2021-06-16 - Design Review minutes (With Ping and Rishi)

General

Overall goals:
- Establish optimal joint speed
- Guide motor & gearbox selection for the comp arm
- Gain design experience for arm
- As cheaply as possible for testing purposes
Main functions of the arm:
- Base loaded with weights to hold the arm down; steel stock or other items
- First axis: turntable and pulley + gearbox
- Second, third, fourth axis similar; pulley and gearbox/motor
- Axis 5 and 6 combined into one gearbox, driven by pulleys
General comments from reviewers:
- Do you have any way to tension any of these pulleys?
  - A: being looked into
    - Q: Is it being reviewed/done right now?
- Tensioning part 2; each place where you have a pulley & tensioner the tensioning will be uneven
- Q from mathieu about overall weight:
  - Will the weight in SW trnaslate to the actual material?
    - In general, yes
- The joints that you do have confidence in, go make them
General to-dos/comments from team:
- looking into cycloidal drives

Joint 1

Q: What's preventing the whole assembly from hitting the table top?
- A: turntable bearing; vertical loads are held by the turntable bearing, wilt the bottom bearing takes the radial force from the moment
Q: How are you going to align the two bearings?
- A: use a thin aluminum plate in between the two bearings, with guiding holes
Q: Are you going with plywood, MDF... ?
- A: whatever is available; can probably find a 3/4 inch board in plywood but not in mdf, or vice versa
- Point: MDF is probably a better solution for this because you absolutely do not want the wood to fail
  - Q (from us): Stack a bunch of MDF boards on top?
    - A/Point: No, you should be able to find 3/4, or even 1 inch MDF at hardware stores
  - Q (from us): Is it worth drilling holes in advance?
    - A/Point: just try not to thread into the wood (may have misheard this)
      - For the turntable, it won't be a problem, but for the 3d printed piece if you put a nut on either side, you might hit your turntable
        Mathieu has a solution for this
        Point: Go to E5 and get precision holes
        Point: For the four corners, you may as well go as far as wooden dowels if needed so it's easily disassembled
        (wooden dowels for aligning... screws on top)
        afterpoint: wooden dowels are not really necessary
        Point: may want to mill the holes for the turntable into the MDF slot
        The small holes in the lazy-susan turntable are for dowels
Q: Can you use spring pins as well for the turntable?
- no answer
Point: Make sure that your belt clears the edges of the black 3D printed part, the belt has height
Point: For the 3D printed parts, you may just want to get PEM inserts (heat them up, insert them, call it a day)
- If you go with large enough inserts, you'll be fine
Q: What plastic are you using for the large 3D printed mounting plate?
- test printing: PLA
- Do not go for the pressed in inserts solution for the mounting plate if you're using PLA
Point: Put a ring above the lower radial bearing, so that it doesn't rise up
Point: Have you talked to the IDEAs clinic for 3D printing?
- Point: we have a 3D printing sponsor that can do pretty much anything
Q: Are SLA prints stronger than FDM?
- A: Yes, and SLS is also stronger than SLA
Point: Rishi and Ping both see a lot of moments on the bottom side of joint 1; you may need to have a second bearing lower on the hex shaft later just to counter the moment
- This is a second iteration problem, unless you have money/time for it
- Look into slew bearings, but Rishi reckons they will be expensive

Joint 2

austin's presentation is godlike, may want to upload here
Large max torque on A2, but want to minimize the mass of it on the second iteration
Q: How risky is motor death?
Austin can add a second belt if it's required
3D printed enclosure for gearbox
Point: If you're concerned about point of failure at the base of A2, you can always make it bigger
Q: Water jet curve correction - for interior features of holes you want to undersize the hole, correct?
- A: Point: Yes, but why do you need such an accurate hole? You can always undersize the hole and use a reamer
- A: Point: You can also use a test piece at the engineering shop
  - Tell them you want to put a bearing in it, and they'll let you know if it's possible
Using tubes in order to move to carbon fiber tubing + adhesive in the future
Point: Motor efficiency
- The leftover % is going into heat, you're not concerned about conserving power so it's fine–just look into a heat sink if needed
Point: Recommend against double-belting, as they probably won't come in the same length-- try to find a bigger width
- buy two. If one breaks, dont put as much load in the second one
Point: Tensioner-- One side of the belt is going to be really tight and another really loose, and it'll push your tensioner in that direction
- Rishi does not believe the tensioner will work
Point:If you're pulling on the arm and moving it down, the bottom side will be tight, and vice versa for the top
Point: If your motor mount is PLA, you'e probably going to snap it right off in the current placement
Point:If you're going to carbon fiber to carbon fiber bonding, fine, carbon fiber to plastic is fine, but Ping is worried about corrosion for aluminum to carbon fiber
- Look into how suspension arms are made in formula SAE-- they mix in a little bit of glass beads into the glue so that the aluminum to carbon fiber interface never actually touches
- Or a fiber glass sheet (austin's suggestion, agreed by ping)
- An anodized layer may be able to be looked into
- In general: carbon fiber to aluminum can be difficult
Point/Q: are you afraid of losing torque at the sandwiched piece of carbon fiber and aluminum joint?
- Look into PEMS (pens?) for this
Point: Locknuts won't give you adjustment, so that may be not fun to assemble/disassemble
- Fine for the first iteration
Point**: Look into a spur gear for the main joint instead of the pulley and tensioner
- two-gear setup instead of pulleys and tensioners
- standard on mcmaster carr
Q from Austin: Wants to avoid spur gearring for high precision, belts as long as they don't slip are good, wont be able to afford harmonic gears, so how bad is the liability of belts under high load?
- A: They're not bad, it's jut from a reliability point of view, especially operating at UTS
  - They will stretch; but that's where the tensioner comes in
  - The gears will have a little bit of backlash
Ping's opinion: A2 is ready!

