LOG.md

DEX

Re-did the machine last week, now much simpler:

...

Some Force Maths (and a .xlsx file)

• want (?) from ex. nylon dogbone
• for this, w/ motor doing 1.5NM, how much reduction ?

Have this spreadsheet already, that's great. The D683 dogbone is ~ 8mm wide, and it would be great to test parts at 4 layers of 0.2mm - 0.8mm thick. That's skinny, but here we are: these are all under ~ 500N. OK.

For that oomph, with a 22T drive pinion, I'll want a 6:1 reduction and a NEMA23 motor (just a short stack) pushing 1.26NM through to a max. 660N linear force. OK, glad to have checked, nice to know about the N23 - I had been thinking of N17s - and will have to watch about getting that 6:1.

uSSM #3

Theres a few new criteria that ussm-3 aims to achieve:

• Manufactured only using simple shop tools, PLA 3d printers, and a standard bed size (24'x 24') laser cutter.
• Pulling force of around 600N

In order to be laser cuttable, acryllic and delrin were the main two materials considered. Delrin was the go to option to avoid shattering.

This design uses mainly the following adapted gantry system: gantry along with this beam design: beam for rigidity.

The main idea is to attach a beam to the linear axis and use it as a carriage to hold fixturing for tensile testing on the top half and compression testing in the bottom half. The gantry system shown above does not supply enough torque, so an adapted version was made shown below.

The uSSM #3 design takes advantage of the beam design by attaching 4 different beams to a larger "O-face", with webs in the corners to prevent torsion. The larger face itself is split into a bottom and top in order to make assembly easier, make each piece smaller to fit into laser cutter bed, and to have the possibility of changing top for larger specimens. The beam design also utilizes custom joinery talked about in the beam repository. This makes the machine take some time to build, but it allows delrin sheets to be used, allowing many fabs with laser cutters to be able to make this machine.

fixturing

Fixturing is being designed to use mainly 3D printed parts. First up, PLA is being in the jaws to see if it can withstand the 600N load alongside a few other McMaster parts and this load cell

Drawingboard Return

With #2 feeling somewhat unloved ('both overdesigned and underdesigned'), I'm back at the basics for #3. There's a few major selections, and decisions to make:

Material Selection

-> ALU, FR4(G10), Acrylic?

While aluminum is my go-to for machine design, and is ostensibly possible to mill on a shopbot by a motivated user (see jens), there is some hesitation to use it.

Material	Young's Modulus (GPA)	Specific Young's	Cost for 6mm x 24x24"	Machinability
ABS	2	?	52	Not Dimensionally Stable, but OK to Machine
Nylon 6	3	2.5	130	Painful
HDPE	1	?	23	Easy
Acetal (Delrin)	2.8	?	89	Breezy, also lasers, and non-cracking
Cast Acrylic	3.3	2.8	46	Breezy, esp. w/ Lasers
6061 ALU	69	22	87	Breezy with WJ, Painful on Shopbot
FR1/CE (Canvas / Phenolic)	6	?	81	TBD, probably WJ Pain and Ease on SB
FR4/G10 (Fiberglass)	22	?	98	Painful on a WJ, Slightly Easier on a Shopbot

data

That said, ALU lands pretty well 1 order of magnitude above Canvas Phenolic ('FR1' or 'CE') for strength, while costing a similar amount of dollars. Fiberglass is a nice candidate, so machining G10 is likely a worthwhile experiment. However, both composites have anisotropic-ness and are sensitive to the size of local features (and to localized loads), making them less favourable.

!TODO: beam equations for the above, to size req' depth !TODO: shear forces for the same, !TODO: cost not-mcmaster, and acrylic, some composite like hydrostone w/ fiber

• ... composite vendors at fabclass website

Transmission Design

How Many kNs ?

We want lots of force, with very fine control of position. This means a nice linear transmission. To estimate the forces we might want to see, I wrote a quick table of forces required to rip apart ~ 3mm square (0.001mm^2) samples of a few materials.

Material	Yield (MPA)	F at Break (N)
ABS	40	360
Nylon 6	70	630
HDPE	15	135
6061 ALU	310	2790
4140 Stainless	655	5895
6AL-4V Ti	950	8550

Brinell hardness tests range from 10N through to 30kN (for steel and cast iron) but non-ferrous materials normally see 5kN only.

So, a ballpark of ~ 10kN would be ideal - this is a big number - off the bat I'm going to estimate that 5kN will be a more reasonable target. 1kN is enough for a complete set of plastics, but that's only allowing for a realtively small sample.

-> Ballscrews, Belt Rack and Pinion, Rack and Pinion

Generating kNs of force is no easy feat, especially when we want to do it very smoothly while displacing very small amounts.

I will start by mentioning that this is dead easy with ballscrews. With a 1605 ballscrew, (16mm diameter, 5mm per turn) and a NEMA23 with 3Nm of torque, we can generate about 3kN of linear force (per motor) - to land at 5kN total no problem.

However, these are somewhat cumbersome and expensive - and they land in fixed sizes. Towards more parametric machines, we can look at a rack and pinion type axis.