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Any tips on cutting TTW fin slots?

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Gunstar

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I'm modding my Broadsword build to have 1/8 inch plywood TTW fins. So I need to cut the slots in the BT-80 tube. I haven't done that before so if there are any tips or tricks to it, please share.
 

bobbyg23

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Measure twice, cut once. New blade in your exacto.
 

manixFan

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Ok, probably not going to help at all, but if I had to do that today I would 3D print a slot guide and a backer for the tube. But, from the 'olden' days of when I used to build with those kinds of tubes:
  • use a backer - either a spare coupler or some other tube to support the body tube from underneath the cut
  • use blue tape to lay out the cut lines - much easier to follow
  • (or if you have a small aluminum angle iron, you can blue tape that as a guide)
  • cut the top and bottom of the slot first - makes it a lot easier to stop at the correct length
  • use good light (surprising how much that helps)
  • use several light strokes to cut through
  • use a new blade
  • use a new blade
  • and, use a new blade (the tip needs to be very sharp not to 'drag' in the material)
No one ever said "I wish my blade was dull", unless of course, they've just cut themselves.


Tony
 

K'Tesh

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After using a fin guide to mark the fin locations, I draw lines, using a mechanical pencil and a piece of angle aluminum (a "L" (in profile) shaped piece of aluminum available from a hardware store), that extend (slightly) beyond the top and bottom of the fin (presuming the fin is not even with the bottom of the body tube. I will then mark the top and bottom of the fin tab. I'll then use the angle aluminum to ensure a straight edge for guiding my X-acto knife (using a new blade) in making several light passes to cut the body tube. I then pick a side of the slits that will be my reference line (typically the right side looking from the rear of the body tube forward). I press the fin against the slit, and mark the line using the mechanical pencil and the angle aluminum again, along the outside edge of it. I cut the second slit, and join them to remove unneeded portion of the body tube. This ensures that the slots are matched to the actual materials I have, and not the given material's official specs (making it neither too tight or too loose). If my slot is too tight (bowing in because of friction with the fin), I sand the right side of the slot with emery boards. If it's too loose, then I can use the removed tab to act as a shim, but I apply it to the right side only.

I'm also not a fan of building motor mounts, and attaching the fins to the mounts, doing all the internal fillets, then sliding the assembled mount and fins into the body tube with slots that extend to the end of the body tube. When the tube is cut like that, those unattached ends can move in undesired ways.

I will do my external fillets before I do my internal fillets (prevents epoxy driping down the side of the fin). To do internal fillets, you can "cap" the motor tube with a lightly inflated round balloon (or condom) to prevent epoxy from dripping inside the motor tube. Wraps of tape can prevent epoxy buildup from preventing the application of the rear centering ring or the motor retainer. I drip 30 minute epoxy into the tube, along the edges I want to do. I typically can do two fillets at one time (a pair of root edges (or body tube edges), or a root edge and a body tube edge. I then lay the rocket on its side, but making sure the epoxy won't drip out, and allow the fillets to cure to the leathery stage (or fully cure) before moving on to the next set of internal fillets.
 
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neil_w

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I'll then use the angle aluminum to ensure a straight edge for guiding my X-acto knife (using a new blade) in making several light passes to cut the body tube.
Yes, exactly this.👆 Cut along the aluminum angle and it's really not difficult, at least for cardboard tubes.
 

prfesser

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FWIW I get superior results by holding the knife at a fairly high angle. Maybe it cuts thru the fibers more readily?
 

Gunstar

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Ok, probably not going to help at all, but if I had to do that today I would 3D print a slot guide and a backer for the tube....
I actually have a design idea in my head for that, with adjustable parts to change where the slot is on the tube and how long it is. I won't have a 3d printer till Christmas however. Once I have it and test my design I'll put it on thingiverse, but I'm not going to hold off on the build till then.
 

prfesser

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As JoePfeiffer says.

I cut very straight slots in a carbon fiber tube with a Dremel. Mark top and bottom of slot and radial locations of fins. Taped the tube firmly to a flat & rigid surface (piece of countertop). Taped the dremel firmly to a block of wood thick enough to put the cutting bit at the right height, in such a way that it cannot wobble on the block. Used a diamond bit of the correct width (from a cheap set at the local flea market). Switch on Dremel, slide it lengthwise on the countertop to cut the slot. Remove tube, turn it 120 degrees, repeat. Repeat for third slot.

For a paper or phenolic tube, a RotoZip type of bit works nicely.

Best -- Terry
 

RocketTree

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I use a hobby knife, while keeping the body tube rigid with an inside coupler or pipe to cut against.

Measure the thickness of the fin tab with a caliper, and transfer it to the body tube, making lines with a length of angle aluminum.

Best of luck.
 

