The Mighty Micro-Mill Drawbar Hammer
Goal: Construct a drawbar hammer for a Harbor Freight Micro Mill, making setups quicker and easier.
Background: The Micro Mill has a quirky metric drawbar. It uses an M10x1.5 thread, topped by an 8mm square head. So you can't use an off-the-shelf hex socket, only an open-end wrench.
Construction: It's a fairly simple project. I was still fighting the 7x10 mini lathe and struggled with surface finish. But it exceeded my expectations in other areas. It seems easier to use than Bridgeport style or similar type of drawbar tools I've used.
As far as materials and sizes go, it's up to your stock on-hand. I first made a strike with a piece of 3/4" brass punch. I drilled and tapped a 3/8"-16 hole, screwed in a bolt, and cut off the head. For the body assembly, I turned a 1"Dx1.5" steel rod, drilled and tapped for a 3/8"-16 to accept the brass strike. Rather than taper the steel, I opted for a 1" diameter Al 6061 disc (about 3/16" thick) and beveled one edge. It's just a chamfered Aluminum washer (spacer).
I considered a square socket opposite the strike, but in not having a broach and considering the extra filing of a drilled hole I opted to assemble the pieces and mark the balance point. Drilled where marked through the body, and tapped for 3/8"-16. Then counter-bored for a 1/2"Dx6-1/2" rod (handle) on one end and for a 5/8"Dx1-1/4" rod (socket) on the opposing end.
For the socket, I milled a 5/16"(7.94mm) wide by 0.375" long slot, 0.275" deep in the side of the 5/8" rod and skewed it ~0.003" on the last pass on a side (8mm finished). Then I turned the opposite end down to 3/8", cut-off, and threaded to 3/8"-16. The handle is also turned down and threaded to 3/8"-16 and all parts assembled. A thin washer for the socket aligns it perpendicular with the drawbar.
Conclusion: It works great! Better than I expected because of the lowered moment arm and angular displacement like an open-end wrench (as opposed to a socket-style drawbar hammer).
Notes: This project gave me a chance to tweak more bugs out of my Mini-Lathe and Micro-Mill. You'll notice my affinity for a 3/8"-16 thread. Because its a sturdy SAE size similar to M10x1.5. I bought a discounted tap and die from Enco. The tap is made in Bosnia and is absolutely gorgeous in quality and finish. I bought them to make another drawbar and standardized tooling. But I'm staying clear of that project until I find a boring head that I like. They're typically a 3/8"-16 thread and I need a #2 Morse Taper (2MT). -1/30/2007-