Machining Plastic

Can anyone please advise on machining recycled plastic? One of the clubs that I belong to, wants to buy recycled plastic for sleepers in a 5"G and 3 1/2"G track. Present method is to use wood with 3 slots cut in them for the steel rails which are rectangular bar on edge. The rails are welded to a steel sleeper at intervals for permancy. The wooden sleepers are more for appearance rather than securing the rails. A horizontal mill is used with a clamping jig for the sleepers.

I think that the plastic could begin to melt unless the cutter speed is kept low or a coolant liquid is used. Not keen to use water with the danger of severely rusting the mill.

Any thoughts/ suggestions please?

If it is recycled it ia almost certainly thermoplastic and may melt. Nothing wrong with using normal metal cutting coolant or minimum concentation soluble coolant. No more risk to mill than cutting metal. Fast feed rates help too.

If you have lot to do it might be worth making a jig with stops for a circular saw. A custom "table saw" with 3 ganged cutters on a spindle might be worth considering as you have a lot to do.

Robert G8RPI

Robert beat me to it - but my first thought was access to a chop saw and set the depth of cut and a series of stops would be a whole lot quicker..

pgk

For milling thermoplastics I use sharp cutters with high rake angles and modest speeds, around 100ft/min. But I use high chip loads, 10 to 20 thou per tooth is not unusual. For side and face cutters I might drop the chip load a bit due to low rake angles. I have never used coolant when machining plastics. Most plastics have poor thermal conductivity so coolant isn't go to do much at the cutting edge. Better to keep the speed down and chip load up.

Andrew

The club I used to belong to used wood circular saws with tc blades.

You should be able to clamp a bunch of them along the mill table and do them quite quickly, if you have power feed that will be nice then just 3 changes of position per batch. I'd go with a high helix cutter meant for aluminium though a sharp 3-flute HSS would be almost as good. Or find a member with a CNC, I've put mine to use doing batches of upto 30 mortice and tennon joints so a few housing joints will be easy.

If you do go for Roberts ganged table saw method which I was surprised at given his posts about electrical safety do check the saws built in electronic braking can handle the extra momentum and it should go without saying rig up some form of guard to go over the exposed crown of the blades. All this assuming the arbor is long enough for more than one blade. Sliding compound or RAS with trenching stop would be a safer saw method but a lot more moving about of the work to get the width of slot

Edited By JasonB on 15/12/2021 08:03:03

At our Scout Campsite we used a sliding chop saw with a blade with the least amount teeth we could find, slow feed and a little WD40 to cut 150mm x 50mm recycled plastic to make some benches. I think the large tooth gullet stopped the blade binding and getting too hot.

Hope this helps.

It might be possible to get the width of slot in one pass using a dado head.

Thank you all for your replies, much appreciated.

Think that I didn't make it clear that club uses horizontal mill with 3 ganged cutters of correct width to cut all 3 slots together to fit rails. Sleepers held one at a time in a clamp/ quick release jig. Slots cut to final depth (about 1/4" in one pass.

Will experiment with doing same with plastic as with wooden sleepers and if it doesn't work, we'll try coolant.

Thanks again everyone for your opinions.

DMB

If it needs to be said, the cutters need to be sharp.

A blunt cutter will tend to rub rather than cut so that the plastic melts with the hear, leaving lots of "rags" to be removed by hand afterwards.

Howard

I am not an expert, but the bit of plastic cutting I had to do I found that a sharp round HSS cutter with good relief all round did a good job on the outside of a round bar making it smaller diameter. I also find that the sharper the point(Not the cutter sharpness) the slower I must feed sometimes very slow to get a good finish.

I also found that if I sharpen a carbide parting blade sharp keeping its relief angles as is, it parts off the plastic well.

So seeing you want to cut grooves, I would look if the cutter will resemble a sharp parting blade angles and sharpness, and start from there with speed and feed and depth of cut.

Edited By Chris Mate on 16/12/2021 05:10:05

Edited By Chris Mate on 16/12/2021 05:10:47

My attempt at making little drawer knobs from black plastic rod worked well, I was amazed at the type of swarf it produced and it piled up like giant Brillo pads all over the place

After turning delrin:

Another reason not to use coolant; dry swarf is easy to vacuum up.

Andrew

I wonder what the effects of sunlight on the sleepers would be if the tracks are outdoors? Some plastic can get brittle with exposure.

By the time that the plastic sleepers are buggered by sunlight, my head will no longer ache, nor many of the other members!

Blimey, Andrew, is that the 'swing a cat' workshop?

Seriously, the "rags" of half melted plastic needn't really be removed, since there are just 3 slots about 1/4" wide and deep, so not much plastic to remove and even less to hang out of the ends of the slots. The sleepers in question are primarily cosmetic, since there are steel sleepers welded to the rails, safeguarding the gauge width. I don't think that there would be much rags to worry about and it's dark chocolate coloured plastic. Most people won't notice odd bits of plastic rags sticking out one end.

Many thanks to you all for your thoughts and opinions.

John