Category: Recumbent Bicycling

Faced with the daunting prospect of converting half a dozen 4-40 brass screws into battery contacts by hand filing, I did what I should have done in the first place: turn it into a CNC project.

It’s quick-n-easy:

• mill the head flat and 0.5 mm thick
• shave off the sides

I grabbed the screw in the Sherline vise, touched off XY on the head (close enough to being concentric for this purpose), and touched off Z on the nut supporting the screw. For the next few, I’ll eyeball the Z touchoff at the bottom of the head, rather than the nut, because the heads don’t quite sit flush on the nut.

They dropped right into place, without any filing or fiddling! Well, the second one did. I had to tweak the dimensions slightly to make the answer come out right. But that’s one of the advantage of hammering out simple G-Code like this: change two lines and wham you’re done.

The heads show some tool marks, but that’ll just make the silver solder stick better. Right?

Herewith, the G-Code…

(ICOM IC-Z1A battery pack shell)
(Battery pack contacts)
(Ed Nisley - KE4ZNU - June 2010)
(Vise clamping on threads, XY orgin on central axis, Z=0 at *bottom* of screw head)
(Tool table used just for Axis previews and to activate "manual" changer via M6)
(Tool change @ G30 position above length probe)

(-- Global dimensions & locations)

#<_Traverse_Z> =        5.0
#<_Cutting_Z> =            0.0

(-- Get started ...)

G40 G49 G54 G80 G90 G92.1 G94 G97 G98        (reset many things)

M5
(msg,Verify XY=0 on screw axis)
M0

(msg,Verify tool touched off at Z=0 on *bottom* of head)
M0

(debug,Verify vise clearance around head)
M0

#<Contact_Width> =        4.1            (X axis metallic contact - minus a smidge)
#<Contact_Head_Dia> =    5.5            (recess for 4-40 head)
#<Contact_Head_Radius> = [#<Contact_Head_Dia> / 2]

#<Contact_Head_Depth> =    0.7            (recess depth - plus  smidge)

#<Mill_Dia> =            1.98            (end mill diameter)
#<Tool_Num> =            20
#<Mill_Radius> =        [#<Mill_Dia> / 2]
#<Mill_RPM> =            5000
#<Mill_Feed> =             50

(debug,Verify #<Mill_Dia> mm end mill)
M0

(debug,Set spindle to #<Mill_RPM>)
M0

F#<Mill_Feed>

(--- Flatten the head)

G0 Z#<_Traverse_Z>

#<X_Step> = [0.5 * #<Mill_Dia>]
#<X_Limit> = [3 * #<Mill_Radius>]
#<Y_Limit> = [#<Contact_Head_Radius> + #<Mill_Radius>]

#<X_Coord> = [0 - #<X_Limit>]

G0 X#<X_Coord> Y[0 - #<Y_Limit>]
G0 Z#<Contact_Head_Depth>

O<Head_Trim> DO

G1 Y#<Y_Limit>
#<X_Coord> = [#<X_Coord> + #<X_Step>]
G1 X#<X_Coord>
G1 Y[0 - #<Y_Limit>]
#<X_Coord> = [#<X_Coord> + #<X_Step>]
G1 X#<X_Coord>

O<Head_Trim> WHILE [#<X_Coord> LT [3 * #<Mill_Radius>]]

G0 Z#<_Traverse_Z>

(--- Trim the sides)

#<Arc_Radius> = [#<Contact_Head_Radius>]
#<Half_Width> = [#<Contact_Width> / 2]
#<Angle> = ACOS [#<Half_Width> / #<Arc_Radius>]
#<Half_Height> = [#<Arc_Radius> * SIN [#<Angle>]]

G0 Z#<_Traverse_Z>

G0 X[0 - #<Half_Width>] Y[0 - #<Contact_Head_Radius> - 3 * #<Mill_Dia>]
G0 Z#<_Cutting_Z>

