Hey guys, where can I buy a prusa hobbed pulley and Grub screw other than from prusa directly?
Prusa wants $36 usd for shipping for just a tiny pulley.
Anywhere else sell the original pulley?
The brass threads for the Grub screw stripped on mine.
Thanks.
Dunno about buying a new pulley but you could also consider drilling out the stripped out screw and retapping it for a slightly larger size.
I’m gonna second @John_Bump 's idea. Easier to take a small drill bit, drill out the old set screw, and tap for a slightly larger size.
I don’t know much about screw sizing or anything like that.
So I’d have to get the tap bit and a screw for that size, then cut the screw down i guess?
Anybody know what size that grub screw is? and what would be the next larger size?
As i said, I don’t have much experience with machine screws.
Take the pulley to the hardware store and try fitting screws into it (or take the grub screw and physically match it.) Machine screws come in four varieties: metric or imperial, and coarse or fine. You don’t care whether your current one is coarse or fine. The whole point is just to find what it is and then go the next size up. Imperial machine screws are generally only available in even sizes these days: #4, 6, and 8 being by far the most common. Metric screws are available in all the millimeters and a lot of half-millimeter increments.
This: http://shop.prusa3d.com/forum/prusa-i3-kit-building-calibrating-first-print-main-f6/replacement-grub-screw-for-extruder-driver-wheel--t3504.html indicates that you have a 3mm x 0.5 pitch grubscrew to replace, so going up to a 4mm x 0.7 screw would be a good choice.
To rethread, you first drill it out with a drillbit that’s pretty precise, then tap it. I’m going to give you some dubious advice here. If the above was correct about sizing, ideally you’d get a 3.4mm drillbit. If you’re in the US you are going to have a horrible time finding one, in which case you can use a #30 drillbit. Here’s the dubious advice: you can also probably use a 1/8" drillbit, because brass is forgiving. This has an error of about 5% extra metal over ideal, and in steel you’d break a tap doing this, but in brass you’ll be okay.
You have to drill fairly carefully right down the centerline of the previous hole. You have to then tap extremely carefully right down the centerline of the hole you just drilled. If you’re off center it’ll feed in and get harder and harder to turn as it tries to drill itself into the solid wall, and then break off, and then you’re mostly screwed (so to speak.) But if you put the pulley on a table and start screwing the tap in and look straight down on it from the top, and straight across from the side, and it looks perpendicular to the pulley on both sides, you’ll be okay. Taps need a tap handle. You can do it with a crescent wrench but it’s way harder to keep it perpendicular. Turn the tap in three turns so it’s good and started, then start turning it in one turn and backing out half a turn, which breaks the chips loose. Four turns in (and three half-turns back) and then unscrew it and clean the chips out of the flutes of the tap, and keep going until it’s gone all the way through or butted against the opposite side of the pulley and you can’t turn it anymore. Don’t force it. Taps are incredibly brittle. If it gets stuck and you feel like you’re about to snap it, just keep turning it back and forth until you can get it to make progress one or the other direction, then unscrew it and clean the chips out again.
@John_Bump Thanks for the well done reply. I appreciate all the advice.
I don’t have a tap bit, so i’ll have to find one. As well as the bit needed to clear out the existing threads. That’s going to be a pain i’d imagine.
Currently the pulley only has a single working thread. I have an M3 bolt from the extras bag that will thread on and hold the pulley, But it’s obviously too long. if i was to cut the bolt down and turn it into a grub screw, it could work, but i’m worried as it only has a single full turn of thread holding the ‘new’ grub screw in. Will that be enough? or will it loosen and fall out fairly easily?
I’m a bit intimidated to try to drill the pulley as i’ve never done that before and i think it’d be fairly difficult to get it perfectly straight on such a small amount of threads.
I wish it wasn’t basically $60USD to order a new grub from prusa.
I suspect a single thread will fail. A rule of thumb is you want at least as many threads deep as the screw is wide, so in your case that’d be 3-4 complete threads. With that said, there’s no harm trying.
I get most of my stuff from JTS Machinery. They’re reasonably inexpensive and have an exhaustive inventory of drillbits and taps. For this use you could go with ordinary high speed steel for the drillbit and a plug or taper high speed steel tap. Brass is almost the easiest stuff in the world to tap.