7x10 CNC on the cheap

[erikjgreen]

Hey, all;


I'm working on cleaning up and CNCing my mini lathe while I wait to get my shop space cleaned out so I can move my Lagun mill into place. Unfortunately I committed a year ago to getting a big boat project done in there and it's not ready to move, so I'm kind of doing the lathe project to get my machining fix in the meantime.

Of course, the boat project is sucking up most of my spare cash right now, and I don't want to spend a ton on the mini lathe since I'm hoping to get a full size lathe to CNC in the next year.

I'd like an opinion on the parts I have collected so far. Mostly they're from salvage and from a local surplus place. I know some of them aren't idea, but I'm hoping they'll work.

Power supply: I have a buck/boost transformer, 20A, with 24VAC output. I have scavenged a large capacitor (forget the exact specs, but I'm sure it's big enough) a 400v/20A bridge rectifier, and the fuse, switches, and resistor I need to put together a basic PS that's about 32v DC.

Stepper controller: I have a 3 axis hobby CNC board I built about a year and a half ago. Long term I'd prefer Gecko boards but this is what I have.

Steppers: I have 2 186 oz-in Rorze steppers, 3A rated. A bit light, so I'll probably use belts to increase torque at the cost of speed. I picked up a 300 oz/in Vexta, but it's one of the 10 wire 5 phase variety, and I don't have a driver for it.

Leadscrews: I picked up a pair of hyper-accurate Universal ACME lead screws, 1/2" 10tpi, with integral thrust bearings and anti-backlash nuts already on. I'm going to use one as-is for the Z axis and cut the other to length for X. These will have more friction than ballscrews would, but at least they'll be accurate. Kinda coarse threading though.

LM Guide: I picked up one LM guide (THK combination ball screw, rails, and trucks) sized for a NEMA 23 stepper. It's a nice little rigid thing, but it doesn't quite fit anywhere in the picture for my lathe. Not sure if I can use it for anything.

Linear rails: I have a THK HSR20 about 20 inches long with two wide trucks, plus some misc rails without trucks that would make decent load bearing surfaces or something. Not sure if it makes sense to replace/upgrade any part of the lathe's rails/gibs/ways system.

Misc: I have many boxes of electronics, cables, switches, etc. I will use to set up limit switches and a tach sensor.

Other: I happen to have some servo motors with encoders (1000 ppr) with these specs:
max terminal voltage 45 V
max cont speed 4000 rpm
max peak torque 180 oz-in (1,27 Nm) at 12 A
max peak current 12 A
max cont current 4,1 A
max cont stall torque 54 oz-in

Which I'd need servo controllers to use. I think I'd also need a bigger power supply.

My first task will be assembling a running electronics set-up, from the parallel port on my control PC to the steppers.

I'm also working on getting the hardware together to upgrade the lathe motor from the standard one to a 1.5HP treadmill motor and DC controller I have.

Pics when I have something to show,

Erik

[erikjgreen]

I dug out the capacitor last night... it's a 23600 uf electrolytic that can handle 40 volts. Should be ok for this use.

I'm building a basic power supply as detailed on this web site:
Step 25: Power Supply Part 1

I've collected all the parts although I need to de-solder the bridge from the circuit board it's on. Probably I will assemble things like it shows on that site, just wiring them together on a board. I'd like to have a circuit board for this but I haven't been set up to make one of those in years. Maybe I should buy some perfboard or something, but most of the components won't mount on the board anyway.

I ordered timing pulleys and belts for the steppers. I'm going with about a 4:1 reduction, so it'll be slow, but I'm hoping that lets the steppers deliver the torque needed to operate the acme screws. XL belts, I ordered 3 sizes so I'd have the one I needed and I wouldn't get stuck on that.

On the down side, I dropped my treadmill motor while clearing some desk space last night to work on soldering, and snapped the tach disk off. Guess I'll need to make a new one. Probably I should mount the tach on the lathe spindle anyway.

I picked up some bearings to mount a jackshaft on for the motor at a local surplus place. Of course nothing fits exactly, including the shaft I was going to use, so I need to set the lathe back up far enough to turn some bushings or turn a new shaft. Once the jackshaft is mounted I'll drive it with a 3:1 pulley reduction (it's a 6500 RPM motor) and that will drive the lathe's gearing using the original drive belt. I also need to figure out a base/mounting for the whole thing so the new motor isn't just clamped to the bench behind the lathe, and I am hoping to flood cool the whole thing, so I need a better pan under it. Maybe I'll build one with MDF, I'll see. Wish I had a sheet metal brake.

Erik

[eartaker]

Good idea on the DC power supply. It will save you lots of money the only thing is some people don't understand how to do it. I am currently doing the same for my ZX-45 mill. The output of the transformer is 60VAC and after I put it through a rectifier it will be around 80-84VDC. This is a little high so I will Drop it to 75VDC with a voltage regulator.

You should have an output of about 32-34VDC on your power supply. Just remember the bigger the capacitor the less of a ripple you will have and a more constant voltage but your cap should work nicely.

