Hex Blind Hole in Polyester Resin

I need to produce a 1/4" AF Hex blind hole in the centre of a 7/16" Dia Polyester Resin rod to a depth of 13/16". Does anyone have any suggestions as to how to achieve this?

I appreciate that there are many ways to skin a cat and that some ot the methods available may be more suitable than others. I assume that rotary broaching is out due to the dia/depth ratio.

Is making a broach from a hex/allen key suitable for PR? I need to make a lot of these (300-500 pcs).

Thanks in advance

Is it necessary for the hex to be full depth or could the lower half or so be a hole of a diameter that is large enough to clear the points of the hex. I have cut 1/4 hex in aluminium to a depth 5/8 inch, with a home made rotary broaching chuck. I can't imagine that you material is particularly hard so you could concave the front of the broach to give a more aggressive cut. Best of luck

Thanks Bernard.

The Hex would need to be at least 75% of the depth so 5/8" may well (just) be enough. I am however, struggling to understand how I could get a larger dia hole beyond the 5/8" hex in the bottom of a blind hole, but then I'm new to all this and finding most of it a struggle!

Do you have to machine existing stock? Another possibility would be moulding them fromliquid resin. You could make a simple multi cavity mould with through-drilled holes 7/16" dia and with a 1/4" hex shaped core down from the top into the cavities. Polyester resin could be filled from the bottom end or pushed in through a gate at the top.

Just a thought. JD

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.

Ed,

Your approach for making the hex with a heated tool will work for thermoplastics, but will not work for thermosets or catalytic cure resins like polyester. Polyester is chemically hardened/cured by adding a catalyst to a liquid resin. Once cured, thermosets and catalytic cure resins are not deformable by applying heat. Thermoplastics are deformable and in fact are re-meltable with added heat after initial cure or moulding.

Thermosets include Bakelite and Tufnol phenolic resins

Catalytic cure resins include polyester, epoxy, and RTV silicone

Thermoplastics include polystyrene, ABS, polyethylene, polypropylene,polyphenylene oxide family (example Noryl), polyphenylene sulphide family (example Ryton), nylon, polycarbonate family (examples Lexan, Makrolon), polyoxymethylene (acetal family, example Delrin) etc.

JD

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.

Thanks for the replies.

Not sure about casting a 'blank', wouldn't I need to add draft to the hex core in order to allow it to release?

If I could find a way to put a 1/4" hex into the end of a 'blank' then that could be turned down to the required end product. The blank would be at least 2" long and have a dia of about 3/4"-7/8".

Would drilling the blank with a 1/4" pilot hole (or maybe slightly larger - 6.5mm), allowa hex to be manually broached?

These parts are actually reproductions of 3/4" dia switch knobs that where originally made from Bakelite.

Edited By Nishka on 26/10/2014 18:38:38

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

Herbert,

I can vouch for that, twenty years ago I used to install three axis robots on injection moulding machines and I and the Fanuc Injection moulding engineer were commissioning three complete systems in a moulding room and we arrived into the department first thing as the operators were starting to heat the barrels, one of the machines had not had the purging material removed from the night before and as it heated up it gave off fumes, the roof was low, eight feet, and everybody suddenly developed sore throats and we were all sent outside and then a check with the company medic.

Martin P

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 you have a drill press and a rotary table or jig that can index 60 degrees. This should work?

Drill a bole slightly less than the hex across the flats measurement.

Make up a small chisel like end that is just as wide as the flats of the hex you need.it could be a bit of silver steel that will fit in the chuck on your drill press. You will have to clamp or wedge under the main pulley on your drill press spindle to stop it rotating.

Now you have a means of driving the chisel up and down. (Set the drill press stop at the maximum depth you wish to cut)

Set your work in the indexing fixture and position the chisel for the first cut and clamp down. after indexing one complete turn measure for size and move out until you get a good fit.

There will be bits of plastic at the bottom of the hole that may need to be cut away you may need to grind and bend up a small tool or an old screwdriver if you have one to cut the bits of plastic at the bottom of the hole, it will depend on the type of plastic, it may just scrape away.

if you have a lathe you can use the same process sideways using the table traverse.

Regards
john

Edited By John McNamara on 27/10/2014 05:43:43

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

Thank you for all the replies, as has been said they are all food for thought.

I think that given the envisaged number to be produced and the facilities I have in hand, that broaching on the lathe will be the way forward.

Thanks Bazyle, John and Jeff, between your answers I image I can get a working solution.

Casting with a draft is not really an option, as the final product has to be a 'snug' fit on a 1/4 hex shaft. Broaching six individual sides would be workable if it was only a small quantity.

It looks like I will have to look into making some 1/4" hex broaches, possibly in increasing size as suggested by Jeff. I also like the idea of the homemade boring tool from Bazyle. It may not be possible to place intermittent relief cuts as the side walls are quite thin, less than the 1.5mm suggested for the boring tool. However the piece tapers outwards towards the bottom of the hex hole so this is a distinct possibility.

Thanks again for the replies. Once I have made a few I will put up some pictures.

Nishka

Edited By Chris Trice on 27/10/2014 11:16:37

The product is primarily for show but must also function in its own right. The original was a cream coloured Bakelite product, so Polyester was chosen as a suitable alternative. Although I am open to suggestions for a more suitable material.

Edited By Nishka on 27/10/2014 12:03:26

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 like Chris' recasting idea. Anyone who has broken a fibreglass tent pole or fishing rod will know that any operation that tries to shear material lengthwise off a GRP rod will probably cause splitting as all the fibres are aligned along the length of the rod.

Neil