Another possibility (theory). Drill a 1/4" hole in the end of your plastic to depth. Make a chuckable metal mandrel with a 1/4" AF hex plug at one end, heat it up and push into the hole in the plastic, allow to cool. Machine the plastic back to size. Removing the plastic from the mandrel might be a bit tricky so maybe apply some talcum powder or some kind of releasing agent before pushing it into the plastic. It's just an idea but I'm thinking of doing something similar to repair a water heater temperature control.

Ed.

Ok thanks JD, sorry for any confusion, always something to learn For my little job I'll be using Acetal which is ok with heat forming.

Be careful with acetal Ed, when you heat the stuff it tends to give off formaldehyde so you need plenty of ventilation. If you get it to about 240C it can change all at once! Copolymers are more stable than homopolymers but they are all dodgy in this respect. It can be a good idea to keep a bucket of water handy to drop it in if it gets a bit gassy. I have seen moulding shops evacuated because of this material.

Bert

Make a hex broach from a new allen key (not an old one that has the end burred over).

Prepare your knob with a 1/4 drill, followed by a slot drill (or D-bit) to flatten the bottom and finally make a small eg 1mm recess at the botom with a boring tool for the swarf to gather. Owing to the depth you could make several such recesses along the hole as nobody will notice if it isn't continuous hex.

If the tool won't shave off nicely use a Dremmel grinding stone to recess the end of the tool to make a cutting edge. Also you might need to grind off alternate points of the hex so it only cuts 3 at a time then rotate 1/6.

To make a suitable boring tool chuck a piece of 1/4 silver steel in the 4 jaw off axis by 1.5mm and put a little offset centre in the end to support it. Leave a bit 1mm at the end full diameter but then turn the next 13/16 down until it is just cutting all round. You can then file the end bit to form a cutting edge on the bit that is sort of left sticking out sideways.

Hi Nishka,

If casting from resin, ideally yes you would need draft on the hex core, which could be stoned or sanded on. One half to one degree per side of draft would be fine. In practice though you may be able to get away with no real draft, just putting a high polish in the direction of draw on the core and using some mould release.

If no draft is permissible you could make a three piece core out of three pieces of hex with angled cuts. The centre piece would be cut wedge shaped and the other pieces clamped to it somehow to align the hex faces. The clamped assy could be moulded into the resin, the centre wedge pulled out, allowing a little clearance to pull out the outer pieces.

Lots of ways to skin the cat, my notes are intended as food for thought.

The broaching ideas may be less work than the moulding idea. Regarding broaching, if several hex broaches were made, each slightly larger than the other by .005" to .010" the broaching force would be reduced and the chips smaller, as well the finished hex hole would be a finer finish.

Good luck whatever method you choose. JD

A thought…
Is this glass reinforced polyester. ?

If so then budget for many cutters unless tolerance on hole is reasonably loose.. ( rotary broach tapers away from cutting face so changes size as sharpened)…
No experience in pure polyester but glass reinforced is hard on tools..

Are you sure you mean polyester and not polyurethane although the same technique will work. Polyester resin is the type used to wet out fibregass matting and can be brittle. Polyurethane can be anything from white to a caramel colour.

Polyester should be fine, its the typ eof resin that is often used for fake ivory, horn, tortoise shell, etc so a bakerlite look would be possible with te right pigments and fillers

Myself, I would take a suitable piece of silver steel, drill it 7/32"down it and then heavily countersink the end. transfer to the milling machine and form a 1/4" hex on the end then slightly smaller hex after the first 5mm or so. Harden and temper then just lightly stone the 1/4" hex

Drill a 1/4" hole in your resin blank and the drive the hollow tool into the hole using a bench drill as a press.

This is much the same as a hollow chisel morticer works except you are doing it in two stages rather thqan one, the waste will find its way into the hollow shaft and can be poked out after each cut or you could try a side hole.

I work in the film business. If I went Jason’s route (which I’m not saying isn’t sound) and invested all that time and material into one 1/4″ hex head with all the potential for foul up, I’d be shot. Drill, pour, insert, leave an hour, unplug, tidy up, job done.

Chris there are 3-500 of these to be made, will you wait an hour for each?