Re: WR90-SMA transition update
Neil Smith G4DBN
I can't get a traditional flycutter to produce a really good mirror finish though. What happens is that when the cutter hits the edge of the workpiece, even with the table locked, there is an audible thump and some signs of resonance, so the surface has some weird waves from the harmonics created. You can just about feel them with a finger, but they are down at the 1 to 4 micrometre level when measured with a Mahr Millimess on my big granite surface plate. More cosmetic than anything, but annoying.
I found that laying some 2mm thick lead sheets on either side of the vice and putting big lumps of iron and/or buckets of sand on top really kills vibration. Also I cast and turned up a 2 kg lead flywheel collar to fit the flycutter spindle, and that helped by increasing the inertia of the tool. However, I had a cheap and rather nasty Chinese 80mm mill and arbour, which was almost unusable because the insert pockets were at varying heights, with up to 15 micrometres of variation. After seeing some advice on a machinists' forum, I tried removing all but one insert and runing a slower traverse speed and it was a revelation. The mass of the head gives a reasonable flywheel effect to reduce the harmonics and wibble in the Bridgeport spindle and drive (Mostly in the varispeed drive I am guessing). I might try try making a lead collar for the arbour, but I think the real answer is to make a copy of the Suburban Tool flycutters with a heavier head, maybe even machining pockets on the back of it to fill with lead, and using a 25mm lathe tool with a large radius insert. The 2HP motor might struggle with really wide cuts, but the biggest things I need to flycut are heatsink faces, and 70mm radius should be sufficient for those. I would need to find a way to counterbalance the off-centre mass of the cutter though, as there is a limit to how long a 25mm insert tool can be, and I'm NOT getting into machining insert pockets, for there lies madness.
I am limited by the spindle speed of the BP, so I have made up a
bracket to fit a 14,000 RPM spindle motor to the quill for doing
really tiny milling with end mills of 0.5 to 1.5 mm, but even that
isn't really fast enough. What I should do is make a coaxial
spindle with offset motor so the single bearings are in-line with
the BP quill, but finding precision bearings rated to 35k rpm that
would fit inside an R8 collet is pretty much impossible. The
bearings I used for the toolpost spindle on my lathe to cut the
0.5 mm (or less) choke grooves on the ends of the 122 GHz horns
were 35 mm OD and cost around £90 a pair, but have sub-micron
runout and can go to 48,000 rpm, which is almost fast enough for
the 0.5 mm end mills. Almost.
I'd certainly trade surface finish for the capability to do
complex curves and 3D tapering forms. Next move though is to get
one of the new large Anycubic Photon Mono X 3D UV resin printers
to make pattern pieces for sandcasting. The I'll be able make
arbitrary shapes for brackets, mounts, feedhorns, enclosures and
suchlike in aluminium or bronze without being dependent on my
On 10/09/2020 07:59, Robin Szemeti - G1YFG wrote:
-- Neil http://g4dbn.uk