DIY MK7 Extruder gear

I’ve been making my own 3d printer and I’ve finally at the  the point where I can make  use the kickstarter. QU-BD  extruder.
http://www.kickstarter.com/projects/qu-bd/open-source-universal-3d-printer-extruder-dual-ext
The machining on these parts was superb but there where some deficiencies in the design, which are discussed here: http://www.fabric8r.com/forums/showthread.php?793-My-very-own-QU-BD-woes

So it sounds like the path I’m going to follow was cut by Mr. Bart Dring (http://www.buildlog.net/wiki/doku.php?id=ord_bot:qu-bd_extruder_improvements) with my little twist being that I’d  make my own extruder wheel,
I found some interesting information here  on making your own wheel:
http://forums.reprap.org/read.php?70,129894,138574

On cutting a worm wheel I found some stuff here: http://www.practicalmachinist.com/vb/general/hobbing-gear-tap-how-160177/ I’ve seen some discussion that the blank needs to be gashed first in an indexer. (I wanted to see if I can avoid that.)
I have a very nice spin fixture/indexer with a 5c collet.  Initially,  my thought was to place the blank in the indexer on the mathematically correct center distance in my mill to the I’m wonder what would happen if I feed the tap axial instead of radial into the piece.  I’m thinking the worst that can happen is that I snap the tap.   I don’t have a large assortment of metric taps but it seem I’m good on inch, so if I’m going to break  one or two or ???, I want it to be an inch tap.  My hope that since I’m the correct center distance the timing thing should just work itself out.  I wound up doing is just feeding in radially and it seemed to work just find.  This approached saved me the hassle of figuring out/setting center distance.  I not sure it would have worked anyway.

I wound up finding some pretty cool gear links, that might come in handy when I want to actually make a worm gear.  I don’t need them for this project but they where interesting links non-the-less.

Ok.. So I found thread pds here: http://www.osgtool.com/_branding/osgtool.com/files/catalogpages/USCTS08%20314.pdf
Also I found some info on worm gears formulas here… http://www.roymech.co.uk/Useful_Tables/Drive/Worm_Gears.html
Ahh… Detailed formulas here for Metric (http://www.sdp-si.com/D805/D805_PDFS/Technical/8050T072.pdf)

Originally I was just going to use a 6-32 tap as a hob. Unfortunately, I don’t own a R-8 collet and I was in no mood to make an adapter sleeve or get a R-8 collet just for this.
I wound up increasing the tap size till I was able to get it to fit in my R-8 collet.  So I wound up using a 10-32? tap.

The Geared wheel on the the QU-DB extruder is a brass spur gear with a circular slot cut into it.  I figure I maintained the OD of the Gear and the Min Diameter of the groove, I should be ok.. (If not.. I let the moths out of the wallet and get a MK-7 tooth gear.

Basically I have a huge supply of 3/4 steel that seems to machine wheel so I thought I’d try that.  I turned the OD down and cut a groove to the same diameter as the QU-DB gear.  I didn’t bother drilling it and I just wanted to see what would happen.  I was actually sort of pleased with the result although improvement is needed.   The thing that I need to do is to set a stop to keep from hobbing too deep.   A couple of tricks, I help the rotary rotate when the tap first started biting, that seemed to help.  I suspect giving it a good bite (not enough to snap the bit probably helps too.

Experiment number two.
Ok.. I’m feeling lucky.. So I’m going to tap using a 10-32 Thread.  I went wrong on my first attempt in that I didn’t add a stop to control my depth…  So basically I need the whole depth of a 10-32 thread.  Machinist handbook give me a internal thread depth of .019
I measured the Qu-bd gear OD ~.457″
The slot diameter ~.416
So if I touch the tap to the OD of the part.  I need a block to set my offset of
.((457-.416)/2)+.019=0.0395      So If I set with a .010″ Feeler gauge I can use a .050 gauge block to set the stop…

It seems that it worked ok..   If someone wants to see pictures… Let me know an I can add some…. But now I’m to other things..