Cutting angles in HDPE

Hello!

I'm in the process of designing and building my first FW bot, but am unsure of how to put the chassis and armour together. This may seem a very fundamental (maybe stupid) question, but what is the best way of cutting the edges of HDPE (or any plastic) at any desired angle?

Basically just how to seamlessly join two parts together like a mitre joint expect with angles other than 45 degrees. I was thinking of using a tabletop circular saw I have but I believe I need a different blade to cut plastics, also I was considering a router or dremel with an angled bit on it. It's most likely going to be 10mm HDPE for the entire bot and I was planning on tapping the plastic so I can use regular bolts.

Thanks in advance, Dan

any wood cutting saw blade will go through plastics no bother. As to joining it, it is possible to weld HDPE together. I'll let Mario take this one over as he has done it on several of his machines.

adjust the angle on a jigsaw base with a screwdriver.
i used a plane but mine is wood!

Remarks on working with HDPE.
HDPE can't get rid of the heat of fast cutting or grinding tools, and will start to melt and spray hot HDPE droplets all over. A few drops of cutting fluid (even plain water works.) or a stream of air will help in this respect.


The Hannibalito's (all 4 of 'm, with the overhauls during the years) have HDPE armor. Bend , bolted and welded. Up to 30mm thick. Obsidian in its last incarnation was also fully covered in angled , welded and screwed HDPE, 15 to 30mm thick.

The tools we use.
Jigsaw.
Disk saw in 3 variants. The handheld, the cut-off and Leo's dad has a DeWalt overhead compound angle sawtable.
These saws use standard wood cutting blades.

Electric planer, also with wood-cutting blades.

In the lathe and mill I use hardmetal cutters for aluminium.

Leinster hot air gun, with adjustable airtemp and HDPE welding tip for 4mm wire. Here a short document from the net on this.

36 box bench with custom bending plates -radius 10mm nose.


I make a CAD (cardboard aided design) mock-up, and use this template to cut the HDPE plates to size.

Preliminary joining is done with long, thin woodscrews. After the prelim joining I start up welding all the connection lines that don't require any removing or moving in the future.
These welds get a crosswelded and screwed/bolted piece of HDPE, because the welds are tad more brittle.
I try in the newer machines to make these crosswelded plates or invisble (on the inside) or decorative.

Parts that need to be re- or movable get steel hinges or aluminium inserts, because HDPE won't support a thread.

So to be clear, does a specific blade need to be used on plastics or not? I'm sure any blade could cut it but its fine as long as the blade is covered in cutting fluid?

I hadn't thought of melting it together or anything, this is only my first bot so it doesn't have to be pristine xD Also will there be any problem also say of using a 5mm tap in the 10mm HDPE (structurally weak?)

Maybe a poorly drawn picture will clear things up a bit : )

http://i215.photobucket.com/albums/c...achi/robot.png

Red is where the bolts would be and blue is the angles (clearly ^_^)

I do have a jigsaw around but I haven't used it in so long I couldn't think if it could be angled or not!

Thanks, Dan

We use a table-saw (with a wood cutting blade) and it works a treat. No fluid needed or anything, though the spray can be quite... uncomfortable, so wear suitable clothing. It's a pretty simple one so we're a little limited angles wise but the cuts shown in your pic would be easy enough. If worst comes to worst, whip out the file and surform!

As Mario said, though, HDPE won't hold a thread. Under stress the threads will just strip. You can use nut inserts or a barrel nut method, which is what we're doing for our machine. Not sure how much 10mm will like 5mm bolts, I'll let someone who has built like that answer.

You could always use L brackets and screw sideways into the material?

If you look at fourth post on this page posted by Maddox:
viewtopic.php?f=111&t=4302&p=119096&hilit=20mm+rod +thread#p119096
He came up with a good solution to the thread in hdpe problem

Ah well I suppose I'll give my table-saw a go then Relentless!

I like the sounds of using a barrel nut or nut inserts. For the inserts I'd guess I'd need to drill a hole a bit smaller than the nut, then heat the plastic and hammer it in? (is a hair-dryer hot enough? xD)

It seems for barrel nuts the typical length is 14mm (will browse more later), so the plastic would have to be at least that thick I imagine. Probably start to have issues with weight if its all that thick.

I don't fancy using L brackets particularly much, I do still have some angled aluminium (2cm x 2cm x couple metres) which I could nut and bolt the plastic too but think it could be a bit much effort perhaps.

Max - That idea does look pretty good (same as the barrel nut idea?) but I'd still have to tap the aluminium myself would I not?

After a google search I've found out what a barrel nut is, so yes it is the same but with the advantage its custom made so it will be the right size for you.

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Originally Posted by Mokichi

Ah well I suppose I'll give my table-saw a go then Relentless!

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Why not? If the blade isn't to worn out, it will go trough like a good knife trough butter.

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I don't fancy using L brackets particularly much, I do still have some angled aluminium (2cm x 2cm x couple metres) which I could nut and bolt the plastic too but think it could be a bit much effort perhaps.

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It's the simplest trick in the book to do the job.

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Max - That idea does look pretty good (same as the barrel nut idea?) but I'd still have to tap the aluminium myself would I not?

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Yep, in the case of the piccy it was an M6. But if you are working in 10mm HDPE, I feel an M4 will be better. And I use my batterydrill for tapping, just putting the clutch in the right position to avoid breaking taps, using cutting fluid and making shure it's straight on the hole.

your bot looks exactly the same as mine :)

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Originally Posted by archie2000

your bot looks exactly the same as mine :)

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Haha, that's just an explanatory diagram but I imagine many bots look roughly like that xD

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Originally Posted by maddox10

Quote:

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Originally Posted by Mokichi

Ah well I suppose I'll give my table-saw a go then Relentless!

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Why not? If the blade isn't to worn out, it will go trough like a good knife trough butter.

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Only thing i would say about table saws and hdpe is that it has a tendancy to wander. It's either a two person job to keep it straight or i find it runs better with a couple of pieces of wood under it (just 10mm runners)

I use a jigsaw with a metal cutting blade and a router for all the groves and angles, works well

I use a circular saw with standard wood cutting disc

I clamp a straight edge (bit of 2 x 2 wood) on the HDPE and run the edge of the saw against that. Gets a nice straight cut
It does create alot of swarf (not sure thats the right term for HDPE but you get the drift) so watch for that.
I also have a chop saw for wood that comes in handy for short cuts
I find hole saws and HSS drill bits work fine as well

For the antweights the pieces are small enough to clear the cuts on a bench grinder (with a light pressure to avoid too much melting)

i thaught i had loads of tools at home