Mill / Captive Drawbar

Introduction

A drawbar is basically a really long screw or bolt which pulls a collet (or other tool) up into a taper. Sometimes the tool gets stuck in the taper, and so instead of the tool (“nut”) being pushed down, the drawbar (“screw”) is pushed up. A captive drawbar holds the drawbar down so it cannot be pushed up, this forces the tool out of the taper.

A captive drawbar is a neat addition to a vertical milling machine because it improves the usability of the machine in a subtle way. When you do a tool change, sometimes you have to reach for a drift or mallet as the drawbar gets “lifted up” rather than the tool being ejected out the bottom. It’s a 5 to 30 second time saver, and not the end of the world, but it would be nice if this never needed to be done.

The explanation

A captive drawbar has a shoulder that bears against a step at the top of the spindle. There is some play, but it is minimal. The shoulder could potentially be a cir-clip. The step is made by turning an offcut of round for a press fit (or threaded / loctite) into the top of the tube that the drawbar usually sits in, and drilling it through for the drawbar to pass through. A head for the drawbar needs to be made to go on top, I opted for a 19mm hex with a flange underneath, and then placed a needle thrust bearing between the flange and the step to reduce friction and I glued the hex on in the lathe with the tailstock to provide some preload. This is not necessary, steel on steel, or even brass on steel would be sufficient, but as I had the bearings it was a case of “smoke ‘em if you got ‘em”. Once the Loctite had set, I also drilled and pressed a cross pin through the top of the drawbar.

The results

The drawbar works quite well. I increased the size of the hex needed to 19mm as 19mm is a common size used around my machines now, and it meant that an odd 18mm wrench could be put away permanently, freeing more wallspace.

Future plans

A future option is to add either a motor or pneumatic wrench to the top of the drawbar (called a “power drawbar”) to make tool changes even faster again. This also is not a necessity, but might be interesting to try if my TODO list ever runs out.