OPEND CNC Mark 1

A first venture into the world of DIY CNC.

Back to part 1

Part 2 Guide rail theory

Three guide rails set each part of the machine to act in one axis only. In conventional XYZ  dimensions there are 6 axes of motion; X,Y and Z in a linear direction, and X,Y and Z in an angular direction. It is the responsibility of the 3 guide rails and the associated bearings to restrict the movement of each part of the machine to one dimension only.



The above picture illustrates the technique used to limit the possible motion to one dimension.
The Red bearing set prevents motion along the linear Z plane.
The Blue bearing set prevents motion along the linear X plane.
The Green bearing set prevents rotation along the Y axis.
The Yellow bearing set prevents rotation along the X axis.
The Purple bearing set prevents rotation along the Z axis.

So with restrictions in 5 planes, the only motion the part is free to move in, is the Linear Y plane.

One problem with this approach is that it requires a perfect spacing between bearing sets and perfectly even thickness rails to ensure that the system acts without backlash.



The above image illustrates a system that follows a guide rail surface, rather than a rail, eliminating the need for perfect thickness and perfectly spaced bearings. Only one bearing in each set is indicated, because, although the other is there, it is spring loaded and keeps the fixed bearings firmly on one surface of the guide rail. This technique enables one to produce a system without guide backlash, and without the need for precision and expensive rails.
It is true that cheaper rails might not guide the system along a perfect path, no surface is perfectly smooth, but the errors will be consistent, and because the errors are repeatable, they can be compensated for in software if an even higher level precision is required.

Further, the blue rail sets 2 axis, 1 linear and 1 axial. No special alignment is needed between the blue and the other rails.
The 2 bearing sets on the Yellow rail fixes another 1 linear and 1 axial dimension and the system is only linked to the green rail by the one  set of green bearings. This means that a miss-alignment of the yellow and green rails will only result in a slight angular shift of the part.

In Conclusion

A system can be produced with no backlash using cheap parts. Only one pair of rails needs to be aligned, and the resultant error can be minimized by designing the system so as to keep the pair of rails as far apart as possible.


Forward to part 3

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