Commit 9c9587c4 authored by Jake Read's avatar Jake Read
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# micro Stress vs Strain Machine
![img](images/20190429-DSC04334.jpg)
**status** machine is designed, fabricated, waiting for adaption of fixturing from 'the generalist' and for squid works controllers.
## Dedicated uSSM
Here I'm using some
Here I'm using some
- Ballscrews, SFU2005 C7 700mm Long [ebay, $150](https://www.ebay.com/itm/SFU2005-Rolled-Ballscrew-C7-Anti-Backlash-Ballnut-L300-350-400-450-500-600-1000/202274175696?hash=item2f187afad0:m:m2SEqt9q_ykYYkrO5ZM9deA:rk:1:pf:0)
- Nema 23 Motors w/ .25" Shaft
- GT2 5mm 6.35mm Bore Pulley 12T for 15mm Belt 'A 6A55-012DF1508'
- GT2 5mm 12mm Bore Pulley 48T for 15mm Belt 'A 6A55M048NF1512'
- GT2 5mm 15mm Belt with 55T 'A 6R55M055150'
- http://shop.sdp-si.com/catalog/product/?id=A_6R55M055150 55T 15mm Closed GT2 Belt
- http://shop.sdp-si.com/catalog/product/?id=A_6R55M055150 55T 15mm Closed GT2 Belt
- http://shop.sdp-si.com/catalog/product/?id=A_6A55-012DF1508 12T 0.25" bore Pulley 15mm
- http://shop.sdp-si.com/catalog/product/?id=A_6A55M048NF1512 48T 12mm Bore
- http://shop.sdp-si.com/catalog/product/?id=A_6A55M044NF1512 44T
- http://shop.sdp-si.com/catalog/product/?id=A_6A55M044NF1512 44T
- http://shop.sdp-si.com/catalog/product/?id=A_6A55M050NF1512 50T
![img](images/uSSM-detail-two.png)
End of the line here is (1) to do small-scale material testing, OK. Next step is (2) to automate material search, doing a kind of real-time simplex where function evaluation is testing and DOF are process and recipe parameters. This is a cohesive, realizeable story.
End of the line here is (1) to do small-scale material testing, OK. Next step is (2) to automate material search, doing a kind of real-time simplex where function evaluation is testing and DOF are process and recipe parameters. This is a cohesive, realizeable story.
I would like to know:
- evaluation: pick: Stiffness and Tensile Strength? some weighted score from both?
- ... no hard yesses, want to be able to pick & make
- ... no hard yesses, want to be able to pick & make
- so far doing tensile strength and elastic modulus
- to add: poisson ratio with vision and actuation
- inputs: if we are to pick, say, 3 process parameters (to have 3D search space), what are they?
- inputs: if we are to pick, say, 3 process parameters (to have 3D search space), what are they?
- water, glycerin, agar
- humidity and temperature / during drying time
- heating profile ... how long the cook is
- heating profile ... how long the cook is
## FEA
......@@ -36,35 +38,35 @@ I ran a quick simulation to see that this flexure was OK. Here it is loaded in t
![fea](images/uSSM-v15-beamfea-5kn.png)
![fea](images/uSSM-v15-beamfea-5kn-faceon.png)
And out of plane, with 250N applied, about 20uM displacement. So 1/10th the load and 1/2 of the displacement, it's at least 5x stiffer in one DOF than the other. Is that grounds for a decent flexure?
And out of plane, with 250N applied, about 20uM displacement. So 1/10th the load and 1/2 of the displacement, it's at least 5x stiffer in one DOF than the other. Is that grounds for a decent flexure?
![fea](images/uSSM-v15-beamfea-500n.png)
![fea](images/uSSM-v15-beamfea-500n-faceon.png)
## The Cutting
0.1mm offset, 0.5mm fillet for test.
0.1mm offset, 0.5mm fillet for test.
## Next Steps
design tasks remaining
- fixturing
- fixturing
- dbner ... in gh
- minor offsets for fits
- measure material carefully
- confirm sdp-si parts fitments
- measure material carefully
- confirm sdp-si parts fitments
- control box ? psu home ?
- fab this thing
- finish waking up new network, new stepper boards / code
- finish hello-world CV on markers
- plot a stress/strain chart
- fab this thing
- finish waking up new network, new stepper boards / code
- finish hello-world CV on markers
- plot a stress/strain chart
- show high resolution stress / strain measurement w/o high stiffness machine
- show machine reference via known material?
## Generalist does uSSM
## Generalist does uSSM
To test requisite parts for a small stress and strain machine, I attached some jaws and a load cell to [this 'generalist' machine](https://gitlab.cba.mit.edu/jakeread/mothermother).
......@@ -82,11 +84,11 @@ We're going to try to track the endpoints with CV, rather than fancy encoders et
# Microphone Stiffness Testing
[Alisya did this in FabAcademy](http://archive.fabacademy.org/2017/fablabsantiago/students/356/project.html) and it would probably be a fun piece of kit / example for the NIST Project. Looks pretty simple... and compelling.
[Alisya did this in FabAcademy](http://archive.fabacademy.org/2017/fablabsantiago/students/356/project.html) and it would probably be a fun piece of kit / example for the NIST Project. Looks pretty simple... and compelling.
