Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

Well, the tiny little drill bits arrived this afternoon, reminding me that I
hadn't figured out yet how to put the RotoZip to work. I also wanted to do
this in a way that every man can do for himself in his garage, sans
Bridgeport. So, eschewing the heavy machinery, I headed out to Lowe's for
common hardware. The short story made long follows, but the end results are
cute little holes, down to 24 mil, through 1/2" MDF, and not a single broken
bit.

I cheated. The whole affair is unreproduceable unless you, like me until
this evening, happen to have a spare 6" linear rail sitting around unused in
your part box. (THK HSR-12RM, 150 mm long, single truck; likely useless for
anything and everything else, but perfect for just this and only this. I
sniped it for well under a sawbuck on eBay last year, apparently planning
ahead for this moment.) Anyway, its attributes are slippery smooth, play
free motion along the rail axis. They're usually used in pairs: two rails in
parallel to absorb lateral torques, and two or more trucks on each rail to
absorb longitudinal moments. The lone truck is more than enough for the
machining forces a tiny hair strand of spinning carbide can generate.
There's still the spindle's wind-up torque to consider; 0 to 30k rpm in
under a second produces a pretty hefty jerk when it starts up. I'm just not
overly concerned, since it runs steadystate rather than on-off when in use.
In short, the HSR-12 is a perfect solution to an otherwise sticky problem.
(Pun unintentional, but very appropriate.)

The structure is made from two chunks of 16 ga mild steel. I wanted .10"
5051 aluminum sheet, but wouldn't you know it? Lowe's doesn't stock it.
You'll probably want someone to cut the steel for you; tin snips just won't
work. One piece about 8" x 8"; a second piece 3.5" x 6.5". I cheated, and
stopped to visit a friend who happened to have a foot shear and press brake.
Odd coincidence, eh? The alternative is a chopsaw if you have one, or an
abrasive disk in the tablesaw, if you have one of those. I saw 10" chopsaw
blades for under $5 there. It'll make an unholy mess of sparks, but can't be
too bad for the three or four straight cuts.

Bend the larger piece into an 8" long L, 2" x 6". This is the post for the
drill. More about that in a moment. Bending heavy gauge steel with the right
tools on hand is absolutely unremarkable. Without the tools, it's a project
in itself. I tried bending a scrap piece in a Record (metal) woodworker's
vise, beating the free leg down with a ball peen hammer. <grins! what fun!>
The first test turned out not too terrible, but it was a short piece, about
4" long. Bending an 8" long piece was a different story. The best try was
with a 1/2" x 3" bar behind it and lots of muscle. It's do-able with a bit
of care. The 2" leg is the important one to protect; clamp that in the vise
to make sure it stays flat. You'll be mounting the linear guide rail to
this. I decided to leave the somewhat straight sweeping radius rather than
deface it with the hammer. It's quite usable in this form.

The "post" is mounted to a chunk of whatever you have on hand. I squared up
a small piece of 6/4 rock maple on the jointer, and screwed the 6" post leg
to one face. The block turned out about 1-1/2" x 4 x 6 by happy
circumstance. The 6" length is the height, and the 4" face sets back a
little from the post face, giving a couple extra inches of throat under the
post. I mounted the post angle 1/2" up from the bottom of the, umm, post so
the pcb can make use of the extra room underneath.

The real piece-de-resistance, the real jewel of a find at Lowe's, was a
1-1/4" to 2" copper grounding clamp for $3.50. This is used in real life to
connect a ground wire to copper pipe, but for me, it's the perfect clamp for
the RotoZip. The RotoZip's throat is 1.70" diameter (sorta egg shaped, but I
doubt that was by design). Toss the tapped through back half, and keep the
slotted eared front half. A pair of 5/16" ceiling fan mount bolts holds this
to a birch ply clamp body. These have lag bolt threads on one end, and 5/16"
UNC threads on the other. The only really nerve wracking operation in the
whole project was ramming the lag bolt into the edge drilled ply. As careful
as I was, and as tightly clamped as it was, I still heard it delaminate. I
couldn't see a crack, but I know I didn't imagine it. Maybe I did; it holds
just fine. Oh yeah... the clamp body is 3/4" birch ply, 3.5" wide, and
somewhat longish. I used a 1-3/4" holesaw to cut the hole, 2.310" from the
edge (the sole critical dimension in the project, and depends on how the
rear lug is mounted), and then split it on the diameter, leaving two flat
surfaces for the clamp bolts. In hindsight, MDF or good dense maple will
work better because of the edge drilling.

That's about it. The rear mounting lug of the Rotozip mounts to small L with
a 45 deg notch to clear the body. Two 4mm bolts holds this to the 3.5" 16 ga
sheet, and four 4mm bolts hold that to the guide's truck. Three #10
woodscrews hold the clamp body to the front plate. Three 1/4" lag bolts hold
the post holder post to a 3/4" MDF work table. It'll need some rubber feet
under it clear the lag bolt heads, but that's a side trip for tomorrow. I
think I might route two short T-slots down the depth of the table, to hold a
guide bar for gridded hole patterns. And by fortuitous chance, the grounding
clamp has a 1/4" ID lug where the ground wire would be screwed. This will
make a handy mount for an LED "headlight", much needed.

Testing was anti-climatic. The spindle glides nicely under its own weight. I
push up to move things around, and guide it down to make a little mound of
dust. A just-right chunk from the scrap box props the head up to change
bits. Changing bits took a few tries to figure out but it's getting almost
second nature already. I hold the depth ring with the tip of the forefinger,
hit the spindle lock with the thumb, and twist the collet lock nut with the
other hand. All by feel, of course, because there's no room underneath to
see. I expected to have to re-align the kajillion and one bolted
connections, but it cuts just fine right off. Not sure how I'll get an
indicator under the collet to swing it, but it doesn't look to be an
emergency this moment.

I started out thinking I would build this as a temporary measure, only until
I could draw up something proper, but it works so well I'm loathe to change
it. The secret ingredient, of course, was the linear guide. It's hard to go
wrong with all that precision ground everything working for you. There are
some things that need addressing. For one, the L post is too flexible; it
tweaks a few degrees when the Rotozip starts up, and stays there until it
shuts off. No side-loads when drilling with hair, so no emergency. The whole
post needs rethinking; maybe build up an MDO box girder instead. For two,
the linear rail is bolted directly on the non-precision sheet metal. I'll
bed it in powdered metal epoxy at some point, but there's no harm leaving it
for the short term. Time enough tomorrow... Speaking of which, UPS tells me
I have a box arriving from Circuit Specialists. Just in time, too, since
Pulsar's box got here today. :)

Thanks for the help, guys. I think I'm well on the way.