Rubber cog help


Hey folks.

I recently had to open up and repair my drum sander. As I was doing it, I found that a rubber cog was really worn out, and needs replacing. I think the technical name is a spider, and it is in the Lovejoy coupler between the motor and drum. It is rubber, about .5"(12-13mm) thick.

I was hoping someone had a good suggestion on a rubber that would laser well for this. Or at least a rubber that can machine well after I make some templates. Maybe a source to?

Or should I just try to source a new generic spider?



That’s not particularly badly worn. Most industrial supply houses should be able to line you up with a replacement.


I could 3D print one in NinjaFlex SemiFlex. I don’t know how well it would hold up as I am not familiar with what it does.


I’ve been trying to find a few good types of rubber for making gaskets. I think it’s totally doable. Might take a few passes to get the inset the way you want it.


For the hassle. i would buy a new one…
The link is specific to spider couplings… but for some reason it just gives the generic listing.


Is the replacement part expensive? (I did a search of the internetwork and it looks like they’re ~$10.) You can probably spend a bunch of time to DIY one, but it might be more effort than it’s worth. Even if you find a rubber material that can be effectively laser-cut, your next question will be “is it suitable for use as a spider?”. Next you’ll have to ask “how much material do I buy?” - by that I mean, are you going to buy enough to make one spider and just hope the first attempt at lasing/machining it is successful, or do you buy extra so that you can make multiple attempts?

Better than lasing/machining might be to try to cast a new one. Cut a mold out of acrylic, weld it onto another piece of acrylic (so that it can hold the liquid rubber) and find some kind of castable rubber, mix it up, and pour it in. Of course, this also warrants weighing the costs and benefits and begs the question about how much castable rubber you want to buy.

If you want to machine it, you could try freezing the rubber beforehand. Here are a couple videos to use for inspiration… (well, possibly inspiration, possibly discouragement)


Only reason I wanted to make my own was I wasn’t sure where to buy, or if it is a “normal sized part”.

@nick07lee up in Canada , I need to not only have a American shipping address, but I think an American credit card or something.

Maybe Ackland Granger or Fastenal might carry.


If you can at all buy the part, do that. Sounds like the industrial supply people are worth searching and maybe even calling. Does the original manufacturer/seller offer any help?

Getting the right material for this kind of thing is potentially way finicky – too soft and it fails or wears away quickly, too hard and it fails by fracture and/or transmits shock loads to other parts that aren’t ready to handle such…


Seconding the recommendation to simply buy one from McMaster. If you need more than a couple dozen of them, your time may be worth it to cast them, but really, don’t build what you can buy cheaper in this case.

I don’t have any experience buying from McMaster from Canada but it’s reported that they’ll ship there these days: mcmaster-carr-ships-canada-now

Re: McMaster-Carr ships to Canada now?
Do a credit card order from the website, enter all your shipping info including company name, don't ask to open an account. At least that's what works for me.

It’s an export restriction on some kinds of industrial products I guess; they don’t have the time or the lawyers to go through their whole catalog figuring out which parts they can ship where.

If that doesn’t work, pipe up in here and let me know which spider you need and I’ll just get one and send it to you.


Had to read this to get an understanding of why rubber. Seems like 3D printing would be best custom solution if you get right material.


That’s the gotcha. It’s got to both be fairly-tough and in the right range of Shore hardness. I haven’t got a lot of experience Ninjaflex and the like, and I presume lots of experimentation would be in order to get the print to a usable state as far as infill, toughness, and hardness are concerned.

If I had to make this spider and was not allowed to buy it, I’d cast it from one of the Smooth-On urethanes. That stuff is awfully tough, not that expensive for more than you’ll ever need unless you’re in the spider-making business, and available in a wide range of mechanical properties that are well-tuned already.


Omg if McMaster-carr is back to shipping to Canada again, life just got so much better… I think they grandfathered in the old accounts before they stopped. I’ll have to look into getting one.

If not @james_scheffler I really appreciate the offer, thank you very much.


What I have found with SemiFlex is if I print it with 100% fill it is quite hard but if I use lower fill percentages it can be quite squash-able. I also have normal NinjaFlex but it seems easier to reduce the infill than it is to swap filaments.

I did print a band for my watch strap with FilaFlex, which is soft like NinjaFlex. It needed to be because it was too thin to have infill so was solid.


Just don’t be afraid to take me up on it! I mean it!


Yeah, as I say, I haven’t got the experience necessary to really make a recommendation there of whether to print or not. if @jordanloshinsky has a 3D printer and filament already available, that’d certainly be the first thing to try.


One day… One day… I am not the most knowledgeable of 3d printing, but I feel it still has a ways to go, and I rarely have a use for it. I know I know, once I have it, I’ll 3d print everything. I think I’m just waiting for the GF of 3d printers to really show.


I hear you. I’ve backed quite a few printers over the years, and self-sourced a reprap build a few years ago when I had a lot of time on my hands and needed something to occupy it. I don’t do much 3D printing anymore because in my own experience at the time, so much of the endeavor was around tweaking the machines, models, and slicer settings to achieve passable results. Once I learned how to cold-cast in proper engineering resins from easily handbuilt molds, I sort of stopped worrying about whether or not I could turn out a Bulbasaur model in Lego-plastic.
Now all of that said, clearly printers have come a very long way in that time. The most recent stuff Josef Prusa has been doing - presaged at OSHS2013 - is really impressive, and I’m tempted to give it another go.
But with the GF and SHaper Origin on the way, I can’t really justify another digital fabrication tool right now.


Out of interest, which RepRap design did you build?


a Mendel Mk2 with some of the Prusa modifications.


I don’t recall a Mendel MK2 but there was a Prusa Mendel MK2. That wasn’t a very good machine, it didn’t work as well as the original Sell’s Mendel. Its short comings are what inspired me to design Mendel90.