Monday, August 18, 2008


Milling plastic with a Reprap machine?

It might not be impractical after all... read more

Hi, Forrest:

I have quite a bit of experience making high speed spindles-- I run them on my mill to make PCBs.

For milling you need very good runout and high speed. dremels typically have pretty bad runout, that tend to break small tools. ( Small defined as diameter 1/32" or smaller ).

I use a custom-made air turbine spindle, which turns about 40-50k rpm and runs from a vacuumcleaner. It makes pcbs quite nicely.

the big problem i ran into with small highspeed spindles was the resolution: even a tiny bit ( like 0.03125 diamter ) does not have the resolution as a reprap. plus, there are major restrictions on the geometry you can make because the depth of cut ( even with many passes ) is very limited on the tool. IE, you get into the "subtractive machining takes a lot more work than additive machining" problem.

I think that the point is to be able to do, more or less, what a laser cutter is doing, not what a really good CNC machine can do.

Obviously, the diameter of the cutter head is going to be quite large. In the case I have at the moment, it amounts to 1/8 inch. That's quite adequate, however, to make something like a Mk II extruder.
True, staying in essentially 2.5 D mode,flat stock, and +/- 1/32" tolderance is do-able for sure.

"feeds and speeds" are tricky with plastics to avoid melting using a rotary tool. you'll need very high rpm.

I think that you may end up needing more speed/torque than a dremel tool.

Good luck with your project! I'm sure that you'll be able to get something going, looking at your progress so far.
"I use a custom-made air turbine spindle, which turns about 40-50k rpm and runs from a vacuumcleaner."

Is it possible that you could blog a bit about your air turbine spindle? It sounds fascinating! :-)

The design is made from pvc plumbing fittings, skate bearings, and a cpu cooling fan from a PC video card.

The spindle is a piece of 3/8" drill rod, turned to fit into the bearings, with threads to load the bearings onto each other.

I use a basic set screw to hold a 1/8" diameter tool.

The skate bearings are fit into an inner tube ( aluminum ), which is mounted inside of the outer sleeve. Air is sucked into the turbine , sucking in dust as well as turning the spindle.

The fan diameter is about 1.5", which seemed to be a good tradeoff of torque vs speed.

Here is a photoset:

There are two sets of pics in there: the ones that dont have the white PVC are an early version with a 1" diameter fan-- it turns about 90k rpm but has less torque than i wanted.

Both designs are basically the same, though-- the main difference is the size of the fans.

SOrry i dont have an exploded view-- i dont want to take it apart right now because alignment is kind of a pain.
Dave, this is really cool! Why don't you blog this in detail in the Builder's blog? This is just the kind of stuff that Reprap needs to really take off!
I have typically resisted posting my "cnc" material here because it seems a bit off topic.

My goals are very much "make an extruder head for a cnc machine", and thus i don't care as much about contraints like "run from 12V source, etc"...

If you think its of value, though, perhaps i'll make a separate blog so that i dont clutter the main RR blog with off-topic stuff.
You might do what I do and just publish a teaser line in the blog and a link to your specialist blog.
yep, good call, that's probably appropriate. I had a similar challenge when i was talking with Zach, who ultimately sold me on posting extruder progress on the builder blog.

i'll make a sep. blog and go that way. thanks for the interest!
For micro-milling purposes the Dremels have better runout for the money than anything else by a wide margin--.001" or better.

However, it is not unusual for the front bearing to become loose in the housing, leading to really awful runout. A wrap of electric or pipe tape to take up any slack can fix this.
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