Is it ethical to get suggestions/corrections about my PhD work from a consultant? I am doing a PhD in Mechanical Engineering. I have little experience with CAD programs, and I have to design an assembly in my project for manufacturing. As my supervisor is busy, and also not a specialist in CAD, I am thinking in hiring a CAD consultant of a company. Her tasks are basically to verify my CAD parts and ensure that everything is fine for manufacturing, and, if it is not, to send correction and comments about what to do to fix the parts/assembly and brief tutorial on how to do it. 

My question is: if I do the work myself and only get suggestions/corrections from someone else, can I encounter any problems later, like having my PhD degree revoked?? I don't want to ask help from my supervisor, as I don't want him to spend 20 hours or more with my work. 

Please tell me if I can do it; and if yes, if I need to inform someone, or put something on the thesis regarding it, or anything else.
You need to do two things here:


You should ask your advisor in advance whether this is acceptable.  The answer may depend on the details of your situation (exactly what is involved in this CAD verification and how central it is to your research), and your advisor is one of the few people who can give you a definitive answer.  I'd bet that it will be OK as long as you do the design yourself, but my opinion is not so relevant: if you end up in a disagreement with your advisor, it won't help if you complain that people on the internet assured you it would be OK.
You must acknowledge this assistance in your dissertation and papers, the same as any other contribution to the work.  This is a matter of intellectual honesty.  However, acknowledging the assistance afterwards can't substitute for asking your advisor in advance.  (In particular, your advisor might say "There's no issue of honesty, since you described exactly what you did.  However, you hired someone to complete a substantial part of your thesis, so you have not done enough yourself to graduate.")