Mortises INSIDE an airframe

Does anyone have any good information about how to cut a mortise INSIDE an airframe? I've used the crude method of drum sanders, burring tools, cutting tools, etc, but just not getting the results I'd like. I don't need extreme precision for my mortise boundary or even the depth, just a suggestion for a bit that might fit a Dremel tool and a way to get that bit perpendicular to the material inside the tube to cut these mortises.

Can someone point me in the right direction? Thank you everyone!

Pictures or diagrams would help us understand better what you are asking. Mortise is not a term used in rocketry much, usually we call it slotting and slotting is usually done outside of the airframe (either before installation or on the exterior of the...well exterior).

Router base for the dremel.

heada said:

Router base for the dremel.

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Sorry, I should have clarified that I'm working on 4" airframes, and I can't get a router base inside a 4" diameter tube. I'm thinking about the 90-degree attachment made by Dremel, but that doesn't give me much control of the cutting bit once inside the airframe...

SammyD said:

Sorry, I should have clarified that I'm working on 4" airframes, and I can't get a router base inside a 4" diameter tube. I'm thinking about the 90-degree attachment made by Dremel, but that doesn't give me much control of the cutting bit once inside the airframe...

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You must not intend to go all the way through the wall. Could you mortise a coupler from the outside and then epoxy it in place?

I should have clarified above that I'm working INSIDE a 4" air-frame. Neither a router base, nor even the dremel unit itself, will fit inside the air-frame...

Steve Shannon said:

You must not intend to go all the way through the wall. Could you mortise a coupler from the outside and then epoxy it in place?

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Sorry for the dual post above - my log-in/log-out activity got me mixed up... :-\

No, I'm not going all the way through the air-frame, just creating a depression where I can put a thin square of brass that will correspond with another piece of the same size on the AV Bay. Once both are in place, I can drill through the brass that's nestled in the mortises and create a guillotine action for shearing 2-56 nylon screws for deployment events.

I've used these in the past with good success, however, I'm REALLY tired of creating these mortises (16 of them per rocket) completely by hand...

SammyD said:

Sorry for the dual post above - my log-in/log-out activity got me mixed up... :-\

No, I'm not going all the way through the air-frame, just creating a depression where I can put a thin square of brass that will correspond with another piece of the same size on the AV Bay. Once both are in place, I can drill through the brass that's nestled in the mortises and create a guillotine action for shearing 2-56 nylon screws for deployment events.

I've used these in the past with good success, however, I'm REALLY tired of creating these mortises (16 of them per rocket) completely by hand...

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Then you must be working on cardboard because fiberglass is hard enough to shear nylon without the brass inset.

I would just soak the shear pin holes in thin CA to harden the material around them and not worry about adding the brass shear plate.

I get quite a few flights out of each rocket (the 4" Warthog shown in my photo) has over 20 flights on it and was built in 2014), so that really doesn't work, the CA in the holes, as the paper/Bluetube around it tends to soften with each deployment sequence.

Flexible rotary tool extension shaft with a right-angle head, if you really want to pursue that particular solution.

rharshberger said:

Pictures or diagrams would help us understand better what you are asking. Mortise is not a term used in rocketry much, usually we call it slotting and slotting is usually done outside of the airframe (either before installation or on the exterior of the...well exterior).

Click to expand...

Here's a few photos of what I've created using too much time to create the mortises:

SammyD said:

I get quite a few flights out of each rocket (the 4" Warthog shown in my photo) has over 20 flights on it and was built in 2014), so that really doesn't work, the CA in the holes, as the paper/Bluetube around it tends to soften with each deployment sequence.

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Then maybe you can make your shear plates round and make one with a slit in it and a stem that you can pass through a larger hole, into the chuck of a drill and then use it to drill a countersink from outside to glue your round shear plate into on the inside.

So, you're using brass strips as shear plates for shear pins. Yep, I've done it. I used .015" brass strips 1/2" wide x 1/2" long. I just laid them in the airframe and marked around them. Then I took an exacto blade and carefully cut into the airframe and peeled out just enough layers to recess the strips in. Then I epoxied the strips in place and drilled shear pin holes. As you say, a PITA, but what you do if using cardboard tube and want perfect shearing of pins consistently...
But 4 per end seems excessive for 4" tube. 3 is plenty for the main and you can probably just use 2 for the drogue. Unless you're really pushing it, and then you'd need to glass it which would negate the need for the strips.

Just wondering why is there a need to recess brass into the tube. I would think if you gllue strip in place than expoy over the whole strip it would work and not snag on anything. When dry just drill the hole.

Steve Shannon said:

Then maybe you can make your shear plates round and make one with a slit in it and a stem that you can pass through a larger hole, into the chuck of a drill and then use it to drill a countersink from outside to glue your round shear plate into on the inside.

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That's an interesting idea for sure. Not sure it will work on a 4" airframe - my shear plates are about 15-17mm x 15-17mm (never get a very consistent size with them, but they do get buried in JB-Weld, then sanded...

crossfire said:

Just wondering why is there a need to recess brass into the tube. I would think if you gllue strip in place than expoy over the whole strip it would work and not snag on anything. When dry just drill the hole.

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The application is between an AV Bay and airframe. With adhesive and the thickness of the brass (about .015"), it's just not enough clearance, and the airframe would hang" on the AV Bay. The BlueTube air-frames and AV Bays fit together pretty snug to begin with - usually have to sand them a bit with 180, then 320 to get them to slide across each other the way I'd like...

