Stripping force for threads tapped in fiberglass sheet

[amiliv]

I attempted to find this online... But the pages I found either assumed non-trivial prior knowledge of the topic (too many terms and/or abbreviations I'm not familiar with) or were charts for various metals.

So, the question for somebody more fleunt in material sciences than I am, if I were to drill and tap a hole for #8-32 screw into 3/16" G10 fiberglass centering ring, what would be the approximate stripping force (i.e. how much force it can withstand before the threads in the fiberglass fail and the screw pulls out)?

The application side for this is simple. I'll be attaching an SC Precision 4" thrust plate for 75mm motors to a 3/16" thick G10 fiberglass centering ring. The thrust plate attaches using 3 #8-32 screws, with generic instructions calling for simply tapping the holes for the screws into the aft centering ring. These screws will basically only experience the weight of the motor while the rocket sits on the pad, and the force of ejection charge trying to push the motor out at apogee. While the motor is firing, there's no force on these screws. There'd be only 3 threads (32 tpi * 3/32" = 3 threads) inside fiberglass for the screw to engage with, I'd assume this should still be plenty. But in the back of my head, I'd still like to know how much of a safety margin there is between the force of the ejection charge pushing on the motor and the force those threads in the fiberglass can withstand before failing. Would doubling aft centering ring (to have more threads engaging) be needless overkill (which I'd assume it would, but I've no data/math to back that up with).

 

[Troy3003]

I attempted to find this online... But the pages I found either assumed non-trivial prior knowledge of the topic (too many terms and/or abbreviations I'm not familiar with) or were charts for various metals.

So, the question for somebody more fleunt in material sciences than I am, if I were to drill and tap a hole for #8-32 screw into 3/16" G10 fiberglass centering ring, what would be the approximate stripping force (i.e. how much force it can withstand before the threads in the fiberglass fail and the screw pulls out)?

The application side for this is simple. I'll be attaching an SC Precision 4" thrust plate for 75mm motors to a 3/16" thick G10 fiberglass centering ring. The thrust plate attaches using 3 #8-32 screws, with generic instructions calling for simply tapping the holes for the screws into the aft centering ring. These screws will basically only experience the weight of the motor while the rocket sits on the pad, and the force of ejection charge trying to push the motor out at apogee. While the motor is firing, there's no force on these screws. There'd be only 3 threads (32 tpi * 3/32" = 3 threads) inside fiberglass for the screw to engage with, I'd assume this should still be plenty. But in the back of my head, I'd still like to know how much of a safety margin there is between the force of the ejection charge pushing on the motor and the force those threads in the fiberglass can withstand before failing. Would doubling aft centering ring (to have more threads engaging) be needless overkill (which I'd assume it would, but I've no data/math to back that up with).

I would just drill a clearance hole and epoxy a nut on the back side providing you have room. Then you have some reasonable assurance it won’t pull out.

 

[Steve Shannon]

Test it. Buy some G10 sheets and test the force empirically. I don’t think it will be much. I would never trust unreinforced fiberglass threads for a recovery anchor.

 

[James Keller]

If you don't want to epoxy a nut on the backside you could also try using wood screws. But definitely do not try to tap fine threads into G10. I'd give that a 50/50 chance of it working.
Edit: I've never tried wood screws hb g10 but it would be better than machine screws

 

[David Schwantz]

Blind nut on back side. Your G10 willfail.

 

[amiliv]

Test it. Buy some G10 sheets and test the force empirically. I don’t think it will be much. I would never trust unreinforced fiberglass threads for a recovery anchor.

Note that this is not a recovery anchor. It only keeps the motor from falling out of the back of the rocket; i.e. think of it in terms of a motor retention. Those are the only force these threads will experience.

 

[NTP2]

Note that this is not a recovery anchor. It only keeps the motor from falling out of the back of the rocket; i.e. think of it in terms of a motor retention. Those are the only force these threads will experience.

