The Saunders spin casting machine, part 1

Saunders Mark V spin casting machine

Fighting 15s does all its casting on a casting machine made by N Saunders Metal Products Ltd.  Saunders appears to have gone out of business in about 2002, but its casting machines live on. A number were apparently used by Games Workshop, and have therefore found their way to other toy soldier businesses: machines are also used to cast items such as fishing weights, and they occasionally crop up on eBay. I recently picked up a second machine that needs attention to get it working properly, and stripping it down provided the opportunity to take pictures and show how it works.

The Saunders is a robustly built, bob-weight machine. It is highly adjustable, and with experience and one simple improvement can reliably turn out high quality castings.

Bob-weight casting machines use weights on the end of three arms to apply pressure to a mould. When the machine is switched on, the platen spins, the weight-ends of the arms rise and the short end of the arm is pushed down, applying pressure to the top plate.

A mould clamped by the action of the weight arms. The protective casing of the machine has been removed for clarity. Never operate a spin casting machine without the casing in place and the lid down. The rubber bands are simply holding the arms in place on the platen for the photograph

Pressure on the mould is varied by the following means: speed of rotation, the heaviness of the arm weight, and the distance of the weight along the arm. In addition, the adjustment screw at the top of the arm affects where the force is applied on the top plate: the location of this is critical, and in part 2 I’ll look at the improvement that can be made to get the best results.

Speed control is rudimentary. The Saunders uses an AC motor whose speed cannot be controlled by a variable resistor – those only work on DC motors. Instead, it uses a variable speed pulley – the belt wheel on the motor’s shaft is split and has a spring, which has a dust cover, that applies pressure to keep the two halves of the belt wheel together. The belt face of this split wheel is sloped, and the motor can be moved in its frame to increase or decrease the tension on the drive belt: the more tension, the wider the split belt wheel is forced apart and the smaller the diameter of the drive wheel becomes, and hence the slower the main turntable spins (a smaller drive wheel takes more turns to turn the main belt wheel once).

Saunders casting machine: the drive belt wheel and pressure spring on the shaft of the motor. There should be a dust cap over the spring mechanism

The mechanism to move the motor assembly, however, is basic and difficult to adjust because it is hard to get purchase on the knob used to adjust the distance.

Saunders casting machine: speed adjustment. The motor sits on a frame that can be moved along a screw thread by turning the knob pictured on the right hand side of this photo. An indication of the speed is given by the pointer at the top of the frame
Close-up of the speed indicator, nominally revolutions per minute. This one is set to about 600rpm and indeed spins at that speed; it also spins at 600rpm when set to 700rpm… The dust cap for the spring mechanism on the motor shaft can be seen behind the belt guard, in line with the pointer.

In practice, because of the difficulty of turning the adjustment knob, you learn to work with the speed the machine runs at and change the other variables to cope. It is possible to convert the machine using a DC motor and a variable resistor to provide better speed control, but the cost might not be worth it.

The arm weight and its position on the weight arm, however, give plenty of control. The system may seem arbitrary, but as the pressure applied depends on the principle of levers it’s quite easy to adjust pressure to suit difference moulds. Even on casting machines operated by air pressure and a ram, discovering the initial pressure required to get figures to cast is a matter of guesswork improved by experience.

Saunders casting machine weights (left to right): 985g (2lb 2 3/4 oz), 260g (9 1/4 oz) and 165g (5 3/4 oz). A set is three of each weight (one for each arm); a full set for the machine appears to be three heavy weights, six mid weights and three light weights

My initial machine came with only a set of heavy weights (985g or so); the new machine has a complete set of three heavy weights, six mid weights (260g or so) and three light weights (165g). The arrival of these weights has given me more flexibility. It is possible to have some machined, which I will be looking into: the arm the weights screw onto is a 5/8in BSF thread (British Standard Fine, or 14 threads per inch). The weights may have accumulated metal around the milled edges and it’s important to remove this to get all the same-sized weights to the same weight.

As I said, how much pressure is applied depends on the formula for levers. And here’s the science bit… If you assume that the rotation speed is constant (it cannot be easily changed anyway), then lever formulae decide the force applied to the top plate. In short, a heavy weight at the top of the arm (nearest the pivot) can apply the same pressure as a lighter weight at the bottom of the arm (furthest from the pivot). The relationship is quite simple, involving mass and distance from the pivot point (fulcrum): M1 x a = M2 x b, where M1 is the mass of one weight and a is the distance from the fulcrum, and M2 is the mass of the second weight and b its distance from the fulcrum. Very crudely, without converting into appropriate units,  a 1,000g weight that is 50mm from the fulcrum is equivalent to a 250g weight that is 200mm from the fulcrum.

