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Milling plastic with a Reprap machine?Monday, August 18. 2008It might not be impractical after all... Back in April and May of last year, Nophead took a look at milling plastic on his repstrap machine. Unlike most of us, Nop simply bought a proper xy platform and went from there. That decision, combined with his very thorough and thoughtful way of approaching problems has made him the treasure of a resource to the Reprap project that he has become in the time since. Parts being almost impossible to come by in those days, Nop resolved to mill his own extruder. I wasn't following his blog regularly at that time and missed much of what he went through. At first he bought a spiral saw which could rev up to 30K rpm. For some reason, though, it proved inadequate, so he acquired a minidrill that could hit 20K rpm from his aunt. He used this one thereafter. That decision made, he proceeded to try to mill some styrene...  ... with indifferent results. He noted at the time that... I might be wrong but I think the snatching is due to the set up not being rigid enough. I had already identified that as a weak point and the new motor mount was aimed at improving it. The first drill was supported at both ends but this one is too short so it needs a much stiffer mount, which is what I was trying to make! Perhaps the end mill bit is not suitable for styrene, or perhaps styrene is not very machinable, or perhaps the RPM is too high, or too low. ...and decided. One thing I can do to improve stiffness is to replace the 2mm aluminium plate with the 6mm slab I already bought for the job. I was putting that off until I got the new mount so as not to have to drill two sets of holes in it. And subsequently managed quite nicely with the string of results we've all benefited from today. It also left us, however, with the impression that has since turned into a received wisdom that it isn't really possible to do milling work on things like plastic with a light framed Reprap machine.
There are a number of other very good
reasons for wanting to avoid getting into milling. Adrian has noted on
any number of occasions that the software that you need to calculate
paths for the toolhead is orders of magnitude more complicated than
that needed to build up an object by extruding a plastic filament. As
well, the majority of CNC milling rigs have several more degrees of
freedom than our simple xyz rigs.
Recently, however, Ian, Vik and Zach, iirc, have got into using laser cutting to create parts sets for the Darwin machine.  These machines look wonderful! Even more recently, a forum poster began a thread describing his efforts towards building a reprap machine which could also do milling. A comment by Nophead caught my attention... If you build a machine sturdy enough for milling you can make an extruder by milling plastic. That is how I make my first one. If it is big enough you could also use a mill to cut the parts for the laser cut design. That wasn't available when I made mine, but should be easier as the parts are just 2D slices. Acrylic is quite easy to mill but you have to do fast shallow cuts to prevent it melting. Heretofore, I'd intended to use a flexible drive cable to mount my Dremel tool on my Tommelise 2.0 reprap machine with the notion of milling my own PCBs. Oddly enough, the notion of milling PCBs was an accepted use for light positioning devices such as mine while milling plastic, for the previously mentioned reasons, wasn't. As the discussion progressed, Nophead reiterated his own experience... Actually, even if you have a light milling head and take shallow cuts you still need a machine with a high degree of stiffness otherwise the cutter snatches and chatters. But Kyle noted the discrepancy in logic... Nophead is right, you shouldn't underestimate the stiffness needs for a good milling setup. That being said, _very_ shallow cuts at high RPMs in relatively soft material may be feasible on a RepRap (witness the PCB milling in another thread here). It will just take someone to try it! That was enough for me. I climbed in the Jeep and got the last Dremel flexible power cable at Orchard Supply and decided to have a go. I had some scrap black HDPE with a knobbed surface (recycled cutting board) and a piece of smooth, transparent acrylic left over from building the Tommelise 2.0 positioning robot. I quickly made a double height straight edge with a recessed bottom layer from two thin pieces of poplar and started doing some of those "very_ shallow cuts at high RPMs", that Kyle talked about. Dremels come in all sorts of models. I have a model 395. It's more expensive than average but is strongly built and also has a speed control. I like Dremels because they even have easily replaceable brushes when you finally wear yours out. They're truly made to last a lifetime. You don't run into tools like that very often these days. In any case, here is a look at what I used.  I began work on the black HDPE cutting board. The knobbed surface gave the cuts a rough look which is really just clinging swarf. I didn't have that problem with the "blue" acrylic. The blue color is actually just a protective film that covers the transparent acrylic sheet. After a half dozen tries to get the feel of making Kyle's "very shallow cuts", I got the hang of it. I then varied the rpm on the Dremel, a variable that I don't think Nophead was able to explore. When I operated at under about 20K rpm I got the "snatches and chatters" that Nop had experienced. When I pushed the RPM past about 25K, however, I stopped getting those as long as I kept the cuts very shallow. The swarf that came off of the HDPE and acrylic was very fine and fluffy, rather like the sort of snow you get when the temperature is just a few degrees below freezing.  Here you can see a 1/8-inch deep cut I was able to make. Although the knobby surface and clinging swarf makes it look rough, inside it is as straight as a die and the bottom of the cut is quite flat. At first glance the cutter that Nop was using looked identical to mine. When I blew up a detail from his pic, however, it appeared to be more of a rasp-like cutter than the one I used.  Mine you can see here.  The "tests" were qualitative, to be certain, and I wasn't able to quantify what sort of lateral forces were involved. I can say, however, that I'm quite a fair shot with the old .45 ACP so I do have a sense of the magnitude of force involved when I'm using one. I don't think I got lateral forces of over an ounce or so after I got the hang of making very shallow cuts and certainly didn't get any snatching or chattering after the cutter speed went over about 25K. I'm almost certain that Tommelise 2.0's positioning robot can handle this sort of work. It's stiff, but not completely rigid. Using Nop's rule-of-thumb... A very rough rule of thumb I found was that if you grab the frame of the machine with one hand and push the head with the other then if you can see it move it is not stiff enough. ...it moves about half a millimeter. Though I haven't built one I suspect that Darwin's positioning robot could handle this kind of milling as well. I'm going to be trying to mill an extruder with the Dremel, as Nophead did, before the end of the month. The point of this is that if we could extend the capabilities of our reprap machines in this manner we could take advantage of the much cheaper, higher strength plastics like HDPE and polypropylene. Right now in the US I can get HDPE sheet for about $3.00/lb and polypropylene for as little as $2.20. ABS filament to do the same job costs anywhere from $6-8/lb. I can get acrylic sheet for about $4.50/lb. If we can mill plastic, it will allow us to make the sorts of parts that we are having to pay an agency to cut with a laser on our home machines. We'll also be able to make stronger, cheaper parts than we can using the extruder toolhead. This could be a very useful extension of reprap machines' capabilities. Trackbacks
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Yes, very exciting new possibilities! I was already excited when I saw the first results of PCB milling. I've been really looking forward to being able to do that myself for PCB prototypes. But using sheet material as construction plastic is also really awesome. If you want to make enclosures, for example, you can use sheet plastic milling and 3D extrusion together...
