Making Timing Belts & Robot Tank Treads Using Scarf Joints: Splicing Openbuilds GT2 & GT3 Belts Using a Scarf-joint or Lap-joint

I want to build some toothed tank-treads for a robot, and I also want to make timing belts. I have some Openbuilds GT2-3M timing belt material, which is a toothed belt 5mm wide with teeth spaced at a 3mm pitch. This tutorial shows how to make a continuous loop of belt from Openbuilds GT2-3M belt (https://openbuildspartstore.com/3gt-gt2-3m-timing-belt-by-the-foot/ ).

Toothed belts are often used in making 3D printers and driving linear actuators, and we use lots of Openbuilds components (www.openbuilds.com) for student projects at the University where I work. I evaluated several processes for splicing these toothed belts together, including testing multiple different adhesives, and surface treatments to evaluate how to make strong loops of belt from toothed belting material.

The GT2-3M belts are neoprene belts with built in lubricant, and a fiber reinforcement layer in the middle. The top layer has rubber teeth, the middle layer is a fiber layer, and the bottom layer is a smooth layer. To make the belt loops, I used a scarf-joint / lap-joint. I used a blade to trim away approximately 6-9mm of material from the top side (2-3 teeth from end #1) down to the fiber layer. For end #2 I trimmed the same length of material (2-3 teeth from end #2) from the smooth bottom layer, down to the fiber layer. These are the scarf joints on the ends of the belt. I prefer using 3 teeth length for a scarf joint. --EDIT: I have also been told that this is also called a lap joint. ---

Now I used IPA (Isopropyl alcohol 100% concentration) to wash out the lubricants that had been baked into the neoprene rubber. Let the exposed scarf-joint ends soak in the IPA for roughly 20 minutes. Now pull the scarf joints from the IPA, and let the alcohol dry away. Now apply Neoprene contact cement onto the exposed & alcohol cleaned ends of the scarf joint, and push the overlapping ends together (use a toothed spacer (either an oiled piece of GT2-3M belt, or use the 3D printed jig included in this tutorial, lightly covered in oil) to maintain the 3mm pitch spacing between teeth.

After the contact cement dries over 24 hours, you will have a loop of belt that is strong and nicely flexible. You can then reinforce the belt using contact cement to glue a strip of fiber reinforced neoprene sheet (ideally 1/16" thick ex: https://www.mscdirect.com/product/details/31937212) onto the back of the belt after it had been cleaned using alcohol.

.

I ran tests using over ten (10) different types of adhesive and selected the best adhesive for use. Please note that the adhesives used were not necessarily used for their intended purposes, and this in no-way reflects the quality of the brand when used for their intended purposes.

References:

a) Alcohol treatment of belt material and use of Neoprene adhesive were taken based on advice from the reprap forum, shown here https://reprap.org/forum/read.php?1,670647,888356

b) Making a scarf joint was taken from this youtube tutorial, by Robert Murray-smith https://youtu.be/1zdl_kSc0Ik

---EDIT: I got the name scarf-joint from Robert's tutorial, but people have informed me that this type of join is actually called a lap-joint. Please watch Robert's tutorial, it is very good.

The basic supplies include

a) Openbuilds GT3 (Openbuilds SKU 626). This is a GT2-3M belt, single sided, 5mm wide, and with a tooth pitch of 3mm (https://openbuildspartstore.com/3gt-gt2-3m-timing-belt-by-the-foot/)

b) exacto-blade or box cutter (ex: https://www.rona.ca/en/product/olfa-l-2-heavy-duty-utility-knife-with-anti-slip-grip-18-mm-abs-plastic-and-steel-yellow-5004-0002042?viewStore=64890&cq_src=google_ads&cq_cmp=11083221249&cq_con=107381541503&cq_term=&cq_med=&cq_plac=&cq_net=u&cq_pos=&cq_plt=gp&&cm_mmc=paid_search-_-google-_-aw_smart_shopping_generic_allproducts_previously_inactive_sku-_-71700000072618402&gclid=Cj0KCQjwjN-SBhCkARIsACsrBz7keEtoj8YY8xWUh5Z8jNjMXYA2msry3ynyqcmQuNziE6V6Qp00ZhAaAsbREALw_wcB&gclsrc=aw.ds)

c) IPA alcohol (Isopropanol) (we used 100% concentration) for surface treatment (ex: https://www.chemtronics.com/ipa-isopropyl-alcohol#:~:text=IPA%20%E2%80%93%20Isopropyl%20Alcohol%20is%20a,Technical%20grade%20isopropyl%20alcohol.)

d) Adhesive (For best results, we recommend gear-aid Aquaseal + neo contact cement for neoprene) EX: https://www.fruugo.ca/gear-aid-neo-neoprene-contact-wetsuit-repair-cement-2-pack/p-54979766-111753845?language=en&ac=google&gclid=Cj0KCQjwjN-SBhCkARIsACsrBz7Qc_zCFPrZIiAzPZ7bw4bvUljBZKCIe7dzA-hIvZhQ8C1-Mt8R-wcaAvLDEALw_wcB

e) cup for holding the IPA

f) self locking pliers or C-clamp for clamping the pieces of belt together EX: https://www.homedepot.com/p/Milwaukee-7-in-Torque-Lock-Curved-Jaw-Locking-Pliers-with-Durable-Grip-48-22-3407/205017479

g) Optional*1: Piece of GT2-3M timing belt covered in a light coat of oil to keep the teeth spaced apart,

h) Optional *2: 3D printed jig with a light coat of oil to prevent adhesive from sticking

i) Optional *3: Strip of 1/16" thick fiber reinforced neoprene sheet. example: https://www.mscdirect.com/product/details/31937212

Measure the length of timing belt needed to fit your application. Cut this length plus 3-6 teeth (9mm to 18mm of extra length) depending on your confidence level with making the scarf-joints). You need to be within +- 1 tooth or +- 3mm max deviation.

