Plate thickness for tapped hole. 5

[okgp]

Hello,

I have a scenario where one plate is parallel to flat face of the bolt and the tapped plate is perpendicular to the flat face of the bolt. Can anyone tell me the rule of thumb for plate thickness for tapped holes? Material is 6061 Aluminum alloy. Bolt is flat head countersink. picture is attached so you understand me.

 

[rb1957]

I hope you're not planning on tapping 6061 (T6?) for your steel(?) fastener. Typical practice is to use an insert (like Heli-coil) which will require a larger hole. And your tension allowable will be further reduced (by the ST direction).

A "barrel nut" might be a "better" option.

As for thickness, my "rule of thumb" would be between 2D and 3D depending on the tension applied.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[okgp]

thanks for replying. I am using stainless fasteners 304/316. it's not too critical, in this application it will be fine. But ok 2D/3D depending on tension. Do you have a guide to figuring this out. like doing the math?

 

[3DDave]

The problem isn't the fasteners - it's the mismatch in strength. Heliooils increase the shear area for the threads in the aluminum without requiring a larger fastener, which would just result in a different amount of mismatch. It's also cheap insurance against cross-threading. Doing it in the shop is much easier than using it as a fix on a job site.

 

[BrianE22]

Stainless into Aluminum could be a problem. Check to make sure those two particular materials aren't sensitive to dissimilar material corrosion.

 

[rb1957]

yes, you have really strong screws in really weak plate. Presumably there's no issue with SS in 6061 ? If not much tension then 2D (no math !!??, just "rule of thumb"). I'd be really worried about the Al thread ... how will you repair a striped hole ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[hydtools]

Ted

 

[dik]

Helicoil... that's a simple question.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[drawoh]

rb1957,

The last time I ran into this problem, my choice was 6[‑]32 with helicoils or 8[‑]32 without helicoils. The structure was permanent, so I went for the 8[‑]32. My plate was[ ]1/4". I just eyeballed everything. On the optical system I was working on, the structural requirement was rigidity. If a structure is rigid enough to keep optics aligned in a helicopter, it will be strong as all hell.

On a similar structure I worked on, the Designated Airworthiness Representative (DAR) did not like tapped holes in the edge of the plate at all. He strongly preferred angles on the side, with bolts and nuts.

okgp,

How structurally critical is this thing?

Flat head bolts into tapped holes is poor design practice. You need some slop to allow for positional tolerances.

--
JHG

 

[rb1957]

yeah, I recently ran into holes tapped into 6061T6. It was a vendor item, I only found it when Design "suddenly" removed the nuts from these bolts. "WTF?" Design happily said "yes, no nuts" Eventually I got the vendor's story ... yes, helicoils, eventually I got the hole size they used (smaller than typical for helicoil) and eventually worked a tension allowable. sigh !?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[okgp]

i can't go with helicoil , constrained on my thickness of plate and bolt size. And we generally use alot of stainless bolts on aluminum parts here and haven't seen much in the way of corrosion with that. will have to be careful when tensioning bolts but thats the only real issue i see here. thanks for the feedback. My first time on this forum and already ive learned alot!

 

[okgp]

im designing a simple bracket to replace the spindle motor of the cnc router. its being repurposed for a chemical testing lab so its a simple enough fixture but i have to use aluminum so the weight of my mount does not exceed the weight of the spindle motor assembly.

 

[rb1957]

If weight is that critical to you, are you machining the plate ? (you only need the thickness near the pins.)

Is welding unacceptable ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[okgp]

welding not really because everything on this cnc router is bolt on design. want to keep it that way. I don't think its a big deal because the max weight is 200 grams plus whatever the bracket weighs which isn't much.

 

[drawoh]

okgp,

I spent thirty five years at an optics company driving stainless steel screws into aluminium, usually but not always (as noted above) with helicoil inserts. We had no corrosion problems.

When I moved to another electronics firm, I was told that stainless steel screws into aluminium were awful, and that they preferred zinc plated carbon steel. The primary problem was galling. Both stainless steel and aluminium are bad for this. My old optics firm used hand drivers to install screws. The electronics company used power tools.

--
JHG

 

[desertfox]

Hi okgp

If I am reading your sketch correctly you should have 1D (bolt diameter) from hole centre to any edge on the plate.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[dvd]

I'd recommend at least as wide as the across-the-flats width of a nut for your particular thread size, and use 6061-T6. Depending on what you are doing, and loading direction, maybe you could mill a mortise groove into the plate to reduce shear...

 

[CWB1]

Rules of thumb are a great way of telling what engineers should be fired for-cause asap, if they rely on any to make design decisions then the answer is yes. I'd highly recommend reading John Bickford's "Intro to the Design & Behavior of Bolted Joints" if you haven't, followed by a proper bolted joint analysis to determine the minimum number of threads to prevent both thread tearout and yielding under the head of the bolt. As to threaded inserts, they're very rarely needed, add significant cost, and create their own set of issues. Typically they're only used on very thin parts and it appears you have plenty of thickness for threads so I wouldn't even consider them.

 

[Tmoose]

Hi CWB1,

Have you had an instance when 2D thread engagement was insufficient for aluminum, titanium, or ferrous metallic structural materials ?

thanks,

Dan T

 

[okgp]

CWB1, a bit harsh i would say. I know how to do a bolted joint analysis... a little context, im a recent graduate (april 2022) and this project is too small for something like that. That's why a rule of thumb is practical here. Thanks for the book however, i got a pdf copy already.