Our lives are based on trust. When we cross a bridge, we trust that the civil engineers and the construction workers did a good job. The building contractor must trust that the concrete and rebar steel grades delivered are as specified. The concrete subcontractors must trust their cement suppliers who in turn must trust their workers in their factories. When you drive your car, you entrust your life to the belief that neither steering nor brakes will fail - always assuming that you can trust the engineers who designed the car in the first place and your workshop who does the servicing. When you undergo surgery, you put your life in the hands of a surgeon. Who in turn must trust his or her clinical equipment, and the medical staff who must be able to use it correctly. The thing is that, in all these cases and indeed almost everywhere in life, much of that work relies on the technical translation that goes on behind the scenes, and so we need to be able to trust translators and their technical translations.
In order to translate a document, the translator should understand it first. Unfortunately, in reality this is not always the case. Translating technical documents requires pertinent technical knowledge. To translate a service manual for a DOHC engine, for example, a translator needs to understand not only on the linguistic surface, but also the ideas behind the language: in other words, the translator should understand how the engine works. It helps if the translator is an engineer, in addition to having a training and a degree as a technical translator - though a double degree would be over-engineering, as a translator is not expected to design the engine.
Nobody knows everything, not even in engineering. Nobody is an “expert in engineering”. Even somebody with a degree in mechanical engineering would be reluctant to call themselves an “expert in mechanical engineering”. The subdomain of combustion engines alone is vast. So an engineer might rather say he or she is an expert in jet turbines or in SI engines or in diesel engines.
Of course, you can study hard and learn almost anything, which is why my teaching program has always included automotive workshops where translation students disassemble or assemble a real car engine. But this takes time we don't really have in real life. Translators with a university degree have been trained to use their research competence to efficiently acquire the technical knowledge they need for a given translation project. There are limits, however, and such learning “on the job” of course impedes the translation process and might impair the quality.
To save your and my time, to spare us both any frustration, and to avoid unnecessary expense, I do not accept translation projects which are outside my sphere of competence.
That means, before I accept a commission, I must have and see the complete source material. Only in isolated cases I might make an exception to this rule.
If you get the impression that I am not desperately keen on getting more translation jobs - you are right. My income and my survival do not depend on translation. For me, technical translation is both a profession and a passion, and I am in the lucky position of being able to decline projects that are not guaranteed to make you and I happy.