Lowcost capable CNC - Make vs Buy?

[Kitwn]

There is almost too much information available for a beginner to know where to start and I simpathise greatly. If you look at other forums and blogs from CNC users you find people cutting wood using machines that would be laughed out of town by MYCNC forum members. Angle iron (or even aluminium) and skate bearings for rails, plywood frames for base and gantry, Makita routers as spindles and so on. If you want to make carved signs, bass relief maps of New Zealand to hang on the wall or other decorative pieces they will work well enough, but my own experience tells me that if you want to cut out pieces of wood that will fit closely together, which I believe is your aim, then you need the kind of machine described on this forum.

Whether that commercial machine is capable of such accuracy I have no idea but some demonstration from the manufacturer before you purchase it would be advisable. You should also confirm that the spindle can cope with the kind of cuts you want to make. I'd want to see an example machine cut out some pieces to my design before parting with that much cash. One of the things you'd expect to be paying for by buying a commercial product is a guaranteed level of performance.

I like the idea of a MYCNC reference design, and there are several close approaches to be found here, but there would need to be at least half a dozen variants to suit different budgets and requirements. The steel v aluminium choice will be different for different people and a wood-only machine can be a little less robust than a machine for cutting aluminium, though the dimensional accuracy still needs to be very good. Probably one of the trickiest parts of the design is deciding how you are going to do the final alignment of the machine and how to design it to make that as easy as possible. You need to keep this in mind from the start and a good reference design would include how to align it for best possible performance from the materials chosen.

Sorry, that turned into a bit of a brain-dump but what else are we going to do stuck indoors every evening? Maybe that should be a topic of it's own!

[Andrewg]

Sorry, that turned into a bit of a brain-dump but what else are we going to do stuck indoors every evening? Maybe that should be a topic of it's own!

No thanks for thoughts - having more time to think about these things and hoping a few of you would too, is why I started my thread now.

You are right I am after a reliable wood workshop tool. It therefore does not need the duty cycle of a production CNC but must be robust and quick enough that when you turn to it, you can set it up and run a cut in a few minutes. Like other workshop machinery it will probably run a for a few minutes 5-15 times a day with a few longer jobs from time to time. I want to cut joinery so 0.1mm accuracy and repeatable must be delivered. I know that issues like safety enclosure, dust collection, fixturing and programming are just as important to get an effective tool, but first I need the right design.

Any reference design ere would have to 'show it's working' so each builder could decide whether to follow, enhance or cut corners but a few things are coming clear (I think) for the first time builder like me:

Aim, to minimise part count, processes (like no milling or welding), tools, time and complexity and so risk of getting to a working machine.

1. Size - You will always want to make it just a little bit bigger, but the challenges get tougher with size. So with ballscrews keep it below 1.5m and ideally below 1m in any axis. The shorter the axis the better for a first machine! So I am shooting for 600x600x125, but the design should scale to 100500x1000x175

2. Frame - Welded steel with triangulation for rigidity is best budget performance for self build, if you have access to welding. Epoxy leveling can give you the precision alignment of surfaces without post machining. However Aluminum Extrusion and 20mm Aluminium end plates is capable for a smaller non-metal cutting machine if heavy duty versions are used. I keen to avoid steel work and welding so am going the Aluminum extrusion route. Ideally plate designs will rely on precision marking and drilling of holes only to provide

3. Linear motion - Hiwin style profile rails should be used on all axis, all be it cheaper Chinese copies are good enough. They not only deliver the essential rigidity in all but the direction of motion, but the rails are a major precision structural element of the frame, don't need adjustment or maintenance. 20mm rails all around are ideal for this size of machine, whilst 15mm rails could be used.

4. Ball screws - Again cheap C7 Chinese ballscrews give high precision, low maintenance, low backlash, high efficiency so should be used throughout. 1610 on X and &m, 1605 on Z or 1610 with 2:1 gearing

5. Y Axis - Single central ballscrew fine for up to 750 width, but beyond that twin ball screws better, and allow for smaller motors (Same as other axis)

6. X Axis - L shaped gantry design. Rails on top/bottom of front of gantry. Ball screw on back. L from two 45x90 Bosch style extrusions allow BHK/BF12 bearings for ballscrew to mount directly to it, as does 20mm rails.

7. Z axis/X plate - Z axis is simple challenge of sandwiching rilsa and ballscrews between two plates, with 20mm raisl 15mm narrower than ballscrew assembly. X plate more complicated

8. Spindle - 2.2kw water cooler spindle seems standard, mostly as the ER20 collet takes up to 1/2in router bits and is quiet for long runs. I'm still wondering if this is overkill for wood cutting where max of 8mm cutters might be enough for most jobs.

9 Steppers - You need power not so much for speed as acceleration. Closed loop are now more affordably and give more power, smoother/quieter running and accuracy/safety as they will error if steps lost rather than spoiling project and/ ripping machine appart in accident! High voltage and low inductance motors allow the power to be delivered, so motors and drivers can be selected appropriate - 3Nm Nema 23 or bigger probably;y required

10 Wiring and control box - Each step is simple but there is a lot of it. Drag chains,earthing, connectors, safe separation of mains and low voltage. Electrical
interference, especially if there is a VFD for the spindle. etc Know little at this time about this area so much more work needed to define.

So before I order my rails+ballscrews set and my steppers plus driver from Aliexpress, my extrusions from Motedis and my controller from UNCNC. I need to be confident I have a design and and a route to the required machined Aluminium plates to pull it all together along with abuild approach (as you pint out, and I have only recently picked up - alignment of rails etc is critical)


SO another too long brain dump - you can see why I need to get this out, ti feels 2/3 complete but mising bits...back to CAD