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I could make up some more oak worktop strips to go between the studs to give the alloy plates some internal reinforcement. Though given the nature of the task I could simply produce some internal squares out of 3/4" birch plywood. These would slide over the studs and axis shaft to provide some extra resistance to any local distortion caused by compression loads from the cross screws.
These
10mm [3/8"] plates are probably sturdy enough that they are unlikely to
bow. Though it is important to maintain the stiffness of the box
structure to avoid flexure. Most commercial mountings use castings for
this purpose.Before rushing off for new saw blades I did some homework online. It seems I need a coarser toothed blade with hooked teeth. Along with kerosene, paraffin or petroleum as a cutting fluid. A 13 mile cycle ride to the shops produced some new specialist aluminium blades from DeWALT. DC2163. DC stands for deep cut.
The new blades worked wonders with a small dab of lamp oil applied with a brush after every 2cm or 3/4" throughout the cut. Much cleaner cutting without jamming or snatching and obviously much quicker than yesterday. I managed the first full length cut of 42cm, 16.5" cut in only 1/4 of an hour. The next at much higher speed in only 5 minutes! Experience showed that less pressure and medium speed worked better than low speed and heavy pressure. A setting of half the available oscillation on the Bosch seemed best.
After smoothing and filing the new cut
edges I clamped up all four sides to check alignment. It seems my
original try-square was off by a tiny fraction. I found another square
and nulled it by reversing the stock after the first sharp pencil mark
on a square plate. Both perpendicular lines matched perfectly. Further
checking of each transverse cut on the wider strips proved the angle to
be slightly out on only two ends. This produced tapered overlaps of the
narrower cladding strips because the wider strips were being slightly
tilted towards the diagonal. The 6" width is slightly oversized when all
four plates are pressed tightly together. More angle grinding to narrow
the overlap.Half way through smoothing and squaring the saw cuts my angle grinder ground slowly to a halt. That meant more expense to buy a new one. [Cheapest DeWALT for about £40/$50] My third angle grinder in a couple of decades and I really don't use them that much. Not to mention the cost of purchase doubling just to buy some flap disks and a plastic backing pad and a few coarse paper disks just in case. They didn't have any mixed packs of sanding disks. The real profit is obviously in the sanding disks and accessories.
Several hours of angle grinding later I had four more plates to complete the PA bearing housing. 40 grit cuts fastest but tends to clog with lumps of aluminium. I tried running steel against the disk and that helped clear some of the built up junk. The flap wheel 80 grade just seemed to polish more than cut.
Finally I could prop it all up to admire the semi-finished appearance.
It looks too top heavy! The RA wormwheel pushes the declination housing
too far above the top PA bearing. The cylinder depth is not a matter of
choice as it is only the length of the vital clamping bush. It looks as
if I really ought to move the RA wormwheel down to the bottom of the PA
to reduce the overhang above the top bearing. This is not so easily
achieved at 55N PA altitude angle compared with lower latitudes. It will
probably require an offset fork base to allow enough clearance for the
11.5" wormwheel. I could reverse the top bearing to bring the inner race
extension inboard. This would alter the top-heavy appearance rather
than the stiffness of the arrangement. Access to the bearing's grub
screws would need consideration. An image rotated to show the 55 PA altitude. The PA does not look quite so understated now. Nor does the RA wheel appear so heavily cantilevered. A substantial base enclosing the PA housing, to allow altitude adjustment, will help to regain the visual balance.
Several hours of angle grinding later I had four more plates to complete the PA bearing housing. 40 grit cuts fastest but tends to clog with lumps of aluminium. I tried running steel against the disk and that helped clear some of the built up junk. The flap wheel 80 grade just seemed to polish more than cut.A made up image with the PA tilted at 55° and the RA wormwheel removed. The inner bearing race cannot be easily inverted to reduce the visual overhang. A small ball is deliberately fixed on the outer race to stop rotation of the spherical outer race. This ensures the lubrication groove matches the placing of the grease nipple on the cast flange housing. Important for a heavily loaded bearing but not for a lightly loaded one which will rotate only about once a day.
Click any image for an enlargement.
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