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Old Sep 26, 2014, 06:53 AM
Themadartist
Australia, QLD, Narangba
Joined Nov 2012
254 Posts
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A question about getting from CAD to CNC ...

A noob question to the RC Groups CAD "Brains Trust" ...

I am considering having a Rhino model made for a 2m composite glider wing. The wings need to be 2-piece, as the fuse has 'shoulders' to which the wings mount.

I am confident the mould can be drawn, but I am totally ignorant of the practicalities of having it machined on a CNC. Here's why ...

I would like the wing moulds to allow the root section to be moulded, as well as having a 'channel' in the mould to facilitate the placement and alignment of the joiner boxes. I hope that's making sense. The attached pic of one of AvB's Angry Bird moulds is an example.

So I'm wondering if a CNC can actually cut the root 'face' - you know, get into the corner, so to speak.

I suspect cutting a plug, to then mould, would work, but I'm unsure if the mould itself would be too 'tight' to do.

Any advice / experience would be very welcome.

Thanks, Steve
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Old Sep 26, 2014, 08:43 AM
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Ward Hagaman's Avatar
San Diego
Joined Mar 2003
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Steve, to make a sharp inside corner with a CNC it is common practice to make a pocket (think trough) at the root and then insert a second piece which makes it square at the bottom. Not a huge deal, but a little more work.
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Old Sep 26, 2014, 09:27 AM
Themadartist
Australia, QLD, Narangba
Joined Nov 2012
254 Posts
Thanks for the reply Ward - I think I get it.

I was thinking that it could be overcut enough for the corner to be formed, then back-filled with a splooge or corian mix.

Cheers, Steve
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Old Sep 26, 2014, 11:41 PM
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Denver, CO
Joined Dec 2005
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Another option it to hand scrape, to remove the radius.
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Old Sep 27, 2014, 01:25 AM
Themadartist
Australia, QLD, Narangba
Joined Nov 2012
254 Posts
I did wonder about that Rob, it just seemed so counter-intuitive to go to the precision of a CNC, and then finish it "by hand".

Appreciate the reply.
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Old Sep 30, 2014, 03:45 AM
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United States, ID
Joined Sep 2011
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You can't cut a square hole with a round tool.

You can "dog-bone", "T-bone", or cut past your corner.

I suggest going back with a smaller bit to clean out the corner. The radius will become insignificant and/or the overcut will become insignificant.
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Old Sep 30, 2014, 09:42 AM
Fool on a Hill
Joined Apr 2014
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Quote:
Originally Posted by Guy97 View Post
I did wonder about that Rob, it just seemed so counter-intuitive to go to the precision of a CNC, and then finish it "by hand".

Appreciate the reply.
I know some professional woodworkers who routinely use CNC machines to cut their projects, and they all do some hand finishing at times.
I know it's common for people to think of CNC machining as a "push a button and it's done" operation, but when dealing with custom shapes, that is far from the truth.
Only in high production operations is it worth the time and trouble and extra tooling to cut parts that are ready to go (or sell), and even then, there is often a QC line that looks over each part and might call for some hand finishing.

What I would do in the case of the OP is to cut to the main dimensions and shapes and then just use a wood chisel to sharpen the corners and things.
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Old Sep 30, 2014, 03:14 PM
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United States, ID
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Quote:
Originally Posted by Depot View Post
What I would do in the case of the OP is to cut to the main dimensions and shapes and then just use a wood chisel to sharpen the corners and things.
Just use a 1/8" or 1/16" bit and dog-bone the corner. The tiny, tiny extra bit of material that will be removed won't matter.

Corners are often where things don't fit. If you machine the parts perfectly you'll have to use a little force to fit them together. If you don't do a little allowance for the corners then they will be at risk of breaking off.

When you dog-bone the corner you get the corner fitting exactly against the corner point, but with a little allowance for the first tiny bit of the corner material.

This way you get a fit that will slightly round off the corner of the inserted part if the fit is too tight, but you won't get a 45 degree break off of the corner.
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Old Sep 30, 2014, 05:24 PM
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Tel Aviv, Israel
Joined Jul 2004
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if you're going to be painting in the mold then Ward's suggestion is the best. polish the mold before setting the fence in.

another way is to end the mold at the root and then add a block to the end of the mold milled to hold your spar piece. more work but if made removable then it can help reduce damage to the sharp root corner by popping it of before demolding the wing.
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Old Oct 01, 2014, 02:14 AM
Themadartist
Australia, QLD, Narangba
Joined Nov 2012
254 Posts
Many thanks ...

Thanks ZAGNUT, jakestew, Depot, RotoRob and Ward,

I appreciate you all taking the time to help out a newbie to the CNC world.

Best regards, Steve.
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