Joining pentagons

[JasonBModerator @jasonb]

Yes you are right Duncan, I had one of the angles wrong two straight cuts each end will work or the triangle to give a more rounded external corner

 

I was right with the 36deg mitre angle but the rotation should have been the complamentry angle of 63.4 so the tube needs rotating 116.565 degrees for the second 36deg mitre cut.

Looks like it goes together though I was having a job totally constraining the positions, one tube to go

Edited By JasonB on 28/03/2023 19:22:15

Edited By JasonB on 28/03/2023 19:35:46

[Andy StopfordParticipant @andystopford50521]

Once you've worked out how to do it, take care welding the tubes – they're usually galvanised and will give off zinc fumes if welded.

[Martin ConnellyParticipant @martinconnelly55370]

If you put the tube at an angle of 20.91° to the horizontal then the two angles you need to cut are 60° from the vertical plane that goes through the tube centreline. It should be possible to make a simple wooden jig to allow them to be marked out like this.

A chop saw with a tilting head set to 30° off vertical and horizontally rotated the 20.91° will also make this cut but cutting steel may be an issue however cutting a wooden saw guide/marking out jig should be easy.

Martin C

[DC31kParticipant @dc31k]

Posted by JasonB on 28/03/2023 19:09:24:

Looks like it goes together though I was having a job totally constraining the positions, one tube to go

Are you able to cut the blue tube somewhere square to its axis and then unwrap (develop) it?

Then someone could take the jpg of the development and enlarge it until the length of the square end matched the circumference of his actual tube and draw on the cut lines.

[JasonBModerator @jasonb]

No I don't have the sheet metal option in Alibre which will do that.

I suppose the OP could wrap some paper around the first tube and cat that accurately then the paper could be wrapped around others as a template to mark out with and then just cut with an angle grinder,

Or itI would not be too hard to develop the shape with traditional methods for a 36 deg cut and then overlay another at the 116deg to make a template

Edited By JasonB on 29/03/2023 07:45:52

[JasonBModerator @jasonb]

Not quite got the unwrapping symmetrical about the ctr line as It's the first time I have used sheet metal with F360 but good enough to prove the principal, just make it double ended, wrap around the tube mark cuts and out with the grinder.

[JasonBModerator @jasonb]

I gave it a go

First marked out the 116.5deg with a speed square, stood the end of the pipe over the guide circle and marked the two angles. Not the most accurate method but good enough to get an idea if it's right.

Over to the SCMS and set mitre angle to 36deg, make a cut, rotate to second mark and cut again

Stick them together, the 108deg external angle is good

Slight inaccuracies in the 116.5 deg marking out and indexing leave a small gap but spend a bit more time getting that right and a tighter joint will be possible. Certainly good enough for welding anyway.

[Andrew Smith 16Participant @andrewsmith16]

Well that has left me dumbstruck Jason.

Thank you – I am not sure what else to say. You've certainly gone above and beyond. As usual.

Can I just send you the tubing now and let you crack on with the real thing?

Truly grateful

Andrew

[Andrew Smith 16Participant @andrewsmith16]

Can I just say "thank you Jason" again.

[Michael GilliganParticipant @michaelgilligan61133]

Nicely demonstrated, Jason

MichaelG.

[SillyOldDufferModerator @sillyoldduffer]

Just in case Andrew wants an extra-difficult, time-consuming and expensive alternative to Jason's excellent solution, it can also be done by making pentagonal dodecahedron end fixings. These are prettier than balls, and complete the motif!

The method is easy, and the dodecahedron fixings could be threaded rather than welded.

All that's needed is 20 steel dodecahedrons big enough to join 2" diameter steel pipe, bored on 3 faces. A mere detail I'm sure! So the next challenge is "How can I make 20 fist-sized steel dodecahedrons in a home workshop?" (With mill, lathe and the usual hand tools.)

Dave

PS.  On a CAD note, FreeCAD did better than Solid Edge on this particular problem.  As far a I can see SE doesn't have a tool for creating dodecahedrons, so they have to be developed from first principles – hard work.   FreeCAD has an add-on workbench for making pyramids and polyhedrons, so they're just a button press.   Trying new things caused the usual CAD grief, which is the software not working quite as expected.   It's easy when you know how!  

