Here's how you can effectively deliver constructive criticism to your team members as a mechanical engineer.

Before delivering criticism, take the time to prepare your thoughts and feedback. Clarify the specific behaviors or issues you want to address and gather relevant examples or evidence to support your points. Prepare to communicate your feedback in a clear, objective, and constructive manner.

Try talking with them as well to understand that you know why they have taken the actions they have taken. It might be that they have a different understanding of the challenge, and that their actions make perfect sense given that understanding.

I've found it crucial to choose the right timing when delivering constructive criticism to team members. One time at work, I waited for a calm moment to provide feedback on a project. I believe that, like materials needing optimal conditions for stress testing, people are most receptive when not under high stress. This approach ensures that feedback is well-received and leads to positive outcomes.

Choose an appropriate time to deliver feedback when both you and the team member are calm, focused, and receptive. Avoid delivering criticism in moments of stress or tension, and instead, look for opportunities to provide feedback in a supportive and conducive environment.

Conduct the feedback session in a private setting to maintain confidentiality and create a safe space for open and honest dialogue. Avoid providing criticism in front of other team members, as this can lead to embarrassment or defensiveness.

Focus on delivering feedback that is specific, actionable, and focused on behavior rather than personal attributes. Frame your criticism in a positive and constructive manner, highlighting areas for improvement and offering suggestions or resources to support growth and development.

Follow up with the team member after delivering criticism to check on their progress and offer additional support or guidance if needed. Encourage them to ask questions, seek clarification, and discuss any concerns or challenges they may have encountered.

Foster an open and two-way dialogue with the team member, encouraging them to share their perspective, insights, and ideas for improvement. Listen actively, validate their feelings, and demonstrate empathy and understanding as you work together to address the feedback.

This is most important - build trust from the outset, with objective and transparent intentions in the best interests of the most relevant stakeholder(s). Set expectations of company culture, e.g. design guides, check-lists, contemporary vs traditional techniques, etc. - do this as a committee of team members but not necessarily a democratic exercise. Walk colleagues through optimisations of their designs, give them opportunities to respond and contribute as well, especially in case we've misunderstood their design intent. Once trust is established and maintained, the probability of misunderstandings, miscommunications and misdirected anger are lower, and we can direct more energy towards the problems and solutions.

You should have been working close enough with the other engineer and talking with them regularly enough about design projects that they are aware of things they need to improve. If they are not aware of these things, then you should look at the nature of the teams and the culture at the respective company.