QUESTION: Fix punctuation: Place the first and last 15 cm on top of each other so that the yarn is the normal thickness and continue to work this is done to make the transition invisible
ANS: Place the first and last 15 cm on top of each other so that the yarn is the normal thickness and continue to work this is done to make the transition invisible.

QUESTION: Fix punctuation: This reasoning is so simple and at first glance so irrefutable that for a quarter of a century it has rendered excellent service to the modern labour movement as a logical weapon against the anarchist phantom and as a means of carrying out the idea of political struggle to the widest circles of the workers
ANS: This reasoning is so simple and at first glance so irrefutable that, for a quarter of a century, it has rendered excellent service to the modern labour movement as a logical weapon against the anarchist phantom and as a means of carrying out the idea of political struggle to the widest circles of the workers.

QUESTION: Fix punctuation: possible use sunscreen to protect the entire face and the neck
ANS: ...possible, use sunscreen to protect the entire face and the neck.

QUESTION: Fix punctuation: During sintering the silicates dissociate thermally
ANS:
During sintering, the silicates dissociate thermally.