[tvb12]

Machining metal parts is pretty loud. How was any of this done in secret?

[defrost]

Camp cooperation - time it with choir practice and|or any other loud activities, cheering during staged boxing matches, use lookouts to warn when guards get close, work under beds with blankets, sheets thrown over to drown noise, etc.

[Doxin]

Using a lathe isn't super loud unless you're doing it poorly or have some horrendous interrupted cut.