G41.1 D#<Mill_Dia>
G1 X[0 - #<Half_Width>] Y[0 - #<Half_Height>]

G1 Y#<Half_Height>
G2 X#<Half_Width> I[#<Half_Width>] J[-#<Half_Height>]
G1 Y[0 - #<Half_Height>]
G2 X[0 - #<Half_Width>] I[-#<Half_Width>] J[#<Half_Height>]
G1 Y#<Half_Height>

G0 Z#<_Traverse_Z>

G40

G30                    (back to tool change position)

(msg,Done!)
M2

Having obtained eyeballometric measurements from the case, the next step was to doodle some shapes on graph paper and pencil in the dimensions. My motivation for not using CAD is simple: it’s easier (for me, at least) to doodle using a pencil.

The outside of the case had pretty much the same features.

The inside, of course, bore no resemblance to the battery pack; the shoulder and whatnot will support the circuit board.

The original battle plan was to build the case in at least two layers, simply because it had to be so deep the Sherline couldn’t reach to the bottom with any rational end mill. It would probably make more sense to glue up four sides on a machined bottom, but that requires actual skill.

This became the Front layer, with Front and Rear faces. The Rear layer attaches to the back of this one. In this picture, the Front layer is on the bottom, taped to the radio.

The two layers peeled apart, with the Front layer to the right. You can barely see the internal shoulder and external tabs.

As described there, I made a fixture and a small plate to hold 2.5 mm and 3.5 mm plugs in the proper alignment for the mic & speaker jacks on our ICOM IC-Z1A HTs. Knowing I was going to rebuild the interface boxes, I made several spare plates and tucked them into a small bag against future need.

Time passes.

Come to find out that the new gratuitously gold-plated 2.5 mm plugs in my stash have a slightly thicker front plate that doesn’t quite fit into the recess I milled in the plates for the old nickel-plated plugs. So I set up a little nest in on the Sherline’s table, snuggled each plate into the corner, and poked a 9/32-inch end mill 1 mm down into the plate. The net change was a 0.5 mm deeper recess. Sheesh.

I’d originally create the recess with helical milling, but I recently uncovered a stash of shiny-new end mills in a box: 9/32 is 7.31 mm, just about exactly what you want for a 7-mm dia plug front plate surrounded by a blob of fast-curing epoxy.

This epoxy just holds the plugs in the right position for wiring and initial testing. After the cable checks out, I’ll smoosh a blob of epoxy putty around the whole thing as before.

The circuit board is 30-mil, double-sided, half-ounce (I think) copper on glass-fiber stock, direct-etched by rubbing ferric chloride solution onto it with a sponge.

Used the CNC Sherline to drill the holes; the G-Code is now tailored for my Sherline mill and tool-length probe station.

The copper layers as a 600 dpi PNG file:

The top copper image (on the left) is reversed so it comes out correctly when you’re doing toner-transfer etching.

I didn’t bother with a silkscreen, because I don’t have a soldermask and there’s no room for text around the parts anyway.

The four vias at the corners mark the edge of the board. Trim it with tinsnips (or a shear if you have one), then introduce it to Mr Belt Sander until the edges pass directly through the middle of those via holes. Round the corners a bit so they fit into the case recess atop the mounting shoulder.

Put Z-wires in the small round vias (the ones that don’t have any other traces) to connect the top and bottom ground planes.

Put Z-wires in the other round vias to connect a top-side signal to the corresponding bottom-side trace.

There are three jumper wires across the bottom; with only two layers I don’t get all bothered about embedding the last few. Those vias are square.

I don’t have any way to do plated-through holes, so solder the wires to both sides of any vias with traces on both planes. I admit I missed two of them on the TT3 ribbon cable.

The big empty space around the positive power terminal prevents the ring-lug connector from shorting to the ground plane. Now that I think of it, there’s no need for an empty space on the bottom copper, but it doesn’t do any harm.