THE PLAN:
 

The Construction of Wedgehog was very 'make it up as you go along'.  We had a vague idea of the set out of the components we had, but no exact designs were ever made.  I find this approach much more fun and much faster as long as you know what you're doing.  If you want to sit down at a table for a week drawing out what you're robot's going to look like then that's fine, but unfortunaely we didn't have the time or the money to do that.  So we just tend to do what works at the time, and it normally does.  This approach also keeps the cost right down as you can use components that you've got already rather than designing something on paper and then having to order all the correct sized nuts and bolts.

We had been looking out for bits and bobs to use on the robot ever since we had first heard Robot Wars was coming to the UK.  Joe's room at home is full of useful rods, wheels, old TV's and most of all, dangerous chemicals.  This is mostly due to a junk shop at the end of our road on in London.  Its called Lianolds and every night they chuck out a new load of junk they didn't manage to sell.  We've picked up hundreds of things from there in the past and as a result have a large stock of weird components ranging from drain pipe to fire extinguishers.  People often stand at the bus stop watching us mauling pieces of machinery, hacking away at solenoid valves or lengths of electrical flex that we think might be useful.

This was also where we picked up the wheels for Wedgehog.  An old dirty 'Mother Care' pram had been thrown out and it didn't take us long to wheel it up to Joe's front door step and strip it of all 8  foam rubber wheels.  We had just received the drills from bosch and so we decided to build a prototype to test their power and battery life.  This involved finding some wood to make a mock up chassis.  A piece of 1/2 inch ply wood was scavenged out of a builders skip from across the road (no expense spared!).  We took the drills, wheels and wood up to the Shadow workshop to begin work.

 

Wheel with axle attached   The first job was to fix the wheels to the axle so that they could be driven by the drills.  The axles where made from two lengths of 7 mm diameter steel rod cut to about 130 mm in length (red in the picture).  Three flats were cut into one end of the rod at 120 degrees to each other to allow the three jaws of the chuck grip the rod with no chance of it slipping.  A 2 mm hole was then drilled radially through the rod to take a 30 mm length of  2 mm diameter steel rod (purple in the picture).  The smaller rod was passed throughout the hole and each end was bent down at 90 degree so that the ends of the rod face the hub of the wheel.  A large 80 mm diameter steel washer (yellow) was bolted in over the plastic hub of the wheel and two holes drilled in it to take the ends of the small rod.  The ends of the rod were pushed down into the two holes and folded over the underside of the washer to stop it coming loose.  This locks the axle onto the wheel completing the wheel assembly.
  Wheel with bracket.

The wheel mountings were designed and made by Joe from 3 mm thick 40 mm wide steel strip (red).  The strips are bent at 90 degrees to create a flange which can be bolted on to the chassis.  The flanges sit on the under side of the wood so that is nearly impossible for the wheels to be pulled off without a massive failure of the chassis.  Joe also added lengths of brass tube (yellow) which were braised into place to act as bearing surfaces.  This seemed a better idea than letting steel rub on steel as since brass is so soft, if self lubricates the bearing surface.  We also found that by gripping the tubes with a pair of pliers they could be bent to the right angle to align them with the axle accurately, look I said it wasn't going to be hi-tech OK!

Here a section through the wheel assembly is shown.  The brackets are hatched in. This shows the flanges fitting in under wood chassis.

   
Using a jig saw, two holes were cut in the ply wood to take the wheels and brackets.  The brackets were at first screwed on to the underside using wood screws.  The drills were fixed onto the ends of the axles the chucks by tightening the chucks up on the flats on the axle.  The test consisted of either me or Joe kneeling on the ply wood board whilst pressing down the triggers on the drills.  This had the result of driving us erratically across the work shop floor normally towards some kind of very sharp looking table leg or broken glass or cliff edge.  Although covered in bruises and with stubbed toes we survived the tests and proved that even without the castors on, the drills were able to push a person around.

The Weapon:

I then started work on the weapon.  I will admit now that it wasn't a great weapon.  It consisted of a pneumatically powered pick axe.  We bought the pick axe in Bricklane market one Sunday morning for £2 along with 5 castors with mounts (20p each).

A pulley was bolted on to the side of the pick axe at one end using 8 mm bolts.  A 6 mm steel axle was passed through the pulley and axe.   Two similar roller bearings were scrounged from the Shadow stores which kind of fitted the ends of the axle and were fitted into two blocks of maple being used to support axe on the chassis.  The maple blocks were then screwed down to the temporary base using 1/2 '' wood screws.  This enabled the pick axe to revolve about the end of its handle back and forth freely.

