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[jee7s]

Specifically addressing this...

Originally Posted by bardd

...Pem nuts. From your posts and the pictures, they seem pretty permanent and hard to remove, and we usually find ourselves making significant changes around day 20...
How easy (or hard) is it to make changes when using this?
How much planning ahead do you need to use them correctly?

Making changes isn't all that easy, particularly if you're off by less than the diameter of the PEM fastener. When the error is that small, you can't make an appropriate hole to get the fastener to clinch.

So, yes, a PEM nut is pretty permanent. Using them requires planning. But, spending a respectable amount of time in Inventor (or SolidWorks, etc) getting all the constraints and tolerances correct is really all you need. And, if you have to make a change on the fly, it's not much more work than drilling a hole for a nut-bolt connection. You just have to make sure you get it right the first time.

Keep in mind that it's not an all-or-nothing decision. You'll use the same bolts in the PEM nut as you would in a regular hex nut. And, you can always use the hex nuts in some places and PEM nuts in others (just make sure you have the corrrect size hole in the correct location). I'd recommend that you start with PEM applications in good, static, well defined things, like chassis construction before moving on to the potentially more complicated parts.