Ideal base plate thickness...?

[danielbriggs]

Basically, as per the title.
I'm after opinions on the thickness of a base plate for my case.
I'd initially thought 12mm (1/2" ish), but then I saw the cost!
I'm going to have the case with 2 'floors':
Bottom: 300x500mm
Top: 180x500mm
With cylindrical support posts on both sides scattered down the 500mm length, probably at 100mm intervals.
Optics + Laser heads downstairs, and power supplies t'upstairs.

It's going to cost around £8 per mm thickness for the overall material required.
So my choices are: 6mm, 8mm, 10mm or 12mm
What would you all suggest?

Obviously 12mm would be the ideal, but I suppose my question really is: could I get away with less at those dimensions, while maintaining a rigid construction?

Cheers,
Dan

[Jem]

Hi Dan

The only thing i'd throw in on this topic is to have the metal laser or water jet cut as it will be much more thermally stable than guillotining.

Personally i'd go for the 10mm or 12mm.

Cheers

Jem

[danielbriggs]

Cheers Jem
Just one question: Any chance you could explain a bit more about the thermally stable part?
I was planning to bandsaw it to close to the right dimensions, then file to shape, and finally mill the angle required on the side.

Ta,
Dan

[Jem]

Cheers Jem
Just one question: Any chance you could explain a bit more about the thermally stable part?
I was planning to bandsaw it to close to the right dimensions, then file to shape, and finally mill the angle required on the side.

Ta,
Dan

Well, speaking from experience... I had my baseplate chopped with a large guillotine at Metal Supermarket in Leeds, this seemed like a good idea at the time. However, when the plate warms up, due to the stress invoked into the metal with the guillotining it tends to warp ever so slightly. Under normal circumstances this wouldn't be a problem, but when you need precise alignment of three laser beams it becomes a problem.

If you perfectly align all your lasers when the plate is cold, when it warms up the alignment shifts slightly. Likewise if you align your lasers when the plate is warm they move slightly as the plate cools. This means that next time you switch on the alignment is off... Can you see a pattern emerging here?

If you carefully cut the metal with a method that doesn't put stress into the metal this doesn't happen and your lasers remain aligned whether the plate is warm or cold. This is one reason why proper optical breadboards are so expensive.

Since I made mine i've also found out that there are different types of aluminium and different methods of extrusion / rolling, all of which have an effect on thermal stability. Obviously the better metal is more expensive... How deep is your wallet?

I'm not quite sure how bandsawing will affect the metal.

Hope this helps.

Jem

[danielbriggs]

Yep, cheers Jem
I didn't twig with that guillotine thing before - makes sense though.
I'm guessing it's the work hardening process going on at the edges. But I wasn't aware this would have such a pronounced effect.
I'll mull over this a little more then over Easter before I jump out and buy £100 sheet of Al. I might have a word with my materials professor to see what he thinks would be the best way to machine a plate and to minimise thermal warping. I might be able to get access to the special cutting machines they use if I ask very nicely!

Again, thanks for the help Jem - much appreciated.

Dan

[Laserman532]

i've built monster projectors with never using more than 1/4" allum. I usually made it very rigid and stable by welding a frame of 1" square steel tube. If you are building a small projector say 1'x2' you might get some twisting but a 1'x1' is very stable.

[Steve Milani]

I vote for 10mm alu baseplate.
Since from my first prototype of RGB projector, I always used 1cm baseplate.
Largest baseplate I made was 60 x 40cm: no problem.... solid like a rock.
Until now I had not ANY problem with this kind of thickness.

[mixedgas]

Steves no twist classic.

One, Ask for jig plate, its polished on one side and destressed.

12" wide by up to 36-48 inches long. allen screw every 3" to hold a 1x1" AL stringers along the long length, at the edges.

____
[] [] cross section view.

with 3/8ths plate (1/4" in pinch) this is remarkably stiff because you just made half a I-beam.

If not, solid , use association channel, but try to make sure its the same alloy as the plate

http://www.industrialmetalsales.com/1895.html

Steve

[dave321]

i have used 10mm aluminium plate, but unless you have a good saw it will be difficult to cut.
i had mine cut to size using a water jet cutter

you could drop to 8mm to save a bit of cash but if you spending loads on laser is the saving worth it at the end ?

dave321

[danielbriggs]

Thanks all...
I tempted to say what the heck and go for the 12mm... I'm only likely to do this once (he says!)
And if it's worth doing, it's worth doing properly
I'll put a word in a uni and see if I can get my plate cut on the water jet cutter.

Ta,
Dan