The View Up Here

Random scribblings about kites, photography, machining, and anything else

Making Big Parts on a Small Mill

Posted by Tom Benedict on 25/06/2010

Or how to get a mill table extension without having to add the extension…

I have a Taig 4-axis desktop mill at home.  It’s a great machine for its size, and I’ve used it on countless projects.  If you’re in the market for a small mill, I highly recommend the one from Taig.

But “small” is the operative term.  On small parts, it’s a blast to use.  I stuck a G540 driver on it, and swapped out the stock 1/10HP motor for a 1HP variable speed DC motor, but otherwise mine is stock.  And for small parts it’ll hog material out and still have the finesse to hold +/- 0.001 tolerance, or better if you’re careful.  On big parts?  Well…  Until recently I’d have shrugged and said, “Get a bigger mill.”

A friend of mine is working on a project and needed some parts made.  He sent me the 1:1 drawings, and without looking too closely at them I said, “Sure, I can do it.”  Then I started dimensioning things and realized I was way the heck out of my depth.  16″ wide?  My mill only has 7″ of travel in that direction!

This is actually a pretty common problem to run into in machining.  And there’s a really simple answer:  Reposition the part, re-indicate, and keep cutting.  Tackling a part piecewise like this, you can theoretically make arbitrarily large parts on even a very small mill.  Of course there are still practical limitations, like the throat depth of the machine and the physical constraints of the room the mill is installed in.  But the idea works.  I’ve used it on manual machines a number of times, and each time it saved my butt.

The real kicker with this approach is that you have to re-indicate the part each time you reposition it.  The parts I had to make didn’t have a lot in the way of features that I could indicate off of, so I knew I had to come up with another plan.

I have a tooling plate on my mill that I use almost all the time.  It has a grid of threaded holes that let me bolt down vises, jigs, stop blocks, or anything else I need to use to make parts.  Step one was to take off the tooling plate and add a grid of precision dowel pin holes.  A couple of hours on a manual mill at work over the weekend took care of that.

The mating part is a second tooling plate with two pressed-in dowel pins on the bottom.  These slip into the dowel pin holes on my main tooling plate, and let me reposition the second plate to any number of positions.  Through holes for bolts that line up with the threaded hole pattern on the main tooling plate allow me to bolt it down once I’ve got it positioned.  The dowel pins are regularly spaced, so plate offsets of 2.000″, 4.000″, or 5.000″ are a matter of picking the right set of holes.

With tooling plates in hand, I gave the big parts a try.

Big Parts

It worked perfectly.  The only issues I ran into were because I was cutting the part out of a sheet rather than working with pre-squared stock.  There are tricks for doing this, such as leaving small tabs in strategic locations to make sure the part doesn’t shift as the contour is finished and the part comes loose.  The tolerances for the outside contour on these parts were pretty loose, so I skipped the tabbing and just cut them out.  I’m very pleased with the results.

I’ve got another project lined up where I might want to make parts larger than the work envelope for my mill.  I still like to design inside the capabilities of the tools I have, but now at least I know I can.

– Tom

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