Questions about making split blocks

While waiting for my mirror cell for my 14.5" f4.3 mirror to be finished I'm moving ahead and want to fabricate the split blocks for my mirrorbox. The truss scopes I've seen that have split blocks fasten them to the mirrorbox like the book recommends, with four drywall screws countersunk into the split block. My mirrorbox is going to be made from 1/2" thick plywood and I don't think the screws will have enough wood to grab them tight enough. Has anyone attached them with the screws driven from the inside of the mirrorbox into the split blocks? Or should I just drill through the holes and use machine screws with a nut and washer inside the mirrorbox?

A friend of mine has an angle jig so instead of shimming the blocks so the truss tubes meet the UTA I'd like to drill the holes at a slight angle. If anyone has make a similar sized scope what size angle did you use? The book says 1.5 to 2 degrees.

Any advice would be appreciated.

If at all possible, drill through the plywood into the blocks. You're right saying the ply wouldn't hold the screws, but the wood in the blocks should, depending on the wood type. Oak and Maple would hold the screws really tight, but soft woods(pine,cedar), they would pull out, so IMO -- go with hardwoods.

I have just completed a 16" (first light anticipated this Saturday, more info to follow) using the drywall screws to hold the split blocks to 1/2" Apple Ply Plywood as directed in the Kreig/Barry dob book. I too was concerned about holding power so I selected drywall screws that were 1/4 inch too long and cut the tips off with bolt cutters prior to installation. I thought this would allow more threaded surface area to bite into the wood and reduces the amount of taper that seemed to me more likely to tear out under load.

Another thing to consider is that there is a significant bolt that will go all the way through the split block and plywood that is used to tighten the truss tube in place. So the only time the drywall screws are used as support is when the knob/bolt combination is not tightened. I have found that with solid altitude bearings and no handles cut into the bearings or box, that the split blocks are the best handholds for carrying the mirror box. In my case the mirror box weighs 65 pounds, so it is advisable that if, like me, you are using the blocks as handles when lifting, that you tighten the knobs first; even if there are no truss tubes in the slots.

That said, mine have been quite solid even when lifting without tightening the bolt first. Probably because the stress on the screws is attempting to shear the screws, not pull them out. However, I would not recommend this if your 1/2" plywood has a softwood core as is typical of what is available at a standard hardware store. I have had these screws strip out on a previous scope using an oak veneered softwood plywood, but it still works because the bolt holds everything in place.

If you decide to use my modified drywall screw method I recommend smoothing the ragged edge of the clipped screw with a file and applying some paraffin to minimize tearing of wood fibers when installing the screws. And don't forget to pre-drill the screw holes, but no wider than the shaft of the threaded part of the screw.

Mark Eschbacher