I feel like engineering isn't conducive to side hustles. That being said, I know some people that have small 3D printing or similar jobs on the side where they use some of their industrial knowledge.

One problem is the high fixed cost of insurance, software etc. Unless you are sure you do an 1,000 hours or so, it really is too high a cost since you pay the full price regardless if you work 1 hour a year, or 2,0000 hours.

Also depends on what type of engineering you do or clients you have. Do they expect you are available to come to a job site if there is a problem?

Get a time machine and go to school for software engineering

I am having a problem visualizing an engineer with "side hustle". I keep seeing an unwilling person on a park bench being accosted by some structural hustler saying "hey dude, need a beam sized? Sure you do man, everybody needs a beam sized! Wattcha want, cantilever, simple span, I go them all."

I know some who do drafting or house plans as what we call "diversified employment".

In the past I have done short manufacturing runs of my own products as well as a ton of general job shop fabrication and machine work in my home shop. In recent years however I have been focused more on home renovation as my wife and I have developed this habit of buying a dumpy house, renovating, then moving and selling for a profit.

I did have one, and now it's my primary employment. One of the biggest problems with a "side hustle," "side gig," etc. is that most engineers with the knowledge to do it are already doing it for a company with no moonlighting clauses in their employee manuals/contracts. I found myself in a fortuitous situation - I was doing in house work for an industrial outfit after working for several years at a consulting/design firm. So I had the knowledge and experience to do the work, and an employer who didn't care if I did it (I wasn't competing with them, and there was no legal avenue for my clients to go after them).

I've seen engineers doing calculations for small precast manufacturers that I suspect was side work (mostly because I ran into them at their regular jobs). Ditto for anchoring calculations for equipment. We require these to be signed and sealed. But the manufacturers don't keep a structural engineer on staff, so they go outside.
I'm lucky enough to be paid (straight time) for extra hours I bill. So why would I risk my primary employment to make some small money ($500 a calculation?), and go through all the hassle? I'll work extra here.

My side hustle was teaching engineering as an adjunct professor at several universities. I was able to add the practice of engineering to the theory I taught; that was really fun and very beneficial for my students when it came time for job hunting. Then I reversed roles and became a full-time professor and worked as a consulting engineer for my side hustle. I burned out teaching and went back to engineering (they hired two faculty to replace me; I reduced my weekly working hours by 50% and got a 50% pay raise at the same time), and I resumed my side hustle as an adjunct faculty member, but only as a senior/graduate project advisor, not teaching courses. Finally, I took a break from teaching and now I'm only working as an engineer. I'll probably resume my side hustle teaching one class a term at some point when I feel my burnout is gone.

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I once had a side hustle. but it was as an employee of that firm. that way I didn't need to worry about insurance etc. I just got paid for the hours they had for me.
My main employer knew about it and was OK.

That kind of ended when they wanted to hire me full time and I decided to stay with my then (and still current) employer.

With being an employee, you overcome the problem with fixed cost I mentioned above.

Obviously make sure your main employer is OK.

I got myself a business number between my last job and my current one, because I wanted to have the option of working as a consultant to others on a gig basis until I found a new full time job. That was nearly 25 years ago. I told my employer about my side consulting business from the beginning, as I was hired on contract at the outset. I have had several occasions when a client at work asked us to do work which was outside our area of interest but related to mine from past employment. I carried out the work privately for the client with my employer's knowledge and blessing.

There is nothing wrong with doing something else for money in addition to being an employee, as long as you are not in conflict with your employer- and you're not hiding anything. My first conversation with any potential side client is, "I'm employed by X, and any work related to X's practice which is areas Y and Z, I can't do for you- it will be handled by X Inc." Of course some employment contracts attempt to restrict any secondary employment including self-employment. I am unburdened by such an agreement. Doing work in contravention of an employment contract is pretty much the definition of "conflict with your employer".

You also need to keep your need for insurance and/or voluntary liability limitation in mind. If you're paranoid about liability, you need to set up protections against that which will mean you will need to really do quite a bit of work just to cover the fixed costs of your side "business". Most full-time employees find that to be a deal-killer right there.

There were times when I did no outside work, because the main job and the rest of my life left me no time. Once the kids got older, it was possible to pick up little bits of work on the basis of relationships and interest. That will continue for me into retirement.

I attempted a side hustle as a "freelance" mechanical engineer when I had too few years under my belt. I only had a few fruitful gigs, and they all ended disastrously. One ran out of money partway through the project, prior to a large deliverable, and wanted to back out of the contract. Another revealed that they had expected me to derive manufacturing tolerances from a competitor's product, using a single example. They also became very angry when I would not sign off on ideas I knew would not work. Others could not wrap their head around the idea that my time was worth much more than $20/hr.

All in all, I'd much rather deliver pizzas after 8-10 hours at the office.

I did almost the opposite: Was working 3D CAD and QA/CAD program testing for a company for 15 years, but their business model was very poorly managed transitioning from large pre-PC computers to the desktop/WIndows/PC environment. Began working steel gates and stair rails and swept spiral stairways and handrails, welding and ornamental iron and repairs. Learned welding and bending and light machining - which translated into power plant field repair engineering/supervision/project management since I had "real experience" in field fabrication and work.

But, as a side business, I was too slow. Simply didn't make enough feet steel in a short enough time to make a sufficient profit as my own boss/welder/fabricator. SO no. Not everybody is able to make enough in a side business to replace a office job - even if you are able to avoid the conflict of interests between both jobs.