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Old Mar 07, 2014, 12:30 AM
MrE
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Kevin, Im sure that you can get good results from CnC if you take the time to set things up and test.

An RCG friend of mine sent me some cores that he CnC cut. He considered them less than perfect but they look pretty good to me. I have now had hands on experience with two different sets of CnC cores and the quality can vary a lot
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Old Mar 07, 2014, 12:37 AM
MrE
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Well, it looks like cutting a very low wire temps has another unforeseen problem.

I just cut another Supra fin after tweaking the templates a bit more - more sanding and adjusting for final size/thickness on the trailing edges.

On the second cut I lowered the wire temp down to the point it was just starting to tear the foam along the fast/long end of the cut.
When I looked at the final piece I saw a good bit of ridges - like the wire had been hanging up a bunch of times. I know it wasnt hanging up - I watched very closely.

After more looking I realized that those small torn bits of foam were hanging on the wire and getting between the wire and template causing the wire to move up off the template. Those little bits of foam were partly melted and pretty hard. Im guessing that because the wire is loosing even more heat into the template, its not hot enough to melt through these bits again.

So, now Im going to play with higher temps.
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Old Mar 07, 2014, 12:39 AM
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I have a question for those of you with normal drop arm cutters.

Have you seen any sign of the kerf being narrower right up against the templates like Im seeing?

Im wondering if this is a universal thing or is it just because Im trying to cut at very low amp/temp setting?
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Old Mar 07, 2014, 01:44 AM
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Originally Posted by MrE View Post
.............................

Have you seen any sign of the kerf being narrower right up against the templates like Im seeing?
.....................................
I have not had that happen. Normal swing arm gravity cutter with .019 wire @ approx 3.5 amps (from memory).
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Old Mar 07, 2014, 05:02 PM
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Quote:
Originally Posted by MrE View Post
I have a question for those of you with normal drop arm cutters.

Have you seen any sign of the kerf being narrower right up against the templates like Im seeing?

Im wondering if this is a universal thing or is it just because Im trying to cut at very low amp/temp setting?
It is quite common. The templates are sucking the heat out of the wire and naturally the kerf will be narrower where there's less heat.
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Old Mar 08, 2014, 12:48 PM
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Formica and phenolic don't do this usually ! FYI !




Quote:
Originally Posted by MrE View Post
I have a question for those of you with normal drop arm cutters.

Have you seen any sign of the kerf being narrower right up against the templates like Im seeing?

Im wondering if this is a universal thing or is it just because Im trying to cut at very low amp/temp setting?
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Old Mar 08, 2014, 05:14 PM
MrE
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I suspect there is some of this going on for everyone, but it may be a small enough effect that's its not noticeable as long as the wire is moving fast enough and running hot enough.

I think in my case, its exaggerated because Im going so slow and cutting at lower amps/temps.
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Old Mar 08, 2014, 05:25 PM
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Originally Posted by MrE View Post
I suspect there is some of this going on for everyone, but it may be a small enough effect that's its not noticeable as long as the wire is moving fast enough and running hot enough.

I think in my case, its exaggerated because Im going so slow and cutting at lower amps/temps.
That sounds logical. I suspect you are correct. I will make some more measurements next time I cut some cores using a straight edge.
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Old Mar 08, 2014, 07:40 PM
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Its easiest to see if you look at the leading edges on a really thin section. Im doing some Supra elevator sections now and even a slight difference in kerf shows up on the really thin tips.

I'll get some pics uploaded shortly.

Im having some good success finally! I added some 3mm depron spacers to to get the templates away from the foam. That combined with a slight increase in wire amps/temp and a slight increase in speed seems to be working very well. The kerf is very small, very uniform and the cuts look really good.
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Old Mar 08, 2014, 08:06 PM
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Here are some pictures of leading edges where I had noticeable difference in the kerf between the template and further inside the cores.
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Old Mar 08, 2014, 08:29 PM
MrE
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Here are the leading edges of the latest Supra horizontal with the spacers added to the templates and a slightly higher wire amp/temp and cutting speed.

As you can see there is no visible dip at the tips of the leading edges. So the kerf is now very uniform all the way through the cut. It is also relatively narrow. I tried measuring the difference in foam thickness before and after the cut. Its not easy to get the foam pieces ligned up perfectly but, it looks like the kerf is about .019 - .02 on the small/slow end and .015 - .018 or so on the longer/faster end of the cuts. Thats with .011 wire.

The best amp setting seems to be about .95 amps and a cutting speed of around 1.5mm/sec on the fast/long end of the cut.

Im going to take a break from cutting cores for a while and do some bagging - so I can actually go fly!

My conclusions so far:

1) This is a LOT OF EXTRA WORK and fiddling around. If your drop arm setup is working fine now, I doubt this is worth messing with.
2) There is always going to be a wider kerf on the slow end of any cut. Thats just the way it works.
3) There is a "best" combination of wire speed and wire amps/temp - but finding it takes cutting a lot of foam.
4) The main benefit of cutting slower is less wire lag which should give better leading edge results. This will be more apparent when using longer bows cutting longer sections. If you're cutting shorter sections and can get enough wire tension, then you dont have a problem that needs fixing - see conclusion #1 above
5) the main downside to cutting slower is kerf differences inside the core. This requires spacers on the templates which could cause problems when joining multi-panel wings. Im not sure that will really be a problem, but the spacers are still more trouble to go to.

As always - YMMV

That said - now that Ive gone to all this trouble - I will probably continue to cut cores this way in the future. This latest stab section looks pretty darn good

This has been an interesting series of experiments and Ive learned a lot from everyone who has posted. I will be back with more results next time I do some more cutting.

Thanks!!!!!!!!!!!!!!!!!!!
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Old Mar 08, 2014, 08:33 PM
MrE
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Oh, here are some shots of the templates with spacers. Im using 3mm depron tacked on with a light spray of 3M77

Ive also included a couple of shots of the cuts in action.
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Old Mar 08, 2014, 09:06 PM
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Good to know you are having some success after all that work and investigation.
What are the philips head screws for? Holding the templates onto the foam?
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Old Mar 08, 2014, 10:59 PM
MrE
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Yes. Im using 2" long wood screws. They hold the templates in place pretty firmly.
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Old Mar 09, 2014, 03:55 PM
MrE
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It occurred to me today that there are a couple of other advantages to using low wire temps and slow/controlled cutting speeds.

1) There is a LOT less smell and fumes. Thats more pleasant and less toxic.
2) I love the CCCV power supply. You can really fine tune the amps and get much more consistent results from day to day with the cutting. It also doesnt seem to matter how far apart t he clips are on the wire. If the amps are the same, then the cutting temp is the same.
3) I like the variable cutting speed and being able to fine tune it.

Between the variable speed and variable amps, you can actually fine tune the cut while its happening. Makes for fewer test cuts to get things just right.

I was just starting a new cut with different foam. I did one pass starting at a low temp and low speed. Then 1/4 way through I upped the temp and speed a bit, then again 1/2 way through and again 3/4 of the way through. Turns out the second setting was about perfect. So I got it dialed in with one test cut instead of several. A lot less wasted foam and time.

This is still a lot more trouble than normal gravity drop arm cutting.
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