December 12, 2012

Pneumatic Power Drawbar for a Knee Mill

Earlier this year, I got a new (to me) vertical milling machine.  I love the machine, and learn something every time I use it.  One of the things I learned early on was that I'm not tall enough.  The machine is almost 7-1/2 feet tall, and the drawbar hex is at just short of seven feet.  I can reach the drawbar, but I can't see it at the same time.  Though I've gotten pretty good at getting a wrench on it by feel, I still have to feel around up there to free and draw up the collets.  I'd seen some examples of power draw bars on the web, both commercial and home-brew.  A DIY power draw bar seemed like a good project to use the mill for, so I started formulating a plan.

Most if not all of the DIY and power drawbars I saw were based on pneumatic impact drivers.  Most also used pneumatic cylinders to lower the driver onto the drawbar hex and to actuate the driver.  I thought the air driver was a great way to go, but I eventually favored a manual mechanism for lowering and controlling the impact driver.  I wasn't afraid of designing the pneumatic control, but felt that I'd get a better feel for engagement with a mechanical linkage. In some ways, the linkage for the manual approach was more challenging.

I'll show the finished device in place first so some ofthe later pics will make more sense:

The power drawbar is actuated using the ball handle that hangs down to a comfortable height.  There is a sort of dual-action parallelogram linkage that lowers the driver when the ball is pulled straight down, and actuates it forward or reverse when the ball handle is pushed forward or pulled back.  It is easy to feel the engagement of the driver with the drawbar hex, and the actuating action is tight and immediate.

Here's an outline on how it was done.

I roughed out and then finished most of the parts from 1/2" and 3/4" 6061 aluminum.  I've been playing around with anodizing lately, so I also anodized them black.

This is a post at the back of the unit that carries the actuator arm mechanism.  The piece of 1/2" key stock is the actuator arm for the impact driver and can pivot on a shoulder bolt fixed to the post assembly.  The springs allow it to pivot either way, but return it to the central position.  The two 1/2 x 1/8 steel pieces act as stops for the springs.

Here is the back post mounted to the base.  Two chunks of 3/4" drill rod serve as slides for the impact driver carrier.

The driver carrier is a piece of 3/4" aluminum bored to accept two oilite bushings to ride on the slides.  The thin rods are to hold the driver in the carrier.

This is part of the control handle and linkage.  It is just welded up steel painted black.

The linkage connects to the actuator arm with that little universal joint.   It's a little hard to see, but the lower arm of the parallelogram linkage bears on those two hex nuts to push the carrier down.  The little rubber caps are on some adjustable studs, and push against the actuator paddle on the impact driver.

This is a keeper machined to fit the top of the impact driver.

I added a miniature inline oiler since doing it by hand would be a royal pain.

I set the built-in "torque adjuster" on the driver to be wide open, and control torque with a dedicated pressure regulator on the wall behind the mill.  Seems to work great.  The only problem now is that it is now even trickier to do the drawbar by hand, and I've got at least one leak somewhere on my compressor, so I can't leave it pressurized all the time, and have to charge it up every time I want to use the mill.

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