The Squidwrench Power Wheels Racer needed a mounting bracket for its DC motor, so Matt handed me a precut steel slab and some drawings. I did a manual layout to get a feel for the sizes:
Yes, it’s slightly rhomboid & irregular on the sides; it’ll be welded to a U-channel. The front edge is the straightest and I scribed a perpendicular datum line over on the right, from which to measure the motor center point.
But then, realizing I’d have to mill the central hole anyway, I did what I should have done from the beginning and lined it up on the Sherline:
With the part zeroed at the center, everything has polar coordinates. The bolt holes are #10 on a 50 mm BCD, which is
G0 @25^[45+90*i]. Rather than writing & debugging a program, I did it all by feeding manual instructions into the interpreter; the
i gets typed as 0, 1, 2, and 3 by clicking on a previous command, backspacing, and retyping, which is both faster and easier than it sounds. The holes are drill cycles:
G81 Z-7 R1 F30
This being steel on a Sherline, the rule of thumb that says you can drill at 100x the drill diameter (in inch/min or mm/min, as appropriate) at 3000 RPM gets derated by at least factor of 10. I settled on 30 mm/min for a #10 drill (0.194 inch = 4.9 mm → 500 mm/min = hogwash) after trying the first hole at 50 mm/min:
The least horrible way to cut out the hole for the motor mounting boss involved chain drilling to excavate the most steel with the least effort. These center drill points are at
G0 @14 ^[15*i] with
i in [0..23]:
I drilled every even hole #27, then every odd hole #28, both at 50 mm/min, to get a thin web:
Then helix-mill downward with a 1/8 inch end mill at 1 mm per pass:
That started at 14 mm from the origin to match the hole circle:
G3 I-14 F100 Z-1
Then I switched to a 3/8 inch = 9.5 mm end mill to bring the hole up to size, ending with
G3 I-12.75 F300
A trial fit showed the hole was slightly off-round, probably due to a few mils of backlash in both axes, and slightly too small, because that’s how I wanted it. Flipped back-to-front, reclamped, recentered, ran the cutter around at 12.75 mm to clear the ovalness, then crept out to 12.8 mm, and it was all good:
That’s an easy fit with maybe 0.1 mm = 4 mil radial play around the boss. Better than that, I cannot do.
Lacquer thinner stripped the layout dye and it’s ready for welding:
Reminders for next time…
The drill feed on a rigid machine with plenty of spindle power is 100 x (drill dia) @ 3000 RPM. On the Sherline, in steel, 10 x dia is optimistic. Aluminum feeds run higher, but don’t get stupid.
Re-centering to the accuracy required for this job is a matter of noting the coordinates where the cutter kisses the perimeter across a diameter along each axis, adding the coordinates, dividing by two, moving to that position, and zeroing the origin. Do that in X, Y, X, and Y and it’s good enough. You could automate that with a touch probe, of course. Hand-turning the spindle with the cutter in place to feel it kiss the workpiece is fine, but use the same cutting edge on both sides of the diameter.
Figure the chain drill diameter thusly:
- Pick a reasonable drill diameter; #10 is about as large as you want on a Sherline
- Drill circle dia = final milled hole diameter – drill dia – 2 mm, round down to lower integer
- # holes = π x DCD / drill dia, rounded down to lower integer
- Hole angle = 360 / # holes
- Hole radius = DCD / 2
Wisely is it written that a man with a CNC milling machine has many friends.