coach bolt hack

Design

The M8 bolt in my BfB extruder hasn't been gripping the filament well. I've tried the standard tricks, sharpening the thread, increasing the force (until I cracked the acrylic), but I'm still able to pull the filament out by hand against the thread. While shopping around, I came across some coach bolts with much deeper, sharper threads. Simple replacement of the M8 bolt was difficult - there aren't nuts that fit and it was impossible to attach the existing gear or provide vertical support. I needed a bolt with m8 at one end for the gear and bearings, and coach bolt screw at the other for the filament bite. So I made this:
From Coach Bolt drive

Construction

I started by cutting off a coach bolt to 55mm and finding a 60mm M8 normal bolt.
From Coach Bolt drive
I cleaned up the cut end of the coach bolt, intending to make a small bearing for the bottom. Seems to work OK without. A small hacksaw was used to cut a slot in each bolt head. A small chunk of old hacksaw blade can be used to help transfer the load from one bolt to the other.
From Coach Bolt drive
I added a nut to help hold things in place, used a couple of spring-tweezery things to hold the bolts together.I then chucked it on my cooker ring and warmed it up. When it was hot, I fed some standard electronics solder into the joint. (Note: if you've got some solder flux or a flux pen, paint it on the areas you want to solder first!)

Then let it cool. I moved it while still hot, so mine isn't quite straight. Doh! (n.b. get it correct BEFORE heating it up)
From Coach Bolt drive
Since the joint is in shear and  compression, a reprapped coupler/or polymorph/or superglue might do as well.( note superglue is not good with shear forces, you would probably still need the slots).
From Coach Bolt drive
It seems to fit OK into the BfB assembly, with a little modification of the bearings. I put one M8 washer over the coach bolt. The M8 bolt now has a nut, a bearing, two M8 washers, a nut, the BfB gear (hex indent downwards) and another bolt.

Rather than a couple of washers in the back of the channel, I trapped a plate from my sacrificed handy hands.  This doesn't move and provides a sturdy plate for the pressure bearing to push the filament against.

It grips the filament lovely now, and hand-winding the gear drives the filament well. I then attached my rebuilt extruder nozzle and the original BfB gearmotor.
From Coach Bolt drive

Testing - ABS - 220C at power 110/255. Score 8/10

I ran up the BfB extruder with the coach bolt drive. Drive is a little inconsistent due to the slightly bent connection, but works (12mm/sec or more). It should be enough to let me build another extruder - i.e. RepStrap itself, with a traditional V1 or V2 extruder.
From Coach Bolt drive
I might try and build a coupler out of polymorph. My solder join is only straightish (a couple of degrees off) because it wasn't held very well while setting. I could also try supergluing a similar reinforcement with the slots - I don't think it will be strong enough on it's own. Mounted it on the RepRap
From Coach Bolt drive
Comments