Yes, you can fabricate new components from COTS parts and raw materials during the access periods and put them on the robot and it would not count against the 45 lb withholding limit, per R18:
Quote:
At an Event, Teams may have access to a static set of FABRICATED ITEMS that shall not exceed 45 lbs. Items made at an Event do not count towards this weight limit.
For Teams attending 2-Day Events, these FABRICATED ITEMS may be used during the Robot Access Period and/or brought to the Event, but the total weight may not exceed 45 lbs. FABRICATED ITEMS constructed during the Robot Access Period and bagged with the ROBOT are exempt from this limit.
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Emphasis mine. The part right before the emphasis clearly spells out the answer to the OP's question, as well.