a lot more on handheld case screws


My new order of screws arrived today and the results are mixed. The #2 Plastites won't fit in the calculators for a reason that I came to understand last week before the supplier received his shipment. I called him right away with my concerns and told him I would buy what I had promised anyway. This weekend I learned more and emailed him to ask that he look into one more possibility.

The #3 X 5/16" "B" thread sheet metal screws have 28 threads per inch but they are noticeably larger than the calculator screws. I'm going to follow my own advice and try them in some scrap material threaded by the calculator screws.

The good news is that I have learned more about screws for plastic and found two types that might be what HP used. One is obsolete and the other is a variation on the Plastite screw that is still available although it will be up to a distributor to order a minimum quantity if they are not in stock at the manufacturer.

First, the reason the #2 Plastites won't work is that they have two intertwined threads which is called a "double lead" or a "twin helix". I mentioned "high-low" screws before which also have two intertwined threads but they have different radius (radii?). The two threads on the Plastite screws have the same radius at a given angle of rotation (they are tri-lobed) but they are 180 degrees apart.

Here is some background information: a sheet metal screw has a circular cross section (viewed from the end) and the profile of the thread itself is 60 degrees. When threaded into a resiliant material, it cuts a broad groove, compressing more material to cut to a certain depth, and being circular, it presses out on the material equally in all directions (this is called "hoop stress"). A #2 sheet metal screw has 32 threads per inch.

A company called Research Engineering & Manufacturing Inc. (REMINC) developed a screw for plastic called "Plastite 60-1" which kept the 60 degree thread profile but had a "Trilobular" cross section and "... a pitch that is generally twice that of a comparable size machine screw". Since a #2 machine screw has 56 threads per inch, a #2 "Plastite 60-1" would presumably have 28 threads per inch (it is obsolete and I didn't find any data tables). Right now I think this might be what HP used on the Classics, Woodstock, and Spice and also the Voyagers, I recently found. The tri-lobed cross section allows the resilient material to flow back into the hollow spaces, reducing "hoop stress" and also providing a lock that prevents the screw from loosening due to vibration. This is why they do the "start-stop" when you turn them. Reducing the number of threads per inch (from 32 to 28 in the case of #2) increases the amount of material anchoring each turn of the thread, increasing the amount of force required to strip the thread.

REMINC later developed a screw for soft plastics called "Plastite 48-2" with the tri-lobed cross section and the thread profile reduced to 48 degrees which compresses the material less, like a sharper knife. It also has the "twin helix" which is basically two threads with twice the thread pich, intertwined. It goes in twice as fast and so takes more torque to turn, so it takes more torque to strip (important for soft materials). Plastite 48-2 is in common use and the thread pitch for #2 is 28 per inch (counting all the peaks in a line, so it has two 14 per inch threads intertwined). This is what the Plastite screws I just received are. I recently discovered that the screws in my 41C are probably #2 Plastite 48-2. They definitely have the "twin helix".

Still later, REMINC developed a "Plastite 45-1" with the tri-lobed cross section, a more acute thread profile of 45 degrees, and a single thread with wider spacing - #2 is 19 threads per inch. I don't know if this was a real improvement or just a new proprietary product for REMINC after their "Plastite 48-2" patent expired.

I know I have glossed over the fact that the Plastite 60-1, which I think might be what we need, is no longer made. But the good news is that at least one manufacturer, Camcar Textron, lists in their catalog "Plastite 48" screws available with either a double or a single helix. Only one thread pitch is given per screw size (#2 is 28 per inch). They say the single helix is for thermoplastics with a higher "flexural modulus" (twin helix, up to 850,000 p.s.i. - single helix, 850,000 to 1,400,000 p.s.i.) whatever that means! (It must relate to the hardness or stiffness.) I asked the supplier to look into the Plastite 48 with single helix.

This link is for a Camcar Textron page where you can download PDFs with data on Plastite 48 and Plastite 45. This link is for a REMINC page that has an overview including the obsolete "Plastite 60-1". If you click on the link that says "Download Pushtite PDF" what you will really get is "TAPTITEII.pdf" which has data on all their current "Trilobular" products.

Here are some measurements of various screws I have which all have 28 threads per inch ( largest diameter of the thread - diameter of the head ) [inches]:

Classic, Woodstock, Spice: ( .088 - .162 ) "single helix"

Voyager: ( .090 - .162 ) "single helix"

My new #2X3/16" Plastite 48-2: ( .086 - .164 ) "twin helix"

HP 41C, long screw: ( .089 - .166 ) "twin helix"

My new #3X5/16" sheet metal: ( .100 - .190 ) "single helix"

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