Posts tagged machine
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.
The primary purpose of this blog is to document specific technical projects that I will have a go at. First up is an attempt at building a low cost but precise CNC milling machine. I will follow the design by Tom McWire shown here: Easy to Build Desk Top 3 Axis CNC Milling Machine.
So now you are wondering, why bother if how to build this machine has been described on the internet already? Because my aim is reproduceability and accuracy. The problem right now is that if two seperate people decide to each build a machine from the instructions, they will end up with slightly different machines that perform differently. For example the dimensions of the machine and which stepper motors used are missing. I want a set of instructions that people can follow to yield predictable results.
Also missing is a description of the kind of accuracy that can be expected. At this point I am not sure if this machine can generate an accuracy good enough for my needs. Hopefully it will. If I can figure out what design decisions to make to achieve the accuracy I need, and document them, then other people will know up front exactly what they need to do to also get the same accuracy. My aim is to be able to engrave commemorative coins. These coins have a small area where a name can be placed, and this will require precision.
I hope to also make some minor improvements here and there, and it might involve a few iterations before the machine works well enough. One of Tom McWire’s aims was to build it in less than $200. I also have the same aim, but if a situation arises where I can spend a few dollars more to increase precision then I will do it. For example I won’t be getting old motors from printers because they are all different and someone else cannot then come along and follow my instructions to get the same machine. Instead the specifications of the motors I use must be known.
(picture from Easy to Build Desk Top 3 Axis CNC Milling Machine)