For a given frame size, the machine will be limited in the size of object that it can work on. So what size frame is needed? First we must look at the design of the machine and see where the limits are.

To maximize the X size, the nut that goes on the X leadscrew should be as near to the center of the X platform as possible. This will allow the center of the platform to get as close to each side as possible. As the center will be on the inside, the platform should remain stable. Also the platform should be able to pass over the top of the X motor. This would allow an object nearly as wide as the machine to be worked on. The movement in the X direction will be limited by the distance between the “legs” of the machine. Also the bit must be located halfway between the sides of the machine.

To maximize the Y size, the bit should be positioned so that it is half way between the back and the front of the machine. The nut for the Y leadscrew can be anywhere from the back of the platform to the center, because travel is limited towards the back by the upright that is holding the rotary tool. Also the platform should be able to pass over the top of the Y motor. If the center of the Y platform moves beyond the front most X rail, then there is a danger that the platform could tip over, therefore the distance between the X rails limits the range of motion of the Y platform. This would allow an object with a depth of about half the depth of the machine to be worked on.

To maximize the Z size the upright holding the Z platform should be tall enough to accomodate the largest object, but also allow the bit to get down to the Y platform for small objects. This can be achieved by using rails of a suitable length to allow for the range of motion. If stability is needed when the tool is at the highest point, weight could be added to the front end of the “legs” of the machine.


So it seems to me that it is important for the rotary tool to be centered in the X and Y directions, which is a feature not present in the McWire original design. Also for a desired object of size A x B, the machine should be built so that is it A wide and B x 2 deep (measured on the inside of the pipes used to construct the base). The X rails should extend half the X platform width on each side past the pipes. The Y rails should extend half the Y platform depth beyond the front most X rail. The X platform width should be at least half the distance between the “leg” pipes. The Y platform depth should be a bit more than the width of the X rails.

The stepper control board I have chosen has support for limit switches which could be employed to stop the platforms from moving too far and either coming off the rails or causing damage to the machine.