Thread: What's a good material for making round knobs?

One of my main interests is in pen making and as part of that I do a fair bit of resin casting using polyester resin.

I have turned quite a few knobs from cast polyester resin it turns very well and polishes up to a good gloss.

There are 2 main suppliers of polyester casting resin in Oz, Nuplex and Barnes. Both these companies also supply predispersed pigments to mix in with the clear polyester resin.

Here is a red knob for my QCTP turned from cast PR

Big Shed, the red polyester certainly does look the part, looks like I'll get a litre of resin from the local fibreglass shop and try it out. Red and black colours would be enough for me, copier toner is a mix of styrene and carbon black so it'll probably work for the black colour. For red though, I could check out some of the paint pigments or maybe the concrete colours: I see they have some bright reds and yellows that are mineral based, they might be a bit hard on the tool steel but should be stable and relatively cheap. I could split a PVC tube lengthwise and wax it to use as a mould for casting bar.

You need castng resin though, if the resin is for fibreglass lay up or repair it won't cast too well, it will develop excess heat because of the thick section, it is pre-promoted.

You could certainly try pigment in powder form, but make sure you disperse it in a small quantity of resin first with a pallet knife and make a thick smooth paste first. Or ask your local fibreglass shop if they have pre-dispersed pigment, then you can just mix it in to the resin, about 5% by weight should do it.

Bunnings sell Diggers Clear Casting resin and also MEKP catalyst, you will need between 1 and 2% by weight.

Yes, PVC pipe works quite well, no need to split it as the resin shrinks somewhat on curing and you can just knock it out of the tube, but keep the tubes short, preferably below 100-150mm in length.

Let it cure overnight and if still a bit tacky on the outside, give it about 10-20 mins @ 75-100C in a toaster oven. Don't use an oven normally used for food, you won't be popular

The red knob above was pre-drilled and tapped, then put on the metal bar and turned in-situ.

Hi Fred,
I would have thought that casting the knob material in place on the handle barstock and then machining it would have been a better way to do it. Have you tried doing it this way?

One I been wanting to try is casting some brass swaf, it is fairly small off some thin plate milling. I was thinking of putting some black pigment in to fill the spaces between the brass might set it of well. Have to be carefully of air entrapment, can always back fill with black resin at worst. May even clear dip it to finish it off.

Russell

Originally Posted by Ueee

Hi Fred,
I would have thought that casting the knob material in place on the handle barstock and then machining it would have been a better way to do it. Have you tried doing it this way?

No I haven't and I can see 2 problems. First is to position the handle barstock accurately, although that can be overcome. Secondly, and more importanty, as I explained the PR tends to shrink and would not be a very tight fit on the bar stock. If the bar stock was threaded, it would be a loose fit.

It is actually quite easy to cast PR blanks and keep them in stock and when needed it can be drilled and tapped to the required thread and turned.

Originally Posted by Big Shed

No I haven't and I can see 2 problems. First is to position the handle barstock accurately, although that can be overcome. Secondly, and more importanty, as I explained the PR tends to shrink and would not be a very tight fit on the bar stock. If the bar stock was threaded, it would be a loose fit.

It is actually quite easy to cast PR blanks and keep them in stock and when needed it can be drilled and tapped to the required thread and turned.

Hi Fred,
As easy as it would be to cast blanks, my memory of physics is that a hole in a solid object acts like it is not there when expansion and contraction is concerned. Ie if an item with a hole shrinks then the hole gets smaller and if the object expands the hole gets bigger. That is why you can shrink fit steel rings onto shafts.

If you're not fussy about colour, I've made knobs out of Delrin (acetyl). It turns and taps beautifully and takes a pretty good polish. I get offcuts of 50mm round from my local plastics place. Black and white are common, other colours less so.

Originally Posted by Big Shed

You need castng resin though, if the resin is for fibreglass lay up or repair it won't cast too well, it will develop excess heat because of the thick section, it is pre-promoted.

You could certainly try pigment in powder form, but make sure you disperse it in a small quantity of resin first with a pallet knife and make a thick smooth paste first. Or ask your local fibreglass shop if they have pre-dispersed pigment, then you can just mix it in to the resin, about 5% by weight should do it.

Bunnings sell Diggers Clear Casting resin and also MEKP catalyst, you will need between 1 and 2% by weight.

Yes, PVC pipe works quite well, no need to split it as the resin shrinks somewhat on curing and you can just knock it out of the tube, but keep the tubes short, preferably below 100-150mm in length.

Let it cure overnight and if still a bit tacky on the outside, give it about 10-20 mins @ 75-100C in a toaster oven. Don't use an oven normally used for food, you won't be popular

The red knob above was pre-drilled and tapped, then put on the metal bar and turned in-situ.

I've been caught out by overheating polyester resin before and get around it these days by using inert fillers such as silica dust, aluminium filings, bronze filings etc. to dilute the volume of the resin which seems to reduce the heating nicely.

