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Old Feb 17, 2015, 07:52 PM
AvB
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Doing a test cut on a laminate of 2 sheets of 12mm Corian. Roughing cut done and just starting the finish cut.This is a wavy surface, which goes through the join line in various places so we can see whether there are any problems with the join (eg particularly the "wafer" of one surface lifting off the glue line).

This was rough cut with a 8mm ball nose, and the finish cut is being done with a radiused endmill - 5mm dia with 2mm corner radius. The finish is incredibly smooth. It's amazing how these high quality SLR photos show more detail than I can see by eye! You can see some fine ridging in the photo, but it's not apparent just looking at it, and can't be felt by hand.

The cut is noisy though. I had heard that corner radius endmills were quieter than flat mills, but this is noisy. I left >2mm material in the rough cut with 2mm stepdowns, so the finish cutter is going thru 3 or 4mm of material ... next time I think I'll go for smaller stepdowns and less material left. The rough cut can be noisy but it's done fairly fast, but the finish cut with fine stepovers takes a long time, so it's an advantage if it's fairly quiet.
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Old Feb 18, 2015, 11:16 PM
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Can I ask why you would use a ball nose instead of a flat end mill for roughing,
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Old Feb 19, 2015, 04:58 PM
AvB
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I won't again!
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Old Feb 27, 2015, 01:45 PM
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looks good, what did you laminate with ?
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Old Feb 27, 2015, 04:02 PM
AvB
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This one was bonded just using Hobby King medium CA. Geoff S read somewhere that the original guy working with acrylics reckoned that CA was probably the best bond. But the glues these days are methylmethacrylate glues, which I understand is preferred, due to the CA's crystallizing over time. We're experimenting with some options for lamination bonding and we have an adhesives supplier sending some samples to try soon, but I wanted to have a look at the CA bond.
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Old Feb 27, 2015, 07:20 PM
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I've been thinking about the theory behind the problem. When the cutting is done, especially is close to the join, there is a lot of heat as the cut occurs with the coinciding expansion and contraction of the material and glue layers. If the coefficient of expansion is different or the hardness of the layers is different, then this will increase the risk of delamination.

I am wondering if mixing the glue with the powder that Tim uses to make his own "Corian" substitute would result in the glue layer having a coefficients of expansion and hardness more similar to the materials being glued. The trick would be to determine how much powder to mix in to get the right coefficients while maintaining the adhesive properties.

Just a brain waffle. Thought it might help
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Old Feb 27, 2015, 07:29 PM
AvB
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The cut I did has undulations that waver above and below the join line, so a lot of the surface is "worst case scenario" with the surface being either a wafer either side, or right on the CA join. It seems pretty much perfect - even once polished, you can't feel the join line and there's no delamination.
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Old Mar 01, 2015, 08:57 AM
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yet ....
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Old Mar 02, 2015, 12:50 AM
AvB
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I've not been doing enough serious mould cutting work. It's been a long time since I did the Scratcho moulds and although I've done some smaller moulds, I'm worried that I could lose what skill base I've already learnt. I've been working through some Rhino tutorials, but you have to work on a real project to build the skills.

So I convinced Dave (Larrikin) to get stuck into CNC cutting some moulds for his Wabbit plank. He didn't take much convincing, and since it all had to be drawn up from scratch, he redesigned the wing and renamed it the Rubicon. Build thread is here: http://www.rcgroups.com/forums/showthread.php?t=2355874

So on Tuesday we got stuck in, initially drawing up the planform to suit Dave's aims. We kept going and by late that afternoon we had the wing completely drawn up and looking good. The next 2 days were busy too - on Thursday cutting, assembling and epoxying the mould working pieces (Hi-Max on ply layers laminated with thick layers of straight and biased glass). Then on Friday fiddling with curves and generating and cutting the toolpaths for roughing and flange facing of the top mould. We had a break over the weekend so as not to upset the neighbors and it was also a big one with my son Nick's 21st party ... then this morning we warmed everything up, rechecked all our zeroes, and ran the finishing cut. It's using a flat 6mm endmill, with 0.6mm increments and is a pretty damn good finish. I also ran small extra cut sections to do the tip paths at an oblique angle to the edge, to avoid ridging the tip edges. Dave's taken it home for polishing, after we did the paths for the trench, alignment holes, and outside profiling.

Still plenty of learning experiences, and lots of head scratching at times, but there's no doubt that by running through the whole process I'm getting to anticipate and avoid the potential traps. And I find myself putting into practice a lot more Rhino skills, and feeling a lot more confident with it, mostly due to lots of patient advice and example from Geoff Sokoll and the Infinite Skills DVD tutorials too.

Bottom side is roughed and flange faced, and I'm hoping to get the finish cut going early tomorrow and have it all finished by late tomorrow arvo.
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Old Mar 02, 2015, 07:33 AM
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Good stuff mate.

Don't want to jinx you but you never forget the mistakes. Their usually a lifetime lesson so if they happen don't get discouraged.

