LPR Technique Question: How to Cut Nice Round Holes in a Body Tube?

brockrwood

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I am having trouble cutting nice, round, clean 1/4” holes in a BT-20 body tube. I used drill bits to cut the holes but I am having trouble getting rid of the “hanging chads”. Any suggestions?

8523E81B-CBAF-4FA9-B317-1FCD40CBA564.jpeg

These are “ejection charge vent holes” in a “strap on” booster tube on a rocket.
 

brockrwood

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Clean them up with an Exacto knife and an emery board. Apogee has a video on drilling holes in body tubes. More tips there.
Thanks for the suggestion! Tim Van Milligan’s video is great!

 

GlenP

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I usually roll some fine grit sandpaper around a dowel, or just roll the sheet with the grit side out, the paper sheet may be stiff enough without a dowel, and spin it around in the staging vent hole itself to smooth out the cut edges of the hole.
EB599327-08A5-4F16-B76C-4F009A6675AC.jpeg
actually, that is not a good example, I think I just used a hole punch on those.
 
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When you are drilling anything, having backing on the underside will prevent breakout. So dowel the exact size on the inside will assist. Put a pencil line so the center of all the holes are in line. A bench drill press and a vice will assist to keep everything in line.
You could wrap some sandpaper around a small dowel to sand the holes out to the size /position you want. Or a Dremel tool with the small sanding drum (they do a 3/16 one). Hardening the edge with a small amount of CA for the final sand can help.
Step drills tend to drill holes more concentrically than enlarging with 2 fluted standard drills. A standard 2 fluted drill tends to make a tri-lobed hole. Especially if hand drilling. There is an engineering explanation why this tends to happen. Too complicated to put it here... :) If you enlarge a tri-lobed hole with another 2 fluted drill, you're probably going to drift off center even further.
If you have access to a 3D printer, they're great for making exact drilling jigs.
Hope some of that helps.
 

rocket_troy

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Yes, standard twist drills are certainly not optimal for that. Hole saws are probably better. I've also used all sorts of backing material to suppress breakout eg. PVC pipe (with tape build up to an extent), plastic rod and even MDF. You could probably even fill a sausage balloon up with 2 part resin paper mache or saw dust and wait for it to set.

TP
 

tsmith1315

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From where you are now, I'd CA the hole and true it up with 1/8" drill bit, small carbide bit, or small grinding bit in a Dremel on slow-med speed. Very light strokes.
 

brockrwood

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Yes, standard twist drills are certainly not optimal for that. Hole saws are probably better. I've also used all sorts of backing material to suppress breakout eg. PVC pipe (with tape build up to an extent), plastic rod and even MDF. You could probably even fill a sausage balloon up with 2 part resin paper mache or saw dust and wait for it to set.

TP
It seems to me that, if I am doing a new build, I can use a spent engine as the backing material. That is if the tube is a BT-5, BT-20, or a BT-50.
 

GlenP

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A forstner bit makes a clean edge flat bottom hole in wood, but not sure if that would work on a curved surface like a small diameter body tube, even with a backing support like a spent engine casing. At 1/4” or do they even make them as small as 1/8” size, it might work?
 

brockrwood

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A forstner bit makes a clean edge flat bottom hole in wood, but not sure if that would work on a curved surface like a small diameter body tube, even with a backing support like a spent engine casing. At 1/4” or do they even make them as small as 1/8” size, it might work?
Sigh. To relive my childhood as a BAR I need to buy every tool in the tool aisle at Home Depot. ;-)

The rocket parts are $10 worth of paper tubes, string, and some balsa.

The cost of tools, paint, sand paper, 6 different kinds of glue, and other buildng supplies? No upper limit. Not if you want the rocket to look pretty. ;-)
 

Zbench

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The best drill bit is a brad point bit which is made specifically for drilling in wood. They have a central spur and two very sharp cutting lips. They are awesome for this application. I usually put some CA on the backside and back that up with a dowel. WL Fuller makes high quality HSS brad point bits.
 

rharshberger

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The best drill bit is a brad point bit which is made specifically for drilling in wood. They have a central spur and two very sharp cutting lips. They are awesome for this application. I usually put some CA on the backside and back that up with a dowel. WL Fuller makes high quality HSS brad point bits.
+1 this method, CA the area of the hole and use a sharp bradpoint bit with a backer inside the tube, the spurs will cut through the tube pretty cleanly and before the body of the drill gets close to the tube. A forstner bit might work okay (maybe) but a brad point would be best here. Even better if possible 3D print a guide for the bit and have a backer inside the tube, make sure the bit doesn't try to pull itself into the tube, a drill press makes the control of the bit much easier.
 
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