Upsizing Helical Inserts

[drawoh]

I am designing a mount for an instrument on the front of a piece of airborne equipment. I want to modify the front panel of this instrument, which is held on by flat head stainless steel machine screws, M4X12, threaded into helical thread inserts. The panel is 3/16" thick, aluminium 6061-T6. I do not have the exact weight of my instrument, but it is a few pounds.

My design must be retro-fittable on existing designs, which are full of the sort of electronics and other stuff that you don't want to be dropping tiny pieces of metal into. I have no way acceptable to management of attaching my equipment other than those six screws. I want to use 10-32UNF pan head structural screws as per MS27039, making me and any safety inspectors out there way more comfortable. The worst case is that I will use M5 screws instead of M4.

I want to extract the M4 helical thread inserts, tap out the holes and insert the new, larger inserts. I do not want to drill anything. The tap drill for 10-32UNF is around .201" diameter. The OD of an M4 helicoil tap drill is around 5mm.

Has anyone tried this? Does it reasonable?

JHG

 

[Heckler]

Is there anyway of using nut plates on the rear of the panel? I would consider using a MS51830-105 a key-locked threaded insert since you have no space constraints. I'm not sure you can modify this without making chips. Is this a depo level modification or are you trying to mod this in plane?

Best Regards,

Heckler
Sr. Mechanical Engineer
SW2005 SP 5.0 & Pro/E 2001
Dell Precision 370
P4 3.6 GHz, 1GB RAM
XP Pro SP2.0
NVIDIA Quadro FX 1400
o
_`\(,_
(_)/ (_)

Never argue with an idiot. They'll bring you down to their level and beat you with experience every time.

 

[drawoh]

Heckler,

Thanks.

I am attaching PEM nuts to the rear of the panel to hold my equipment. My panel is attached to tapped holes in the end faces of aluminium plates. I don't see an opportunity to use nut plates or other such inserts. Even a right-angle bracket would be a challenge.

JHG

 

[strokersix]

Look into using thread forming screws or thread forming taps. Watch the hole size, they are more sensitive than cutting taps. You might be able to do this without creating any chips. Can you test it on scrap before doing the real thing? Also, if the aluminum is anodized that might complicate the thread forming operation.

 

[dimjim]

What is wrong with the M4 screws that you are using?
Can you go to a higher strength M4 screws. Any other
modification is going to create some micro chips of
aluminum floating around.

 

[dewd0916]

if a nutplate is not possible, perhaps just a nut clip would be a fast and easy solution. requires no additional drilling, so no chips, doesn't enlarge the hole size an slides on easily for ease of replacement at a later date

 

[Tunalover]

I believe you should carefully rethink your approach. Have you proven that the M4 screws are insufficient or are you going on intuition?


Tunalover

 

[drawoh]

tunalover,

I am going on intuition.

Our stuff periodically gets inspected to see that it is safe to install inside aircraft. I have asked the approving engineer what kind of fasteners he likes to see and he said that the structural screws MS27039 (pan head) and MS24694 series (flat head) are highly approved, and that they do not like to see anything smaller than 8-32UNC.

The company is trying to go metric, but I use the structural screws whenever I can get away with it. An M4 screw is approximately equivalent to a similar 8-32UNC screws. More of the M4 screws would be a good solution if I did not have to drill more holes.

The part I am attaching sticks out a lot, and inevitably will be used as a handle to lift the entire system.

JHG

 

[MikeHalloran]

Bigger screws? More screws? How many Gs are you required to design for? How much load can the subpanels accept?

Ex: Six x M4 screws should be sufficient to lift a small car.

Run the numbers.



Mike Halloran
Pembroke Pines, FL, USA

 

[dimjim]

What is the male screw size of the heli-coil?
Can you use that thread for the screw and apply
loctite to the screw threads?
Just brainstorming.

 

[Heckler]

If this plane falls under FAA requirements you will need to consult FAR 25 Airworthiness Standard. I've done rack and component design for a certain plane that fell under FAA requirements. The FAA - DER requested that I do a full crash safety analysis of all the critical interfaces which included mechanical testing. The analysis was IAW FAA/FAR Section 25.561 General Emergency Landing Conditions. Here is a website with some general information.

http://www.flightsimaviation.com/data/FARS/part_25-561.html

Best Regards,

Heckler
Sr. Mechanical Engineer
SW2005 SP 5.0 & Pro/E 2001
Dell Precision 370
P4 3.6 GHz, 1GB RAM
XP Pro SP2.0
NVIDIA Quadro FX 1400
o
_`\(,_
(_)/ (_)

Never argue with an idiot. They'll bring you down to their level and beat you with experience every time.

 

[drawoh]

Heckler,

We have rarely had detailed calculations done on our stuff. Most of our stuff is optics. By the time you have a structure that is rigid enough to keep optics aligned under vibration in an aircraft, you are well beyond what you need to survive a crash. I have had at least one certifying engineer come in and state that a structure I had designed was safe "by observation".

This is someting I did not design originally, and we are doing stuff to it no one had originally envisioned. It looks like I will have to follow MikeHalloran's suggestion and crunch the numbers.

JHG

 

[drawoh]

dimjim,

Thanks.

The male screw size of a helicoil is the male screw size of a helicoil. It does not resemble any other fastener. You might be able to screw a 10-32UNF screw into the tapped hole for a 6-32UNC helicoil, but I cannot imagine that this would be good practice.

JHG

 

[dimjim]

I was hoping it was a standard metric bolt size.
That would have made it too easy like going
from a M4 x 0.7 to a M5 x 0.7 pitch thread.
This is equivalent to 36.2857 threads per inch
and a #8-36 would only be marginally better
than the M4 x 0.7
Looks like there is not a practical way other
than redrilling and retapping.

 

[mighoser]

See ASME B18.3.1 These are grade 12.9 bolts which are one of the highest strength grades avaiable. Another option is to make hardware from even higher strength material $$$. Aeromet 100 or Custom 465 is probably all you can do for increasing bolt strength.

 

[drawoh]

mighoser,

I am not a Professional Engineer. I asked the PEng who was approving my stuff for use in aircraft, what kind of screws he likes to see, and he said he approves of the MS24694 and the MS27039 structural screws. It always pays to agree with approving engineers, especially when they work for our customers.

I did a search on the internet to figure out what Europeans do when they want approved screws on critical applications like aircraft. From what I can figure out, Airbus has its own set of fastener specifications, and they use some English hardware.

Tunalover probably is right that the M4 screws are strong enough. Meeting a standard that an approving engineer recognizes is more important than being high strength.

I have ordered grade 12.9 metric screws, opened the box and found grade 8.8. I don't know if our production people would notice this. I have been pretty successful with the MS structural screws.

JHG