small mill for machining steel. Max spend £500 including computer

[Andrew Wilding]

The ways were built up with PTFE. This was sheet purchased from E bay and had an acid etched back to allow it to be bonded to the ways using epoxy resin.

The PTFE was then machined an scraped to alignment

[Andrew Wilding]

Ball screws are the obvious choice for a cnc mill but alas the budget did not extend this far. I opted to make the best out of conventional screws. I selected 16mm dia by 4mm pitch trap leadscrew stock. This has a max pitch error of 50 microns per 300mm (2 thou per 12") which should be sufficiently accurate to meet my accuracy target. I may experiment with lead screw mapping later on. The 16mm stock is sufficiently stiff over the lengths I am using to only require supporting at one end. At the supported end I am using a preloaded pair of angular contact bearings to reduce support backlash as much as possible.

I am using wear compensating antiback lash nuts from delrin. There are made as a main nut and slave nut biased together using a torsional spring made from piano wire.

Below shows the nuts being made on the small lathe. the thread was roughed in the lathe to ensure parellelsim with the nut. These were finished using a tap made from some of the stock.



The slave nuts were castellated together on the mill. Note the DIY dividing head. This was made when I only had a lathe and uses change gears to provide most of the divisions required in my shop.



This shot shows the three lead screws, bearings and nuts assembled on the bench



Total cost of leadscrew assemblies about £25 as opposed to at least £200 for similar pitch accuracy ball screws.

[Jonathan]

Looking good.
I'm interested in your nut design - do you have a drawing that you could post here?