Today, the final piece of the puzzle fell in to place. Or rather, the final parts arrived. The actual falling in to place will probably be more of a sweaty, frustrating, angry microadjustment-with-sledgehammer-redo-everything-piece-of-fucking-shit thing over the course of a few weeks. I can hardly wait to get started :).
The plan is this: I’m moving back to Uppsala this weekend, and hopefully by monday I’ll be able to start building. I’ll have a few days to build, before uni starts and I’m going to Gran Canaria. Because fuck uni, that’s why. That means that this is the last boring post with no acual building to show. I’ll use this time to go over the parts in a bit more detail (but still a very watered down version, do you have any idea how many books I could write on the subject of hobby cnc by now?) and post a more or less complete BOM (Bill Of Materials).
The frame is built completely out of aluminum, with a variety of alloys. The machine will weigh around 120 kilos, which isn’t very much. I could fill the profiles with dry sand later on if I wanted to increase the weight and get rid of some vibrations. Then again, I wont be able to move it as it is. Haven’t really figured out where to put it in my tiny apartment yet…
Anyway. I’ve ordered a grand total of 23 pieces of aluminum plates from motedis.com and maskindelen.se. The plates are 20mm, 15mm, 5mm and 4mm. The majority is of course 20mm. Most of the plates are 5083, a few 6060-T6. 5083 is almost as rigid as the very common 6061, but should be less susceptible to vibrations. Also great if I’m using the machine in seawater.
The gantry crosspieces are 80×80 alu profile, the rest 60×60. The table is made up of 4 pieces of 160×16 profile that is surprisingly massive. You should be able to see some alu angles that connect the profiles. Those are bought from china, and I deliberately made sure that no real stresses would be put on them. Turns out they were excellent quality and rigidity, I could have used them alot more than I did. Pleasant surprises all ’round (in fact, I’ve bought a lot of items from china, mainly using aliexpress.com, and every single piece has far exceeded my expectations).
The parts are fastened by means of drilling, tapping and applying screws. Very high tech. Oh yeah, you know those hammer-head specialty nuts that you use for profiles? Also bought those from china at practically no cost. Excellent quality. Insane prices here in Europe.
The beauty of the frame isn’t really in the parts though, it’s in the design. Not going to talk about that here, but one of the main considerations I had when designing was to avoid dependecy on the accuracy of the cuts when I ordered the parts. Having measured them now, motedis claimed a 0.2mm deviation from the ordered lengths, and that criterion was met. However, the width and thickness of the plate parts were consistently 0.3-0.4mm too thick and wide. Doesn’t matter in the slightest for me, but something to consider should you buy from them. Apart from that, I’m very pleased with their cuts and packaging, I’d definitely recommend them to anyone needing alu.
The linear units
The linear units were bought from fy bearing on aliexpress. Wendy was a great seller, easy work with and great engrish. My order was completely custom made, and most of the things I ordered wasn’t even in his store. You can see from the pictures up top that the packaging was slightly damaged from shipping, but none of the items were harmed. They were pretty dirty from the cuts, so the first thing I did was to clean them up and oil everything up with some machine oil. The rails are, as far as I can tell, actually genuine HIWIN parts. That’s pretty fucking impressive. Next up we have ballscrews. C7 quality 1605 ballscrews, they appeared to be perfectly straight, or at least as near as I could tell, the machining was looking good as well. I’ll just throw in the entire parts list here:
2 x HIWIN HGR20R 1000mm linear rail
2 x HIWIN HGR20R 700mm linear rail
2 x HIWIN HGR20R 300mm linear rail
12 x HIWIN HGH20CA block (He actually sent 2 used blocks, which I didn’t notice until after I’d voided my purchase protection. When I told him, he didn’t believe me, but would have a talk with his employees. 1 day later, he sent 3 new ones free of charge.)
2 x SFU/RM 1605-1000mm ballscrew
1 x SFU/RM 1605-700mm ballscrew
1 x SFU/RM 1605-300mm ballscrew
4 x 1605 ballnut
4 x ballnut holder/housing DSG16H
4 x BK12 ballscrew supports
4 x BF12 ballscrew supports
4 x 8mmx10mm flexible ballscrew/motor couplings.
Can’t think of anything more that wouldn’t be too detailed at the moment. But you know, there’s a comment field if you would like to know more.
There is a literal fuckton of chinese spindles scattered over fleabay and aliexpress that look virtually the same. Protip: They aren’t. Your garden variety chinese spindle probably boasts something like “german import ceramic bearing, more longer life very good”. Inside you’ll find 3 cheap bearings that might of might not be angular contact bearings. The connector will have 3 or 4 pins, the fourth will probably not be connected to the housing. It’s a gamble if you get a good one.
I’ve spent a lot of time finding a good supplier, and I finally did. What you really want is 4 bearings, which will increase the service life alot. An earthed housing is pretty much essential as well. My spindle appears to be perfect, though I haven’t yet put it to the test in the machine. In fact, the clamping nut for the collets even has two drill holes that indicate that the spindle has been balanced.
