Benonymous
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| posted on 21/1/08 at 02:50 AM |
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So you want to build a plug....
Hello all.
I was re-reading Dennises site detailing the building of the DP-1 ( http://www.dpcars.net/ ) and I was suitably horrified at the cost quoted to him
for machining the plug for his bodywork. I also was intrigued with Fred WB and his Lola project but the way he built his plug seemed very involved
to me. Anyway I started noodling ideas and came up with something that other builders may find useful.
My idea is to first build a buck that is a series of simple planes. All of the planes come to within three inches of the finished exterior.
Then a series of foam panels can be shaped on a simple 3 axis CNC router, as opposed to a multimillion dollar 5 axis robot. The parts would need to
be sectioned up so that each one had a flat base. This makes for less setups by the operator (cheaper)
All the parts would be made from high density foam, which is easy on the machine and tools (cheaper).
After the parts are machined, they are simply glued to the planar buck. Fairing and surface prep would be the same as any other HDF plug and it would
probably destroy the foam once the mould was pulled.
The images are a very quick lashup on SolidWorks. The sides could be sectioned and have foam parts put on as well to make the profiles required.
Any feedback would be appreciated, tell me how it wouldn't work....
[Edited on 21/1/08 by Benonymous]
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RazMan
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| posted on 21/1/08 at 05:43 AM |
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The main problem with a 2D base is that the profile of the bodywork will not have any curves going 'east / west' and you will therefore
start off with a very slabby base and any side profiling will go straight down through the foam and into the base.
I think Fred's method is by far the best method to achieve detail.
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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Fred W B
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| posted on 21/1/08 at 08:14 AM |
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I think Ben is saying that the add on bits will be deep enough to get all the 3d details on. Depends on how much tumblehome the sides have - always
wanted to use that word
I like it a lot - see no reason it won't work, you'll just have to make sure that the base blocks are exactly correct to size
otherwise the "skin" bits won't match up.
Would be ideal for a more 70's slab sided style like what I want I to do next
cheers
Fred W B
[Edited on 21/1/08 by Fred W B]
You can do it quickly. You can do it cheap. You can do it right. – Pick any two.
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mcerd1
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| posted on 21/1/08 at 10:06 AM |
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It reminds me of a rapid prototyping technique that some people I was at uni with were working on (although they were making small things from steel
using robots to assemble it / weld it )
I see no reason why you can't add as much detail as you like - but obviously the more complex the shape the more bits you'll have to get
made - while at the same time maintaining the accuracy
If it was me doing it I'd start with the final shape you want - fill it with cuboid shapes as best you can - then work on the triangular and
curved bits
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02GF74
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| posted on 21/1/08 at 10:17 AM |
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Lego?
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Benonymous
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| posted on 21/1/08 at 10:22 AM |
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quick lash-up
I feared that my quick lash-up in SW would give the impression that it would only be a useful method for simple shapes. If I spent a bit of time on
this, I'd give something like Freds Lola a go 
The important thing here is that the foam parts would be CNC machined and thus precise, plus, done on a much cheaper machine. A decent DIY 3 axis
would easily do the job. I would make the planar buck on the CNC too. Very precise and Locost.
Additionally, It wouldn't matter what the shape was, you would include parting lines and so forth just like you would with any other mould
making process for GRP.
I wont be making full bodywork for my car, just a bonnet and some passenger/firewall mouldings. But if I was going to have a go at a full-cover body,
this is how I'd make a plug.
And it wouldn't cost me "a new Porsche 911" to quote Dennis.
[Edited on 21/1/08 by Benonymous]
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Rogue Se7ens
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| posted on 21/1/08 at 04:59 PM |
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Benonymous, that is a good way to make the plug, I've used a similar method many times. Most of the parts have been smaller (bonnet...) but am
currently working on a new design (see avatar) and will be making a plug for it. Just a couple pointers: Being that the design will be modeled (I
too use Solidworks), be careful that your structure is built to the proper dimensions and square and plumb, as when the time comes to adhere the foam
to it, minor errors will accumulate. I typically use 1/2 inch mdf for the structure. Urethane foam has many advantages as it can be sanded to shape
and styrene wont attach it. Also watch that the glue doesn't come up between the blocks of foam since the glue doesn't sand well and it
can be difficult to get the joints smooth. There are many more things you will learn as you go, but it looks like you are on the right track.
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Puk
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| posted on 23/1/08 at 08:34 AM |
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Benonymous that seems like a sensible approach - if it could be made to work consistently it would open up opportunities for making a far bigger
variety of cars (boats, planes...)
So a cheap machine makes 90% by volume of the male back and an expensive one is used to make the surface contours. Could a router create a surface
that is smooth enough to take a mould from?
Have you any idea what it would take to create a set of body work for a 7 - something like Rogue Se7en's avatar?
Would there be any advantage in using a home built router
(check
out this thread) or are the cheap DIY routers capable of doing the job?
(I know nothing about routing, is it showing?)
Puk
[Edited on 23/1/08 by Puk]
[Edited on 23/1/08 by Puk]
Before you judge a guy, walk a mile in his shoes. Then when you judge him, you're a mile a way and you've nicked his shoes.
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Rogue Se7ens
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| posted on 23/1/08 at 11:17 PM |
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I've downloaded the plans for the router and am planning to build one when I get the time and money, you need the controllers and CAM also.
http://roguese7ens.com/rogueroadster/
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Benonymous
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| posted on 29/1/08 at 12:58 AM |
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Plug
Hi Puk. I have no idea as to the cost of doing it this way I just know it'd be cheaper than using a 5 axis machine. A 3 axis machine would be
perfectly capable of cutting complex, cmpound curves and shapes, if it was fitted with a ball-end milling bit. The only drawback I have noticed in my
plan so far is that some of the foam pieces on my lash up model have undercuts on them which would either require two set-ups (more cost) or they
would be excluded from the model and added by hand later. I know that this would increase the possibility of errors but hey, we're going for
locost here. Locost will almost always involve more hours/skill.
As to the finishing, as Fred WB is doing on his plug and Dennis did on the DP-1, the foam is sealed, sanded waxed and polished to get the surface
suitable to pull a mould off. The mould will always require further filling and finishing.
Making anything from GRP is a pain in the arse, messy and time consuming. Don't launch into it if you need things to happen quickly and cleanly
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Puk
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| posted on 29/1/08 at 08:16 AM |
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Its the sanding and polishing part which puts me off! In fact that was one of the prime drivers for my opting to build a aluminium honey comb chassis
in 7 dimensions, so that I could prove the method of chassis fabrication without spending months covered white powder!
I was hoping that OZRodder (who contributes to the
thread
in my previous mail) had come up with a way for minimizing the manual input:
quote:
Once the section is machines I spray it with a hibuild undercoat (spray on body filler) and machine it a second time. This leaves very little work to
preparts the plug section for creating a mold.
Go on - burst my bubble
Before you judge a guy, walk a mile in his shoes. Then when you judge him, you're a mile a way and you've nicked his shoes.
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RazMan
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| posted on 29/1/08 at 08:21 AM |
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I have to agree that the 3 axis machine should be capable of doing all but the most complex shapes. If undercuts are required then this will entail
more complex moulds too, adding further costs. 5 axis is a bit of a luxury really and a bit of hand finishing at the moulding stage shouldn't be
too demanding as you can always put 'finishing lines' into the buck for reference points to make things easier while still maintaining
accuracy.
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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