[HBR] THanks for Offers of Parts and Opinions

[Walt Hutchens]

Scott said:
> I don't have a drill press, and I do dislike the hole quality and
> inaccuracy of my hand drilling. I may need to buy a small drill
> press.

A drill press will give you the best accuracy if you are just marking
the chassis and drilling. If, however, you center punch your holes
first you can get even better accuracy with a hand drill, and I don't
mean the electric kind.

The reason?  A hand drill will center itself on the punch mark while a
drill press will tend to put the hole where the drill comes down and
generally 'walk' a little even if you get it exactly right.

You can do fine work with a hand drill for the holes that must be
accurate and a 1/4" hand held variable speed electric when you have a
bunch to do and they don't have to be too precise, as when making the
cutout for an Eddystone dial.

A few thoughts on chassis metalwork:

1. Use only aluminum chassis and panels: Steel doesn't begin to be
worth the trouble. Cover the chassis with paper. Get it on TIGHTLY so
it won't move and keep that sucker dry until the holes are all
punched.

2. Arrange the parts. You need more space between tubes and front
panel than you might think, especially around switches, pots, etc.
Double check conflicts between stuff on top and what goes underneath.
Dual section tubes need more space than single, oscillators, mixers,
and product detectors need more than amplifiers. Don't forget tie
strips and grommeted holes for wires to go from above to below, for
the panel, transformers, etc. How does the power get into your
chassis? Fuses? Antenna and speaker connections?

Triple check layout around meters and loudspeakers: The large size and
irregular shape can make trouble.  It's best to allow a little extra
space in case you have to replace one of these parts with one that's
not identical.

Using a beam tube such as a 7360 or 6JH8? NO MAGNETIC FIELDS NEARBY!
That means no meter, speaker, power transformer, output transformer,
or choke.  A panel mounted loudspeaker goes at the opposite end of the
chassis from the local oscillator and even then you may have trouble
from vibration.

Do you have enough room for filter capacitors? Variable cap rotors
when open? Long shafts and shaft couplings? Room to grasp and remove
plug in coils? Extra space around oscillator coils for stability?

It's good to check even established designs that you are copying: You
probably don't have exactly the same parts.

3. Measure the locations of the parts adjusting slightly so things are
lined up and make a dimensioned sketch. From that sketch lay out the
paper covered chassis.  Either put lines down the left-right and
front-rear center of the chassis and make all measurements from them
or always measure from the same side and the front or rear.

You can do layout for socket holes in two ways: Either mark just the
center and use the socket itself as a template for the mounting screw
holes, or carefully make three 1/8" holes in a scrap as a jig and lay
out all the holes by marking through the jig. 'Easier' vs. 'more
accurate' but both can work well.

Sockets are not precisely made so the 'socket as template' method will
often require adjustment of the holes with a small file.

Lay out the panel and mount it to the chassis temporarily: Clamp it on
(pad the clamps so you don't mar the panel) and drill a tight fitting
hole for one machine screw so you can maintain the same alignment.
Then put a couple more screws in locations that will later be occupied
by controls for a rigid setup and take off the clamps. Make
measurements and do layout for the dial mechanism and tuning cap.
Check this layout several times: If these parts are not perfectly
aligned you will have problems with binding and/or backlash.

Flexible shaft couplings will take care of almost invisible shaft
misalignment but NOT the real thing.

OLD PAPER TEMPLATES (AS FOR THE EDDYSTONE DIAL) WILL BE INACCURATE.
CHECK THEM AGAINST ACTUAL PARTS MEASUREMENTS.

Remove the panel and center punch all the holes. Put a block of
hardwood or metal against the inside of the chassis right under the
spot you're punching so you don't dent the chassis. When drilling put
a block of wood up inside the chassis so that you don't dent the
chassis when you cut through.

I generally drill the panel first, then do the matching chassis holes.
For the Eddystone dial cut necessary clearance holes for the mechanism
and flywheel. Put the panel on the chassis. Temporarily mount the
front panel parts. Then before starting to drill the rest of the
chassis holes put all the parts on the chassis AGAIN, over their
respective punch marks. Better to have a spare punch mark or two than
a tube that won't fit!

4. Write or sketch each part on the chassis paper so you'll know what
you're drilling for. Use your hand drill to drill everything 1/8"
diameter unless a smaller hole is required.