Joint 3

Q: Do you have any hardstops for the rotation?
- A: currently 45 degrees, could be altered
Q: Would you recommend the hex shaft or keying?
- A: whichever is easier-- if you can readily buy a hex shaft, go for it

Joint 4

Heat inserts for the top and bottom
Looking into changing the pulley system to direct drive
Q: Do you need a bearing at the output of the enclosure?
- A: bushings/flanged bushing swork
Q: how is the black enclosure mounted together?
- A: Heat inserts on both sides
  - Rishi and Ping recommend something additional; right now you'll shear the mounts on the bearing plates
    - Look into injection moulding enclosures (how they do ribs on the ends, pockets on the corners)
Look at direct drive for iteration 2; solve your current issues of clamping
- If the current pulley system fails, go into a spur gear configuration like mentioned for joint 2
Why not use retainer rings instead of shaft collars?
- weren't sure if we had machine shop access
Main concern: the block and how it holds together
Put aluminum plates around the main block
May want to look into the width of the face aluminum holding A3 and A4 together; currently 1/8 but may change

Joint 5 & 6

Point: Ping foresees alignment issues for all the drive units

Point: Rishi doesn't like the lapping of the two metal pieces; why not just make it one piece
- It's easy to bend that metal, take it to doro
Point: Because of the alignment issues, a force will be induced on one of the sides (if the axis of the motor is off by even a bit)
Point: Thickness of the walls is too low, they will twist with any kind of side load
- Even with the standoffs
Point: Rishi would just machine it out of a block of aluminum for high precision
- Leave a wall of aluminum instead of the standoffs - ping
Point: not too difficult to make on a mill, do not go for the lapping of the two metal pieces
Any movement on the bronze bearing(s) is going to translate to the box tube, but if the gears don't maintain their spacing it will cause issues
Motor + gearbox assembly is only fastened on one side, which results in the sheet metal bending
There will be times when one motor is working in the opposite direction (of the other?) but this will cause significant bending
May want to look into a worm gear
The alignment issues are difficult to resolve
Put a u-shaped piece of sheet metal around the motors to secure them, like a lid
Try adding internal flanges on the two side plates to give them internal stiffness
Do not use hex standoffs for the encoder mount, or at least use more/make the pieces wider and more rigid
Point: They definitely see the sides where the pulleys are, where the bronze bearings are, flexing away from each other
Run a smaller shaft between the two gears to keep them aligned
- connect the two differential gears in some way
Point: add washers... everywhere, except for shaft collars
Change from box tube, or use a smaller box tube
Put a lid on the box tube, and then you'll be able to add additional support for the end effector however you want
Ping likes the current design (3 gears) over the other design mentioned/showed (2 gears), just make sure to connect the two shafts (instead of the current shaft going from the middle gear to the shaft collar)
Ping was worried about some type of enclosure, but as long as it is not run in a dirty environment it is fine
TL;DR:
- Anything that adds more internal stiffness is good
- Pulleys are supported in one direction, creating a cantilever
- Get rid of the overlapping place
- Get rid of the standoffs, turn into blocks with dowel holes
- Put flanges on the end of the ...
- Use dowel holes for alignment
- All of the "white spacers" are just saying that the box tube can be smaller.
- Turn standoffs into blocks with dowel holes
- Slots for tensioning will be easier, but even with slots, it's difficult to support the other side of the tensioning/motor