No-hob extruder

by profezzorn, published

No-hob extruder by profezzorn Aug 13, 2012


It's an extruder that uses a dremel bit instead of a hobbed bolt. The sharp edges of the dremel bit makes this extruder design very strong.

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If you're using a sin() that takes it's input in degrees rather than radians, then you need to use sin(180/10) instead.

Thanks a lot.  I'm having trouble getting the math to work out to what you suggest above though, maybe I'm missing something?

For 200 steps, 32 micro:

steps * microsteps * 47/9 = 200 * 32 * 5.222 = 33422.08
(sin(pi/10) * 10 * 7/32 * 25.4) = 
0.0054830861 * 10 * .21875 * 25.4 = 0.3046539714

33422.08 / 0.3046539714 = 109705.0527403694

Quite a bit higher than 1939.  Thanks for your help, I've got mine printing but am having a lot of trouble getting the steps/mm dialed in.

Weird, now that I retry the calculation I get 1939 rather than 1946.

Either way, here is how I got that number:

47/9 = teeth on large gear / teeth on small gear
sin(pi/10)*10  = circumference of a decagon of size 1 (the bit has 10 teeth)
7/32 * 25.4 = the diameter of the bit

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Required hardware:
o 1 Dremel bit #196 (high speed cutter)
o 1 m3x25 screws
o 6 m3x10 screws (If you have a 6mm one, it will simplify things)
o ~6 m3 washers
o 4 m3 nuts
o 1 20mm 8mm threaded rod
o 1 608 bearing
o 2 r2zz bearings
o 4 m4 nuts
o 2 m4x50 screws
o 1 nema 17 stepper motor

Please note, the STLs are all configured for 1.75mm filament. If you want to use this for 3mm filament, please download the source and export your own STLs.

Print the four parts, main block, idler, large gear and small gear. I put the main block and idler in a combined STL, but I was using a printer with a 10x10cm print area, so it didn't fit. I included the idler and main block as separate STLs as well. After printing, make sure your holes are big enough, otherwise drill them. Some holes are covered by filament and must be drilled. Also snap off the hinge support.

While it's printing, you can work on flattening the end of the dremel shank. Don't bother trying to use a file, it won't make much of a dent on the hardened steel. Instead you'll need something to grind it with. Something with a moving sandpaper for instance. (See attached picture) Oh, and if your motor shaft is not flattened already, you'll need to do this for the motor shaft too. (See reprap wiki for details on this.)

Once the preparations are complete, you'll need to mount the small gear on the motor. If you're lucky, and have a long motor shaft, you can invert the gear before you print it and mount it with the set screw away from the motor body. For me, this did not work, and a standard m3 10mm screw is too long. (Hits the motor mount) However, a few minutes with a file will easy convert a 10mm screw into a 6mm screw. You can also file the head down a bit to make sure it doesn't hit anything. Now drop an m3 nut into the slot (requires some force) and then fasten the small gear to the motor shaft.

Drop an m4 nut into the slot in the base under the motor mount and tape it in place. You can do this later, but it's just easier to do it now. Put the motor in place with 3 x 10mm m3 screws and washers, but don't tighten yet.

The large gear has two nut traps, this is because we need a lot of force to hold the dremel shaft, and using a single set screw causes the gear to wobble. Push m3 nuts into the nut traps of the large gear and put set screws in.

Take the two r2zz bearings, push one in from each side of the main block. Put a m3 washer on the dremel shank and push it through until it starts to poke out the back. Put as many m3 washers as you need to align the gears up on the dremel shaft (2 for me). Then put the large gear in place and push the dremel bit in all the way. Now tighten the set screws of the large gear, make sure one of them hits the flat part of the dremel shaft.

Now adjust the motor and tighten. Make sure gears are working smoothly.

Now for the idler.. Put an 608 bearing on a 20mm piece of threaded rod (8mm) and snap it into place in the idler. This might take some force. Then hold an m3 nut in place inside the idler hinge while you insert an m3x25 screw from the outside. After it is screwed half-way into the nut, put the idler in place and screw the screw in all the way.

Drop m4 nuts in the nut traps on top of the block, put springs, rubber grommets or whatever you have handy on the m4x50 screws and screw them into the screws, don't bother tighten until you have some filament in there. Springs are nice, but rubberized washers works too, as you can see from my picture.

