Free Play of Wooden Screws

This post is in response to Gary’s comment on the November 12 post “Using the Tap”. Yes, an amount of “Free Play” or “Looseness” is important when making wooden screws.

One advantage of the thread making method covered in these posts is that the fit of the screw to the threaded hole is adjustable. A loose fit will prevent the problem of presses that become ‘stuck’ when the weather changes. Using large threads makes it possible for the threads to be quite loose. As seen above the screw can ‘wiggle’ in the hole. The hole is drilled at exactly 90 degrees; the screw can wiggle a degree or two in any direction.

The loose fit also allows the screws of the press to be adjusted independently (to a limited degree). This, I believe, is the issue that Gary was interested in.

This is a 4 TPI screw showing how it fits the internal threads. One might wonder if this loose fit compromises the strength. In my experience it does not…

I attach these two photos to illustrate this. As an experiment I tried to break a thread. I was curious to know what would fail first. In this case the handle split where the screwdriver was used as a lever. I tried again after putting on the hose clamps and was unable to break anything. The friction increased so much that the screw could not be tightened further, even with the use of the two plastic washers that I used in an attempt to reduce the friction. If this were done with two screws in a real press the amount of force generated by this extreme tightening would likely be ‘unhealthy’ for any book that was clamped there.

This shows how the handle split.

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