[knudsen]

Generally speaking, unless you have a regulator(s) on the circuit boards you are powering, you want the regulate the DC. Unreg drops with load. If it's just DC for the steppers, it probably won't need to be regulated. When designing a power supply, you want the components to be rated to handle 1.5 to 2 x the expected maximum current. This is designed into good commercial supplies. Like if it's rated at 5 amps, it should be able to handle 7.5 Amps. Bad power is the number 1 cause of electronic woes. Overkill is just enough

[knudsen]

Oh, yah, on the cap, you want the working voltage to be at least 1.5 x the expected voltage of the circuit. If it's an electrolytic cap, like the ones shown on your linked page, or other polarized cap (ie has a + and/or - marked on it), be sure and observe the polarity. Exceeding the working voltage of the cap, or wiring it backwards, can make it explode. Bigger the cap, the less the danger. Big ones tend to just swell up and ooze capacitor pus. I saw a student wire a small electrolytic backwards and it blew through a plastic project case and made a hole in the drop ceiling tile. Real big ones have vents. Electrolytic is what you want for a PS, though. Some just have a red dot indicating +, some a plus by the solder lug, some a white stripe on the side pointing to minus. If in doubt, chicken out.

[erikjgreen]

Yeah, the PS is just for the steppers. I'm pretty confident it'll work well because the guy on that web page is using the same board with this power supply design, and Dave R. at HobbyCNC uses pretty much the same circuit.

My cap is speced at 50VDC 65 Surge, 23600uf, with screw lugs and a marked + post. PS will be about 34-36 volts DC output. I should be ok.

My major worry at this point is the steppers.. the leadscrews are accurate but stiff, so I'm gearing the steppers down to up the torque and setting them to full step. Hopefully that will get me the torque for smooth motion without dropping speed to something slower than doing it by hand...

Erik

[knudsen]

That should be a good cap. The safest thing to do would be measure the voltage before connecting the wires to the cap. It will actually be rectified AC at that point, but the voltmeter will read the DC average. Those big screw on caps are usually vented (small rubber plug around the screw terminals), or they just ooze, but you don't want to have to go get a new one... With the screw on cap, you almost have a solderless kit, which is actually good even if you can solder, because if it heats up, it won't melt the solder and drop a hot wire onto ground. Probably a moot point and lets not test that theory

Wonder why it's stiff?

[erikjgreen]

I was thinking of this earlier today.. I checked the voltage output from the transformer and bridge to ensure I had the wiring right before I connected everything. The cap is fine, no problems at all. Just a hair over 34 volts from the supply as expected.

I'm going to pick up a better switch and power cable for it tomorrow, and buy or find a case for it. Other than that I'd call it done for now.

If all goes well I'll power the board tomorrow and set the stepper current levels, then connect the steppers.

I spent time over the weekend getting a computer built up from parts - about $200 got me a Pentium 4 3 Ghz with a 500GB drive and 1 GB ram. I downloaded EMC2 and installed it, then set it up for the HobbyCNC.

So far, so good.

Erik

[erikjgreen]

Oh, and I'm thinking the stiffness is due to the precision of the leadscrew... it's precision ground with a zero backlash nut.

Of course, it could be due to sitting in storage for a while, dunno. Here's a link to the manufacturer:

Lead screws and screw assemblies, by Universal

Mine are 1/2" standard precision screws. Looking over the specs again, I probably need to find some oil to put in the nut. I'm guessing I won't be able to afford to buy it from the vendor %) so maybe a light machine oil?

I also came across some cheap ball screws that might work, but I'm building with the acme lead screws for the moment.

Erik

[eartaker]

Use some chainsaw bar oil... its the same as way oil but much cheaper.

[erikjgreen]

So, I've been hitting one of the local surplus stores pretty hard lately. It's a fun place, I always find more things than I can afford to buy there.

This time I found two sets of really nice 5 phase stepper motors. They're Vexta PK699AUHA models, which have about 700 oz-in of holding torque. The reason I wanted these is that they're a "high speed" type that can provide that torque out to about 600 rpm in half step mode, which is about twice as fast as the non high speed version. There's a driver feature to cut holding torque back when standing still to keep heat down, pretty sure I'll leave that on unless I have problems.

Since the motors are 5 phase they'll run smoother than bipolar steppers, and along with the motors themselves I got the factory matched drivers and controllers. I'll probably just use the drivers, since they can accept step/direction input in one configuration. They also have individual power supplies so I can just feed in 110v. In case anyone's interested they're UDK5128NA units.

So I'm going to get 'em running and test the steppers on my PC parallel port, then order some couplers to go from the 1/2" steppers to the 1/4" acme screws. Then I'll start a teardown of my lathe and clean it up, followed by work to install the new leadscrews and mount the steppers.

I also picked up a cheap VFD from the same surplus place, so I'll be running the spindle from a 1 HP 3 phase motor and using spindle speed control rather than the DC motor I was planning on.

so much for cheap, though

Erik