You're over complicating things. I've used this on BT60 sized cardboard tubes up to 4 inch.

https://www.rocketryforum.com/threads/shear-pin-shear-plates-in-your-cardboard-tubes.136706/

jd2cylman said:

So, you're using brass strips as shear plates for shear pins. Yep, I've done it. I used .015" brass strips 1/2" wide x 1/2" long. I just laid them in the airframe and marked around them. Then I took an exacto blade and carefully cut into the airframe and peeled out just enough layers to recess the strips in. Then I epoxied the strips in place and drilled shear pin holes. As you say, a PITA, but what you do if using cardboard tube and want perfect shearing of pins consistently...
But 4 per end seems excessive for 4" tube. 3 is plenty for the main and you can probably just use 2 for the drogue. Unless you're really pushing it, and then you'd need to glass it which would negate the need for the strips.

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Yes Adrian, I'm using them in that way - you understand what I'm trying to achieve EXACTLY! I like consistency in my deployments; this helps give me that consistency and peace of mind. I had not considered using the Exacto knife to cut small slits in paper air-frames (what the 4" in the photo is made of), but I'm using BlueTube for a stretch now, and it's not too easy to cut into. All in all, I am pretty clever when it comes to figuring s**t out, but the long-term solution has eluded me on these mortises. I am going to give the Exacto a shot - working on three 4" rockets right now (one a repair to my 4" Warthog that has 20+ flights)......... Thank you! Sam

Banzai88 said:

You're over complicating things. I've used this on BT60 sized cardboard tubes up to 4 inch.

https://www.rocketryforum.com/threads/shear-pin-shear-plates-in-your-cardboard-tubes.136706/

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For smaller rockets and using motors in the F to maybe H range, I thing that would work just fine. I like a little more strength in my shear plates though...

SammyD said:

For smaller rockets and using motors in the F to maybe H range, I thing that would work just fine. I like a little more strength in my shear plates though...

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This handles everything through the entirej range of L2 motors, especially seeing as materials aren't level of motor sensitive, and a bp separation charge at L1 altitudes is just as energetic as at L2 altitudes. This will do 2-56 and 4-40 screws, no problem, with reliability and longevity.

A 6 foot tall 4 inch diameter 54mm mount madcow rocket hardly cares if it has a G or a K in it when it seperates for deployment.

Unless you're blowing things up repeatedly......carry on.

SammyD said:

Yes Adrian, I'm using them in that way - you understand what I'm trying to achieve EXACTLY! I like consistency in my deployments; this helps give me that consistency and peace of mind. I had not considered using the Exacto knife to cut small slits in paper air-frames (what the 4" in the photo is made of), but I'm using BlueTube for a stretch now, and it's not too easy to cut into. All in all, I am pretty clever when it comes to figuring s**t out, but the long-term solution has eluded me on these mortises. I am going to give the Exacto a shot - working on three 4" rockets right now (one a repair to my 4" Warthog that has 20+ flights)......... Thank you! Sam

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To be honest, I've never used Blue Tube... But I have done the same thing in Phenolic tube, just cut slower and peeled harder... Banzai88's idea is sound and will work well with blue tube and phenolic. AND the plastic nose cone shoulder... That's the toughest to recess the strips into IMHO. Although my Yank 4" cone had the two ribs which let me just glue the strips in between the ribs.

jd2cylman said:

To be honest, I've never used Blue Tube... But I have done the same thing in Phenolic tube, just cut slower and peeled harder... Banzai88's idea is sound and will work well with blue tube and phenolic. AND the plastic nose cone shoulder... That's the toughest to recess the strips into IMHO. Although my Yank 4" cone had the two ribs which let me just glue the strips in between the ribs.

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I've found that with flex phenolic the airframe tube doesn't need anything more than CA because the head of the screw prevents it from being pulled through. The coupler/ nose cone side is the side that needs to cleanly shear the shearpin, I imagine Bluetube is similar. I've had good success using 1/8" pop rivets pressed through from the inside of the coupler, glued in and covered with a layer of glass. Remove the steel shaft before pressing it in then file it smooth with the coupler then drill through with the correct size number drill. For plastic nosecones I like to cut the plastic coupler part off and fit a phenolic coupler tube to it, glued and screwed to a plywood centering ring inside....then do the pop rivet thing

To the original question- you're needing to do some linear broaching. You would need to make a custom tool to do this. It's kind of a scaping action inside of the tube.

I did something a little different on my Tyrannosaur. I used some pieces of aluminum inside the airframe for the sheer plates, edges profiled to blend with the bodytube. And I attached small pieces of aluminum angle to my bulkhead with enough clearance to clear the sheer plates in the bodytube. Then just drill and tap as normal. Leaves room for error and pretty simple. Alignment of the plates and angle is the toughest part but is still pretty easy. Rough picture of what I mean, dark warehouse at work. The plates will never touch the coupler, as they will be 1/2" or so farther in the tube. And the pins will go through the plates and then the aluminum angle.

Frankly, I don't bother with shear pins on 4" and smaller cardboard tubes. I have one that is 9 years old with 46 flights. Friction fit has always worked just fine. The key is don't use the "blow it out or blow it up" method. Size your charges correctly.