I’ve searched CD for poly cord welding info and I think I have the general technique down but they are based mostly on textual descriptions. Here is the technique I’m hearing.
Take care, use gloves, and don’t get melted urethane on your skin.
Cut poly cord to length of about 3-5% shorter than length of run, even though mfgr says 10%.-- 10% is overkill and puts more tension on things than you want and just makes things more of a pain.
Joint cuts can be square cut or angle cut (not sure if it matters)
Heat/melt both ends by pressing against piece of aluminum, and heating opposite side of aluminum with torch. Or use heat gun, but that would seem to be more messy.
using angle alum as a guide to align, press ends together firmly for several minutes, then let set for several hours before using. Seems like a small u-channel or tubing that matches diameter may be even better for aligning.
A couple of questions:
Some writings seem to imply that the tubing should be stretched while welding. Not sure what is meant by this-- the only thing I can think of is that you put it around your pulleys and weld it into its final installation, stretching to make the ends meet-- makes sense if you can’t install it otherwise after it is a loop, I guess, but in our case, we can weld separate from the bot and then install with little difficulty (I hope, or am I missing something?)
One poster suggested using a block (of alumninum?) and drilling a hole to match cord diam. and then splitting the block to aid in alignment.
It would be great if someone who has proven success with welding solid core could post a easy to follow video on how to do it (without buying a welding tool for several hundred dollars!)
Try a laboratory hot plate for the heat source (use an aluminum plate to protect its surface).
Aluminum structural extrusions (e.g. Bosch Rexroth) provide channels of approximately the right size for alignment.
I’ve never tried to weld it in place. The robot was designed so that the belts could be installed later. We just guessed long with the length, and would have re-welded it if it was too loose.
You could also use a hot razor blade, place the two ends oposite each other on the blade, apply inward pressure until the two ends are melted and remove the blade, keeping the 2 ends pressed together for 5min, or whatever your belting requires.
We made a welder that works very well. Basically it’s a piece of 8020 extrusion, 2 short pieces of angle, and two binder clips. Rubber band puts pressure on the two ends, they are aligned and slide on the 8020, and a soldering gun with a small aluminum plate bolted to the tip for heat.
I posted a pic to the CD-media, will link to it when accepted.
Generally angled cuts are stronger because there is more surface area for the joint but they are a bit more difficult to make because they are harder to line up and harder to keep the edge from melting away. With angled cuts, you also have to make sure that both ends are cut at the same angle and so that the cord doesn’t have to be twisted to complete the band.
4: Use a thin flat plate or blade that is hot (like a soldering iron tip) to put between the cut ends, then just pull it away when you have good melting. Heat gun won’t work.
5. Once it reaches room temperature it is as strong as it will ever be. Don’t need to wait hours. After pressing together, cool it with a little water, no need to wait minutes.
What we do is take a drill bit a bit larger than the thickness of the polycord and drill into a piece of solid aluminum with it and then cut that piece in half at the point of the hole. We then cut the polycord to length, melt the ends, place it between the 2 pieces of aluminum and squeeze it into shape. We then let the polycord rest in the mold which is held together by a clamp and when it is done we cut away the excess.
We have used polycord of all sorts in our last 3 robots so feel free to pm me with any of your questions. I would recommend using the round 2 inch pulley polycord, we have never had any problems with it and I have even seen a team use it on their drivetrain(I do not recommend this).
I’ve had good luck with a “Hot Knife For Her” from Michael’s arts and crafts store. We custom made a bigger blade for it and made a jig to hold the belt ends and keep them aligned. No need for an angled weld. If done properly, you’ll never break it with a normal butt weld.
The past several years I have used both a jig and without a jig. The jig I used was a simple piece of wood with two clamping latches on it that held the to ends tight. Then I melted the ends with a lighter or soldering iron and pushed them together. After that year, I have just held them together with two pairs of pliers then heated up the ends and pushed together. Note: second method requires two people.
Our team chose to use the inserts because it would be faster to fasten them together and my teacher thinks that welding it is crazy. And it seems easy to repair if broken or easy to replace if we ever need to becuase you can connect the polycord while on the robot. If i am wrong with anything please explain…
Thanks
My text-book understand of polycord is that the inserts require a large pulley radius to work properly, something on the order of 4"-5", which is annoyingly big.
The McMaster Carr catalog pages for the cord show hollow cord, with the minimum pulley size, and based on that we bought the hollow 1/4" cord and the aluminum connectors to use on 2 inch OD “pulleys”. So far it works fine. If we do have problems with the connectors, we could weld it together.
We’ve had polycord break during a match (got snagged and broke). Fixing it with welding while on the robot is easy - 1) grab each end of the broken cord, 2) heat ends until gooey, 3) press together until hard (about a minute).
If a blob forms when pressing together, it can be easily trimmed to remove the blob with an x-acto knife.
(edit: BTW, I don’t recall ever seeing polycord snap at a weld. I’m sure it has happened, but the weld seems just as strong as the rest of the cord.)
I did a few polycord connections for a roller claw, I just used a lighter (wear gloves for it), melted the 2 ends, and put them together with out a jig. Also, don’t put any tension of the poly cord while you melt/cool it. Also make slightly angled cuts at the connection ends of your poly cord so you have more surface area on the connection (the angle should extend about 1/8 in from the edge of the cord)
We purchased the minibutt welding unit… never had a weld or a cord break…
I am sure you can do the same by hand but the little clamp in that kit makes it easier to push the ends together properly…
You can purchase the clamp by itself and use your own heater and snippers to trim back to diameter.
I like the clamp… it is easy to use and costs about $80
You can certainly make your own…
Teams do this lots of ways… practice a little bit and you will be fine…
We simply use a spackel knife, and a torch.
Secure the spackel knife in a vise. Heat it with the torch. Touch the ends of the polycord to the hot knife until they melt. Slide them up the knife and press them together.
Easy and effective. We’ve never had a cord break during competition.
