Stress riser

[Darick]

I've only seen one quantifiable explanation of a stress riser and that was in the AC43-13 on acceptable repair methods in regards to nicks on propellers. I'd have to review that again but for what I remember it doesn't address typical cutting marks produced by saws, files and whatever. Other than that, I've only heard very general things like "if you can catch your fingernail on it" which I find to be way too inadequate for something my life depends on.

Does anyone have a better guide for what is and what isn't a stress riser...possibly related by something as simple as sand paper grit? Example...60 grit produces an unacceptable roughness or the surface finish produced by 320 grit is acceptable? If we really want to get technical, a profilometer standard in RMS units would be dandy.

[Bryan Cotton]

Darick,
In the helicopter world we specified an RMS finish for critical machined parts - but never for sheetmetal. Smooth to the touch is what the workers shoot for and Q/A will inspect to. I would be ok edge finishing with 220 grit but I prefer the red scotch brite. You can polish up edges nicely with it. My grey scotch brite wheel on my bench grinder is a miracle for straight parts. The little grey rolloc type pads are good too.

In general I would say the edges cut in the kit are too rough and need polishing via your favorite method. The machined angle parts have a great finish and I have left those edges alone, though I am a nut.

The fact that there is no RMS requirement in AC43.13 should be taken as evidence that it is not required. I suspect your standards are higher than the average OEM, which is not a bad thing.

[Darick]

Thanks Bryan. That's good news in that I know my airplane will never have a failure due to a stress riser. It's bad news in that it is taking me too long to build, so perhaps I'll loosen my building tolerances just a little bit...no more F9 on every part....JUST KIDDING!

[SvingenB]

I use a good file, similar to a vixen file but much finer, on all straight lines of sheet metal. A couple of runs and it is as smooth at it gets. I use a vixen file for all thick and straight parts followed by the finer file. A scotchbrite wheel is also essential on most thick parts.

The point is to get it smooth to touch. Saws and scissors does not produce smooth to touch edges. If you have lots of nick marks from a file, then you are using the file wrong.

Smooth to touch = no stress risers.

[Ronsip]

A great resource is the Hints for Homebuilders EAA page. http://www.eaavideo.org/ Search the videos for stress riser.

[Rynoth]

My standards are:

1) Grey(fine) scotch brite wheel finish on any part that I can get to the wheel.
2) Maroon scotch brite hand pad on anything else.

There are 2 products I've found that have been immensely helpful in making quick work of most deburring and finishing jobs (excluding rivet holes), by bridging the gap between cutting/filing and fine finishing.

1) A coarse scotch brite wheel like this: http://solutions.3m.com/wps/portal/3M/en_US/3MIndustrial/Abrasives/Products/~/Scotch-Brite-EXL-Unitized-Wheel?N=7581702+3294944821&rt=rud

I got mine new off ebay for $30 (ACS doesn't carry them. Here's a link: http://www.ebay.com/itm/1-Scotch-Brite-6A-CRS-EXL-Tan-Unitized-Wheel-6X3-4X1-2-/320755717762) It practically eliminates the need for filing on any part that can reach the wheel. I used it in place of a vixen file for nearly all of my fabricated (band-saw cut) angle components. It eliminates stress risers and preps the part perfectly for a quick final pass on the fine scotch brite wheel.

2) A heavy duty scotch brite hand pad like this: http://www.aircraftspruce.com/catalog/cspages/3mscotcheavydutyhandpad.php?clickkey=95717

This makes short work of the edges of sheet metal, and bridges the gap between a filed edge and the maroon scotch brite pad. Does a great job helping to clean up any cuts I've made in sheet aluminum.


Also, these small coarse 2" hook/loop scotch brite discs (in a die grinder) have been quite handy for hitting hard-to-reach spots that a file couldn't quite reach. They can remove material at a pretty good rate while providing a smooth edge, much like the coarse scotch brite wheel above. https://www.grainger.com/product/3M-Hook-Loop-Disc-4ZR51?searchQuery=4zr51

[Ronsip]

Hi Ryan, which (part #) grey fine scotch brite wheel are you using? Thanks

[Rynoth]

Hi Ryan, which (part #) grey fine scotch brite wheel are you using? Thanks

I'm using this one from ACS: https://www.aircraftspruce.com/catalog/topages/scotchdeburrwheel.php

The 2 part numbers are for 1" or 1/2" diameter center mounting hole.

This wheel turns aluminum into buttery-soft goodness.

[Ronsip]

Hi Ryan, which (part #) grey fine scotch brite wheel are you using? Thanks

I'm using this one from ACS: https://www.aircraftspruce.com/catalog/topages/scotchdeburrwheel.php

The 2 part numbers are for 1" or 1/2" diameter center mounting hole.

This wheel turns aluminum into buttery-soft goodness.

Cool Thanks

[Bryan Cotton]

Worth every penny!