MadVac engraving on aluminium Y axis cover plate

First cuts

  The Madvac engraving (top) was the very first engraving job done with a beat up Craftsman 90 degree V bit in a 6061 aluminum sheet. The holes for the screws were also milled out rather than drilled.
Proofing the rouging operations in MDF before plunging into expensive aluminum. Here you can see the different machining strategies (as done in Visual Mill) to remove the material with relatively even chip load and efficient material removal path.

My day of reckoning came early on when milling aluminum molds which align to each other by three ground pins. The machine milled three 0.375"deep x 0.2502" diameter pockets spaced in a triangle about 5" and 20" apart into each half of the mold shell. Then ground steel pins of 0.2500" dia were slip fit into the holes and THEN the two halves of the mold were brought together. A major nerve wrecking moment ended in exaltation. Both halves of the mold came together is a nice and tight slip fit fashion resisted mostly by the air cushions trapped under the pins like air under a piston in a cylinder. This was the final proof to satisfy myself that the machine could mill to "half a thou" accuracy even with a piece of $#!^ router. With the pins nearly air tight in the holes, any compound misalignment or backlash outside of 0.0005" in X and Y direction and Z axis out of square would prevent the mold haves coming together. This is one of those moments when you thank higher powers for the providence to abandon the "acme screw idea" and going way out on a limb for precision ground ballscrews. If you are in this for a hobby, by all means, get acme screws, rolled ballscrews or rack and pinion but if you plan to make accurate fitting parts, bite the bullet and "buy right, buy once". You will rejoice when precise parts come off the machine.

Picture on the left shows the matching halves of polished 3D female molds. The surface was first surfaced flat with 1/2" mill. The rest of the milling takes place using quite a few of Visual Mills machining strategies using various sizes of flat and ball mills. The plate is a 2024-T6 aluminum (copper rich). It is a superb material to dry machine (with a wood router to boot!), almost like machinable wax. It is harder than common 6061 grade of aluminum, doesn't gum/gall and takes on perfect finish and polish. You cannot make this type of thing from 6061 material since it will not take polish well, is not homogenous and I did try.
Fair and smooth mold reflection of hydrodynamic rudder blade showing the quality of the all the software and hardware participating in its genesis. The foil curves were imported from CAD into Rhino which generated NURB surfaces, processed by Visual Mill CAM and imported into Mach2 for milling. Only a few years ago, this was possible to do only in large companies possessing expensive machinery and software. Now you can do this in your garage!

This is the ultimate reward of having your own CNC. The possibilities are limited only by your imagination (software and precision of machine components).

Tips on dry milling aluminum:
1) The best quality of finish can be achieved with the 2024 and the less common 5000 grade of aluminum. The 6000 grade which is most common is softer but it gums and galls and is pain to machine without low RPM and lubrication. Fortal which is a version of the 7000 grade Al is better than 6000 as well.
I had a very interesting experience where the mill got gummed up with 6061 aluminum and instead of cutting the material, it melted it by friction as the bit plowed throught the aluminum like soft plastic - the router did not like that very much.
2) Use the largest, most rigid bit suitable. It deflects less, chatters less, breaks less and achieves better finish. Use lower speeds with a router.
2b) use the shortest flute length that will do the job. This increases rigidity and reduces deflection, chatter and poor finish.
3)Tap Magic tapping fluid is excellent lubricant for "dry" machining. Better than WD40 in my experience. No galling with little smear of TM.
4) Use high positive rake high helix mills. I use Fulerton 3 flute carbide mills (ball and end mills) most often since they cut very well and do not chatter as much as the standard 2 flutes or even 4 flutes. Fulerton carbide bits are of superb quality (better polish and tolerances than I have seen on more expensive bits).

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Last page update: 12 May 2005