[Obi-Wahn]

Hi!

My name is Andreas and I'm toying around with the idea to build a CNC for ages. I've worked with CNCs in the past (old machines with SD floppy drives built in to transfer data) but I've never built or designed one.

Naturally I'd like to build a machine that does everything, but due to monetary restrictions (e.g. I'm not Trump) I'd like to start it kind of simple but keep it flexible to upgrade.

My biggest problem is that I'm frightened about all the calculations about steppersizes, screws, etc., so I'd like to have some help, please.


What I've got in mind:

So one citeria is the size of plywood sheats from my local building supplies store. Another one is my targeted materials I'd like to work with, which would be mostly wood and wood products (plywood, particle board, etc).

I've read many articles in the last weeks which recommends these ballscrews and these steppers, but in the end, without calculations it'll be just guestimation.


Long story short, this would be a description of my CNC:
Workable area: 2600 x 1500 x 750 mm
X- and Y-Axis: linear rails, rack and pinion drive
Z-Axis: linear rails, ACME screw
Bottom construction: steel, partly welded and screwed together for disassembly in case of moving, 50 x 50 x 3 mm box tubing
Top construction: aluminium system profiles, 160 x 40 mm
Axis: 3 + 1 at the beginning, in the end 5 + 1
Spinde: chinese 2.2 kW watercooled spindle
Targeted materials (ltr prioritized): wood, plastics, non-ferrous metals, steel


Now a few of my minds about these specs:

Workable area: Full plywood sheats in my local building store are about 2500 x 1250 mm, standard building height to code is about 2500 mm, so I'd be able to route entire sides of a wardrobe at once.
Rack and pinion drive: Due to the lenght of the axis (x and y) I think it would suite best. I think a screw this long would introduce wobble or would sag.
ACME screw: I've read that an acme screw would be more likely to hold the weight of the axis in case of a power loss.
Steel subframe: cheaper
number of axis: what I've read, a slave drive for the long axis would be necessary. For the beginning 3 axis (maybe a fourth one on the table) would be sufficient, but in the end I'd like to build a 5 axis machine with an additional on the table.
system profiles: got 4 + 2 near perfect slots where I could mount the rack, linear rails and a dust cover.
spindle: I'll take a risk for a cheap one, I can upgrade later
materials: steel would be nice to work, but is entirely optional. If it wouldn't work out with steel to route, I would probably replace the top with a water bed and try to use the cnc as a plasma cutter for steel if needed.


Summary:
I'd appreciate your input about my targeted specifications and also your help to calculate what steppers, rails, racks and screws I'd need. I'd estimate roughly the ganty to about 75 kg without stepers, rails and rack. Side plates of the gantry are calculated with 20mm aluminium. The system profiles have about 8.7 kg/m.

Thank you in advance
Andreas

[NIC 77]

My biggest problem is that I'm frightened about all the calculations about steppersizes, screws, etc., so I'd like to have some help, please.

You're using steppers? Nema 34, low mH inductance values, good quality drivers, either gecko, or leadshine. 2:1 or 3:1 belt driven gear reduction for X and Y spring loaded into the pinion. You can either use a commercially available carriage or make your own.

For the Z, 10mm lead ballscrew, depending on spindle weight and if you want to counterbalance it pneumatically. 5mm would also work, but not as fast. If you buy a premade linear stage from EBay or somewhere else to use as a Z axis, then a leadscrew is fine. Just be careful, you want 2 bearing blocks and two supported rails per side. The cost of a 16mm or 20mm ballscrew isn't very expensive so if you make your own Z axis, do that. Unless you already have free parts somewhere. It's not critical.

I've read many articles in the last weeks which recommends these ballscrews and these steppers, but in the end, without calculations it'll be just guestimation.

I do performance calculations with graphs and including inertia sometimes. This is a pretty straight forward build if I understand it correctly. What you are talking about is like one of the many CNC Router Parts builds, or Fineline Automation Saturns. If you were using servos with different gearing and heavy weights, and some very specific performance targets in mind, then yes, it's worth it to calculate.

