Hobbed Bolt

Hi guys.

Living in Argentina makes hard and expensive to source some parts for a printer.

Now im dealing with the hobbed bolt.

Iv seen that i can build one with a dremel, but i have a friend who owns a machine shop, so i can have access to a cnc lathe and stuff like that.

We where talking about making a clone of the hyena hobbed bolt, but we are having some trouble with the grooves. How many of them should the bolt have?, how deep?

Do you have any recommendations for the hobbed bolt?

If you have access to a lathe, it should be pretty easy. Have a look at the various methods on this page: [reprap.org]

Yes, i just want to avoid using a tap, so i can have consistent grooves that goes through the whole central waist.

Thanks

Using a tap is the easiest way, and works fine for just about everybody. I make mine on a bench drill press.
If you want to cut it with the dremel blade, you'll need an indexer like further down the page I linked above, or a tool like this one [www.thingiverse.com] (but you'll need a printer to print it). This lets you turn the bolt accurately, so the cuts are consistent.

This is a cut and paste from an old post I made to the developers forum. It is yet another method of cutted teeth on the axle using an end mill and a CNC rotary stage:


May well have already been covered but I thought I would share a method that I experimented with this weekend to make the friction drive wheel for an extruder. Based on some reprap research, looks like people are using the hob method (use a threading tap to hob teeth into a bolt...sort of like hobbing a worm gear) and others, using a press in insert that has gripping ridges.

I have a CNC taig mill with a fourth rotary axis (I built the rotary access...hobbed the main drive gear using a tap). My approach was to chuck the axle rod (bolt equivalent) in the rotary axis and mill a groove into the circumference of the axle using the edge of a simple straight 1/8" endmill. Looking at the end of the rod, the mill "kisses" the rod at 45 degrees so it results in a groove cut with an included angle of 90 degrees. Cut one groove, rotate the rod axis and repeat until you go all away around the circumference. For this test, I milled grooves every 10 degrees on a 0.25" diameter rod to a depth of 0.015"...so a total of 36 grooves. The attached pictures show the test cut and the mill with rotary axis. The resulting teeth are very sharp and straight...I worry a little about the hob approach since it generates a spiral cut...does it tend to twist the plastic filament? For the real extruder, I mill mill into stainless steel for wear...brass was just easy to cut for a test.

Yeah, that's one way to do it that we have in mind, the thing there is that theres no slot.

Isnt the slot necesary to avoid filament lateral movent?

Mars, "I worry a little about the hob approach since it generates a spiral cut"

Do you mean the method of using a tap to produce the hob?

The spiral cut caused by using a tap to create the hobbing does NOT cause the filament to rotate. Honestly, it's the easiest way, and produces a slot to avoid filament lateral movement if you use a M3.5 or M4 tap, which works fine for 3mm and 1.75mm filament, so long as you don't go too deep with the hobbing.

Thanks for the clarification droftarts. I bought my first one premade, but I presume I'll need to make one or more in the future as I begin to design my own extruders. Good to know.

the groove is for more surface area. Cheap PLA will rip apart with the bolt above. it is perfect for abs plastic.

How frequently does everyone clean their Hobbed-bolt to avoid clogging while using PLA?
Is it possible to have an extruder design that auto-cleans the Hobbed-bolt?

Regarding your question of "don't you need a slot", in my extruder head, I have a wheel that presses the filament against the drive axle. While the drive axle does not have a "slot" due to the way it is fabricated, the drive wheel has a groove (slot) about 1/2 the diameter of the filament deep around its circumference that serves the purpose of keeping the filament "centered". I made the wheel out of a flanged ball bearing with a press on brass "tire". I turned the tire on a lathe to press fit the bearing and turned the groove on the outside of the tire. Seems fine.

Given that it appears most people have good results using a tap to form the "teeth", I am sure it is a good way to go. Regarding the concern I expressed that a tap fabricated bolt might tend to twist the filament, it was only a conceptual concern, not something I experienced.

I think the analog to the tap fabricated bolt is a worm gear (filament) with its corresponding worm wheel (tap fabricated bolt). At many typical "pitches", worm gear sets are not back driveable i.e. if you try to spin the worm wheel, the worm gear doesn't spin (due to the gear ratio being very high and the friction). If the analogy holds, this (no spinning) is what is desired in the filament drive; i.e you don't want to "back drive" the filament i..e spin the filament...you want to push it out. So no twist...too high an effective "gear" ratio and too much friction. Anyway, fun to think about...practically, many different ways to go with tap fabricated bolts a proven design.

Thank you all.

I will head to the machine shop, so, if we have time to make something i will post pictures.

I have made a hobbed bolt using the tap method to "hob" the bolt, i have used a dremel and i have made one on a cnc mill with the 4th axis indexer. The tap method works but i wasnt satisfied with the depth of the teeth that it left. The dremel method works great and leaves nice deep teeth. Milling on a cnc such as described above is the best solution IMO, but not many people have access to this so if you do id say go for it. If you want to try the dremel method id suggest searching thingverse for the dremel jigs that can be printed out.

I'm interested in your method for the cnc 4th axis as I have a Taig w/ 4th axis.

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How frequently does everyone clean their Hobbed-bolt to avoid clogging while using PLA?
Is it possible to have an extruder design that auto-cleans the Hobbed-bolt?

In normal use I don't clean my hobbed bolts. They produce a small amount of dust, that's all. If I have a jam for some reason then it will strip the filament and fill the teeth. In which case I remove it and clean it with a wire brush, but when everything is working properly the teeth stay clean. They may have some loose dust on them, but that is not the same as being clogged with torn off plastic.

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[www.hydraraptor.blogspot.com]

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nophead
In normal use I don't clean my hobbed bolts. They produce a small amount of dust, that's all. If I have a jam for some reason then it will strip the filament and fill the teeth. In which case I remove it and clean it with a wire brush, but when everything is working properly the teeth stay clean. They may have some loose dust on them, but that is not the same as being clogged with torn off plastic.

ditto.

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Bob Morrison
Wörth am Rhein, Germany
"Luke, use the source!"
BLOG - PHOTOS - Thingiverse

After I printed myself a spool holder, I do not have to clean the hobbled bolt anymore.
Cleaning of the hobbled bolt seems necessary when I frequently feed filament sections into the extruder.