Re: Sanity check for homebrew CNC milling machine

Arvid, take a look at some of the designs offered from the likes of
www.campbelldesigns.com and others if you haven't already. I think
you could build a better machine from aluminum extrusions for not
much more money than MDF. You want the rigidity on the bridge, but
not the weight as F=MA.

I have the following drive/servo motors for sale that seem like a
better match for your system, especially considering the efficiency
or you leadscrews:

4 Yaskawa SGDA-04AS servo packs, 4 SGMP-04A3A4 servo motors, 4
DP9320089-2 servo pack to motor encoder cables, and programming
software for sale. All are brand new. The motors are 400W (0.53HP),
3000 RPM continuous, 4500 RPM max, 181 oz/in continuous, and have
2048ppr encoders. You can get more info at www.yaskawa.com or contact
me and I can email the specs in PDF format (large file). A power
supply is not needed with servo packs as it's built in. $1300 takes
everything.

Best Regards,

~petev
--- In CAD_CAM_EDM_DRO@yahoogroups.com, "bitnick78"
<arvidpublic@h...> wrote:

> I'm about to buy some of the hardware needed to build a bridge type
> CNC machine. I'm going to use it for routing and engraving
> (wood/plastics) and also want to be able to use it for some light
> aluminium work. Before I buy though I wanted to check my ideas with
> someone who's got a clue if/how well it will work - that's you
> people! :) Perhaps you can give your ideas on what kind of
> performance to expect from a machine like this.
>
> For those of you who aren't used to the metric system (numbers are
> rounded):
> Force: 1 Newton [N] = 0.2248 [lbf]
> Torque: 1 Newtonmeter [Nm] = 141.612 [oz-in] (Nm is written just

like

> that, and not N-m or N*m)
> Distance: 1 millimeter [mm] = 0.001 meter [m] = 1/25.4 [in] = 0.039
> [in]
>
> This is what I had in mind:
>
> * The frame of the machine will be constructed out of 19 mm MDF
> (medium density fibreboard). I will build 150 mm deep and 150 mm

wide

> U profiles out of the MDF to get maximum rigidity for the bridge,

and

> use lots of extra pieces for supports to get a machine as heavy and
> rigid as I can. The work area of the machine will be 600 mm x 420

mm.

> (The reasons for building with MDF are - it's cheap and I can

machine

> it. I don't have access to a machine shop.)
>
> * Linear guides: for the main axis I already have linear

ballbearings

> and shafts. These are 1100 mm long and 20 mm dia solid steel bars

and

> linear ball bearings. Unfortunately they are not the "supported"
> kind, but I will support them at the middle as well as at the ends.
> For the other axes I haven't made up my mind yet - I thought I

would

> evaluate the guides I've got first before buying anything more.
>
> * For leadscrews I'm going for 20 mm leadscrews with "Anti-Backlash
> Supernuts" (BS&A XCT20x4M). The lead is 4 mm/rev and they are
> designed for a load of 1100 N (250 lbs).
> Now, to get 1100 N linear force, I need about 1.7 Nm of torque

at

> the leadscrew. To get this I used:
>
> M = F*p/(2*pi*Eff), where
>
> M = torque [Nm],
> F = linear force [N],
> p = lead of screw [m/rev],
> pi = 3.14159..., and
> Eff = efficiency of lead screw, in this case 0.41.
>
> * Motors: I found a source (Brigar Electronics) for surplus PM
> synchronous servo motors, 0.33 Nm at 2.6A continous / 0.99 Nm peak

at

> 7.8A. I'm going to couple these to the lead screws with a toothed
> belt, with a reduction of 2.5:1, which will give me 0.8 Nm

continous

> and almost 2.5 Nm peak torque at the lead screw. The maximum RPM of
> these motors are 7500 RPM, which is way more than I need (even 2500
> RPM gives a linear speed of 4000 mm/min or close to 160 in/min!). I
> am however concerned about the available continous torque - will it
> be enough or do I risk overheating the motors?
>
> (And yes, I know these motors will be a lot more difficult to drive
> than normal DC servo motors - but I will manage somehow because I
> think they're cool :)
>
> I'm sorry for this long letter but all these things (together with
> end machining of the lead screws and angular contact ball bearings)
> is a considerable investment for me as a hobbyist. I hope to get a
> homepage on this machine going so that others might get some
> inspiration and help also though.
>
> Any thoughts on this - hints, corrections of mistakes, things I
> haven't thought about, even "yes, I think it will work!" - are more
> than welcome!
>
> Best regards,
> Arvid Brodin