The actual spindle is a 2.2kW water cooled three phase asynchronous motor, 0-400Hz. The water cooled motors are a bit more work than the air cooled ones, but it does pay off in the sound level and the ability to maintain low rpms without overheating. I can tell you, when running my motor at full speed (24000 rpm) I can hardly tell if it’s even on. With my wood router, I needed double ear protection and it still hurt to run it at those speeds. To run this kind of motor, you need a VFD. I went with the Huanyang VFD, though a lot of people recommend getting a quality vfd; Hitachi, ABB or similar. I got a set of 14 collets for the spindle, ER20, meaning I can use up to 13mm bits.
I also got a 75W water pump that I intend to use with distilled water to cool the spindle. Since it’s an induction motor, it doesn’t produce a lot of heat until you put some load on it. I could probably mill circuit boards without using any cooling at all.
It’s very important to use shielded cable for the spindle power, since it’s really dirty and will mess up your limit switches and steppers. I used 4 core 1.5mm² CY cable. Got it from meterorelectrical.co.uk. CY cable is practically impossible to get your hands on in Sweden. I was going to buy the cable when I was in London, went to a CEF but they were out of stock. Bummer.
The steppers and drivers
When buying steppers, what you’re looking for is primarily the form factor (usually NEMA), the winding inductance and the holding torque. Larger motors are alot stronger, but comes with a higher inductance. NEMA 23’s are usually a good compromise. Preferably you’d want an inductance at near 3 or lower. The reason for this is that steppers are really strong when standing still, but loose torque at higher speeds. This effect can be mitigated by running them at a higher voltage. Gecko recommends sqrt(inductance)*32 iirc. Most of my motors are 3.8mH, which would mean 62 volts as an optimum level. Now, I’m going to use the steppers and drivers from my old cnc. These steppers are NEMA 23, 3Nm, 3.8mH. The drivers are DM542A, analog drivers with a maximum voltage of 50V. My power supply is at 36V. This means that my machine will be far from the speeds it could be with good digital drivers and a proper voltage.
Since I’ll be using a fourth motor this time, I’m taking the chance to try something better. My new motor is a NEMA 23, 4Nm, 3.0mH (cnc4you.co.uk). I’m driving this with a Leadshine AM882 (aliexpree), which is a really good digital drive, capable of up to 80V. This could produce perhaps triple speeds, with less noise and stall detection. Should I fall in love with this combo, I’ll probably replace the other drivers and the power supply eventually.
The cable is a 4 core 0.75mm² CY cable. Shielding is important, yo. Didn’t use shielded cable in my last machine, and I couldn’t use limit switches despite RC filters and such.
Now this is an exciting chapter. Should probably make an entire blog post on this subject alone. Maby later.
First of all, stepper drivers take 2 input signals to make shit move. Step and Dir. Dir sets direction and one pulse on Step means the motors move one step (or microstep). One step is usually 1/200 of a revolution, or 1.8° (a microstep can be almost impossibly small, the AM882 supports 512 microstepping. That’s 100 000 steps per revolution or 49 nanometer per step with a 5mm rise ballscrew). Usually, you have mach3 or linuxcnc generate those step pulses and send them through an old fashioned parallell port to a breakoutboard that sends the signals to the drivers. The thing about mach3 is that it runs on Windows. Windows is a great operating system when it comes to most things. Realtime operations is not one of those. Not being a realtime OS means that Windows doesn’t really care all that much about when things happen, only that they happen fast. Things like microcontrollers and FPGAs aren’t that fast (compared to a modern processor at least), but they are great at doing things at precisely the right time.
A motion controller will talk to mach3, in its own time. A little bit of information here (pun intended), a little there. Usually by USB or ethernet. The motion controller then makes its own step pulses, with blackjack and hookers. Properly timed pulses means a happy stepper motor. A happy stepper motor can be run alot faster without stalling. A cnc operator likes that. Everyone is happy with a motion controller.
Actually chosing a motion controller is another story. There are quite a few. The smoothstepper is perhaps the best known, but other contestants like planetCNC, CSMIO/IP-S, PLCM-E3 and EdingCNC are also avaliable. The thing about all those (except the CSMIO, but it’s insanely expensive) is that they all need a separate breakout board. A breakout board can be had on the cheap from china. A cheap board will mess up your signals though, which kinda defeats the purpose, ye? I was looking around, trying to find a good board with integrated breakout of the signals. In my desperation I stumbled on to purelogic.ru and went google translate on their asses. Lo and behold, I found this thing called PLCM-E4, a motion controlled that is reasonably priced with intergrated breakout. I contacted them and managed to buy one, with some persuasion. I have it in my hands now, currently untested (a bit nervous I have to admit). I think I’m probably the only one west of the iron curtain that has this particular board. Pretty mindblowing, but it might also mean support might be a bit hard to get. Then again, I’ve heard that purelogic has great support, we’ll see if I need it. When I get it running, I’ll make a review of the board and perhaps introduce it to the rest of the world.
I’ve got a shitload on my mind, and there’s probably tons of items that I’ve bought, or planned to buy, or will discover that I need when I start building. We’ll see.
I’ve bought 5m cable carrier, e-chain, w/e. Should look real nice to get the cables sorted in them. A pretty small size though, we’ll see if I can fit the stepper cables, spindle cable, water pipes and limit switch cables in it. I’ve made some calculations, and it’s tight as a 5-year old.