# Design Spec / Notes
The goal here is to design and build a machine, which can be fabricated in $250k size fab labs, that can generate stress-strain curves for a wide set of materials as well as perform hardness testing.
The goal here is to design and build a machine, which can be fabricated in $250k size fab labs, that can generate stress-strain curves for a wide set of materials as well as perform hardness testing.
![img](images/sketch.png)
![img](images/detail-ish.png)
......@@ -106,9 +108,9 @@ We want lots of force, with very fine control of position. This means a nice lin
| 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.
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.
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.
To generate the force required, we're going to need some motor / transmission oomph. Here's a list of typical NEMA size motors, and the torques they can generate. The [atkstepper](https://gitlab.cba.mit.edu/jakeread/atkstepper23) can supply enough current to power any of these.
......@@ -122,17 +124,17 @@ To generate the force required, we're going to need some motor / transmission oo
### Racks and Pinions
We'll be using two linear stages (one on either side of the platform), so, from our N34 156mm motor with 13Nm of torque we'll need 26, 5.2 and 2.6mm lever arms respectively for 1, 5 and 10kN total force.
We'll be using two linear stages (one on either side of the platform), so, from our N34 156mm motor with 13Nm of torque we'll need 26, 5.2 and 2.6mm lever arms respectively for 1, 5 and 10kN total force.
Considering practical limits on pinion diameter (with a shaft of 14mm, and 19 3mm-pitch teeth, we'll have an 18mm diameter pinion - 9mm lever arm) we will only realistically achieve 1.44kN of linear force per motor with direct-drive rack and pinion on a Nema 34 size motor. This makes a 3kN machine, but to add some safety factor we're at at 2kN goal with this approach.
Considering practical limits on pinion diameter (with a shaft of 14mm, and 19 3mm-pitch teeth, we'll have an 18mm diameter pinion - 9mm lever arm) we will only realistically achieve 1.44kN of linear force per motor with direct-drive rack and pinion on a Nema 34 size motor. This makes a 3kN machine, but to add some safety factor we're at at 2kN goal with this approach.
The next step would be to check against tooth shear stress for 3mm pitch.
From reasonable WEDM (time) and Waterjet (washout) limits, I expect the thickness of any fabricated pinion to have a limit around 12mm. To make this all simple, I'll say 10mm. For the waterjet, this is a bit of a stretch (get it?)...
From reasonable WEDM (time) and Waterjet (washout) limits, I expect the thickness of any fabricated pinion to have a limit around 12mm. To make this all simple, I'll say 10mm. For the waterjet, this is a bit of a stretch (get it?)...
To guess at resolutions, we'll take our sample above in aluminium having a length of 100mm - with elongation at break being 12%, we're interested in a 12mm 'long' stress / strain curve. For 1000 pts on this curve, we're interested in a step size of 12 microns.
To guess at resolutions, we'll take our sample above in aluminium having a length of 100mm - with elongation at break being 12%, we're interested in a 12mm 'long' stress / strain curve. For 1000 pts on this curve, we're interested in a step size of 12 microns.
From our 18mm diameter pinion having a circumference of 56mm, this means 4667 counts per revolution. In a 200-step motor, we would need 32 microsteps per revolution - most drivers will go to 256 - but microstepping isn't exactly linear. To do this really well, we will want to finish work on closed loop stepper driving, where we can use a 14-bit encoder to control around ~ 4096 counts reliably.
From our 18mm diameter pinion having a circumference of 56mm, this means 4667 counts per revolution. In a 200-step motor, we would need 32 microsteps per revolution - most drivers will go to 256 - but microstepping isn't exactly linear. To do this really well, we will want to finish work on closed loop stepper driving, where we can use a 14-bit encoder to control around ~ 4096 counts reliably.
All in, a direct rack and pinion drive can land us at a 2kN machine with some desired resolution, but we're at or near most of the limits here.
......@@ -140,13 +142,13 @@ All in, a direct rack and pinion drive can land us at a 2kN machine with some de
![img](images/belts.jpg)
Since I was spinning this up to test on a belt-driven gantry system, I was able to confirm that this is a bad idea. We defeat the elastic stretch of the belt by measuring at the specimen (not through the machine's structural loop) but a meagre GT2 belt profile skips teeth pretty quickly as we approach any tens of kilos of load.
Since I was spinning this up to test on a belt-driven gantry system, I was able to confirm that this is a bad idea. We defeat the elastic stretch of the belt by measuring at the specimen (not through the machine's structural loop) but a meagre GT2 belt profile skips teeth pretty quickly as we approach any tens of kilos of load.
### Ball or Leadscrews
Racks aside, a ballscrew is the obvious way to do this. Ballscrews can be had for less than hundereds of dollars. Besides transmitting motion smoothly, their ratios are favourable. For example, with a 5mm pitch ballscrew having an efficiency of 85%, we can drive 14kN with our 13Nm motor - so 28kN for the machine.
For the same ballscrew, to achieve 12 micron resolution we'll only need 416 steps in each rotation - this is easy to get.
For the same ballscrew, to achieve 12 micron resolution we'll only need 416 steps in each rotation - this is easy to get.
## Step Two: Force Measurement
......@@ -162,4 +164,4 @@ I believe I want my ADC to have the green line 'on top' and white on the bottom,
![img](images/jaws.jpg)
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