Onyx 3D printed gears - hex wear

Technical Technical Discussion
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Heya,
So wev’e had a Markforged for a while now and we love it, but one particular problems seems to happen quite a lot.

Many of the 20DP gears we print (onyx only) wear quite quickly in the 1/2’’ hex hub part. The teeth hold quite well, but once the hub becomes more circular, the gears are worthless.
Was wondering what solutions other teams have for that.

Thanks

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I’ve never used these, but I saw this on the Thrifty Bot update page. Seems like this might work for you:

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we’ve used these and they work amazing, we where having the same issues as you and then we tried these and they solved it.

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While the Thrifty Inserts definitely work, I believe the OP is in Israel. If there’s a desire for a multi-team bulk order to ship items over feel free to contact me and I can combine orders to ship together.

Some other good ideas include 3D printing your part with a geometry cutout of an existing 1/2 inch hex or 8mm keyed gear that you already have in hand and press it into your part. It will do the same thing that the Thrifty Insert does and you may already have some old gears laying around that will work for this.

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I believe some teams use the following 3/4 hex tube:

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Can confirm 33 has done this with onyx printed pulleys etc over the last 3 years with success. Just chop to length and press into the pulley. You can add some epoxy if you really want to make sure it never comes out.

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We like to 3D print a lot of our pulleys, we use this setup. Just two laser cut plates that seat into the print, works like a charm. These are effectively just the TTB product, just easy and cheap if you have a laser in house/sponsor. (Idea taken from a post by a 4414 person from a bit ago.)

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Good to know this is working for 33. Do you ever have issues with loose fit to hex shafts?

McMaster-Carr data for this tube indicates generous tolerance on the inside dimension:
image image

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What material are you making these with?

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6061 Aluminum, the ones pictured are just 0.09"

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I never personally never noticed any issue with this.

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If there is excessive backlash, you can buy shim stock from McMaster also to tighten it up. This is my go-to solution after Aren enlightened me in 2016…more teams should be using it on rotational mechanisms.

-Ronnie

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Out of curiosity, how many layers for vertical walls and top, bottom layers are teams using? We have printed many HTD pulleys over the years in both nylon CF and PETG. With a .6mm nozzle and 5 to 6 top, bottom and vertical walls , they have held up. We go for a slight press fit. On thunder hex we print the thunder hex profile. Also when possible for the high torque applications, we extend a hub to give more surface area. Another problem with Nylon CF printed power transmission parts is moisture in the filament. With nylon 6, 6/66 or 66 any moisture will greatly decrease the material properties. Once wet nylon is difficult to dry. I usually dry at 165 F for 24 hours then into a dry box for printing. The difference in a dry printed pulley and a slightly wet one is significant. Another thing we are in the process of dialing in is printing high loaded parts in pure polycarbonate. Not a alloy of poly like Polymaker PClite. Printing Poly is very difficult. And almost impossible for larger parts with out some form of heated chamber. But, the parts are amazingly tough. Nylon CF has very high tensile strengh and stiffness. Poly is not as stiff but the surface hardness and toughness are better then Nylon CF. I wouldn’t give up on printed pulleys except for the very high torque and shock load applications. Those may need the metal. Look at your printing and design methods first before declaring that you have to put metal in the pulleys.

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In the past we used to mill out a solid 1/2" 6061 insert.

But we have lately been using the 3/4" Hex from McMaster and haven’t experienced any issues so far. If we do, we will go back to our milling.

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The nylon is not wearing, it is creeping from high stress and low modulus of elasticity. Also the Onyx is likely to be mostly hollow if you are using the standard printing options. Consider changing the print to solid or change the design to have more roof and floor layers around the hex hole. Test Report for 3-D Printed 20DP PinionsGears.docx (15.1 KB)

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Are you using standard 1/2" hex or 1/2" thunderhex shaft? IMO Thunderhex geometry mated with Onyx in medium and heavier torque applications is just asking for heartbreak.

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