Press fit hex into round hole?

[Hawkeye]

What about this? Drill and tap a hole from the side of the knob for a 10-24 nylon screw. Then drill and tap your 5/16 hole.

Install a nylon screw long enough to extend into the 5/16" hole. This should lock the knob in place. As the nylon screw wears out, tighten or replace.

[Prototyper]

If this is a one off, your press fit nut concept should work fine. This may be obvious, but you should machine the hole with stepped diameters. The nut should be pressed in to a shoulder, preventing the nut from pulling through the part in use.

For a production part, you should take a look at locking helicoils. They have a deformed thread in the middle of the coil which grabs the screw. I work on components for Formula 1 cars, and they use these extensively as they are light weight, re-usable, and very effective under high vibration conditions. They use standard Helicoil taps, and installation tools. Get them from McMaster carr.

[airsmith282]

For aluminum make the hole diameter the same as the dimension across the flats. For locking, use a Stover nut and grind the "corners" so the vertices of the flats are sharp and pound it in. A Stover nut is all metal and has a built in washer on one side so it won't go in too far.

One problem with press fits of steel into aluminum is that if it gets much warmer than the temp at which it was assembled the steel part will fall out.

a stover nut is a one time use nut...

[darryl]

Combining a couple of the suggestions you have a short length of the hole bored large enough to center the nut, then a smaller ID in which to press it. I'd probably go a tad larger than the dimension across the flats for the smaller bore. Part of what might become a problem is if the nut cuts the aluminum as the corners are pressed in. If they do, then the nut will want to fall out when the temperature of the part rises significantly. If the corners can skid into the bore under pressure, the aluminum will become stretched around the nut and a lot more of a temperature rise will be required to make the nut come loose. It is entirely possible that by using a lube you can avoid the first circumstance.

The bolt being inserted will only require as much torquing as the nyloc insert decides. It's not that high, and the inserted nut should be stable and not rip out. If there's going to be side forces against the bolt or threaded rod piece being used, you might bore the hole deeper for the inside end of the bolt or rod to nest into a short way.

[Ian B]

Can you mount the locknut on a piece of threaded rod, turn the hex to a cylinder and thread the outside? Then, thread the hole, loctite & screw the nut in. This will be more secure than a simple press fit, and will resist the pulling forces when in use.

Ian

[Evan]

a stover nut is a one time use nut...

This isn't holding on an aircraft wing.

[Evan]

Part of what might become a problem is if the nut cuts the aluminum as the corners are pressed in. If they do, then the nut will want to fall out when the temperature of the part rises significantly.

It really doesn't make much difference either way. If the nut is pressed in and only distorts the aluminum the aluminum will be at or over the elastic limit. If the temperature goes down the the aluminum will stretch as it tries to contract. The forces involved in linear expansion/contraction are much greater than the elastic limits of any particular material. In fact, they are greater than the absolute tensile limit which is why castings crack when cooled.

Press fits of metals with widely differing Coefficients of Linear Expansion usually do not work well. Aluminum has twice the CLE of steel.