elasticity

2 Articles

Enormous CNC Router Uses Clever Tricks To Improve Performance

December 16, 2020 by 32 Comments

CNC machines made from wood and 3D-printed parts may be popular, but they aren’t always practical from a precision and repeatability standpoint. This is especially true as the machines are scaled up in size, where the compliance of their components starts to really add up. But can those issues be resolved? [jamie clarke] thinks so, and he’s in the process of building a CNC router that can handle a full sheet of plywood. (Video, embedded below.)

This is very much a work in progress, and the videos below are only the very beginning of the process. But we found [jamie]’s build interesting even at this early point because he has included a few clever tricks to control the normal sources of slop that plague larger CNC machines. To provide stiffness on a budget, [jamie] went with a wooden torsion-box design for the bed of his machine. It’s the approach taken by the Root CNC project, which is the inspiration for this build. The bed is formed from shallow boxes that achieve their stiffness through stressed skins applied to rigid, lightweight frames.

Upon the torsion-box bed are guide rails made from commodity lengths of square steel tubing. Stiff these may be over short lengths, but over the three meters needed to access a full sheet of plywood, even steel will bend. [jamie]’s solution is a support that moves along with the carriage, which halves the unsupported length of the beam at all points of travel. He’s using a similar approach to fight whip in the ball screw, with a clever flip-down cradle at the midpoint of the screw.

So far, we’re impressed by the quality of this build. We’re looking forward to seeing where this goes and how well the machine performs, so we’re paying close attention to the playlist for updates. At an estimated build cost of £1,500, this might be just the CNC build you’ve been looking for.

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Measuring The Stiffness Of 3D-Printed Parts

October 12, 2018 by 16 Comments

How do you choose filament when you want strong 3D-printed parts? Like most of us, you probably take a guess, or just use what you have on hand and hope for the best. But armed with a little knowledge on strength of materials, you might be able to make a more educated assessment.

To help you further your armchair mechanical engineer ambitions, [Stefan] has thoughtfully put together this video of tests he conducted to determine the stiffness of common 3D-printing plastics. He’s quick to point out that strength and stiffness are not the same thing, and that stiffness might be more important than strength in some applications. Strength measures how much stress can be applied to an element before it deforms, while stiffness describes how well an element returns to its original state after being stressed. The test rig [Stefan] built for the video analyzes stiffness by measuring the deflection of printed parts under increasing loads. Graphing the applied force versus the deflection gives an indication of the rigidity of the part, while taking the thickness of the material into account yields the bending modulus. The results are not terribly surprising, with polypropylene being the floppiest material and exotic composite filaments, like glass fiber or even “nanodiamond” reinforced PLA coming out as the stiffest. PLA, the workhorse filament, comes in around the middle of the pack.

[Stefan] did some great work here, but as he points out, in the final analysis it almost doesn’t matter what the stiffness and strength of the filament are since you can easily change your design and add more material where it’s needed. That only works up to a point, of course, but it’s one of the many advantages of additive manufacturing.

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