Where the Saunders limits this is the length of the weight arm, which is around 110mm, so you can’t use a mid-weight at one end of the arm to be the equivalent to the big weight at the top of the arm, because the arm isn’t long enough. With the locking nuts above and below the weight in position, the maximum distance a weight can be positioned along the threaded arm is about 75mm, or about 105mm from the pivot point allowing for the unthreaded elements of the weight and arm assembly. It means the heaviest weight at the top of the arm (it’s about 50mm from the pivot point) is, crudely, 980 x 50 = 49,000 “force units” – the same as two of the 260g mid-weights positioned 94mm or so along the arm. You also have to allow for the weight of the nuts and where the actual centre of mass is for the total weight if you want to be 100 per cent accurate.

Saunders casting machine: combined weights. A 260g weight and a 165g weight together, providing 425g in total – plus the locking nuts. Positioned about 50mm from the pivot point, I find this is ideal for most 15mm infantry moulds. Horses, which have a broad edge towards the outside of the mould, require a 990g weight at about the same distance to seal them effectively. The machine spins at around 600rpm – a higher speed would require less weight. The line marked on the weight and the top nut allow the number of rotations up or down the arm to be counted when adjusting the weight to suit different moulds

In general, once you find out the correct weight and distance along the arm for one mould, that setting will work for most other moulds of the same type. There may be slight differences, and more or less pressure may be applied by turning the weight down or up the arm respectively. Marking each weight with a line allows you to count the turns up or down, so you know how many are required to restore a weight to its initial position. If nothing has cast on a first spin you may be 10 or more turns off: slight flash takes only one or two turns to cure.

If you’re starting with a Saunders for the first time, having the weight as far away from the pivot point as it can go will reduce the likelihood of spraying metal all around the insides and over all the adjustable elements. Getting metal off the parts may lead to the sort of damage my second machine has suffered.

Using lighter weights gives finer control over pressure. That levers formula means that a light weight has to be moved a greater distance to have the same effect of moving a heavy weight a small distance. So one turn of a light weight is equivalent to a fraction of a turn of a big weight.

Finally, it’s important to get the adjustment bolt at the top of each arm set to come down at the same distance in from the edge of the top plate to ensure that pressure is even.  The top platen has three hard metal discs screwed onto it and on which the adjustment bolt lands: the right position is approximately in the centre of each of these discs. The discs are directly above the locking pillars that are screwed into the bottom platen and which have location studs onto which the top platen fits to stop it spinning off when the machine starts up. Basically, to set the adjustment bolts you hold the arm onto the top plate and adjust the bolt so it is central. Doing this ensures that even an uneven mould is evenly clamped by the machine. It’s a bit hit and miss, and the improvement I’ll cover in part 2 deals with how to do this quickly and reliably.

Saunders casting machine: arm adjustment bolts. Each adjustment bolt in the top of the weight arm is set so that when the bolt touches the hard disc set into the top plate, it lands roughly centrally. This places it directly over the centre of the locking pillar located between the bottom and top plates. The rubber bands are just there to hold the arms in position for the picture. Ignore the spirit levels – they are part of the dodge for part 2!

Lastly, a look at some of the basics of the machine. The Saunders should have an industrial machine stop-start switch. However, it’s what is known as a  “dangerous start” switch, because it’s possible to start the machine simply by inserting the plug if the stop-start switch has been left on. However, my Mark V also has a lid-operated safety switch that means it only operates when the lid is down, so replacing the stop-start switch is not a concern.

Saunders Mark V casting machine: on-off switch. This is not an NVR switch, and the machine will therefore start if the On button is left pressed in and the power is switched on…
Saunders Mark V casting machine: lid-operated switch. The lid-mounted bolt (upper left) presses down on a rod in the side of the machine that goes down to a safety interlock switch under the base – and therefore safely away from hot metal. In essence, the lid must be down for the machine to run, and lifting the lid cuts power to the motor

The machine I am stripping and rebuilding, however, has lost its original stop-start switch, and has been fitted with an immersion heater switch. It also does not have a lid safety interlock switch. I will be replacing the main switch with a no voltage release (NVR) switch as a priority, and adding a safety interlock switch to the lid later.

Replacing the switch: although the immersion heater switch will cope with the load, it’s not a safe machine switch. The new NVR magnetic switch will eliminate the possibility of a “dangerous start”

Next: modding the Saunders to make life easier

Update 22 July 2017: Included the term “variable speed pulley” under speed control as that is the technical name for the sprung, split belt wheel.



3 thoughts on “The Saunders spin casting machine, part 1”

  1. Hi, I have acquired a Mark 4 Saunders, found in an abandoned garage and in poor condition. Your blog is the only way I could identify what it was! I’ve put it on eBay in case you are interested 🙂

    1. Glad it was useful. I’ve forwarded the eBay link to people who might be interested. I have three and don’t need any more. One day they may be found in a similar condition to your one, next to my dessicated corpse. 🙂

Leave a Reply

Please log in using one of these methods to post your comment: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s