Re: [CAD_CAM_EDM_DRO] Long Acme threaded rod

Unfortunately, you are off track.
Your dimensions are WAY too small for anything except engraving.

I do understand your needs re:costs, but ..

Cutting aluminium with small end mills and low pressures, might mean
using about 20 kg of lateral pressure, needing approx 1/3 Hp of spindle
power.
At that pressure, the structure must not vibrate, leading to chatter and
broken end mills.

Ballscrews are not necessarily that expensive.
Neither is steel structurre - in fact that is the cheapest easiest
material to fabricate stuff from.

I used 0.750 ballscrews for a similar size.
For routing alu, ok ...
My machine is for milling steel, and now I am installing 32 mm ballscrews.

I am using guideways ways of over 200 kg mass per axis, with linear rails.
Preloaded steel bases, 50x50x5, with 25 mm linear guideways.
Just to give you an idea.
Total machine mass is 2000 kg for 1600 x1600 x 400 work area. Its still
ultra light, but works ok.
Spindle is a Bp Mhead, 3/4 Hp.

The needed stiffness goes up by the 3rd power of the free length.
You can use acme screws ... but you should look at screws in the 1-2"
size for 1500 mm length.

Likewise, building a so-so linear track solution is possible.
Otoh, you can buy a cheap v-guidway in steel that is likely to be very
much better, and 2 orders of magnitude stiffer (worst case) which is
what you need to build for. Something similar to bishop visecarver.
Generic stuff is cheap.

You 8020 should be in the 4 inches size to work well. At least.
A lot better, cheaper, easier to build in steel.
You can have steel cut to size and flycut flat first, quite cheaply.
Or buy thin flat ground stock and bolt it to structural steel.
Only a drill (press) is needed.
Structural steel is nowhere near flat enough.

> Try not to laugh...
>
> I'm working on designing and building a CNC mill/engraver. I'd like it
> to have a cut volume of approximately 1000x1500x100mm (about 3ft by
> 5ft by 4 inches) that can cut aluminum plate and hardwood, at most. I
> expect to take multiple passes to cut through an aluminum plate.
>
> I'm basing the design on the ShapeOko machine sized up, using 8020 in
> the 25mm size with Openrail hardcoat and grooved steel ball or
> delrin bearings rather than ground rods and linear bearings. The Y
> travel will have two steppers driving it.
>
> Something like this, except the Y rails will be fully supported by
> another piece of 8020 extrusion bolted alongside and the 8020 along
> the front and back will be raised so I can slide a longer sheet in:
> http://powercordlabs.com/1200mm-x-1200mm-shapeoko-build/
>
> Openrail:
> http://openbuildspartstore.com/openrail-hard-coat/
>
> The ShapeOko uses timing belts to move things, but for the size I
> want, I am worried about stretching. So I'm planning on using Acme
> rod. However, I'm worried about a 1500mm long Acme rod whipping. I
> already have some 6ft 1/2-10 and there are anti-backlash Delrin nuts
> available, but that seems thin for such a long rod.
>
> I have considered ballscrews, but the price is fairly high and it is
> cheaper to get a more powerful stepper to make up the power loss.
> Although if there will be a significant performance difference, I'll
> save a bit longer. I am not trying for 0.5 thou cut accuracy. I'll be
> cutting aluminum panels for equipment builds (slots for slide pots,
> holes for controls and buttons, holes for displays), and
> engraving/2-1/2D machining of plywood, hardwoods, machineable wax,
> plastics, etc. Probably the parts requiring the most accuracy will be
> wooden gears for clocks, driven by a stepper motor.
>
> Am I completely off track? There are also steel wire reinforced timing
> belts available.
>
> Steve Greenfield AE7HD
> http://www.linkedin.com/in/stevenjgreenfield._,___
>

--
-hanermo



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