Spindle Crank

Being calibrated in imperial units, my lathe has difficulty turning metric threads. Although using my 'changegear' program gives me the closest pitch using my existing gears, I cannot disengage the halfnuts during metric threading without losing registration. The leadscrew and chuck will only be in sync every 127 turns!

Having seen mention of a spindle crank in various publications, and having a need for metric threads I knocked up a crank. If I recall correctly, this was the middle of the night some 3 years ago when I needed M16 threads rather urgently as the article needed to be shipped the next day.

The first foto shows the knocked up version. I used this until yesterday, seeing as it functioned perfectly even though the finish was somewhat rough.

However, having just finished the slitting saw arbor I spotted the crank and decided to upgrade it. There were three particular problems with it. Firstly, the studding handle is a bit hard on the hand that turns it. Secondly the studding sticking out of the main shaft would get caught on my sleeve during operation and I had not wanted to shorten it as I planned to put a longer nut on it. Thirdly, seeking out the spanner every time I wanted to mount or remove it was getting to be a chore. I use the crank for removing and mounting chucks/backplate all the time, as it gives a convenient lever for undoing the chuck and then an easy way to unscrew it too. Thus I might mount and unmount the handle several times within half an hour.

Having completed the knurling tool I decided to make a knurled knob for the taper drawbar. A bit of scrap BMS with a pre-existing 12mm hole in one end looked just the job. However, this proved to have too little leverage to get the taper tight enough to allow chuck removal, despite my chucks never being much more than hand tight. Therefore I cross drilled the knob for a 6mm tommy bar.

As per my usual stunt with studding, I 'polished' the threads where the nut runs with a wire wheel in the drillpress. This smoothes out the plating and makes the thread run very nicely under pressure. I also put a brass washer under the nut to reduce the turning effort.


The knocked up version. The only critical parts are the fit of the split part in the spindle (5/8" for ML7) and the angle of the taper plug. My first try was too steep and I had to rework it down to 20 degrees included angle. The plug and the seating for it must be a very fine finish so they can slide across each other easily. If they tend to bind, you can rub pencil lead all over the plug to aid release. I do this on my woodworking router collets and it works very well.

The flat bar is a bit of aluminum extrusion that was to hand at the time. It actually has a T section, making it very stiff for its size. You could use 20x5mm aluminum plate, or 20x3 steel plate, which I guess woudl be stiff enough.

Last edited February 20 2013 09:52:46.

This is the main shaft. It is drilled through to clear the M6 studding. The right hand end has a tapered section that matches the plug. The plug is pinned to the end of the studding and has a 10 degree taper (20 degrees included). Do machine the plug first, then the tapered hole, without changing the topslide setting. I used a junior hacksaw to cut the shaft into 4 sections so it can expand. The longer the slits the easier it will expand.

Last edited February 20 2013 09:52:47.

The vamped version. The changes include

1 - loose handle on the studding. The bit of tube from the scrap box turned out to be stainless despite coming from the brass box. This makes the thing a lot more out of balance so I'll have to be extra careful not to start the lathe with the handle in place. Yes, the locked nuts on the end of the handle ARE thinner than normal.
2 - knurled knob with some 15mm of thread and removeable tommy bar
3 - brass washer under knob
4 - seating for the knob and handle end were milled flat. These were previously filed to a 'it will work' state and I only has to take off 10 thou to true them up.
5 - the arm was shortened about an inch.

Note that the main shaft is pinned to the aluminum plate to prevent it turning. Although the steel to ali joint provides enough friction for it to work ok most of the time, now and then it would just turn, preventing either tightening or loosening of the taper. I put in a 2mm pin from a poprivett, which promptly sheared off. I then added two 3mm diameter stainless pins.

Last edited February 20 2013 09:52:46.

This is a very useful addition to any lathe. I use it for chuck changes, metric threading, threading fine threads to a shoulder, and tapping. I also use it when setting work up on the faceplate and in the 4 jaw chuck as I then have a quick way to flip the work 180 degrees to get 2 indicator readings which I then average. Move the part till the indicator reads that 'average' reading all around, and you're done.

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Last modified: February 20 2013 07:52:34.