October 27, 2020

Milli a new composite mill kit

2 min read
Hi All – This maybe a bit early to start but here goes. I’m building...

Hi All – This maybe a bit early to start but here goes. I’m building a CNC router to build the parts and moulds for this Mill. The parts will be made from infused fibreglass, steel fibre reinforced epoxy or aluminium grit epoxy. I make these materials using a vacuum casting method, look up Tetrium-S. Choosing the best material as we go. The router build can be found by searching for Brevis-HD build. I shall build Milli using MDF for the proto parts and use those parts to make the moulds for the actual parts. The aim of the first round of design is to determine major flaws in geometry or function. The mill will be a lifting gantry design to remove the usual issues with bearings and geometry being crammed into the saddle. Other work has been done (in another thread) on using twin rails or single rails per side to improve rigidity with the single rail seeming to be OK. The lifting gantry is required to allow high Z axis for mould work and other high Z applications. It also removes the saddle from the design, so the tool plate can be very stiff as it only moves across the gantry not also in the Z. There are a couple of lifting gantry designs in the forums… 500x500x500mm as a round figure envelope is the starting target. It aims to be as stiff as a standard benchtop machine being able to cut steel and aluminium, It will use a BT-30 spindle… The BT-30 maybe a bit big, opinions on the spindle would be appreciated as its my first mill build…

I have started by using YaGs parts as they exist and morphing the sub assemblies into Millis geometry. Having many years experience with testing laminates means I have a very good data base of mechanical properties for the fibreglass and carbon fibre. Generally its stiffer and stronger then the quoted values of epoxy granite so will be a very good material to use. I can also use carbon fibre if more stiffness is needed. For example a general fibreglass laminate will have E=25GPa a bending optimised laminate will be E=35GPa and a carbon fibre laminate will be 50GPa general and 80GPa optimised for bending. Both carbon and fibreglass test well over 500MPa in tension/compression and flexure so are stronger than mild steel and high strength aluminiums. Moulding the parts allows for geometric optimisation and combining parts into less parts. It get me away from using standard size tubing and sheet metal bending as I have done in the past. The CNC thread journey begins… Peter

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