel and box.

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