Sunday afternoon and into the evening, I was over at my friend’s house building orrery parts.  It was pretty exciting in a very 21st century way: it was the end-point of a process that has never existed before.  That is, if you can dream it up on a computer, you can build it for real and the parts come out just magically fitting together like a model kit. This is the first time in history that ordinary folks can do something like this.  The price of entry is still pretty high but the Fab Labs that are springing up all over are attempting to take away that price of entry for ordinary folks.

Well, almost.  It was not all perfection as you’ll see.

My ill-fated attempt at gluing up some pine to make the base and top led me to just go buy a sheet of MDF in the interest of getting things done.  I’ve glued up many panels in my day and this one actually had biscuits to hold the pieces in alignment.  How the pieces ended up non-flat like they did is beyond me although I suspect that I put one of the clamps on funny and since I wasn’t in my normal shop (I was squatting on the floor of my son’s room because it was so cold in the garage) I probably just failed to notice.  I will rip it down the middle and start over soon and at the same time, add another piece to the side so that it will be big enough.  It had another slight flaw that I had refused to acknowledge:  I had jointed the two edges with a hand plane and had a slight bit of non-straightness near one end.  I will correct that also.  If I had been using a powered jointer, I’d call that ‘snipe’ but is there such a thing with a hand tool?  I don’t know.

At any rate, all last week I had tinkered with all my part designs; making all the hole sizes correct and separating all the parts into separate files that corresponded with different material sizes.  I then took all this over to my friend’s house.  On the way, I stopped for the aforementioned MDF.

The Machine

A ShopBot CNC Router

This has all followed a very typical project build for me (and for everyone else).  Nothing ever goes perfectly the first time you do it.  We had always assumed that I’d use a teeny router bit and because of that, it would be short.  I therefore would have to buy thin material and make several versions of it and glue them together.  As it turns out, we believe that the gear teeth will mesh well enough with a 1/8 inch router bit which is about an inch and a half long so I could make all this out of ¾ inch material if I want.  But I had already bought the thin stuff except for that sheet I purchased to make the top out of.  No matter; the beauty of CNC is that if you need to make one part out of several thinner parts, you just draw several alignment holes on your part.  The machine drills them and voila!  You line them up, shove a dowel in the holes and you have a perfect composite part.  We did this with my gears and it works swimmingly.

It took awhile to get things from CAD to CNC though.  There are a few things that have to be done and which just can’t be done until you get to the software that drives the machine.  For example, you have to define what side of the line to cut on.  Then there’s how deep to make the cuts.  You have to tell the machine how thick your raw material is and let the machine know where it is.  Do you have the machine cut out waste that you don’t want or just turn the entire waste sections into sawdust? This all takes time and the more small parts you have, the longer it takes.  I have dozens of parts and so it took a lot of time.  There are six planets with two gears each.  Laminating them out of thinner material doubled the amount of actual objects required. I was using a small piece of plywood that was only 12 x 24 and it still had about twenty small parts on it.  It took about an hour and a half to mill all that out and the router was moving pretty swiftly.

Anyway, you get the idea.

Almost done cutting

Almost done cutting the first set of gears.

We first put a piece of ¼ inch Baltic birch plywood down onto the bed of the thing.  We tried spraying it with Type 77 adhesive and sticking it down.  This worked but not well enough.  After about half the parts were cut, the adhesive started to give up and the thing started to work loose.  We had to extemporize some clamps.  Then we realized another small issue.  We measured the plywood exactly with a caliper but then in order to make sure the parts actually got cut out, we had the thing cut more than the thickness of the plywood.  This is standard practice but we must have told it too much since it cut deeply enough that we actually didn’t leave any tabs of wood to hold the parts and keep them from coming loose and being flung by the router bit.  This made things a little ‘sporty’ near the end of the session when the material (or what was left of it by that time) kept trying to lift up away from the bed and the small parts frequently getting cut loose from the material and sometimes being flung in random directions.  We made adjustments for the next session.

And when I say ‘we’ I mean my friend of course since I only came up with the design – he’s the expert on the CNC process.

My first Part

My first part

There is also the issue of the type of router bit you use.  I think we were using a ‘spiral upcut’ bit; mostly because that’s what he happened to have.  Upcut bits have spiral flutes in the side of the cutter that not only cut but pull the sawdust up and out of the hole.  This also tends to leave fuzzy bits around all the edges when you’re milling plywood.  This is easily removed with a few swipes of sandpaper but it accounts for the hairiness of the parts in the photos.

We then cut the top.  This went perfectly since we applied all the lessons learned up to this point.  Plus, the top was a lot simpler in design than a gear so there was less to go wrong.  We clamped the wood to the bed in the old fashioned way (with bolts) and it stayed put throughout the session.  Also, the depth of cut was set more carefully and the tabs that were defined were actually left and held the cutout pieces in place as designed.  You can see the photo of it on the left there.

The Orrery Top

The top is done!

He’s not vacuuming the dust out of the channel for photographic purposes.  The dust extractor was not pulling all the sawdust out of that narrow little gap; the sawdust from MDF is notoriously dusty – a lot like snuff or baby powder.  The secret to long life of router bits is getting rid of the heat.  If sawdust builds up in a hole, it acts like insulation and allows the heat from the friction of cutting to build up to surprising levels.  You have to continually watch for smoke in projects like this if your dust extraction is not working as it should.  So he’s making sure we don’t start any campfires on the way to our final product.

MDF is just smashed together sawdust and some sort of binder.  There is no grain to it therefore no fibers to lift up on the edge.  Hence the perfect edges right off the machine.  There is a spectrum of wood-based materials; some natural, most of them manmade.  They go from OSB (otherwise known as ‘chipboard’ or ‘waferboard’), to particleboard, to MDF, to HDF (i.e., Masonite) and something like that but harder that they make clipboards out of.  These latter are all called ‘engineered materials’ and there are way more than this.  They all have their advantages and disadvantages.  The greatest disadvantage to me is that they’re ugly.  They have a great advantage in than they tend to machine really well.  I view this first article from MDF as a prototype.  If everything fits, I’ll go back and make one out of some real wood.

The final product lifted off the bed easily and the cutout pieces popped out just like they were supposed to after a little persuasion with a pocket knife.  Just like the pieces of a model kit.  The final product looked exactly like what you get when you cut something on a bandsaw and then carefully sand all the edges.  In other words, what comes out is a final part that needs no further work except maybe to clean up the bits left from the tabs.

We had a substantial amount of scrap left over but he has a woodburning stove in his house and all scrap ends up heating the house so we don’t worry about that.  It certainly never ends up in a landfill.  He has one of those stove types that has a catalytic converter in the flue so that the smoke is burned up as well; it puts very little into the atmosphere so at the end of a project like this there is literally nothing left over.

Unfortunately, by this time, I had to leave and go pick Melissa up at the church.  Otherwise, we could have cut every part in the plans.  But I’ll get back to do the rest.

Now I need to start assembling the parts: glue up the composite gears, assemble them on the shafts, assemble the lantern gears, and a number of other things.

Stay tuned.

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