So aluminium and steel work seems the norm for really major sized CNC machines.
Next is wood for the hobbyist machines of a smaller nature...what about Perspex/Plexiglass for the Gantry and y axis? is this material taboo for the smaller desktop build?
I'm talking 20mm to 25mm for the gantry and Y axis components.
its been used, there are some examples on small mills. But perspex has a very low modulus of elasticity so is approx 1/3 - 1/2 the rigidity of wood and 1/15 - 1/25 that of aluminium so has to be that much thicker and it tends to crack
Last edited by irving2008; 16-07-2010 at 10:38 PM.
Yes it does have a tendancy to crack...but 24mm?,you'd have to give it a hefty wack with hammer to crack it.
I have some here,10mmI think? it bends but it won't crack don't know what grade type it is?
These guys used 1/2" acrylic thats a real nono,looks fantastic though.
Last edited by GeorgeD; 17-07-2010 at 06:10 AM.
perspex crack at stress points... it is striong in sheet form as you say, but drill a hole in it and you get cracks radiating from the hole, just as you do with any material, but perspex, as I understand it, cracks easily once its solidity is compromised
Don't forget that although different materials have different physical strengths and as such they also need to be used in way that best utilizes them, just swapping perspex for Ali in an design that was intended for Ali is likely to fail.
Metal has to be bolted (large parts to allow this) as welding can distort the shape but the advantage of acrylic is that it can be glued, (therefore design a monocoque structure) If you can machine small rebates for thin plates to sit in then and really triangulate the beam I would have thought that help reduce the problem with localized stress points that Irving pointed out. Hope this makes sense, I'm not very good at explaining things .
there is also the option of creating a composite beam. acrylic structure with thin sheet metal like a flitch beam or just adding steel rods to the tension side like a concrete lintel.
I need to brush up on plastics, not really used them, is there a difference between Acrylic and perspex?
Of course This is in the context of a small desktop machine
Ok,I've got 3 sheets of 500x500x24mm how about if I was to weld it together and get some 1 or 2mm plate aluminium off ebay(quite cheap and I don't need a lot) and use 1/2" M6 seltappers to screw the plate to the sides of the gantry,that should strengthen it a bit.
Will use either Superglue orAraldite or even Dichloromethane ifI can get my hands on it?
Everybody is trying to tell you "don't do it", but it seems that you need to try it yourself in order to believe it :). From my experience, drilled, cut and/or glued perspex is cracking like hell when mechanically stressed or vibrated (wich is exactly the case here). If you would use polycarbonate, there would be a different story, but still not matching the aluminium or even wood build. Now, if your machine will be very lightly used, it might work.
However, we all can learn, that's why we are on the forum, so if you do it, we might learn something...
All the best,
Well its an interesting idea George... though you might want to look at the weight of that by the time you're finished compared with other materials. Taking aluminium as 1, mdf is 2 and Perspex 2.5 in rigidity ratio for a square section beam, the weights are 1, 1.5, 2.8 in ratio, i.e. to replace a 10mm sq ali bar in perspex you'd need one 25mm sq and it would weigh nearly 3 times more...
How will it work in practice? No idea, never used plastics that way.
If the machine is vibrating,Florin then there's something amiss. :naughty:
But that's my opinion and I can be wrong. Not trying to discourage you, each of us needs to make his own mistakes, that's the way to learn and progress. Myself I am coming on the forum to learn a bit easier, and I am lucky to find here ways to not repeat other's mistakes. I will be among the first to congratulate you if I am wrong, but meanwhile, if you ask for advice, I am posting my opinions.
All the best,
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