Bolt hobbing advice

I'm building my first printer (prusa i2) and just hobbed my first bolt, following the instructions on the Wade's Geared Extruder page. The resulting bolt looks like crap compared to commercial ones I've seen–very shallow teeth, and the teeth weren't totally lined up between different turns with the tap. However, when assembled in my Wade's accessible I'm unable to strip the filament by hand. Do you think it is "good enough," or should I get another bolt and try again?



For background I'll summarize my procedure:

1. Chuck the bolt in a hand drill and file a half-circular groove with a 2.8mm file. I tried shallower grooves but couldn't get the tap to turn smoothly. My taps all have wide flutes which make it difficult to make a smooth cut. Tap:

M3x.6tap.jpg

2. Clamp 2 608 bearings in a vice with the bolt. Chuck a M3x.6 tap in the hand drill. Other sources suggested M4x.7 or M5x.75, but I couldn't find rattail files with the proper diameter for the groove. Slowely turn the tap in the groove counter-clockwise, letting the bolt turn slowely towards the drill. I used a lot of pressure, but still had problems getting the cuts deep enough. I let the bolt spin around ~5 times over ~15 minutes (too slow?).

Most pictures I've seen have a fairly shallow groove, much less than the half-circle I had to use to get any hobbing at all. Do I need to look for a tap without flutes, so that it doesn't require filing a groove before tapping?

Doesn't look too bad - mine looks worse and has printed tens of kg without stripping. Provided it feeds the filament evenly it'll be fine.

Don't cut so deep with the round file, I've had luck around 1 mm deep. Then cut with the tap, with cutting oil. It is hard to get the cut started. I probably have the bolt turn about 20 times. When the cut is good you should be able to get the bolt to turn with the tap with less pressure than when cutting.

Hi there,
If you put some scrap wood between the vice jaws and the bearings that hold the bolt, you can push the point of the tap into the wood as you begin to cut into the bolt at first. This helps the tap stay in the one spot on the bolt and start its cut. Apply firm pressure on the tab, but not too much because with the point of the tab now constrained you could snap the tab. Once it begins to cut in, a little drop of light oil will help the cut also.

If your bolt in the picture grips the filament then use it to get started, but it could be a little shallower and with more defined teeth.
What size filament are you using?

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@james Thanks, that makes me feel better about pushing forward with my build.

@NumberSix Using a wood scrap sounds like great advice. I'll give it a try once I get another bolt to practice on. I'm using 3mm filament.

I almost feel like just a 3mm groove with some rough scratches would hold the filament tight enough, even without threads to dig in. That would be a whole lot easier for new reprappers to get started.