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Tool Capacity

Posted by Tom Benedict on 14/01/2012

Recently I had an odd lesson in tool capacity. Before launching into what I learned, I need to go back a little:

Around 2003 I made some vacuum feed-throughs for one of our instruments. Because of some space constraints and the decision to use a flex print rather than discrete wires to carry the signals, it wound up having a really oddball shape on the outside so the connector could be tilted at an angle. This let the flex reach the inside surface of the connector without getting crimped.

The design required a 3D contoured surface on the outside. At the time I didn’t have freedom of the shop at work, which includes a large CNC mill, so I made the parts at home. I’ve owned a Taig benchtop 4-axis CNC mill since about 2001. This was before I did all of the rework on the control electronics, so making those parts really stretched the capacity of the tool, but it worked. I made the two parts and swore I’d never do that again. Since then I replaced the control electronics, so doing contouring at home is well within the scope of what my mill is capable of. Nonetheless, I never did make more than those two original parts.

A recent problem with the electronics on one of the feed-throughs put this back on my plate: I need to make four spares. I hadn’t even opened the files since 2003, so this involved some head-scratching. But eventually I got everything back in order. These days I’m the only machinist at our headquarters location, so I do have full run of the shop. I know I can do these at home, but what a great project for the big CNC mill! It has a much bigger spindle than my Taig as well as a larger work envelope. The combination of servo motors and ball screws give it better backlash characteristics than my Taig, which uses open loop stepper motors and lead screws. But the real kicker is that it has flood cooling, whereas I have to stand by my Taig to squirt coolant at the tool while it runs. The mill at work would be perfect for this!

Or so I thought…

My first surprise came when I tried to load the finishing toolpath onto the mill. Because it’s a contouring job and because it’s a finishing path that moves the tool in many many small steps, the file wound up being 3.4MB. No problem for my little Taig at home! I use USB memory sticks to move part files around, so I never even batted an eye when I ran the part in 2003. But the mill at work uses floppies. 1.44 MB floppies. Oh… The file wouldn’t even fit.

The mill at work has, in almost every way, a larger capacity than my mill at home: work envelope, spindle horsepower, maximum slew rate, etc. But it’s limited by how large a part file it can run. In that respect my little Taig, which I can pick up with two hands, wins by a landslide. The only limit to the size part file I can run on it is the size of the memory stick I use. And if those are too small, I can finally get around to running a network cable out to the shop so I can just transfer part files via file sharing.

I found a work-around for the mill at work. I’ll be able to finish the parts in plenty of time. But it made me laugh when I realized how a minor problem like this can bring a project to a halt. And I realized how nice a tool I have at home. Now all I need to do is upgrade the big CNC at work so it can compete with my Taig.

– Tom

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