Tuesday, December 7, 2010
Quick CNC video
Monday, December 6, 2010
Cogs
Thought I’d try making some gears. The first one’s not so good - the mill bit’s first cut was too deep into the material, and the pressure was too much for it to follow the pattern closely. The next cuts were better but that first cut left a bit of a step on the edge of the teeth:
The next one was better. It’s better to start the mill well above the surface - “cutting air” - rather than risking the first cut being too deep:
Much cleaner.
Wednesday, November 24, 2010
CNC mill improvements
CNC machines normally need limit and home switches to tell the computer where the various moving bits are. Limit switches stop the computer from trying to run the machine off the end of any of the runners, and home switches give the computer a datum point — a way to say “OK, we’ll call this place 0 on all the axes, so do all your measurements from here from now on”.
I’m only putting home switches on my machine because it’s always going to be run attended, and even if noone’s there to stop it, it’s not capable of doing itself too much damage as the motors are pretty weak.
Here’s the switch on the X-axis - you can just see the carriage on the upper right of the picture, and as it moves left, it hits the switch and the computer knows it’s home.
Harder to see but the Y-axis home switch is similar:
The Z-axis is trickier, as there’s not a lot of travel up and down so the switch probably needs to be adjustable, plus it’s in a pretty confined location. Could do with a little bracket:
The machine took about 10 minutes to mill it out:
… and me about 2 minutes to sand the edges:
Little switch attached:
And finally, mounted in position:
Yay - you can now tell the computer to “Home all axes” and it moves each axis of the machine back until it hits its switch.
To make things a little more stable I added an extra bearing to the bottom of each side of the platform to stop it twisting up and off the rails:
I added a little light behind the Dremel spindle cos everything looks better with lights on:
That’s it for now. Next job is to sort out the work platform - make sure it’s level, and create some little clamps to hold the work down as the mill is working on it.
Friday, November 19, 2010
Amateur CNC mill show and tell
Thought I’d have a go at something a bit mechanical for a change. I bought a cheapy pillar drill from Screwfix a while back - around £50 if I remember right. It’s a lot of machine for the money:
Mmm… precision drilling… mmm…
OK, back to me: CNC mills are all the rage at the moment - loads of people seem to be building them, from huge ones through to tiny little ones you can knock up for very little money. If you can get the parts.
I’ve got loads of electronic junk in boxes, so I had a dig around. These are full of useful bits:
It’s a scanner - a disco/nightclub light that can change colour and, using a motorised mirror, shine the beam around the room in carefully choreographed patterns. The motors it uses are stepper motors:
These things are the heart of a CNC machine - you can control precisely how much its shaft rotates, unlike a normal motor that spins freely when you give it power. These ones are a bit small… still, worth a go.
I managed to scavenge a load of bearings and rods from old printers. Old dot matrix printers (if you can find them) tend to have lots of beefy bits and pieces in them, from linear bearings/bushings to chunky stepper motors. My ‘Never Throw Anything Away’ ethos is finally paying off…
I bought a few lengths of leadscrew from Marchant Dice (yay! UK supplier of CNC bits! Not as cheap as you can get stuff in the States, mind. Sometimes seems like every house in America comes with a fully equipped machine shop and every town has a specialist hardware shop…)
I also bought a matching tap, to make the nuts that’ll be driven by the screw. Cheaper in the long run than buying a load of pre-made nuts. Here’s one being made - cut off a bit of nylon:
Drill a hole in it:
And here it is after being tapped to match the screw:
A few more threaded holes so I can fix the nut to the moving bits:
Here’s one threaded onto the screw and mounted on a motor. This will be part of the Z-axis, moving a drill up and down over the thing I’m milling:
First: X-axis stuff
Here’s the X-axis, so the drill can move left and right over the workpiece.
One of my dodgy motor couplings:
These are the main source of problems on my mill so far. You’d think it would be fairly straightforward to put a little hole all the way through a bit of nylon, then drill the hole out to a larger diameter for half its length. Not for me - no matter how hard I try (and believe me, I’ve got through a lot of nylon) I can’t get the holes to be perfectly in line, so the leadscrew is always slightly wonky; it’s not a big problem but if means the motor has to work harder and it makes the machine a bit noisy.
One thing that helps mitigate the problem is having nice solid linear bearings. The rails and carriage here have to carry a fair bit of weight - the Z-axis carriage, motor and drill all have to be mounted on it, and I didn’t want it twisting or binding.