We managed to find a pneumatic piston in the Shadow stores to use as the actuator.  The piston is about 50 mm in diameter with about 150 mm of travel.  We found that at about 5 bar it could lift me up quite happily.  The piston also contained a return spring which acted to recoil the piston when the air pressure was released.  This was great as it meant that the hammer could be made to reset its self by using the spring.

By rapping a piece of rope around the pulley and pulling, the hammer could be made to fly back and forth.  We needed 180 degrees of travel in the hammer for full effect and we quickly realized that 15 cm of travel from the piston wasn't going to translate into 180 degrees of rotation.  This lead to loads of arguments about what we should do.  At this point everyone was getting very pissed off with each other.  We were all very tired and nobody wanted to listen to anyone else.  I had already previous built a system that worked fine but Hugo wanted to build his own version. What ended up happening was that both me and Hugo made our own versions of the hammer.  My idea was to put a linkage in on the end of the piston to increase the range of movement but reduce the force provided.  It acted just like a lever with the piston end attached close to the pivot and the hammer end further away, therefore increasing the velocity ratio and travel in the hammer but reducing the static mechanical advantage.  Hugo's idea was to use the piston directly but put a smaller pulley onto the shaft of the hammer, with a smaller pulley the rotary movement is larger for the same lateral movement in the rope.  Both ideas should have had the same result with the only difference being in the size.  My design had already been built and we knew it worked but it was much larger than Hugo's due to the extra space needed to accommodate the linkage.  When my design was laid down on the floor it was about 20 cm wide, Hugo's was only slightly wider than the cylinder saving about 50 mm.  As we wanted to make the robot as compact as possible Hugo's design sounded good.  So he went ahead and built it.  When they were both finished we tested them against each other.  For some reason that I still can't work out my design seemed to be more powerful  (can anyone figure out why?).  The two systems should have been identical in the amount of force they provide to the hammer but something strange was happening. It may have had something to do with the velocity of the rope as in my design the end of the linkage is moving much faster than the piston, whilst in Hugo's the rope moves at the same speed as the piston.

A compromise was reached when I had the idea of having my linkage running vertically not horizontally.  This meant that the mechanism was higher but much thinner.  It also meant that the drills could almost be put bang up against to each other with just enough room for the rope to pass between them.
 

Holes were cut for the wheels and the brackets fixed on.  We found that with the new chassis material the screws holding the brackets on originally were too weak and pulled out easily.  They were replaced with bolts going all the way through the wood to large washers on the top side.   The wheels were put in and the drills attached to the axle stubs.

    All the varioius other parts were bolted on to the chassis such as the radio and speed control gear, weapon module, castors and of course the armor.
   

This is a list of all the parts we used in Wedgehog, where we got them from and how much, if anything they all cost.
 

 

PART	MATERIAL	NUMBER	OBTAINED FROM	PRICE
GSR 9.6 VES-2 Drills		2	Bosch	Free
Wheels	Foam tire, plastic hub	2	Dumped Pram	Free
Axles	Steel rod	2	Shadow Stores	Free
Chassis	1/4 Inch Ply		Local DIY Store	£2
Armor	Aluminium Sheet 3 mm (British Rail Information Sign)	1	York Way scrap metal	£2
	Dexian Shelving	1	Dumped at end of road	Free
Rivets	Steel / Aluminium	145	Anchor Supplies	£6
Castors	Plastic / Steel	2	Bricklane Market	40p
Pneumatic  Piston	Steel/Brass	1	Shadow Stores	unknown
Pick Axe	Steel	1	Bricklane Market	£2
Solenoid Valves	Various	2	Taken from old washing machine	Free
Wheel Mounts	Steel 3 mm Strip	1	Shadow Stores	estimate £3
General Wood	Maple/Pine/Ply	Lots	Skips / Shadow Stores / Joe's  Dad	Free
Compressed Gas	360 ml Butane Canister	3	Photographic Suppliers	£12
Nuts 'n' Bolts	Steel	Many	Shadow Stores	estimate £10
Pulleys	Aluminium / Plastic	2	Proops/ Shadow Stores	estimate £5
Radio Control Set	Futaba Skysport 6A	1	Ripmax	£140
Speed Controllers	Sung-Ji RF 10	2	Ripmax	£70
Channel Mixer	Various	1	S.M. Supplies	£20
Wiring	Mains Flex	Lots	Home	Free
Connectors	Copper	Lots	Shadow Stores	estimate £3
.	.	.	.	.
.	.	.	Total	£275.40