I promise I will stop asking daft question soon. Ish.

[D.C.]

I know I said I would do a cheap mdf machine, but the more I look at it the more I trouble I have. I don't think mdf will give me a decent 3x4 cutting area for doing useful work on and at the same time be able to mill new bits for itself reliably. The amount of mdf needed for a strong structure of that size would also end up as scrap when I came to upgrade. I realised that I don't have a problem with building boxes out of wood or steel I have a problem with all the cnc fetish talk of whipping, backlash, ballscrews and the unhealthy obsession with industrial strength vibration.

So steel it is for the frame and gantry.

I am intending to use 80x40x4mm box for the legs and the top of the longest axis held together with plates and M8 threaded rod. For the rest of the frame I'll use 30x30x3mm box. I'll be sheathing it in mdf so that I can use the underneath as a cabinet.

My longest axis will be 1.5m of rail and the gantry will 1.25m of rail. I am intending to construct the gantry out 30x30x3mm box backed by 3/4 inch mdf and also use 3/4 mdf for the gantry base & z-axis plates etc. Later I plan to machine new plates from 20mm aluminium to replace the mdf.

I'll be using sbr20 supported rail for all the rails and 20mm antibacklash ballscrews/couplers driven by nema 23 3.1nm motors driven by a cw8060 with a 600w 48V 12A power supply.

Each side of the longest axis will have a single sbr20 rail mounted directly to the 80x40 and ballscrew above, the gantry and z axis will have 2 rails and one central screw. I'll be ordering an extra long z axis but not using the full amount until I install the 4th axis.

The steppers etc I plan to get from cnc4you and the shiny metal bits & connectors from Chai.

Now for the questions...

1- Is 80x40x4mm & 30x30xmm mild steel good enough for the frame and gantry and will the threads/plates hold up ok without welding?

2- Will steel & mdf provide enough stiffness without cross bracing? (the frame will have a cabinet floor of 30x30x3mm near it's base)

3- Will the sbr20 mount ok directly to the 80x40x4mm or do I need a cunning plan to attach it?

4- Should I drive the two threads with a belt system or just slave an extra motor?

5- Anything horribly flawed with this masterplan?

I need this build to deal with light use, 3D carving of upto 4 inches of laminated ply/mdf, accurate cutting of inlays on veneered wood, occasional soft metal upto 20mm thick (only need that thickness for cutting new machine parts for it) and cutting/engraving on acrylic/pcbs.

Thanks for all the help!

[D.C.]

Oh, and one last question does Chai sell the water cooled spindles?

[Hobgoblin]

Oh, and one last question does Chai sell the water cooled spindles?

yes, inverters too. no mounts though :(

[Jonathan]

1) Depends how you use it, but I'd reccomend using something larger than 30x30 for the gantry - don't worry about the weight. It's easier to mount the SBR20 rails on something that is at least as wide as the rail base.
2) MDF wouldn't be my first choice, but it will still add stiffness.
3) Yes it will be fine on the 80mm surface.
4) 6 of one... could spend forever discussing this. I'd probably go for two motors, Jazz will say one motor and a long belt. Price wise there's not that much in it.
5) Use 16mm diameter, 10mm pitch (RM1610) ballscrews, not 20mm diameter. You wont gain anything by using the larger size ballscrew since those motors, especially when run from only 48V, will not be able to get good accelleration and speed with the larger ballscrew, especially if you're thinking of the RM2005 from Chai. Get RM1610. Yes the critical speed is lower, but that's only an issue on the 1.5m ballscrew and even so if comparing to 5mm pitch screws it will still go much faster as the rpm is halved for a given feedrate. For the Z-axis the best option is RM1605. Also you should use timing belts/pulleys to get the best performance...couplings are notorious for snapping or being annoying to align, whereas belts are trivial to align, allow you to change the ratio to gain speed or resolution and also help suppress resonance.

Oh, and one last question does Chai sell the water cooled spindles?

Yes, but he's one of the more expensive sellers for spindles on eBay. I got mine from solar.jean on eBay, and know a fair few other people who have and it's been excellent.

no mounts though :(

[Hobgoblin]

Are they from chai jonathan? As when I asked him last week if he could supply he said no :(

[Washout]

Sees spindle/motor mounts and wants ;-)

If you're selling those Jonathan and can take paypal, don't be shy and send me a PM (2.2Kw water cooled spindle 80mm dia attaching to a 160mm wide Z plate BTW) :-)

[martin54]

I'm guessing thise mounts are from Jonathan DC

[Jonathan]

Yes they're all ones I've made. I need to make one for someone else on the forum asap, but I'm very short for time currently since I'm at university and have another project that needs finishing. However if I can make a few to the same design that would be pretty efficient...the one in the second photo from the left is easiest if made from 1/2" thick aluminium, to this drawing (can change the hole spacing if required):

[D.C.]

Thanks for all the help 80x40 it is then and I'll plan for using belts.

As far as the ballscrews go, would a 20mm 5mm lead not offer better accuracy, I'm not intending to do large volume runs of stuff so speed isn't that important (as long as it doesn't case any material problems) but precision for engraving is.

And last two questions, is there a good online resource for explaining the basic maths and mechanics for working this stuff out properly so I can stop vaguely throwing combinations of parts around in the hope that someone will work it out for me? :)

Is there a good all-in-one type book (real dead tree type) that covers all the basic aspects of cnc preferably with emc2 examples?

Ta very much!

[Jonathan]

As far as the ballscrews go, would a 20mm 5mm lead not offer better accuracy, I'm not intending to do large volume runs of stuff so speed isn't that important (as long as it doesn't case any material problems) but precision for engraving is.

Yes, but overall you're still better off getting 10mm pitch ballscrews since you can use pulleys and have a 1:2 (motor:ballscrew) ratio to get the same resolution as a 5mm pitch screw on 1:1. If you got 5mm pitch then the ballscrew has to spin twice as fast, so unless the critical speed is twice as great you will be limited. The critical speed is just proportional to the diameter, so using 20mm will only gain 25% speed hence there's no point. If the 20mm diameter screw was 10mm pitch I probably still wouldn't use it since the required torque to accelerate it is much greater.

Also the resolution you get from 5mm pitch is hardly ever required. I cut this with (roughly) 10mm effective pitch:

The area shown in that image is roughly 50mm wide...

online resource for explaining the basic maths and mechanics for working this stuff out properly so I can stop vaguely throwing combinations of parts around in the hope that someone will work it out for me? :)
... preferably with emc2 examples?

Er, me? But seriously, the information on this forum is all you need. If you've not already then read as many of the build logs as you can as the same things come up regularly, so it shouldn't take long to find out what the best way to each part is. The fact you're intending to use LinuxCNC doesn't make a lot of difference to the process - in most cases it'll be just selecting a different option when you export the G-code from the CAM program.