Then as long you don’t have 1kg of BP in there it will be fine.

Edit I think…

 

[Titan II]

There'd be only 3 threads (32 tpi * 3/32" = 3 threads) inside fiberglass

? You said the G10 was 3/16".
If so, use a sharp tap and you will be fine.

 

[David Schwantz]

Even though it is not a recovery anchor, it still is an important aspect of the flight. You do NOT want your motor, case , falling out of the rocket. Think of it hitting someone, or landing in a field of crops. Pretty sure farmer does not want it in the combine. Very disappointed in your reply.

 

[Steve Shannon]

Note that this is not a recovery anchor. It only keeps the motor from falling out of the back of the rocket; i.e. think of it in terms of a motor retention. Those are the only force these threads will experience.

Good luck.

 

[mtnmanak]

I am not sure I even understand the point of having the thrust plate on there if it is just going to be screwed to the centering rings... The point of a thrust plate is to transfer a portion of the thrust directly to the airframe. If it is just screwed to the centering ring, then isn't the motor mount assembly still taking all the force of the thrust? And why would the thrust plate be the basis of your motor retention?

I think pictures of what you are trying to do would probably be more helpful than trying to explain it.

Nobody here wants your rocket to fail and I am sure you would get some good advice, but you probably need to more clearly show the problem you are trying to solve in order to get the advice you need.

 

[Titan II]

I am not sure I even understand the point of having the thrust plate on there if it is just going to be screwed to the centering rings... The point of a thrust plate is to transfer a portion of the thrust directly to the airframe. If it is just screwed to the centering ring, then isn't the motor mount assembly still taking all the force of the thrust? And why would the thrust plate be the basis of your motor retention?

The plate "is" against the body tube. Google SC Precision 4" thrust plate for 75mm motors and you will see how it is installed. Their retainer screws to the thrust plate. I use them and they work great.

 

[sharkbait]

I attempted to find this online... But the pages I found either assumed non-trivial prior knowledge of the topic (too many terms and/or abbreviations I'm not familiar with) or were charts for various metals.

So, the question for somebody more fleunt in material sciences than I am, if I were to drill and tap a hole for #8-32 screw into 3/16" G10 fiberglass centering ring, what would be the approximate stripping force (i.e. how much force it can withstand before the threads in the fiberglass fail and the screw pulls out)?

The application side for this is simple. I'll be attaching an SC Precision 4" thrust plate for 75mm motors to a 3/16" thick G10 fiberglass centering ring. The thrust plate attaches using 3 #8-32 screws, with generic instructions calling for simply tapping the holes for the screws into the aft centering ring. These screws will basically only experience the weight of the motor while the rocket sits on the pad, and the force of ejection charge trying to push the motor out at apogee. While the motor is firing, there's no force on these screws. There'd be only 3 threads (32 tpi * 3/32" = 3 threads) inside fiberglass for the screw to engage with, I'd assume this should still be plenty. But in the back of my head, I'd still like to know how much of a safety margin there is between the force of the ejection charge pushing on the motor and the force those threads in the fiberglass can withstand before failing. Would doubling aft centering ring (to have more threads engaging) be needless overkill (which I'd assume it would, but I've no data/math to back that up with).

Put a small dab of JB Weld or Locktite thread locker in your hole before you put your screws in, done this on what your talking about many times with zero issues

 

[mtnmanak]

The plate "is" against the body tube. Google SC Precision 4" thrust plate for 75mm motors and you will see how it is installed. Their retainer screws to the thrust plate. I use them and they work great.

Ahh - It is stepped so it sits over the edge of the body frame.

And I can see from the instructions they do tell you to screw the plate into the rear centering ring, which makes the OP's question more clear.

I would have to agree on adding some kind of nut behind the centering ring to make this method likely to succeed. I would also JB weld the whole thrust plate to the airframe and CR.

 

[mrwalsh85]

For what it's worth, I thread G-10 and my Filament Wound Body tubes all the time. I don't use backers.

