I continued to play around with design ideas but a new topic became an hot issue.. This was whether to use a ballscrew or rack and pinion. There are a number of pros and cons for either system, but after a long learning curve really it became obvious that if you are serious about your build, then 1 to 1.5 metres is a reasonable and practical limit to using Ballscrews in this application. I have seen many a build at 2.5 metres long using 16 or 20mm diameter ballscrews, and any physics tells you that this is not adequate. the potential for whipping is practically inevitable if you think along these lines.

As my thought train rolled down the rails, I increasingly became passionate about building a machine correctly with the equipment fitted to it, specified correctly for the application. But at this stage in the project I was still playing with ideas. I now turned my attention to the design of a Z axis. The idea around this was that i could use the assembly as a an excercise to A build part of the build and B test what level of precision and fit I could achieve with just basic tools. I had a floor mounted Sealey Radial Pillar Drill to work with for the drilling of assembly holes etc.... I shopped around a bit and settled on a compound slide table to use on the Pillar drill to help make minor location adjustments. I could also easily index dimensions for drilling holes reasonably accurately..

A pattern was developing here and that was that you continue to obsess about the design and if you lose or gain some working footprint. As i moved towards a finalised concept for the project, the thought of losing 50mm in Y did not bother one little bit if it was for the right reasons.

Key Learnings....

• Work backwards with your design from the foot print you require. You might be tempted to draw a 1metre square assembly and then become surprised at the end that you are only achieving a 600x400 worktable footprint.
• Something that I will never understand aslong as I have bones in my feet is why some people design Z axis assemblies that are 800mm tall when the base of the gantry is only 200mm from the top of the machine bed. Really, please someone tell me whats going on there. Don't do it. You are just creating a large wobbly weight on the top emphasing every little bit of flex.
• Consider revolving your design around the purchase ground tool plate or bar. This way you can be sure that the widths and thicknesses are accurate and consistent.
• Consider using dimensions that you can replicate easily with stephensons blocks. These are very accurate and make first assemblies very easy to get very accurate the first time round.
• Don't ever think that your rails are going to help you provide rigidity. This is not how they are designed. They are not design to add strength. Design your Z assmebly correctly and plan for limit switches and dust/coolant protection early if thats what you want...