In your firmware, set the steps per mm to:
steps * microsteps * 47/9 / (sin(pi/10) * 10 * 7/32 * 25.4)
With 200 steps and 32 microsteps, this comes out to 1939.


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flimshaw on Sep 24, 2012 said:

Could you explain a bit how you calculated your steps per mm?  I'm having trouble getting the same answers you did with that equation, and my stepper has different values.  Thanks!

profezzorn on Oct 1, 2012 said:

Weird, now that I retry the calculation I get 1939 rather than 1946.

Either way, here is how I got that number:

47/9 = teeth on large gear / teeth on small gear
sin(pi/10)*10  = circumference of a decagon of size 1 (the bit has 10 teeth)
7/32 * 25.4 = the diameter of the bit

mkruse on Sep 18, 2012 said:

is the Qty (1) 602 bearing supposed to be 608? I bought several of the 602's by mail and they are way too small.

profezzorn on Oct 1, 2012 said:

 Oops, yes it's supposed to be a 608 bearing.
Sorry about that.

Anonymous on Sep 18, 2012 said:

did you intend to say the assembly requires a qty (1) of the 608 bearing? i bought several 602 bearings in the mail, and they were way too small! The 608 is 8X22X7 which looks like it is about the right size.

MangoT21 on Sep 4, 2012 said:

That's very slick.

scotty1024 on Aug 25, 2012 said:

The Dremel 115 High Speed Cutter is 5/16" and $5.95 on Amazon.

profezzorn on Aug 26, 2012 said:

The 115 bit should also work with some minor modifications to the scad file. It may produce a smoother feed since it has more teeth. I went with the 196, which produces a stronger feeder since because of the smaller diameter. I haven't noticed anything uneven about the feed and I'm very happy with the extruder so far.

nanonid on Aug 25, 2012 said:

dremel bit 196 is $6.52 with prime shipping from Amazon.

Still a hobbed bolt is only $8 from Printrbot store.

bstott on Aug 25, 2012 said:

dremel bits are throw away items. even dollar stores have sets for next to nothing... You can even choose your style..

This is a real DUH! Or is that DOEY?

GREAT! I spent too much time worrying over this item and here I've got twenty or more dremel bits around the house. This IS COOL!!! :-D

duckythescientist on Aug 16, 2012 said:

I love it! How can I get it to work with a bowden system?

jamesarm97 on Aug 19, 2012 said:

I would like to see this work with a bowden system as well (Rostock). I don't like the setup I have with the Airtripper. Just needs different mounting and an adapter to hold the nut / tube.

smartroad on Aug 15, 2012 said:

Which stepper are you using?

profezzorn on Aug 15, 2012 said:

It is a nema 17 stepper. The actual model is no longer in production though.

JuliaDee on Aug 15, 2012 said:

Excellent. This is very similar to the stock feed drive gear in the Up! printer (but the Up! has a fixed-position roller bearing instead of a pivoting spring-pressure thing). The nicks in the filament do not adversely affect extrusion.

takigama on Aug 13, 2012 said:

that is a brilliant idea... cheap bit and available practically everywhere.

takigama on Aug 13, 2012 said:

actually, one thought occurs, put the 608 bearing back in and add a "chuck" (with some a bit of a lip) to the top of the large gear so when you push the large gear thru the 608, the 608 compresses the chuck and the lip sits on the "fillament" side of the chuck

heyarn on Aug 13, 2012 said:

hahaha i love it

cerberus333 on Aug 13, 2012 said:

Very good idea!

Sanjay on Aug 13, 2012 said:

Wow, very interesting idea.

Can you post images of the filament after its been pulled through the extruder, to show the 'Bite marks'?

Awesome work!

profezzorn on Aug 14, 2012 said:

Getting a good picture of the bit marks is not easy, but here is *a* picture at least.

bitflusher on Aug 13, 2012 said:

Seriously interesting development. These bits are most likely more consistent in both quality and diameter. If I ever get annoyed with my hobbed bolt this will be considered, thanx!

jasonwelsh on Aug 13, 2012 said:

Would genius be to strong a word?

Running to lowes when it opens!!!!!!! need dremel bit!

bstott on Aug 25, 2012 said:

I think genius should be spelt Genius!! ;)