Forrest, I'll be following your progress really close! Hopefully I get time to play with this myself soon...
I have tried some milling with my RepStrap machine and was lucky enough to find out that a Milling bit is made for the Dremel tool besides the rasp and the cutter shown in the above photos. So far, it looks like the milling bit is only available in 1/8 inch diameter. Also, I suggest that you use the Collet to hold the bit instead of the Dremel keyless chuck, as I had the bit become loose and start slipping out. (and cutting too deep)
"I ... was lucky enough to find out that a Milling bit is made for the Dremel tool besides the rasp and the cutter shown in the above photos."
Was it this one?
http://www.hobbylinc.com/htm/dre/dre561.htm
No, that's not it. I used this one: http://www.widgetsupply.com/page/WS/PROD/dremel-router-bit/D-HJ24 My mistake about calling it a Milling bit, I should have said Router Bit. However, I have cut plastic and Teflon as part of my extruder build.
Great milling work! If milling plastic works, we could make gears and rack & pinion parts.
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For milling software, Ralith mentioned the open source milling package GCAM: http://gcam.js.cx/index.php/Main_Page
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Also in theory for simple shapes, you could subtract the desired shape from a surrounding box, then use slice and dice software to make the infill gcode pattern for the subtracted shape, and drive the milling head with that gcode.
Good ideas, Enrique! I'd been wondering how to approach the problem of converting shapes into milling instructions. :-)
Look up the phrases:
Feeds and Speeds,
Climb Cut, and
Conventional Cut
for an appreciation of the issues to be solved.
Take a look at Cut2D from Vectric for some sw that does the job reasonably well.
- Leland
Thanks, Leland. I appreciate the search terms. One of the biggest problems I have is not knowing what to call things. I'll certainly make use of them.
It's good to hear that this idea has some potential. I agree that it opens up some interesting possibilities.
Of course, while milling is incredibly useful for RepStrapping I don't think it will be too common on RepRaps once they're working reliably. As a posessor of a CNC milling machine, I can say that not only are the algorithms an order of magnitude more complex, but it takes an order of magnitude more time and effort to get the build set up correctly and it's much harder to automate everything. It also requires an order of magnitude more time/money to service, with the higher loads/fast spinning things/dust getting everywhere. And it cannot produce nearly so flexible geometry as an additive fabricator can.
Of course, I'm still rooting for you. This definitely complements FFF technology (that's why I have a milling machine!) while admittedly not replacing it.
It does allow for milling PCB's though... orders of magnitude less complex than prepping a reprap for extruding metals?
Yes, I think it will be. A lot of people have already used light duty positioning robots to mill PCB's so there is already a lot of experience with that. I won't be doing anything in that regard that hasn't already been done many, many times before. I know Adrian wants to actually print conductive tracks and circuits on plastic with low temperature eutectics like Field's Metal and the like and then drop in components. My feeling is that the really low temperature eutectics like Fields are going to melt and run off the first time a circuit board gets a bit warm. Some time ago, I tried to print ordinary Sn/Pb solder on HDPE and succeeded, to an extent. It certainly adheres to the HDPE very nicely. The only practical problem I ran into was that the solder has pretty low viscosity when it is molten. It tended to run right out of my extruder barrel through the half millimeter orifice. The old Mk II extruder design definitely wants rethinking if we are going to print metals.
there is a bit of an issue if you move towards reprapping a dremel though...
I've been lurking around reprap pretty much since the start and imho the focus on reprap as an 'all in on' is flawed.
A 'family' approach with a reprap, an assembly 'robot' arm, a mill and a lathe in a production line allows for many more parts to be created with tighter tolerances.
A specialized mill could use a stationary cutter, which could then be bulkier and thus constructed of softer material
Next step of course is the interaction between metal-working and plastic- working families, electronics production lines with photographic etching and foil winding machines....
so maybe it's not what reprap is all about, after all :)
a spinoff perhaps?
anyways, glad to see that you got this to work, have a dremel + extension lying around but no motor skills to speak off.. ;)
"glad to see that you got this to work, have a dremel + extension lying around but no motor skills to speak off.. ;)"
That's no excuse. I can barely hang on to my tea mug. You just have to be patient and determined.
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