I am assembling a Pete I just got as a birthday self-gift. (I’m soooo thoughtful!) I figured that the problem I am encountering could also happen with the generic SES community, so here I am.
The Problem:
I have assembled the leg joints per the instructions and the diagrams therein. From outside to inside, the bearing side is Screw, Bearing, C Bracket, Lock Washer, Universal Bracket, Nut. The problem is that the 645 servo’s mounting tabs will not sit flush on the Universal Bracket because the Machine Screws are all too long. The bottom of the servo case is hitting the axial screw on the bearing side of the joint. This is effectively pushing the servo horn out further than it should be and the C Bracket is slipping off of the bearing. Basically, it’s unworkable from the materials and instructions as provided.
I returned to the instructions. (When all else fails RTFM!) I noticed during initial assembly of the first joint that the instructions call for 8mm machine screws, but all of the ones I received are all the same length and their thread length seemed longer than 8mm. I haven’t broken out my micrometer to verify this, but my guesstimate is that they are all closer to 10mm thread length. The screws extend through the nuts to be over 1mm proud, judged by my Mark 1 Mod A Eyeballs.
Did I:
- use the wrong screws?
- screw up the assembly somehow?
- receive the wrong length screws?
and/or
- spend over $1k on a kit that includes all of the hardware and electronics required, but now have to file down at least 14 screws to make it work?
Thoughts?
Honestly, I’m a bit confused by the whole situation. I am also, admittedly, more than a little miffed that a kit that is more expensive than a good deal of the others on the market and more than 13 years old would have ANY surprises or require any hand modification of what are represented as drop in parts…
Thanks for any help…
Cal
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@caltain Welcome to the RobotShop Community. We’re here to help. Pete is a nice kit so we’re looking forward to seeing what you do with it.
Can you confirm you mean step 7 in the assembly guide here:
https://www.lynxmotion.com/images/html/build089.htm
And the issue is that the bottom of the screw protrudes into the base of the 645MG servo:
If this is the case, it might be that a shipment of screws received were a millimeter or so longer (any shorted and there are not enough threads for the nut, and any longer means they push against the servo’s base. The easy work-around if this is the case is to flip the screw and nut such that the head of the screw is next to the bottom of the servo.
Do you have any way at all to measure the length of the threaded part of the screw to know if you simply received the wrong ones (10mm)? Can you provide a photo showing how long they protrude after the nut (without the servo mounted)
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Similar to this assembly found in another assembly guide:
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Yes, that’s the page of instructions. Actually, it’s steps 3, 5, 7, and 9, but yes, that is the specific assembly diagram that was used.
I broke out my micrometer. I was off by 0.09mm, so I will have to put my guesstimator in for calibration and recertification! Every screw in the 9 bracket packs used for the leg assemblies is 10mm threaded length.
The extra length past the nut varies some, but is approximately 1.2mm. Picture below…
I considered swapping ends on the screw, but that solution is unacceptable for a number of reasons, including, but not limited to:
- Disassembly for modification or replacement would require the removal of the servo every time.
- Clearance between the legs. The area between the bracket arms is static and already “occupied” because of the shape of the bracket. Nothing inside that space will interfere with movement or range of motion. Worse, the screw ends are perpendicular to the path of motion in a “conflict of space” event. That means servo damage is much more probable, as the parts have no opportunity to deflect past each other as with the rounded screw heads.
- Neatness counts!
- Potential wear to and shorting of the servo leads. Potential strain on the leads if they become trapped. The servo leads are nontrivial to replace, unlike nearly every other part on the bot.
Mr. Murphy is a dear and close personal friend. We have spent many, many long years living and working side-by-side. He has taught me to always consider failure cases to be far more probable in any development environment than success cases. That, and when your multi-million dollar project executes an uncommanded a Scrap Heap Transformation or Yard Sale Maneuver, “it’s always the 39-cent part.”
So, anybody got any nice 8mm screws they don’t need, or do I need to start rounding up my Implements Of Destruction?
Thanks again!
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RobotShop will be happy to send you the 8mm screws ASAP. Can you provide the order number and how many screws you’ll need? They’ll also check other kits and make the necessary corrections. Sorry for any inconvenience! When you reply with the details, this community thread will be converted to a ticket assigned to the RMA department.
Happy to have you on the community!
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Order 100283127
As for the quantity, I assume it will be 1 for each of the 22 servo joints, but I’m not opposed to having a few spares if the tolerance is as tight as you indicate…
Thanks for the quick response and replacement parts. I’m betting I just discovered a supply chain alternate silently introduced by your supplier or their supplier. 4th time it has happened to me, actually. Sorry for the tedious fix required to correct your stock. Most of the screw/split washer/nut sets were in heat sealed baglets inside the resealable bag, so they should mostly be easy to keep together and possibly easier to spot through the outer product bag. Been there and done that, so I feel bad for finding a worse problem than my 22 screws.
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This topic was transferred to internal RobotShop Support.
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