Good to see your getting better faster results on your tool paths once you get your magic speeds, feeds and tool types fill us in
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Old Mar 02, 2015, 01:33 PM
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really cool Andrew, seriously cool. Always follow with great interest. SO good of you to do this for others
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Old Mar 02, 2015, 06:01 PM
AvB
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With the Corian type stuff, mostly around 14,000rpm and feedrates around 3000mm/min. The 2 flute cutters are really good for getting chips out if you're ripping deep, but mostly I'm using 3 flute flat end mills. The finish planar cuts are generally 6mm flat end mill with 0.5mm stepover, but I do smaller region cuts at the tips that hit the LE pretty much square on, which really makes a big difference. The biggest difficulty for me is the deep profiling when we cut the final shape out. I busted an expensive cutter doing that yesterday. Still a bit to learn there.
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Old Mar 04, 2015, 05:41 AM
AvB
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Dave's Rubicon moulds are done, and he's taken them to insert and glue the brass tube for the alignment pin holes, and do the rubbing back and polishing of the 2nd half. He spent lots of time on surface finishing of the first (top) half, and it's a bit disappointing that despite all the block wet sanding, the polished surface definitely has a slight texture to it. This is something I noticed with the Scratcho moulds which were made from Corian with a really marked stone texture. (a very common theme for kitchen solid surface materials!) This sheet of Hi-macs is a light colour also with a stone texture. It seems that there is variation in surface hardness associated with the stone texture. To put it in perspective, you certainly can't feel it, and you have to get the right sort of light at the right angle to see it. If you pulled out a painted wing and gave it a wet rub with 2000 grit and then polish, it would be all gone in 10 minutes. So it's not going to have any real effect on the potential of the wing, but considering the awesome technology that I have at my disposal, I do expect better! Unfortunately a lot of the Corian-type materials we got have some sort of colour texture. Next time I will try one with coloured flecks rather than the stone look, and see if it comes out smoother. The plain white sheet that I used for the Curvy Scratcho tail moulds came up just totally perfect, so I guess if we get desperate, we can pay the bucks to order plain colours.

I'm really glad to see Dave go off with a set of accurate moulds and I look forward to seeing him DSing the final product!

Just raving on here ...

my big aim with the CNC has been to develop the methods to directly cut good moulds without needing to make handmade moulds from plugs. It's been great for the wing moulds, but fuselage moulds pose several challenges.

Nose cones ... fuselages with nose cones need a step cut around the fuz where the nose cone butts up. Apart from the very bottom of the mould cavity, the step can't be cut nice and square because as the cutter works the more vertical wall it cuts a radius rather than a square corner. I have come up with 4 options: 1) The traditional way would be to cut separate moulds for the fuz up to the nose cone, and for the inset nose cone area, (called a "boat") and it's sleeved and glued into the fuz. 2) The whole nose cone section could be milled as an insert into a pocket in the mould. 3) You could cut a narrow shallow trench flush with the step, right around the circumference of the mould cavity. Then fill the trench with polishable gelcoat and trowel it off level and square to the step, and sand and polish smooth. I reckon it's worth a try, and if it works, it offers the simplest build. 4) Another possibility suggested by Justin the machinist is just do the step the best you can with a small cutter and then get into it with a cut down triangular file … he reckons machinists actually cut inside corners to some steel bearing seats etc this way and it should work fine.

The other big issue with fuselages is filletting, necessary to cut back drag at the wing/ fuselage join. As far as I know, this is generally managed by making a complete fuselage plug and shaping up the fillets onto the side of the wing saddle by shaping and sanding putty, splooge, bog etc - and then laying up a conventional gelcoated mould from the plug. I'd like to see if we can draw up a design that shares the fillet shape between the wing and the fuselage, and use small engraving cutters that would allow us to direct cut the matching fillet shapes.
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Old Mar 04, 2015, 05:53 AM
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Going great Andrew , well done so cool to see thing's coming out of your new shed.

Talk to spencer about the fillets on the kinetics. The K2m plug ( aluminium ) had nice little fillets. Interested how he did those ?? Must have been a tinny 2mm bit or something.
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Old Mar 04, 2015, 10:46 AM
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Quote:
Originally Posted by AvB View Post
Dave's Rubicon moulds are done,
I'm really glad to see Dave go off with a set of accurate moulds and I look forward to seeing him DSing the final product!
I have thanked Andrew personally, and now I thank him publicly.

One of the greatest benefits of rc soaring is the type of people that are attracted to it. And time has a way of filter the nuggets from the tailings. Take this as a complement, Andrew ... You're a nugget Thank you.

Quote:
Originally Posted by AvB View Post

my big aim with the CNC has been to develop the methods to directly cut good moulds without needing to make handmade moulds from plugs. It's been great for the wing moulds, but fuselage moulds pose several challenges.

The other big issue with fuselages is filletting,
I'll run this up the flagpole, would there be any value in starting with milling a plug first? My logic says that it would give the opportunity of getting your head around the CAD/CAM side of things, albeit in reverse, and if something unexpected occurs then the plug can be hand finished. It's much easier to finish a convex surface than a concave one.

D.
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