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L-Bracket for motor mount, milled component for motor mount
This is final design type of issue, for now 3d printed is fine, milled is better because it can hold tighter tolerances -
Take out chamfers of the front and back milled components
Cost little to nothing, some chamfers are essential -
Make the nut carriage simpler to manufacture
costs 100 on fictiv, similar to the other brackets - Ream holes on laser cut pieces
- Add anti-seize to BOM for pin joints
- Add 3/16 id shims to bom
- specify spot welds instead of weld fillet (help retail temper on the sheet metal parts)
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Misumi shaftmight not be able to get the steps required, or will require a post op -see if this can be machined in house first
a tight tolerance step at the minor diameter of the lead screw is required to interface with the f-loc gear, might bot beable to post op machine that out of an existing screw - Adhesive backed rubber
- Try different rubbers - sorbethane
- add loctite 660 to bom - mcmaster
- redo the spring location - interface is fine but location is wack
- spring calculations and iteration
- make drawings for all of the components
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Things to be done still: hardware bom this includes stuff from mcmaster, KHK and one part from misumi. The front and end bracket also need to be machined and the lead screw needs to be modified once it arrives.
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Alot has happened since the last log, parts arrived, the mechanism was assembled, and the mechanism went through some preliminary testing.
Manufacturing
Due to timelines, we had to remove the anodization from the order. The parts received were impeccable. One issue encountered (which was my fault - not the manufacturer) was the washers did not fit the clevis pins out of the packaging, and each of the spacers had to be filled in order to fit. This is because these spacers were the only parts not specified to be reamed, and when cut, the parts had a massive bur. The ID specified for this part was 5 thou larger, but this was not enough clearance.
The front and end lead-screw mount plates that were manufactured in house were incorrectly machined. The screw holes are 1mm too close and the width is 1mm to thin. This did not cause any issues during assembly thankfully but in the next revision this should be corrected. It also shows that the parts can be further optimized for weight
Assembly
Assembly went quite smoothly, although there were a few bumps which are now described. First off, the loctite 660 and loctite 380 both worked phenomenally to bond the desired components together, I accidentally bonded the sorbothane to the weldment whilst it was assembled, causing it to seize... thankfully i was able to take it apart and scrape out the adhesive without causing permanent damage
One thing that was frustrating, was that all of the clevis pins have about 1mm of play in the axial direction. These are specced correctly on McMaster, but the parts have a 1mm extra usable length compared to the drawing given in the catalog. This does not give us any issues, although the mechanism appears to be loose. the OD of the pin fits perfectly with the ID of the laser cut parts, but the length has play. This can easily be corrected for with some 1mm spacers, but should be noted when using clevis pins in future designs.
The spring pins were also a massive pain in the ass to insert, maybe they are always like this, or maybe the hole was too small, but it was not an enjoyable experience. The holes given in the drawing for the part matched the specifications on the McMaster catalog, so it could be that the holes were incorrectly machined but this was not validated.
When the mechanism was initially assembled, there was a lot of binding in the lead-screw drive portion. What I suspected, was that the front and back support plates screwed down before they were correctly aligned. The solution to this was to loosen all 8 fasteners, align the plates such that there was no binding, then slowly fasten all the fasteners ensuring there was no binding throughout the process. This worked like a charm
Testing
Overall the mechanism works great... with the adaptive links locked. the main selling point of this mechanism was found out to not work, so for the next iteration, the adaptive capabilities will be removed from the mechanism
First thing that was noticed was the snails pace of the end effector. This is actually a good sign as the mechanism was calculated to move slowly. From the testing we can actually see how slow is slow. For the next iteration, we will design for a faster mechanism by changing the gear reduction, as the mechanism is over design with respect to torque.
Nothing broke during my limited testing - gears all survived, links all survived rubber survived so this is all good news.
The biggest issue → the adaptive links. Note that the rubber bands/springs used to control the adaptive features, were the only piece of the puzzle not calculated for... The plan was to just test different spring elements during the testing phase, but no rubber band seems to provide sufficient force. The temporary fix for this was to solder a loop of wire tightly around the adaptive links where the rubber bands would have gone. This worked surprisingly well. A replacement part is in the process of being manufactured.
The main star of this entire assembly was really the sorbothane pads used for grip. The pads are so sticky and squishy that it can pick up literally anything. It also has extremely good performance for moment loads (picking up the end of a hammer, not the head) the coefficient of friction used in calculation was 0.6, but based on feel it was easily more than 1. This means that the torque exerted is most likely more than required. More testing will have to be done to confirm this. This info can be used help reduce the weight as well as increase the speed of the mechanism. More testing should be done.