5. Now go around and re-drill the holes that need to be larger, up to
1/4" or so. It is very hard to drill perfect holes greater than 1/4"
diameter in sheet metal without a drill press and more rigid setup
than is available in the usual home shop.

Up to ~1/2" a taper reamer alternated with a rat tail file works well.
Start with a 1/4" hole, go slowly, use the reamer from both sides, and
clean off the burrs as they form with a pocket knife.

For still larger holes up to 1-1/2" chassis punches can't be beat: use
the reamer to enlarge the center hole for the punch bolt but of course
nice work isn't needed. Many punches have marks on the sides that can
be aligned with lines through the center hole to get precise
placement. Note that octal sockets are of several different diameters.

You can do just as nice work without punches at the cost of an extra
10 minutes per hole. Instead of drilling the 1/8" center hole, ring
the c. punch mark with a Magic Marker for the approximate diameter
then use a compass with a sharp point to scribe the proper size circle
in the MM black. Then use an electric drill repeatedly, staying just
inside the scribed line until you can cut out out the piece with a
pocket knife. Work very carefully with a half-round file to clean up
the hole. When you get within 1/32" or so, take a dowel or piece of
PVC pipe not much smaller than the hole and wrap in 60 grit sandpaper,
work this around in the hole trying the fit on your part regularly
until it is both smooth and the correct size. For the last tiny bit
switch to 150 grit.

The drill-around-the-edges technique is also a good way to make
rectangular holes. These are cleaned up with a flat file. Wrap the
file in sandpaper (as above) and slide laterally back and forth to
finish. For large holes (that darned Eddystone cutout!) use a wood
block wrapped in sandpaper for finishing.

The expensive sandpaper works FAR better than the cheap stuff and will
only cost you a few $ extra for the whole job, with lots left over.

Alternative for small rectangular holes: grinding disks in a Dremel.
Watch out for the chuck hitting the surface and chewing it up.

I don't like metal nibblers. The nibbling goes very slowly, it is hard
to do precisely and the tool mars the surface.

6. I like to finish with 300 grit sandpaper followed by 600 grit
watersand used wet, then a 'brillo' pad. Some people use clear plastic
spray on the top of the chassis but I've never been able to get the
stuff to stay on.

7. Sheet metal screws are rarely satisfactory for chassis work: Use
machine screws and nuts with lockwashers.

For mounting shields and very small items you can drill and form
threads for 3-48 machine screws in almost any aluminum chassis. Buy a
couple of 3-48 'thread forming taps' (Google ...): These push the
metal aside rather than cutting it away. This does two things: (a)
thickens it, and (b) work-hardens it around the hole. Those scrap
command sets will furnish all the screws you need. Drill well
undersize, chuck the tap in your hand drill and use a lubricant --
margarine works well. DO NOT REMOVE THE UPSET METAL OR BURRS.

This sounds flimsy but it is PLENTY strong and with a screw every
1-1/2" or so, extremely rigid. For example it is excellent when
mounting a tuning cap on a bracket.

Holes in the part to be mounted must be considerably oversize (say
1/8" for 3-48 screws) to allow for the upset metal around the hole.
Best technique for shields and brackets is to lay out and center punch
the holes, clamp the part in place, and drill the (small) tap holes
through both pieces. Then remove the part and enlarge the holes in the
non-tapped piece to 1/8". Parts assembled this way go together as if
they were milled and doweled in place.

Alternative when clamping isn't practical: PRECISELY lay out two or
three well separated holes on both parts. Finish these up and
temporarily mount the part, then proceed as for clamping.

Try every new technique on a piece of scrap once or twice before doing
it 'for real.' Yes, even the layout for a couple of parts, center
punching and drilling.

An HBR-scale project might require 100-200 holes; the first time you
do one you might need 20-40 hours for the chassis and front panel
metalwork. After you've done a couple you can get down to half that.
$100 worth of hand tools (some from fleamarkets) is enough to do
essentially perfect chassis and panel work if you practice each new
technique first and don't rush things.

And yes, you can do it on your kitchen table. Take a 2'x4' sheet of
1/2" plywood and temporarily nail four strips on it to keep your
chassis from moving while you're drilling (etc.) When you're ready to
wire, fasten on a couple of softwood pieces to support the chassis
upside down without damaging topside parts.

Walt
KJ4KV