Hobbling a 6mm bar with a 3mm tap

[raynerd]

Guys, for my reprap I want/need to hob a 6mm bar with an m3 tap to create something like this:



In all honesty, the profile of the hob is relevantly unimportant, the idea is that it chews up the bolt enough that when a piece of 3mm PLA filament plastic is pushed against the hobbed section, it grips it and pulls it around with it. The M3 profile simply matches the 3mm filament size, really the chewed section is the important bit. I'm just generating teeth to dig into the plastic.

I've no doubt I could do this without posting but although I have the equipment, I don't always use it in the most useful way so thought I'd post up before setting out.

I could cut it like a pinion for a clock - most of you know my exploration in clock making. But this is tricky setup as I'd have to cut a cutter!
If I hobbed, I'm guessing I'd need to make a rolling fixture to allow the hobbed bolt to turn. Could I make use of my CNC rotary table on continuous turn at aprox 15rpm and offer it upto the tap mounted in the spindle and allow the rotary table to drag the tap around in the unpowered spindle?

Other than that, I guess it requires a supporting fixture.

Ideas welcome!

Chris

[chipenter]

Looking at the raidius of the teeth the are 3 to 4 times 3mm , that will be more ridgid I don't think you will be able to use a 3mm tap without breaking it .

[ChrisC]

Hi Chris,
I've built half a dozen extruders for repraps, and I always just use a medium knurl on the drive roller(bolt). I've never had any particular problems with slippage. Most extruder problems are related to the hot end design. Some of the very simple hot end designs need quite a lot of attention to keep them working properly. The first symptoms are that the force needed to push the filament through increases. A good hot end design seems to overcome a lot of problems.
Chris

[Noitoen]

Eyeballing that thread does not look like 3mm at all. It looks the same as the bolt's thread. You could take a block of metal, drill 2 holes at 90º with an intersection, Stick the bolt in one and the tap on the other. It might work

[picclock]

Just a thought 

As a possible alternative how about fixing a dremel to the cross slide. The indent for the filament could be machined then  use the dremel cutting wheel to cut the grooves as needed. 

Alternately, stick to your tap idea. Turn a 2.5mm dia hollow into the bolt shaft before you start so you only need to cut the thread part. Put the tap in the lathe chuck and the bolt in a bearing mounted vertically on the cross slide so that it will freely rotate. You can then introduce the bolt to the slowly turning tap and cut as needed. If you cannot support the tap or it keeps breaking it just use a long cap screw with slots cut into the threads with a  . These are quite hard and will easily work for a one off.   

Good Luck

picclock

[JohnHaine]

The way I did this was to put the bolt in the two ball bearings that it's going to run in, with a nut to locate it axially.  Then I mounted the bearings so they straddled the centre slot of my mill table (the slots are bevelled which means they acted as mini V blocks) and clamped them lightly with strap clamps.  I pulled the races apart against the bolt head and the nut so the bolt couldn't move axially though of course it could rotate in the bearings.  Mounted the (M5 tap for M8 bolt) tap in a chuck in the mill spindle, moved it gently against the appropriate point on the bolt, job done in 5 minutes.  If this description isn't clear I can try to recreate the setup and take some pics for you.

John