Mike Haberer

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Measure twice, cut once. New blade in your exacto.
A lot of folks don't think the gold exacto blades are worth the money but for stuff like cutting slots they are money. Instructions for King Kraken said to use a new blade for each tube fin. I used one goldy for all four fins and am still using it months later.
 

John Kemker

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No one ever said "I wish my blade was dull", unless of course, they've just cut themselves.


Tony
I was always taught that a dull blade is much more likely to cut you than a sharp one. If your blade is sharp, you're much less likely to bear down too hard on it, causing it to resist until finally giving way unexpectedly. A sharp blade cuts with minimal pressure and you're more in control than with a dull blade.

At least, that's what I was taught.
 

Gary Dow

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This may help
Tube Slotting and Cutting Guide
 

manixFan

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I was always taught that a dull blade is much more likely to cut you than a sharp one. If your blade is sharp, you're much less likely to bear down too hard on it, causing it to resist until finally giving way unexpectedly. A sharp blade cuts with minimal pressure and you're more in control than with a dull blade.

At least, that's what I was taught.
Well, not to belabor the point, but actually, you are more likely to cut yourself with a dull blade (for the reason you described), but the cut with a sharp knife can be much worse. A sharp blade can cut you much more easily and deeply with lighter pressure or an accidental brush with the blade, and worse, you may not notice because the cut is so clean. I have a fair collection of knives and can tell you that just during general handling the super sharp ones are far scarier than the dull ones. I was at a gun show once and watched a guy sharpen a hunting knife with a very high end sharpener (I think it was a Wicked Edge). Before he gave the knife back to the guy, he told him "do not test it with your thumb, you will cut yourself". Of course, the guy gets the knife, runs his thumb along the edge, and says he can't feel it. The knife sharpener hands him a paper towel and the guy looked confused. He was already bleeding and did not realize it yet.

Dull knives are bad tools, sharp knives are silent.


Tony
 
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lakeroadster

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I make a wrap around template and then just free hand cut them. A good sharp x-acto knife blade and patience are the keys to success.

And for 1/8" fins I use an emery board nail file to clean up the cut edges. Makes for a perfect fit.

Good Luck!

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o1d_dude

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Don’t try to cut all the way through the tube wall in one pass. Multiple light passes will work much better.

Mark your slot cut lines slightly narrower than the fin thickness and sand to fit.

Some folks mark the cut lines by laying down strips of masking tape and cut along the edge of the tape.

Me? I buy slotted tubes except in emergency situations (the night before the launch and “go fever”).
 

OverTheTop

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A mill with a rotary head makes life easy. I have been known to mount a dental drill in the quill. At over 400k rpm the dental burrs make a nice slot in anything.

If you don't have a mill then a sharp knife or a Dremel can do wonders. Just take your time whatever you do.
 

XrayLizard

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A mill with a rotary head makes life easy. I
I don’t know about that haha. The setup time and trouble hardly seems worth it.
Even the fixture that holds the tube looks overly expensive lol

jcokers router setup looks like winner
Plus a router spins a heck of a lot faster

just my not humble highly opinionated thoughts !!! Lol!!!
 

OverTheTop

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It is a little (!) overkill for cardboard tubes. The tube does need a center bung to stop it collapsing in the jaws, and there needs to be a rest on the other end as well. Setup is not overly difficult with a dial gauge.

Maybe John Coker's jig with a dental drill would be a winner?
 

Woody's Workshop

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I don't have expensive tools, and if I could afford them I wouldn't have room for them here in the apartment.
So what I've done is use Estes Tube Marking Jig. I put a narrow piece of Aluminum Duct Tape over the edge so I don't cut the plastic of the jig.
A sharp hobby blade (I prefer the larger #1's) with a freshly sharpened blade.
Stick it full hilt, and as you draw the blade through the BT slowly raise the blade out.
This gives it more of a cutting action through the wall of the BT.
45 degrees from the BT to the edge of the blade is the steepest you want to cut.
I usually go lower to 25 or 30 degrees. The lower the angle, the less chance of tearing the layers loose.
I could spend time making a jig to use my Dremel tool with, but I could slot a 100 tubes or more while I'm building said jig.
Working with my hands is about all I got left, and really enjoy it. I miss my machines and wood shop.
But I'm learning a lot more about hand tools, which I kept all of mine instead of selling them.
You can't imagine how much pleasure you get out of hand cutting dovetails that fit tight without a router & dovetail jig.
 

RocketTree

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Probably the most simple method...

I cut the TTW fin slots with Mastercraft folding razor knife.

After measuring fin tab thickness with caliper, mark it on the body with angled aluminum to keep it straight. Double check the markings for accuracy by using a fin. Good to use an old coupler, motor mount or something in there to keep the body rigid. Ready to cut, making 3 or so passes on each.
 

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