Rock Maple Soft Fixture for Boring Thin Parts

**Paul Jones** · Dec 31, 2014, 01:13 PM

The cylinder head for a small compound steam engine required thin cylinder liners made from silicon bronze. This operation required machining the low and high pressure cylinder liners to 1.250" and 1.500” OD respectively and then boring the liners to final ID. The low pressure and high press cylinder liners will be press fit into the compound steam engine cylinder block with an interference fit just under 0.0005” requiring close tolerances.

Using rock maple, I created a faceplate fixture for holding the cylinder liners for boring. The fixture requires the bronze cylinder OD and ID to be concentric and to grip the part uniformly without distortion. I first cut the rock maple fixture on my table saw and also cut a centered slot for part clamping. Boring the fixture to 1.501” using a face plate with clamping straps (1/8" thick x1/2" wide and held with M6X1 machine screws) ensures the concentric OD alignment of the bronze parts when boring the ID. I also bored a 0.25” deep by 0.1 wide bottom step in the rock maple fixture for holding the bronze part a safe distance away from the faceplate (and also bored additional bottom-hole clearance at the center of the fixture for boring tool bottom relief). Note, a temporary thin piece of wood added to the center slot allowed rough boring of the fixture and then removed, and the fixture final bored to size for clamping the bronze cylinder. The 1/4-20 machine screw is for clamping the part and easier removal.

In case you are wondering, I made another slotted “cylindrical liner” from aluminum for holding the smaller diameter high pressure cylinder liner in the larger diameter rock maple fixture. I plan to reuse the fixture for building more of these engines but could have machined the smaller liner first and then re-bored the fixture for the larger liner for a one-time operation.