Can I bring an 18x6x6 inch Metal Box on Flight?

This is more of a relevant anecdote that might give you an idea. I can only say my trip went smoothly and in the future I think I should engage TSA in advance and not rely on luck.

I flew on several international and domestic flights (Chicago-Taipei-Manila-Tacloban-Butuan) in early 2020 with the pictured case which contained 6 Rasberry PI Zeros and electronic parts (no batteries), the purpose being a 2-week Linux+C+circuits session for elementary students in a rural area. It filled one compartment of my carryon backpack and measures about 9x13x2 inches. More electronic parts were in my checked luggage. It never occurred to me that I might have trouble, but didn't get any, and didn't even remove it from the backpack during the many screenings I had to go through. This is a soft case and not a metal one, and I'm sure all those wires showed up bright and clear on the monitors. Your handmade metal box will certainly be more suspect (and opaque to the xrays) and require some explaining.

The screwdriver would have to go in your checked bag if it were longer than 7 inches, but under that size it's allowed. Make sure you have a bit of margin...

The metal box shouldn't be an issue, as long as they can see inside it and any compartments, and it has no sharp edges (including if you disassemble it). Since it will probably not be very friendly to scanners, they will most certainly have to inspect it manually. The risk they take it for a bomb is extremely low.

The size of the box is well within the usual sizes allowed for hand luggage, but of course you probably will have other stuff with you, as well as packaging to protect it, make sure the overall dimensions fit the limits set by your airline (they vary slightly from airline to airline).

Don't forget, though, that if your hand luggage it on the larger end of the allowance, it may be "gate-checked" (i.e. put in the hold) in at least the following cases:

• the flight is very full and they are short on space in the overhead bins or they expect to be
• the flight is operated with a smaller aircraft (usually regional jets or turboprops) which have very very limited overhead bin space.

Having hand luggage that is under the dimensions to "fit under the seat in front of you" usually guarantees you'll be able to keep it, provided you're not on the first row of any section or the emergency exit rows.

This assumes that you carry on meets the dimensional limits of your airline.

I'm certain that TSA agents have pretty well seen everything when it comes to people hand carrying electronic parts and systems. However, there seems to be no mention of miscellaneous electronics on the TSA's What can I bring site

But the TSA has this helpful advice:

For items not listed here, simply snap a picture or send a question to AskTSA on Facebook Messenger or Twitter. We look forward to answering your questions, from 8 a.m. to 10 p.m. ET weekdays; 9 a.m. to 7 p.m. weekends/holidays.

I would do that as a first step anyway.

The TSA also says

Planning ahead and packing properly can facilitate the screening process and ease your travel experience at the airport. Know what you can pack in your carry-on and checked baggage before arriving at the airport by reviewing the lists below. Even if an item is generally permitted, it may be subject to additional screening or not allowed through the checkpoint if it triggers an alarm during the screening process, appears to have been tampered with, or poses other security concerns.

So even if you get an approval from their customer support, I would still expect that your carry on will be subjected to a secondary screening, and that you should definitely arrive earlier at the airport than usual in order to allow for the extra time.

As for worrying about being a bomb, the TSA has access to chemical analysis equipment and can easily take a swab and process it in a matter of minutes. So unless you work in a chemical/explosives factory you shouldn't be worried about that aspect.

Amusing anecdote. A colleague of my brother in the UK did work with various chemicals and once took his work laptop on a flight one day. It had so much chemical residue on it that it pegged the measuring device. The security staff were confused by this and came to the conclusion that the machine was broken, and let him pass. He of course kept his mouth shut as to why the measuring machine was acting strangely.