I always clearance drill the hole accordingly for the tap, then tap with a nice, sharp tap. After that, I saturate the threads with thin CA and allow that to wick in. After that, hand-twist the clearance drill through the hole. Then I run the tap through again. This way I can get a nice clean tapped hole with reinforced edges. I have flown rockets with more than ten flights on the same holes. I secure av bays to my airframes in this manner.

YMMV.

 

[David Schwantz]

I believe the OP is talking about holding the retainer to the thrust plate with just 3 screws. He Is not talking about the mounting of the thrust plate to the airframe. He is worried about the ejection charge blowing the retainer off the thrust plate.

 

[Astrobuf]

I attempted to find this online... But the pages I found either assumed non-trivial prior knowledge of the topic (too many terms and/or abbreviations I'm not familiar with) or were charts for various metals.

So, the question for somebody more fleunt in material sciences than I am, if I were to drill and tap a hole for #8-32 screw into 3/16" G10 fiberglass centering ring, what would be the approximate stripping force (i.e. how much force it can withstand before the threads in the fiberglass fail and the screw pulls out)?

The application side for this is simple. I'll be attaching an SC Precision 4" thrust plate for 75mm motors to a 3/16" thick G10 fiberglass centering ring. The thrust plate attaches using 3 #8-32 screws, with generic instructions calling for simply tapping the holes for the screws into the aft centering ring. These screws will basically only experience the weight of the motor while the rocket sits on the pad, and the force of ejection charge trying to push the motor out at apogee. While the motor is firing, there's no force on these screws. There'd be only 3 threads (32 tpi * 3/32" = 3 threads) inside fiberglass for the screw to engage with, I'd assume this should still be plenty. But in the back of my head, I'd still like to know how much of a safety margin there is between the force of the ejection charge pushing on the motor and the force those threads in the fiberglass can withstand before failing. Would doubling aft centering ring (to have more threads engaging) be needless overkill (which I'd assume it would, but I've no data/math to back that up with).

Your biggest problem will be the second time you use this. The resin will wear badly wear each time you run the retention screw in and out on the the fine threads. Further, you will surely cross thread the screw given you only have 3 threads engaged in a very soft material.

I think the advice to use an insert is wise. I'd look at McMaster-Carr for low profile nutserts/rivnuts. These are commonly used in aerospace for addind usable threads to sheet metal structures.

Astrobuf

 

[mtnmanak]

I believe the OP is talking about holding the retainer to the thrust plate with just 3 screws. He Is not talking about the mounting of the thrust plate to the airframe. He is worried about the ejection charge blowing the retainer off the thrust plate.

In the first post, he says "I'll be attaching an SC Precision 4" thrust plate for 75mm motors to a 3/16" thick G10 fiberglass centering ring. The thrust plate attaches using 3 #8-32 screws, with generic instructions calling for simply tapping the holes for the screws into the aft centering ring. "

I thought he was referring to mounting the thrust plate itself.

 

[David Schwantz]

By God, I think I read it wrong also???? I should just stay off these forums until I've had my glass of OJ

 

[David Schwantz]

Ok, so I went back and re read OP first post and also looked up the plate, of which I have many. If OP is so worried about the 3 screws stripping out, why not just use blind nuts on the back side or just drill 3 more holes for more retaining screws? Although I will say that mine are used on birds that use no motor eject, so the 3 screws have been sufficient. But they do have blind nuts on the back of the ply CR.

 

[JohnCoker]

I like to use press-fit nuts ("PEM nuts"). They're probably overkill, but the provide a nice solid anchor and the fewer pieces of loose hardware you have to deal with at the launch site the better.
https://www.mcmaster.com/products/p...-press-fit-nuts-for-soft-metal-and-plastic-7/
Here's a picture of them installed in an aluminum plate, but G10 is pretty much the same:

Astrobee 500

 

[Belrock]

My thrust plate