A chuck holder for use in block trimming prior to ultramicrotomy on an ultracut E ultramicrotome. It comprises 3 prints; a main block, a collar with tang and a locking lever-handle. A cap head machine screw with the handle fitted onto its cap pushes against the tang on the collar to grip the chuck in the block sufficiently to allow block trimming. Rotation of the screw is limited by 2 metal posts which stop the handle rotating. Even if printed in PLA, it is robust enough for the job (lasted several years so far). This can easily be adapted for other ultramicrotome chucks by adjusting the internal diameter of the collar.