Long story short, this would be a description of my CNC:
Workable area: 2600 x 1500 x 750 mm

Why 750? Did you not mention you want to cut sheet products? Like plywood? I assume those are travels because it looks like you want to cut an 8' x 4' sheet with this?

Bottom construction: steel, partly welded and screwed together for disassembly in case of moving, 50 x 50 x 3 mm box tubing

Yes on being able to disassemble. I am doing that on my build. only way to move it in and out of a basement. No on the 50 x 50 x 3 box. It's too small, and will distort too easily when welded. Try to get some larger sections. Way easier. Even go to your local steel scrapyard and see what they have.

Axis: 3 + 1 at the beginning,

How you incorportate the 4th axis will need to be well thought out from the start.

in the end 5 + 1

Nope. Make up your mind now if you want to design a 3 axis or a 5 axis machine. Was this the reason for the 750mm? You would really need to design the entire machine at the start. 5 axis machines are hard to do and expensive. That is why they are rare. You mentioned cost twice in your post. Perhaps this isn't for you.

If you want some better feedback, then you will need to draw up what you want to do in CAD and post some pictures of your designs. That's the best way to get relevant help when designing.

Steel subframe: cheaper

Yes and no. It depends on alot. You usually have to get the steel flat either by machining or using epoxy leveling, so there is some time and money expense there to consider. Aluminum typically comes flat enough already.

but in the end I'd like to build a 5 axis machine with an additional on the table.

So you want to build a machine that you can make longer in the future? How do you wish to accomplish this? And by how, I mean, please post some pictures of your design, drawn in CAD, showing how. I don't see this working out.

spindle: I'll take a risk for a cheap one, I can upgrade later

You will end up upgrading the entire machine so there are three original bolts left and a piece of steel tube by the time you're done upgrades and you haven't even built the original yet.

materials: steel would be nice to work, but is entirely optional. If it wouldn't work out with steel to route, I would probably replace the top with a water bed and try to use the cnc as a plasma cutter for steel if needed.

So you want a 3 axes machine that will has a cutting area of 2600mm x 1500mm with a rotary axis.

That you can easily convert to a 5 axis machine keeping the additional rotary axis.

That you can easily upgrade the spindle to a larger one.

That you can add an addition on for longer movement later.

That you can also use as a CNC plasma machine.

Did you mention a budget? LOL! Let me guess. Is it $2000 to $3000 USD? Am I a mind reader?

With a gantry weight of 75 kg. No, this is too light for a 5 axis with a long movement. Your Z axis assembly will weigh twice as much as your gantry!

You forgot 3d printer. Just saying.

I'd appreciate your input about my targeted specifications

At this point, it's not looking very good. You will not meet all of those expectations. I'm being kind with the way I worded this.

Buy a kit or use plans from a popular manufacturer and limit yourself to 3 axis and perhaps a rotary. That's the easy way.

Or, if you seriously want to pursue this, draw it out in CAD, post some pictures of exactly what you want to build, then you will get specific relevant advice, and I'll be happy to throw in my 2 cents.

[NIC 77]

Oh, I see, just reread that.

An additional axis ON the table.

Not additional movement.

Off to a good start, already struck one off the list. now we just need to strike off the 5 axis and the plasma, and you'll be golden.

[awerby]

What would a 5th axis "on the table" be - a trunnion? In a build like this, you save headroom - and thus increase rigidity - by putting the 4th axis (and the trunnion, if any) IN the table rather than ON it. If the center of the A axis is at the same height as the top of the table, then the Z axis can be shorter, since it doesn't have to lift up so high to clear the workpiece.

[ger21]

Forget about the 5th axis, and reduce the Z axis travel to 200mm. If you don't, you'll be rebuilding your machine as soon as it's finished, because it won't work very well. Unless you spend about $100,000 on it, a DIY 5 axis won't work very well when used as a 3 axis machine.