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30-12-2011 #1
Thanks routercnc for the spreadsheet and the comment re the X axis bearing placement. I've seen them placed like this on another design, but somehow felt that vertical placement would be better for carring the vertical weight of the gantry and resisting racking. For the gantry I will be using 20mm plate sides with 120x40mm heavy extrusion (8.8 kg/m) for the y-axis on the basis that the box section resists twisting from the cutting loads better than plate and is lighter, reducing the forces required to accelerate and stop the gantry.
BTW I too am a model flyer (Gliders and power) and have in mind to build a 1:3.5 scale (4.25m) "Reheinland" glider with all the bits CNC cut - once I have done my brother's guitars!
Kind Regards
Mike
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30-12-2011 #2
It's going to be tricky to calculate as it relies on the strength of the bolted joint between the round rail and the support. The aluminium surface at the joint is a V-shape, so the rail only makes two line contacts. There's not many bolts (and if you don't threadlock them they can work loose) which overall means the round bar can, and do, bend in that direction upon the rail. Check it with a dial indicator and you should be able to see the effect.
See you mention making the frame very strong, which is great, but with only one ballscrew you're kind of defeating the object of doing that. Not saying don't make the frame strong, as you can't change that later (without wasting material/money) unlike adding an additional ballscrew which is obviously easy enough.
I bought a 500 size helicopter recently, so I'm also almost a model flyer. Not dared actually fly it yet! Couple of bits to cut from carbon on my router first. So far I have just stuck to model cars.
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30-12-2011 #3
Hi Mike,
Good to see a fellow flyer on the forum. I'm sure you know this already but Chris Williams writes a good 'big glider' column in QEFI magazine. Once you cnc cut parts from CAD designs there's no going back to carbon paper and all those hand methods!
Hi Jonathan,
Good point you make about the rail itself deflecting - I was think only of the block itself which is of course open to allow the supported rail underneath. That makes things more complicated and either needs FE, or someone to do a quick test!
If you've never flown a model heli before then join a club and get trained because otherwise you will crash it in seconds! Lots of PC simulators out there which are worth a go because you can often connect your own Transmitter (using the buddy lead).
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