Archive for September 19th, 2010
The scrap pile disgorged a chunk of aluminum plate exactly the correct size for a heat spreader that will mate eight power FETS to that heatsink. The catch: a 1-1/4-inch deep hole tapped 1/4-20 for about 3/4 inch at almost the right spot along one end. Rather than sawing off Yet Another Chunk from the original plate, I figured it’d be more useful to just plug the hole.
Note that this is somewhat different than the situation described there, where I screwed up by putting a hole in the wrong place. Here, I’m just being a cheapskate by making a piece of junk good enough to use in a project, rather than having it kick around in the scrap pile for another decade.
I turned a 3/8-inch diameter aluminum rod down to 1/4 inch for the threaded part and a bit under 0.200 inch to fit into the partially threaded end.
A real machinist would single-point the thread, but I just screwed a die over it. The narrow end is slightly larger than the minor thread diameter, which helped get things started. Then a trial fit, saw off the excess on the skinny end, and apply a touch of the file to shape the end to mate with the hole’s drill-point bottom:
I buttered up the plug with a generous helping of JB Weld epoxy and screwed it in. Toward the end of that process, the air trapped in the end became exceedingly compressed, to the extent I had to stop after each quarter-turn to let it ooze outward; eventually the hole gave off a great pffft as the remaining air pooted out. Unscrewed slightly to suck some epoxy back in, screwed it tight, and let it cure overnight.
Sawed off the plug, filed the rubble more-or-less smooth, then squared it in the Sherline mill. The heatsink prefers to sit on a nice, smooth metal surface, so I flycut the other side of the block to get rid of a few dings and the entire anodized layer while I was at it.
The epoxy ring doesn’t have a uniform width, because you’re looking at a cross section of the thread. The skinny part is the crest of the plug thread, the wide part is along one flank. Barely a Class 1A fit, methinks.
Locate the midpoint of the block’s end, center-drill, then poke a new #29 hole 20 mm deep (I really do prefer metric!) for an 8-32 screw. The plug didn’t move at all during this process, pretty much as you’d expect. The chips came out of this hole in little crumbles, rather than the long stringy swarf from the solid aluminum on the other end.
Using a simple peck drill canned cycle is just downright wonderful:
G83 Z-20 R1 Q3 F100
The rule of thumb is 3000 RPM with a feed 100 times the drill diameter. In this case, the drill is about 3 mm and calls for 300 mm/min, but the Sherline is happier with slower feeds. Maybe if I was doing production work, I’d push it harder.
A real machinist would have a milling machine with a servo-driven spindle for rigid tapping, but I just screwed an ordinary hand tap into the holes.
A bit of futzing converted a pair of solderless connectors into clips that capture the hooks on the ends of the heatsink’s springy wiffletree to secure the spreader to the heatsink. You can see the flycut surface peeking out from below the end of the heatsink. I should hit it with some fine abrasive to polish it out, but I think heatsink compound alone will do the trick.
The next step: drilling-and-tapping eight more blind holes along the sides for the FETs. It’d be really neat to have a servo spindle…