You had mentioned in your answer to the question as to why Paul shaved his head in Cenchrea recorded in Acts 18:18, that it was because of the Nazarite Vow (Numbers 6:2-21)? The Nazarite Vow does require that "no razor touch your head" (Numbers 6:5). How do you explain this?
"All the days of his vow of separation no razor shall pass over his head. He shall be holy until the days are fulfilled for which he separated himself to the LORD; he shall let the locks of hair on his head grow long" (Numbers 6:5).
During the time a person is under a Nazirite vow, he is not to cut his hair. Most Nazirite vows were limited in their length. People under lifelong vows were rare, such as Samson or John the Baptist. Historically we know the typical vow lasted about a month.
At the end of the vow, a part of the requirements was to cut off the hair that grew during the vow and offer it to God.
"Now this is the law of the Nazirite when the days of his separation are fulfilled, he shall bring the offering to the doorway of the tent of meeting. ... The Nazirite shall then shave his dedicated head of hair at the doorway of the tent of meeting, and take the dedicated hair of his head and put it on the fire which is under the sacrifice of peace offerings" (Numbers 6:13,18).
The mention of Paul cutting his hair because of a vow he was keeping (Acts 18:18) would indicate that he was keeping a Nazirite vow and had completed it when he was in Cenchrea.