Mini-Lathe Spindle Crank

Threading and tapping stock is one of the uses where a lathe excels, indeed this outstanding feature can be exploited even further by using a fully manual control­—­ a crank in this case, allowing for safe, minute, and precise operations. A spindle crank is thus a handy accessory to have that can allow the operator to perform tasks which can be nerve- wracking whilst under power, and safer for both man and machine. Most of these devices are homemade affairs and range from functional but plain to a rather elegant, swan necked apparatus. A good design should be secure, easy to install, have plenty of leverage, smooth operation, and finally, be easy to detach. I chose to add a knob instead of a bolt for tightening the tool, it gave a cleaner look and eliminated the need for a wrench.

Ensure power to the lathe is removed before using the spindle crank, hazardous centrifugal forces may be generated if the machine is energized while this accessory is installed.

A recumbent bicycle skeleton picked up roadside gave me all the inspiration I needed to envision the crank and handle portion of this project. The arm and pedal axle are made of a forging, which is superior to anything cast or bent up since by design, they are expected to undergo rigorous and punishing use. The spindle which drives a lathe chuck is typically a hollowed tube which allows the passage of round stock through it for machining, and has no native means to be externally driven other than the integral motor. The most popular method to attach a spindle crank is by inserting an arbor rod and camming it internally, using either a slip wedge as in a typical bicycle handlebar- to- fork mounting stem or an expansion wedge as used in French bicycles. Either method works well, and I chose the slip wedge as it is less complex to fabricate then the expansion type yet works just as effectively.

While I had the gearbox cover off, I also enlarged it's spindle access hole to accommodate the crank arbor's hub which was larger then the factory bore allowed for. Using a hole template to draw the enlargement, and a rotary tool with a straight burr mounted in it made short work of this task.

I used 1” [25.4mm] steel rod for the body of the clamping arbor, turning down the shaft to a slip- fit of only a very few thousandths clearance, the closer to the spindle I.D., the better the action. In the middle was a hub that would provide a stop for the wedging action. I also turned down a portion of the opposite end to approximate the dimension needed for milling 4 flats that the handle would engage. I finished up this process by center- boring a hole sized for a 5mm thread and tapping it about 1” [25.4mm] in length from the short wedge end, the rest of the rod was generously oversize drilled to clear the draw-screw which will move about and produce the clamping action.

I mounted and trued a grinding wheel on my radial arm saw in preparation for grinding 4 flats that would mate with the crank handle's square drive end. Turning down to near- net dimensions saved a lot of tedious effort to fabricate this feature, and so after grinding the first facet, I ensured a right- angle grind on the next by using a square to position the ground shaft flat vertically, and so all remaining ones were done sequentially in similar fashion.

There is a slight taper from hub to outboard end, which I estimated at 1- 2 degrees, so I tilted the head after fashioning the squares and made that happen by grinding too, although a file could also have been used.

I cut and threaded both ends of a length of 5mm silver steel salvaged from an old printer to serve as the drawbar which acts upon the wedge end and, through an adapter, terminates in the drawbar knob. Now the risky part, cutting off the wedge head. No problem whatsoever, I just marked out an angle of about 30 degrees and hacksawed it off, finishing up by smoothing the sliding surfaces with sandpaper, cleaning, and applying a very light touch of grease, being careful to wipe any excess from the shaft body which should remain in a high- friction dry state.

Use of a washer between the knob and shaft end greatly reduces friction when tightening the lock knob, and makes the action a toolless one. A ¼- 20 setscrew was added to the crank hub to lock it to the shaft, and a handle was fashioned and installed, an Instructable was published on that aspect of the project and so won't be repeated here.

My design was based on internet postings and assets in hand, and is as unoriginal as a clothespin, however it is a sorely needed adjunct to my lathe operations and will be put to hard use for the rest of it's life, so I wanted as robust an end result as possible with no potential for troublesome maintenance, and I think I got what I wished for.

Instructable member qthurtle posted his version on January of 2014,and is well worth a look also, if one is contemplating making their own:

https://www.instructables.com/id/Lathe-Spindle-Hand...