To test it all worked, I made up a quick and dirty motor-controller circuit with an Arduino and a darlington array:
It’s extremely simplistic but it works well enough to drive the motor backwards and forwards. This is the first real milestone - seeing something move under computer control! Woo!
Next: Z-axis stuff
The drill has to be able to move up and down over the work, so it runs on two rails scavenged from old printers. To make sure the rails are absolutely parallel, I drilled the holes for both ends of the supports at the same time:
The rails stick in the holes. Nice and simple, perfectly parallel. These rails are 6mm diameter rods taken from an old scanner.
The motor mounts on one end and turns the leadscrew which will lift and lower the drill:
The bearings that run up and down the rails need a larger diameter hole to mount them, and no matter how carefully I measured and drilled, I couldn’t get the holes in the right places; they were always out by some little fraction, and wouldn’t run smoothly. So I had to bodge it: I drilled holes larger than they needed to be, fitted the bearings to the rails and put it all together, then cemented the bearings in place with some two-part metal putty:
The carriage runs nice and smoothly up and down the rails.
I made an extremely crude mount for a Dremel mini-drill - it’s just a slot hacked out of some plywood, but it seems to grip the Dremel tightly enough. We’ll see.
I mounted the drill to the Z-axis carriage and mounted the whole lot onto the X-axis carriage:
It’s bloody heavy. I’m pretty astounded it actually moves - they’re tough little motors.
Y-Axis stuff
So - the drill can now move up and down and left and right - all that’s left is forwards and backwards. Rather than move the drill forwards and backwards, I’ve chosen to move the workpiece instead: it’ll sit on a little platform on runners. I picked up some cheap bearings from eBay and fixed them to a bit of angled aluminium:
Each bearing needs a nut and bolt and loads of washers to space it correctly:
A complete one (there’s two of these runners, one for each end of the work platform):
They sit nicely on top of a round bar. Again, scavenged from a big printer :)
I initially thought it would all work fine if I had the runners sitting over the bar at 45 degrees (as I’m holding it above), but once I had the machine going it was too easy for the motor to pull the runners off the bar sideways. I ended up redesigning the platform so the runners could sit at 90 degrees, like this:
This way the platform and bearings are kinda jammed inbetween the two support rails, stopping it from twisting when the motor tries to pull or push it. It’s not perfect, but it works. The other end:
I really struggled with the motor coupling for this axis: if the leadscrew isn’t mounted perfectly in line with the motor shaft, it tends to wander in a circular motion and push the platform up and off its runners. I managed to codge a flexible coupling together:
The white stuff in the middle is polystyrene foam to act as a shock absorber and spring. It works better than nothing, but it’s damn noisy. Sounds like something’s grinding - it’s just the threads of the little screws rattling against their holes but it sounds much worse :/
Up and running!
So the machine is complete!
Complete to a minimum level of functionality, that is: it needs limit switches so the computer driving it can tell where the carriages are, and it could do with having various probes and tool-depth sensors before it can be used properly. But still - I loaded up TurboCNC on an old laptop (after spending a day and a half trying to get the laptop to wake up in DOS rather than Windows) and connected it all together and did my first computer-controlled engraving:
Mmm… circular… [drools]
And just to test it can handle more complex stuff:
So that’s where I’m at. Gotta say a huge thanks to everyone who’s put so much information about their own CNC mills online - I knew next to nothing about these things before I started, but there’s a ton of info online if you want to have a go. It’s unbelievably exciting watching it work and knowing that I made it :) Yay!!!!!!1111!1!!!!!!1!!!PONIES
More updates on this on my blog here…
Thanks to:
http://makeyourbot.org/ - The site that inspired me to have a go, and convinced me that plywood could work.
http://buildyouridea.com/ - This guy is astounding - he not only knocks up CNC machines but he casts his own parts from aluminium too. The site is full of interesting stuff, you’re likely to lose a few hours there.
… there should be a load more links here but I can’t remember them all. Google ‘DIY CNC’ and you’ll find loads of stuff to get you going :)
Steve Jobs quote
When you ask a creative person how they did something, they may feel a little guilty because they didn’t really do it, they just saw something. It seemed obvious to them after awhile. That’s because they were able to connect experiences they’ve had and synthesize new things. And the reason they were able to do that was that they’ve had more experiences or have thought more about their experiences than other people have.
Steve Jobs, Wired (March, 1996)
(as seen on http://buildyouridea.com)
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