Harbor Freight Drill Motors

I hit a stumbling block last week working on a robotics project.  Without getting into details, the motors I was using were just not well suited to the task.  After much googling  I discovered that “Battlebots” forums and blogs are amazing resources.  Lots of trial and error, engineering, and budget shopping has already been done by the teams who build these rugged RC vehicles ranging from lightweight to a couple hundred pounds.  I personally have never tried to build a battlebot, so I never understood the level of sophistication that goes into these things.

I found that it’s popular to harvest motors from cheap Harbor Freight drills.  (Actually I need to give credit to hearing about this first from a fellow Asylum member.)  The common drill used has a 3/8″ chuck.  I decided to pickup the larger 1/2″ chuck model that has a higher torque rating.  The motors are larger as I expected – 775 size as opposed to 550.  The drill is item 68850.

For $40 per drill I got a motor, 2-speed gear box, clutch, NiCd battery, charger, and plastic body with trigger that I’ll probably use in a future project.  The disappointment is that the gearbox is plastic and the clutch isn’t easily disabled.  Also, the drive intentionally has maybe 10-15 degrees of free-floating movement (slop) where the gearbox output is mated to the chuck shaft.  I assume this is for a small amount of hammer-like action that might help the drill impart torque to a fastener.  The plastic would be fine, except there’s no good way to mount it to anything (except the drill body it was designed for).

Parts I will replace with machined aluminum

Here you can see what a full motor-gearbox assembly looks like.  The first drill was more fully disassembled so I could measure all the parts with calipers – you can see the motor and, in the background, part of the clutch assembly.  The parts are all very similar to the smaller 14.4v drill, which is covered in great detail here.

The arrows show the two plastic pieces I will replace with machined aluminum parts – the housing behind the clutch assembly and the motor mount plate.  Last night I machined two of the housings.  Below is the WAD drawing I went by (whiteboard aided design ;).  One thing to note is that the middle section was actually machined to 1.448″ so that the metal ring gear in the final planetary stage could be pressed in.  This gear is the part that is captured by the clutch and allowed to slip under high torque.  By pressing it in, I hope to keep it from turning at all.  I may also insert a pin as extra insurance.

One thing not shown is that I also drilled and countersunk 3/16″ holes in a 1.625″ frame so I could mount this gearbox to a bearing housing that will support my drive axles.  I plan to screw the whole thing together with 8-32 fasteners and some Al tubing, much like the Magnum 775 Planetary Gearmotor is assembled.

I’ll post updates as I machine the other parts and put it all together.



A mathematician, hacker, fabricator, and all around do-it-yourselfer. I like building my own tools, learning new technologies, and breaking stuff.

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