CNC facilitated bribery

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ijv
Posts: 71
Joined: Sun Apr 06, 2014 10:41 am

CNC facilitated bribery

Post by ijv » Fri Aug 07, 2015 2:03 am

After spending all the time & money to get my SO2 up and going, I thought it would be politic to get the kids on side ;). So my first big project was a dolls house and matching furniture.
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IMG_20150806_155041.jpg (494.96 KiB) Viewed 1487 times
The designs are from MakeCNC http://www.makecnc.com/welcome.php. I used the 3mm stock thickness version of their plans for the house, but adjusted all the joints to fit 4mm plywood, as I thought that 3mm ply was a bit too flimsy. This resulted in a bit of sanding and other touch ups to get it all to fit together. If I was doing it again I'd either stick with 3mm ply or scale the whole house up for 4mm stock thickness and work out the fixturing to cut the biggest parts as even at 3mm scale they are too big to be cut on a standard SO2 in one go. The furniture is cut from 3mm ply and fitted together first time with no need for adjustments at the CAM stage.

CAM was done using CamBam, works great for 2.D work like this, I used the roughing clearance options to make the slots fit 4mm thick stock without have to redraw the design.
Cut using 2mm 2 flute down cut bit, rpm=12,000 using the Inventables Quiet Cut spindle, feed=200mm/min, plunge=150mm/min, Depth of cut=0.5mm.
Engraved using a Dremmel 1mm ball nose engraving bit, rpm=12,000 feed=600mm/min, plunge=300mm/min, depth=0.4mm.

One problem I had was fairly consistent chattering problems with the cutting (I think it was chattering is was screechy enough). I think part of it was due to the center of the plywood sheet being free to vibrate. But does anyone have any advice on feeds and speeds for this kind of work and material? I used some of the info on the Wiki as a starting point but otherwise it was testing and guesswork, with no one set of parameters really standing out. What do other folks with the Quiet Cut spindle use for their rpm setting?

Anyway, the kids consider themselves suitably bribed and are keen on extra furniture and things like custom door signs and the like. While I'm torn between just cutting more stuff and upgrading my machine. Homing and spindle speed control would be really nice :D.

ijv

Gadgetman!
Posts: 1061
Joined: Sun Aug 25, 2013 12:44 pm
Location: Norway
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Re: CNC facilitated bribery

Post by Gadgetman! » Fri Aug 07, 2015 7:17 am

It should be able to cut Plywood at that speed.
Please examine your machine for signs of wobble in the spindle/endmill, and for general looseness.
(Improperly tensioned belts can cause a lot of weird problems)
If you don't find any issues, drop the speed to 175mm/minute and see if that helps.

When I cut large pieces of thin Plywood, I use a 2.5Kg (5lbs) iron weightlifting disc that I place in the middle of the Plywood.
In your case, with big cutouts, I would have drilled holes and placed hold-downs where possible.
Weird guy...
Shapeoko 2014F: 1000mm X/Y, 300W Quiet Cut spindle, Arduino Uno/G-shield and GRBL 0.9i
15x30 drag chains, custom spindle mount, 9mm belts, 8mm endplates, 6mm motor plates.

ijv
Posts: 71
Joined: Sun Apr 06, 2014 10:41 am

Re: CNC facilitated bribery

Post by ijv » Fri Aug 07, 2015 11:16 am

Thanks Gadgetman, I haven't really re-tightened or re-squared my machine since I built it. I think you might be onto something with using weights to damp out vibrations. Any advice on suitable spindle speeds with the Quiet Cut Spindle?

Gadgetman!
Posts: 1061
Joined: Sun Aug 25, 2013 12:44 pm
Location: Norway
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Re: CNC facilitated bribery

Post by Gadgetman! » Fri Aug 07, 2015 11:58 am

If you have a speed controller, try cutting a scrap pice, and while cutting try dropping the speed a little bit for a moment.
Don't drop it more than about 1/8 total, though. (If it improves, adjust the feed speed on the next try)
It may be cutting just too fast, and the second edge comes around too quickly, so that it hasn't moved far enough to actually cut into the material, but just slides on it.
Oh, and measure how wide it cuts.
(wide cuts = too high feed speed, in my experience)

2mm endmills can be a bit tricky sometimes.
Weird guy...
Shapeoko 2014F: 1000mm X/Y, 300W Quiet Cut spindle, Arduino Uno/G-shield and GRBL 0.9i
15x30 drag chains, custom spindle mount, 9mm belts, 8mm endplates, 6mm motor plates.

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