Re: [CAD_CAM_EDM_DRO] Sanity check for homebrew CNC milling machine

Arvid, I understand you are in Sweden. I can maybe help you, I have a small shop with a lathe and a mill.
Contact me off-list at halle(at)telia.com.

/Thomas

----- Original Message -----
From: "bitnick78" <arvidpublic@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Wednesday, October 08, 2003 10:53 PM
Subject: [CAD_CAM_EDM_DRO] Sanity check for homebrew CNC milling machine


> 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
>
>
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