Additive manufacturing: Terrible tolerances

[briankb]

A smaller endmill with longer flutes might be better, and you could definitely run at at least a few hundred RPM without melting problems.

Exactly what I was trying to say earlier. Milling (metal) is generally under 2500 rpm where as Routing (wood) is at 12-22k. I have a bunch of catalogs that offer specialized bits with single flutes, reverse flutes (pushing the material down), and specially angled cutters specifically made for plastics.

626 I agree if you used a router, even at the lowest speed of say 10,000 rpm, the friction even with a specialized single fluted end mill would probably melt or just rip the piece right off the bed.

Of course all of this is mute if we can just get a better outcome on the PLA/ABS and although I keep missing the mark, I've seen prints at the Orlando MakerFair that look perfect.

Has anyone seen one of the giant 3DPUnlimited printers in person? A week or so they released an upgraded servo they are calling "SurePrint". From what I've read it seems to have active feedback on the motors to give a finer control.

[Generic Default]

Some metals like aluminum and brass don't really have a speed limit on machining. Others like inconel and titanium can only be cut so fast even with state of the art cutting tools.


Some of the prints I've been getting recently look close to perfect, and come off the printer with flatness +- 0.002 across a few inches of surface. The 0.002 is mostly from stepper resolution ripples. I need to upgrade to 0.9 degree steppers to keep up with my ball magnetic arms.

The flatness is great and indicates a well aligned and calibrated machine, but the overall print tolerances are still +- a few thousandths of an inch because of nozzle offsets and end effector momentum overshoots on sharp corners.

[Krinje]

That video: "Using unusually slow speeds to show chip formation along... "

Some of your worries are unfounded. as long as the CPT (chip per tooth) stays up then melting shouldn't be an issue. 800+ SFM (Surface Feet per Min, or RPM * Cutter-Dia * Pi/12, or roughly RPM*Dia/4) wouldn't break a sweat (for this example: 625 SFM 10,000 RPM .25" Dia). If the Feed rate is kept to more than say .001" per tooth it'll be fine. It'd be happy taking .003" per tooth. That works out to a feed of 120ipm for a 4 flute tool. If the feed goes too slow and gets babied b/c of worries then you'll have a rubbing effect for several teeth before there is enough 'meat' for it to take a cut, rubbing = friction = melting. EG: 20ipm would be .0002" per tooth, not enough to actually cut, so it rubs for 3-5 teeth. If all goes to plan most of the heat is carried away by the chip itself.

The real crux of your problem is that the cutting forces are simply to substantial for A Delta, or really any printer not purpose built for this task. You would see backlash and wiggle and wobble, that you simply thought weren't there. Whatever the speed it still takes a certain amount of force to peel a .001" chip off of something. Printers just aren't built with forces at the hotend in mind, from the ground up everything about them is wrong for that, that also makes them affordable too.

Add on top of that the lightest of cuts at any significant height off of the bed would *pop* it right off.

Best solution: retrofit printer components into an existing mill. even a small hobby mill like a converted Grizzly would work. Then figure out what this special built plate needs to be to hold on to it.

[JFettig]

They clearly state the speeds are for demonstration purposes. Cut plastics at highest speeds you can. Create a proper chip and keep the tool cool and it'll cut great with very low cutting forces.

When I cut on my cnc, I only torque the vise to 6-20ft-lbs depending on what I'm holding, however that is still 500-2000lb of clamping force, when cutting plastics, I'd torque with an inch pound torque wrench.

[Krinje]

I'd bet dollars to donuts that even the best effector could be moved in a very significantly measurable way with 1 finger, more than .01". The rigidity just doesn't exist.

I want to see it now.

Could someone with CF/balljoint arms please set up an indicator parallel to the bed, touching the hotend and push on the opposite side? (with the steppers on ofc)

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I have sloppy stock arms and I know this would be easy to do.

I completely agree if you create a proper chip (maintain chip load) it cuts great.
Don't forget clamping has many benefits including being close to what your cutting, in this instance a tall part needing machining work could be up off the heated bead, unsupported, 10" or more.