I bought a kilo of pre promoted resin and a 1.75 Kg of black cement pigment which seems to be some form of iron oxide as it's magnetic. The resin and black filler was mixed together and left to sit for a while to allow the resin to soak into the pigment, after a few mixings over 15 minutes or so it seemed pretty smooth and consistent. I added the catalyst and gave it a good mix and stuck the mix in a plastic bag like cake icing and squirted it into some ping pong balls.....should be fun removing the hardened resin from the mould later on. There were bubbles but tapping the ball moulds on the table allowed the bubbles to come to the surface which was surprising.

Next experiments will be bronze dust filler and casting the balls around a polished, buffed and waxed bolt thread.


On a related note, I see blue synthetic aquamarine pigment at Bunnings for cement pigment about $76 for a kilo: I'm wondering how finely it's ground as it may be usable for spotting blue.

Edit: It's "Ultramarine Blue" according to this datasheet: www.cementaustralia.com.au/wps/wcm/connect/website/packaged-products/resources/55940b80461d875d94a695b796eb3285/PDS-OX-Rev-2-170212.pdf

Well the first batch of ball were under catalysed and are still soft, so this morning I weighed the resin, added the iron oxide powder and gave it a good long stir to try and get rid of lumps, probably sitting overnight would also be a good idea. This time I used the recommended amount of catalyst and stirred it in very thoroughly then stuck it in a ziplock bag and squirted it into some ping pong balls with holes made in the top with a soldering iron. The resin mix was about the consistency of honey so it was still liquid and the whole process was very very messy and gave me a lot more respect for the confectioners trade. I had disposable gloves on hand and did need them changed from time to time as things got all messy. I finally got six balls filled with the resin mix and tapped them on a hard surface to get all the bubbles to the surface, after a few hours they needed a top up of resin as the level had sunk a bit. The temp is about 25 degrees C and the resin took about four hours to go off fully and like Big Shed mentioned they warm up due to the type of resin: about 40 degrees C, if the balls were much bigger it would have been a lot hotter too.

The polyester resin did not stick to the ping pong balls at all which was a big relief and I just peeled them out with sidecutters to get a matte finish ball. They are hard, dense, bubble free and have all the properties of the knobs you see on machinery and are satisfactory in every way. I lost two balls while buffing as the buff mop flicked them down hard enough to split one in half (see picture). The second one suffered the same fate and is in the shed somewhere hiding, so for the third ball I drilled and tapped it, and used a bolt as a handle. It drills easily and taps tightly without lubrication, squeaking all the time to leave a shiny smooth thread, both drill and tap showed signs of wear from the abrasive oxide material. I gave the ball a light sand with 240 grit to remove a lot of the equatorial ring from the ping pong ball mould seam. It's a hard tough material that takes a fair bit of buffing, so I expect it to be wear resistant too.

This process is a complete success as far as I'm concerned, it is messy however. I'm going to make up a big batch next time, assembly line style to offset the mucking around and have a stock of these balls for various projects, first of which is replacing those crap knobs on the drill press with some real ones!. Thanks for all the advice and suggestions everyone!

Very nice work, Mark. But erm, you do know plastic knobs are readily available right?

Nice work Mark
I know what you mean about the mess, once you start it is just impossible to stay clean. I scored a big roll of 3' wide brown paper i use for things like this, just roll a bit out, work on it and then throw it away. Paddle pop sticks from craft stores are also really handy as are take away containers.
As Bryan said you can buy knobs but where is the fun in that?
I had heard somewhere that round knobs where bad for your hands in the long term and that knobs like Fred's red one where better. Not sure if its true though!

Originally Posted by Bryan

Very nice work, Mark. But erm, you do know plastic knobs are readily available right?

I was going to make the same comment but in reality Mark has forked out far less dough making his own than buying them. Small Parts and Bearings want about 8 bucks for a 44mm dia. black phenolic ball.

Knobsballthreadedphenolic - Small Mechanical Parts And Components

Mark showing us how easy it is to make your own would make the idea of designing and casting some custom handles an achievable thing.

BT

Thanks guys for the comments, stuff is a lot harder to buy here 1800Km's North of Brisbane: nobody keeps much stock of anything and they want roughly double the cost of a capital city due to lack of competition .

The photo's can't describe the weight and hardness of these mineral filled knobs which duplicate the feel of knobs on older machines. Judging by the black brown residue on the buff mop, early plastics makers must have used similar fillers, it feels like a ball of granite, you would not want one thrown at you. My total cost is going to be around $36 for about 30 odd knobs (2.75Kg of filler and resin + moulds). Bob is right: I have plans for a few different products made from this material, about the best way to describe it is the heft and feel of Corian (acrylic) not to mention being able to make items of any colour I choose.

Ueee: Before I read your post about the ergonomics, I'd just fitted three to the drill press in place of the originals which are identical to Big Shed's red knob on the tool post: they spin in the heel of your palm as you rotate the handle, the originals seemed to dig into your hand. I can see where smacking a round knob with the palm of your hand would be painful after a while though.