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AdeV:
Well.... it happened. Again.

Yep - a new tool for the workshop  :lol:

This time, I threw the awemawson Book of Machinery Buying (Scale: Large) away... and bought a diddy little thing - a CNC3018 "Pro" engraving machine from China. £140 delivered to my door. After a couple of impatient weeks waiting, it finally arrived last weekend, so I immediately assembled it (didn't read the instructions, naturally, keep reading to find out why that wasn't such a bright idea), and to my delight, it worked right away! Of course, I immediately crashed it four or five times, whilst trying to get used to the control system...

Anyways... I spent a little time tweaking the speeds & feeds, so now I can crash it REALLY fast  :zap:

The attached picture of it is as it stands right now.

 :proj:

I've got a bunch of diddy microswitches on order (they should have arrived by tomorrow), so Saturday will be spent making the first modification: Limit switches. That way I can set up the homing function, and the hard limits should stop me crashing it on quite such a regular basis.

Second mod isn't really a mod... when I put the table together, I bolted the linear bearing pillow blocks on so they were as far apart as possible, for maximum stability. So I lost a full 2cm of Y-axis travel.... so I'll whip the table back off & fix that.

Y, Z and leftwards X travel are all excellent, but there's a fairly nasty graunching noise when the X-axis is travelling to the right from the left-hand-side. I'm not 100% sure what's causing it, I suspect it's an iffy linear bearing. I've tried adding lithium grease, but it just made a big mess & no discernable difference. I have about 20 litres of way oil, I may try a little of that, but I don't have high hopes. So a future mod will be to replace the plastic Z-axis carrier with a proper metal one, and fit some better linear bearings.

Finally, you may be wondering why I got this thing when I have a perfectly good* Bridgeport CNC machine... well, actually, I don't plan to do any engraving with it, or milling (it's not rigid enough for milling anyway) - actually, I plan to use it as a CNC PCB drilling machine. Allegedly the spindle motor does 10kRPM. I don't think it's doing anywhere near that frankly, but it should go faster than the 4kRPM that the Bridgeport can manage. And when it comes to drilling PCBs... the faster the better, especially with really small holes. With much less weight in the table/spindle than the Bridgie, it will move from place to place a lot faster; and with no side forces from the drilling, the lack of rigidity shouldn't be an issue. Plus PCBs only have to be drilled +/- maybe 0.1mm or so accuracy, and that should be well within this machine's capabilities, even with the spaghetti ACME screws/linear guides.


* Also there's a bit of a problem with the Bridgeport: 18 months of doing nothing seems to have caused some electronics rot, when I powered it up last week, all seemed to be going well until it tried to start the spindle, then a bunch of magic smoke came out from one of the circuit boards  :(  I've identified one blown capacitor, but there's some fizzing/crackling going on on a circuit board I can't see, it's behind another board. So now that's become a project too. My plan was always to use the machine as-is until the controller blew & then retro-fit a new controller (LinuxCNC seems favourite), but if it's just flakey caps as I suspect, I'll try to fix it as-is first.

Anyway. It's Saturday soon, so with a bit of luck I'll be making limit switch brackets for the new toy! :beer:

efrench:
You could try replacing the balls in the linear guides.  I did that to several mgn15's and it cured the ratcheties :med:. The ball size was 2.778mm.

ddmckee54:
Silly question, but when you added the grease did you just add it to the rail, or did you take the bearing off and pack it with grease?  If you didn't pack the bearing, then the wipers on the bearing probably did their job and wiped the grease off before it got inside the bearing where it would actually do some good.  You need to pack the grease around the balls and into the race.

I had the same issue on the Y axis of my 3D printer.  My printer was a fairly low-buck Prusa I3 clone, so they probably went with the lowest cost linear bearings they could find.  I figured that they had to cut every expense they could in order to make any money off the printer.  I replaced the bearings with something that cost a little more than $50 per hundred.  I didn't get the high priced Fafnir's at $30 each, but I only spent $2-$3 each for the LM8UU bearings that I got.  I wanted something a little bit better, but I didn't want to break the bank.  They made a BIG difference.

You hate to do it with a new machine, but you might want to consider replacing the linear bearings on the X axis, and maybe even replace the bearings on all of the axis. 

Don

AdeV:
Hi Don & efrench,

Thanks for the replies - no, I didn't pack the grease inside, and yeah, I probably should do... I used pretty lightweight lithium grease, if I'm going to dismantle it all, I might look at something a touch heavier. On the other hand... I can probably live with the crunchies, I don't plan to do any (much) actual milling/engraving with this machine. Rigidity is not it's strong point...

The motor carrier/X-Axis unit looks suspiciously like 3D-printed plastic, it's pretty easy to deflect just with moderate hand pressure. I think I may (at some point.... maybe possibly) make an aluminium unit, at which point I'll put better linear bearings in, and maybe even ballscrews, depends how flush and/or motivated I'm feeling at the time. Rather than fixing up this machine, though, I may use it as inspiration to make a better one from scratch, & just re-use the electronics/motors.


So, onwards and sideways.... at the weekend, I managed to spend a little time working on some limit switch ideas. Unfortunately, the microswitches I chose need itty bitty M2 screws to mount them. Something I don't have, and something kinda hard to come by at 6pm on a Saturday, and definitely not at all on a Sunday. Not wishing to abandon the project entirely, I made do with some hot glue for the time being. Remarkable stuff...

So, first, a couple of glamour shots of the machine as it is now (attached).

AdeV:
The wiring is very much temporary at this point. Whilst in China I discovered you can buy that "chain" stuff they use on real machines to keep the wires under control. Various people have made various 3d-printed plastic end pieces/mounting brackets for this stuff. So I foresee some of that in my future (but I'll have to mill brackets for the time being).

Anyway... the goal of the weekend (finished today) was to put some basic limit switches in place. Only one switch per axis just now, the plan is to have switches on the far end as well, to prevent any chance of accidental crashes. Putting the 3 switches on, however, allows GRBL to run it's homing cycle; add soft limits, and it' just stops instead of trying to drive the x/y/z axis into the wall, door or ground, if you press the wrong button or send it some iffy G-code.

First up, the Z-axis. For this, I hot glued (will eventually fit with proper screws) a microswitch to the top of the carrier. A simple plastic block was milled to size, an M4 screw thread tapped in to one side, this is the adjuster screw; and an M6 at 90 degrees to affix it to the machine. This worked OK, except the M4 thread in the plastic really didn't work at all well. It ended up so loose the bolt could be pulled right through it! As a temporary solution, adding some sellotape to the screw has given it enough bite. If it every fails, I'll re-do that part in aluminium.

To adjust, simply run the Z-axis Pretty Damn Close to the top of it's travel, then adjust the screw until the microswitch just clicks. A couple of tiddly tweaks got me pretty much the maximum travel that I'll get. Total travel is about 44mm, but I lose a couple of mm to the homing pull-off, which is the same setting for all axes. And, one slightly silly (IMHO) thing, if you move the machine back to it's newly read origin, the switch clicks & it locks out until you re-home it. Personally, I'd have liked to have seen it zero itself at its pull-off co-ordinates, so I could tell it the full travel available to it (minus the bit lost to the switches) & have it stop without alarming. But, hey ho. Maybe I'll have a dig in the code one day & sort that...

Attached picture shows the Z-axis adjuster block & microswitch. Please avert your eyes from the huuuge blob of glue. Me trigger happy  :palm:

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