Came close to destroying a spar with the hammer when it slipped off the C-frame set. I bent the spar flange a little but was easily fixed. Whew! Turns out the anvil base screws had backed out on my new Avery unit and the loose set up became unstable. Lesson learned: check those anvil bolts before each use. Also, lay a sheet of plywood or similar across the spar to prevent bashing your parts!