Lowcost capable CNC - Make vs Buy?

[Andrewg]

Your way way over thinking this.!! . . .

That is what I need - Keep me focused on the real issues


Yes this is what I aspire to:Neat, clean, capable looking machine!
From your previous posts I am assuming I am looking at
- 45x90 extrusions on Y axis with two for the L gantry.
- 12mm Plate for ends and gantry sides
- 20mm linear rails all axis
- 1610 ballscrew, 1605 on Z driven via belt and pulleys so steppers can be tucked out of sight, 1: 1 gearing? is it worth added parts?

A little worried about the X/Z carriage getting complicated. If rails on front face and ballscrew on top, then couldn't it be a single plate again?

I am assuming this design cannot cut beyond the table, but by removing the gantry sweep back and maybe mounting the rails on the face of the gantry (and removing the control panel) I could create sufficient over run.

The smaller closed-loop motors (<=3nm) all tend to be 3 phase

The ones I was looking at for price were like these appear to be just steppers with encoder and therefore not 3phase though these look similar but are described as 3phase and are £40 more so guess your advantages of 3phase closed loop will cost a little more. Know little about steppers besides frame size (NEMA) and torque (Nm) plus a vague idea that inductance needs to be worried about.

Don't use 15mm for the Z-axis as it makes more difficult to build regards clearances and they are fiddly. 20mm cost very little extra and make for a much stronger and easier to build Z-axis.

Again feedback from real experience appreciated. How much should I be worried about keeping Z axis weight down? I was assuming that on a small machine, I want to keep the weight being flung about to a minimum to ensure good acceleration and performance whilst allowing modest (lower cost) motors and drivers to be spec-ed.

Working on a Z axis, need to upgrade the rails to 20mm!

[JAZZCNC]

From your previous posts I am assuming I am looking at
- 45x90 extrusions on Y axis with two for the L gantry.
- 12mm Plate for ends and gantry sides
- 20mm linear rails all axis
- 1610 ballscrew, 1605 on Z driven via belt and pulleys so steppers can be tucked out of sight, 1: 1 gearing? is it worth added parts?

Yes 45 x 90 with 10mm slot Bosch Rexroth or equivalent with this style of machine. This is type and size is required to allow easy fitting ball screws because the slots match up with Bk12 bearing blocks so no extra plates are required.

12mm could be used for the end plates but I wouldn't use it for the gantry sides and Z-axis. Normaly I would use 15mm for the end plates and making motor mounts etc and use 20mm for the gantry sides and the Z-axis.

A little worried about the X/Z carriage getting complicated. If rails on front face and ballscrew on top, then couldn't it be a single plate again?

Well yes and No.
Yes could be done but with this design, it compromises the machine in other ways. Rails on the front face only mean the distance between the rails is narrow so there's less support to the Z-axis rear plate.
Next the ball screw on the top means extra plates will be required so the bearings can be mounted, this means extra work and it also raises the screw high up the Z-axis rear plate causing an imbalance in how the forces are applied.

I am assuming this design cannot cut beyond the table, but by removing the gantry sweep back and maybe mounting the rails on the face of the gantry (and removing the control panel) I could create sufficient overrun.

No, it can't, well it does but not by much. This design doesn't suit cutting past the front. If you want to cut past the front then use a gantry design like what's used on the other machine along with a slightly different base frame for rail placement.

That machine uses ITEM style profile 120 x 80 with an 8mm slot. It makes a very stiff gantry and if you look closely you'll see one rail is on the front face and the other is on the top.

If you like I could quickly knock you up a model of how it would look.?

The ones I was looking at for price were like look similar but are described as 3phase and are £40 more so guess your advantages of 3phase closed loop will cost a little more. Know little about steppers besides frame size (NEMA) and torque (Nm) plus a vague idea that inductance needs to be worried about.

Those would work reasonably ok even thou 2 phase. The more important spec to look for is the voltage the drives can be run at. I
inductance is important but it goes hand in hand with voltage and I won't get too deep into why now, let us just say more volts allows higher rpm's and negates slightly if inductance is on the high side.

Now those drives allow Max 60vdc but that is the maximum and you never want to run at or near the maximum so we allow roughly a 10% tolerance on the voltage. So the ideal voltage for those drives is 54Vdc which will still allow the motors to give decent RPM.

However, if I was going with 2phase these are what I would use. The extra Nm means you can push the machine harder but more than this the extra Voltage range allows for higher speeds. Another advantage and money saver is that they will run on AC or DC so if you run on AC you only need a transformer. Whereas with DC you need to build the PSU with capacitors and bridge rectifiers which costs more money.

https://www.aliexpress.com/item/3279...69e73421AESKcL

Again feedback from real experience appreciated. How much should I be worried about keeping the Z-axis weight down? I was assuming that on a small machine, I want to keep the weight being flung about to a minimum to ensure good acceleration and performance whilst allowing modest (lower cost) motors and drivers to be spec-ed.

With 3Nm with 50+ Vdc or the motors, I suggested then for normal woodworking conditions with a typical 2.2Kw spindle, etc you don't need to worry at all.
Only if you have special needs or are going to use the machine for work like 3D which has lots of high-speed positional moves and attach a heavy spindle-like ATC spindle will you need to consider weight or fitting higher rated motors that can handle it and give you the speeds you need?

[Andrewg]

If you like I could quickly knock you up a model of how it would look.?

If you have the time it would be very helpful!

So the X/Z carriage with rails top and bottom and ballscrew behind, ends up being an inverted G rather than C - do you have a picture of the back?

Lichuan 5.5N.m closed-loop step motor
Ok those are seriously stronger motors than I was thinking off, but I guess it is good you are revising my expectations for when I finally try and calculate what power I need/can afford! I wont worry about weight then, as I can see you are directing me toward a seriously solid solution.

[Andrewg]

Ok, Just banged out a rough budget.
I'm sure there is much to add and adjust, but for now we are still near enough the £2k mark (£2,112 as I post this)

It is Google Spreadsheet here

Break it to me gentle what I have missed or under estimated

Also: Links were just what I used for budget numbers, and does not indicate intention to use supplier or endorsement of them