Rosson offers advice on tracking your tension culprit.
Frame dimensions: Do the previous concerns mean that wood is an unsatisfactory material for constructing frames? No, they do not. But the problem with many wood frames is they are built of wood that is not sized appropriately for the job. Frames traditionally are built with typical lumber-yard stock, sold as two-by-fours, two-by-twos, etc. However, these dimensional labels are really used as a matter of convenience rather than to accurately define the lumber's true width and depth. For example, lumber that we call "two-by-two" actually measures 11/2 x 11/2 in.
In my experience, I have found that a 23 x 31-in. frame constructed with 11/2 x 11/2-in. wood can hold screen tensions in the neighborhood of 35 N/cm, provided that the frames are kept dry and at a consistent room temperature. The frames' capacity to maintain this tension level will decrease substantially if the frames are wet or too warm.
The best wood frames are engineered rather than just built of convenient materials. While 11/2 x 11/2-in. boards are adequate for constructing 23 x 31-in. manual press frames, they are out of the question for building automatic press frames of 23 x 31 in. or for 25 x 36-in. frames (manual or automatic). These frames should have sides that are a minimum of 11/2 in. deep by at least 21/8 in. wide. Such frames have more than twice the strength of frames with a conventional 11/2 x 11/2 -in. cross section and are less likely to bend inward when tensioned mesh is affixed.
Screen attachment and deflection
Some factors that can bring about tension loss are not reserved for wood frames alone. Although metal frame types, including rigid and retensionable frames, do provide a more stable platform for the screen, they can also present opportunities for reduced tension. The two sections that follow detail a few things to keep in mind regardless of the frame system you're working with.
Screen attachment: An obvious step you can take to reduce tension loss is to make sure the screen fabric is fixed securely to the frame. Wood frames offer several fabric attachment possibilities. One approach is the rope and groove method, which is as antiquated as a buggy whip and not worthy of much comment.
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