Edited By SillyOldDuffer on 29/03/2023 11:28:55

[JasonBModerator @jasonb]

Dave, does whatever CAD you are using do sheet metal? I can't get the free F360 version to output a pdf or DXF that Andrew could use as a template. Might even unfold those 20 dodecahedrons to something like this

[SillyOldDufferModerator @sillyoldduffer]

Posted by JasonB on 29/03/2023 11:15:41:

Dave, does whatever CAD you are using do sheet metal?

I have FreeCAD and Solid Edge CE. (And F360 in the background). They both have sheet-metal workbenches, but I'm not fluent with either. FreeCAD's sheet-metal workbench is markedly less capable than SE.

If you put the F360 file on Dropbox, I can see if SE or FreeCAD can read it and produce a template for Andrew.

I guess Fusion360 not supporting the export is a restriction on the hobby version designed to nudge folk into paying for a full licence.

Dave

[JasonBModerator @jasonb]

I just can't export the flat sheet, comes up with an error message when I take the only export DXF option rather than the usual message we want your money.

I can do a STP of the tube if that helps, you may need to "cut" a slot down it's length so it can be rolled out flat

Edited By JasonB on 29/03/2023 11:47:59

[Michael GilliganParticipant @michaelgilligan61133]

Posted by JasonB on 29/03/2023 11:15:41:

Dave, does whatever CAD you are using do sheet metal? I can't get the free F360 version to output a pdf or DXF that Andrew could use as a template. Might even unfold those 20 dodecahedrons to something like this

[…]

.

Or just print from the link in my first post here

MichaelG.

MichaelG.

[JasonBModerator @jasonb]

Hopefully Dave won't see this and will keep beavering away

In the mean time this is a PDF that should be 1:1 for 50mm tube. Suggest you try it out on 3 short lengths before cutting up all the tubes.

Seems I was trying the direct export from sheet metal which is in DXF format. but I tried F360 drawing and it gives me the options to do a drawing of the flat sheet or rolled/folded so printed that to a pdf printer.

[JasonBModerator @jasonb]

Revised template, the previous was for 1mm wall tube which was a bit short to wrap around the outside, this one should suit paper or thin card

 

Edited By JasonB on 29/03/2023 12:20:45

[Andrew Smith 16Participant @andrewsmith16]

Thanks again for that template Jason – I appreciate you couldn't have done more to spoon feed me a solution.

I'll cut metal over Easter and report back.

Got to love this forum,

[SillyOldDufferModerator @sillyoldduffer]

As cruel Jason hasn't told me to stop, I'm still plugging away at this! (Have to give him my lunch money too.)

The problem turns out to be a good way of extending my CAD skills. Thanks to Andrew I've found a simple way to get FreeCAD's Part and Part-Design Workbenches to cooperate, plus found useful features in the previously unexplored Arch Workbench.

Going to attack it with Solid Edge next, which I expect will fill more knowledge gaps. And I've realised I need to grip CAD sheet-metal tools. All useful progress, even though Jason and Andrew have long since answered the original question.

Dave

[Martin KyteParticipant @martinkyte99762]

Stop prevaricating and get on with the clock.

🙄

regards Martin

[JasonBModerator @jasonb]

It will swing better with a dodecahedron shaped pendulum anyway

[Andrew Smith 16Participant @andrewsmith16]

Now Dave is just making me feel guilty. I can use Freecad to create small scale railway bits for the 3d printer. Now it looks as though I might have been able to find the answer in there. That outcome seems grossly unlikely . . .

I am still struggling to come to terms with the fact that each part is symmetrical. I'd convinced myself one end had to be rotated relative to the other. The answer is obvious now but should have been all along. Surely I haven't been this stupid all my life. Sigh.

[Andrew Smith 16Participant @andrewsmith16]

I am almost embarrassed to post these pics but given the help from members of the forum in sorting out what should have been a blindingly simple problem I owe a debt of honour.

The welding is truly shocking but luckily flap wheels are pretty cheap. That task will fill a few hours.

Rushed the marking out resulting in some poor relationships on the end of some parts that needed a bit of filling. First attempt with gasless mig and I struggled to get the settings sorted for what turned out to be 1mm wall thickness.

Wife hates it and wouldn't have it near the garden. So a nice idea poorly executed and not liked by the intended market. At least I am consistent.

[Ady1Participant @ady1]

Nice, but cleaning up the welds is going to be a pain. That's what apprentices are for!

35A and 17volts using tackwelds would do it, in-out in-out

[JasonBModerator @jasonb]

Thanks for posting the progress, I'm sure you could sell it on e-bay as modern art.

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