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« on: June 16, 2009, 04:27:58 PM »
I'm in the process of adding a toolchanger to my BP clone mill. First step is a power drawbar. Rather than use the very common impact wrench approach, I wanted something that would leave the collet in place, and simply pop out the tool. I will be using Tormach TTS toolholders. So, taking a page from the design of many VMCs, I've created a design that tensions the drawbar using a stack of Belleville washers. The drawbar is initially tensioned in the normal manner, by tightening with a wrench, to compress the Belleviille washers enough to create the desired drawbar tension (about 2000 pounds, as it turns out...). To release the tool, all that's required is to compress the Bellevilles a bit further, then push the drawbar down to pop the collet free of the taper, and the tool simply falls out. The trick is to provide a means of compressing the Bellevilles that does not place that 2000+ pound load on the spindle bearings. To accomplish that, a small arm is rotated in under the Bellevilles, to take the downward load.
Since, on my first attempt at this, I was largely shooting in the dark as far as the required forces, and some of the other side-effects of the design, I've found I need to make a few minor modifications to the design to get it working properly. But, the first attempt, shown in the below pictures, does, in fact, work pretty well. The biggest problem is I under-estimated the drawbar tension required, and the air cylinder in the pictures is not quite up to the task. A larger cylinder will be here in a day or two, which should do the job nicely. The other "gotcha" is that the long drawbar extension sticking up above the head "whips" at speed - not good. So, it will be surrounded by a cylindrical guide, rigidly attached to the spindle, that will rotate with the spindle and prevent any whipping.
The assembly is fastened to the head, by bolting an adaptor ring to the flange that retains the upper bearing outer race. The upper bearing inner race retainer is replace with the drawbar guide tube. The adaptor has two guide rods rigidly attached. The rest of the assembly "floats" on these guide rods. The guide rods have concentric springs on them such that the whole floating assembly is spring-loaded downward. To do a toolchange, the quill is run fully up, which causes the top of the drawbar to contact the plunger in the center top of the drawbar assembly. This compresses the springs, placing a downward force on the drawbar assembly. This is the force that is used to pop the collet free, and will probably be on the order of 100 pounds - hardly enough to harm the spindle bearings. A small air cylinder then flips an arm in under the flange that lsupports the bottom of the stack of Belleville washers on the drawbar. Once that arm is in position, the large air cylinder is activated, which compresses the Bellevilles between that arm, and the plunger. Since the whole assembly is floating on the guide rods, none of the Belleville force is transmitted to the spindle - only the initial downforce of the springs on the guide rods is transmitted to the spindle. Once the Bellevilles are compressed sufficiently (0.030-0.050"), the collet pops free of the taper, and the tool falls out. Once the new tool is installed, the air pressure is removed from both cylinders, and the collet is once again locked by the tension in the Bellevilles.
This is a totally bolt-on design, which requires absolutely no modifications whatsoever to the machine. Drawbar tension can be adjusted over a very wide range by simply changing the number and type of Belleville washers used, and, if necessary, the strength of the air cylinder. Collet-popping force is adjustable by adjusting the pre-load on the downforce springs. Initial position is set by simply adjusting the length of the link on the air cylinder. All very simple and flexible. Tool changes become almost instantaneous - literally less than a second to engage or disengage the drawbar.
For the toolchanger, I am taking a very simple route, since I only need a few tools - I will have a single-row rack of tools mounted to the T-slot along the front face of the table, holding perhaps 8-10 tools, or perhaps 4-6 tool racks mounted at either end of the table. The tool pick-and-place will be handled entirely by moving the machine, with no additional mechanism required. This does cost a small amount of useable table travel, but since I have a 49" table with 34" of travel, I am more than happy to lose a few inches of workspace in exchange for the huge increase in productivity I'll get from the toolchanger.
Regards,
Ray L.
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