Ex, if your belt is going to have a perimeter of 300mm (100 teeth), then you need to cut 309mm minimum (100 teeth + an overlap of 3 teeth/9mm).

People sometimes bungle making the top-cut of the scarf joint. If you think that you might make a mistake and get things wrong, then add an additional 3 teeth/9mm of extra length in addition to the overlap so now your 300mm belt now requires a strip of belt 300mm + 9mm overlap + 9mm extra = 318mm.

Cut the length of belt. Cut between the teeth. The belt length needs to be the length of the perimeter of the belt plus a 3-tooth overlap (9mm in this case for a belt with a tooth pitch of 3mm) + 3 extra teeth in case of a mistake when doing the top scarf-cut. .

Ex: If the belt has a perimeter length of 300mm, then

a) we would need 300mm + (3 teeth* pitch between teeth) = 309mm exactly, (includes overlap)

b) to be safe, we add an extra 3-teeth in case we make a mistake on the top scarf-cut so we cut a length of

= 300mm + overlap+buffer length = 300mm + 3 tooth overlap + 3 tooth extra

= 300mm + 9mm overlap (3 teeth) + 9mm extra (3 teeth)

It can take a few tries to make good scarf cuts that go down to the fiber layer, but which do not cut through it. This means that it would be useful to practice both top scarf-cuts (where you cut away the teeth, cut away 3-teeth/9mm), and bottom scarf-cuts (where you cut away the smooth bottom layer, cut away 3-teeth/9mm) on a spare length of belt, without cutting through a lot of the middle fiber layers.

Practice doing a top scarf-cut, and a bottom scarf cut until you are confident that you can do it reliably over the length of 3 teeth (9mm), while leaving the middle layer mostly intact.

--EDIT: Once again, I have been informed that this type of join is actually called a LAP-Joint--

If we know the required length of the belt (ex: perimeter length of belt)

Make the top scarf-cut by removing the top 3 teeth (9mm) down to the fiber layer. Remember that the belt was sized with a 3-tooth overlap/9mm longer than needed

(ex: belt length is supposed to be 100mm, then the exact belt length is 109mm (exact length needed) or

118mm (with 9mm overlap, and 9mm extra length in case we make a mistake doing the top scarf cut, and need to cut away the botched 9mm length). I find the top scarf-cut to be the hardest cut to make, this is why we include the extra length. It is trivial to trim away extra belt length if needed.

If the top scarf-cut (the one that cuts away teeth) was done correctly the first time, then the safety length needs to be cut away before making the (easier) bottom scarf-cut. Trim away the safety length (now length = Perimeter length P + 3p of overlap. Ex: if Perimeter length is P=300mm, and belt pitch is p=3mm, now belt length should be 309mm.

Now after verifying that the safety length has been removed, make the bottom scarf-cut. It should be 9mm long. Remember NOT to cut through the fiber layer (some scraping is OK).

Fold belt into a loop and use the jig (either 3D print, or length of belt) to verify that a pitch of 3mm can be maintained between the teeth of the belt over the scarf-joint. The teeth of the belt loop over the scarf-joint should easily interlocking with the teeth of the jig.

it is OK if there is a small gap where the smooth sides join up.

if the teeth of the belt will not fit/interlock with the teeth of the jig over the scarf joint, then the end of the top, or bottom scarf joint will need trimming. See wherre the belt has gaps, or collisions, and trim the problem-end accordingly. (please use pictures as guide). Trimming should be less than 1mm worth of trimming.

Wash the ends in alcohol by soaking them in IPA for minimum one hour. It is OK if they soak overnight. Remember to agitate/shake at least once after 20 minutes to clear surface of the scarf-joints and allow fresh alcohol to soak at the surface. We used IPA @ 100% concentration.

Apply a light coat of a light oil or mold-release compound to the jig (either the length of belt, or the 3D printed jig) to ensure that the neoprene adhesive does not stick to the jig.

In this case we used cutting oil, because we had some in the shop. Vegetable oil could work as well.

After removing the scarf joints from the IPA, let them dry, then apply neoprene contact cement to the top scarf-joint. Overlap the top and bottom scarf joints (using the jig to ensure that the ends are properly spaced to maintain pitch) and clamp.

Leave the ends clamped overnight or for 12-24 hours.

If any of the edges of scarf-joint are lifting off, then it may be necessary to re-glue that area. Simply clean away any oil from the area using alcohol, then reapply glue and re-clamp.

If you are worried about the strength of the belt, you can cut a strip of 1/16" thick (or 1/8" thick) fiber reinforced neoprene and glue it to the back of the belt.

Cut the strip to an appropriate size (same width as belt, length = 3p = 9mm), soak in alcohol/IPA, then apply thin coat of neoprene contact cement and glue to the smooth side of the belt.

Now you have a durable belt or tank treads.

We also tested cyanoacrylate glues including

a)      Pro-line tire glue 6031-00

b)     Loctite black 480 prism instant adhesive

c) Starbond Black Medium-Thick High Performance Premium Cyanoacrylate

They were able to bond to the timing belt, but the bonded area was much stiffer than using the Neoprene contact cement. They did NOT require alcohol cleaning to work though. This may be of use to someone.

I just happened to have submitted this instructable at the same time as the "Make it Stick" contest was announced. Since I tested over 10 different adhesives under 20 different tests (with and without) alcohol, I'd appreciate it if you voted for this entry in that contest.

Thank you.