How To: 4 Jaw chuck. –Clutch basket re-tooth
This weekend’s session was spent playing around with the 4 jaw chuck on the lathe. If you have a part that is not round, and you want to turn it, you’ll run into some trouble. A conventional 3 jaw chuck has three jaws, which all move inward to clamp onto a round or hex part. A scroll (basically a rotary cam that looks like a plate with threads) engages the jaws, and pulls them inwards uniformly. Thus you can simply turn a single chuck key and be sure that your part is held concentrically. The drill chuck in your 18V Makita or drill press uses a similar operating principal. It was invented in 1902 by WI Jacobs, and Jacobs brand chucks are still among the best you can buy.
So, if you have a square part, or a rectangular part, or some other irregular shape, you mustn’t use the typical 3 jaw chuck. But a 4 jaw will work well. Each of the 4 jaws can move independently of the other three jaws.
The two parts shown on the left have threaded portions on the right that could be done on a lathe with a 4 jaw.
Here are two quick views showing how they could be gripped:
If you look at the first image of this article, the 4 jaw is larger than the 3 jaw chuck. That is why I was using it, as the sprocket that I needed to cut was too big for my 3 jaw. This new sprocket had 36 teeth, which was a nice number, as it is divisible by 3 or by 4, and could have fit onto either chuck. But the 3 jaw was too small… It could have been mounted on the faceplate, but the 4 jaw was faster and simpler to use. Also, it is better practice to reference the teeth than the center hole when locating a sprocket. We don’t really care where the hole is, but the location of the teeth is important. (hard to measure teeth location though..)
The only problem with the 4 jaw is getting the part to be concentric, this is not automatic. But with an indicator and a light touch on all 4 of the jaw adjusters, it was soon to within .001”. That runout is fine; recall that a single strand of your hair is about .002” or .003”.
My 4 jaw chuck has removable jaws, which can be inserted backwards to grip parts from either the part inside hole, or the outside surface. In these photos, the jaws are set up to grip from the outside.
The video shows the part spinning slowly and the indicator holding a steady reading.
You might recall from last time, the Eclipse clutch basket had sprocket teeth that were worn out. The process of cutting off the old teeth, then modifying a new sprocket is the same as shown here. One difference was that the basket had male threads, so I cut female threads to fit. (about 6” x 20tpi). After the parts were threaded together, I used an end mill to cut a 1/4” hole and installed a roll pin to keep the two parts from unthreading. (a roll pin is much like a dowel pin, but it is made up of bent sheetmetal, and it will fit into a hole that is slightly too large or small for a tightly-toleranced dowel pin).