ROOMBA DISCOVERY

HANDLING THREAD-FORMING SCREWS

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Thread-Forming Screws

Almost all of Roomba's parts are fastened together by thread-forming screws, (TFS). The screws in use, look like 'sheet-metal screws' but, with blunt-tips. Roomba's thread-forming screws do not cut the plastic, but rather "push" the plastic-material aside to form shallow, vee-form helical grooves in the hole of a mating 'screw-boss'. This, then, effectively becomes a threaded 'nut' for the screw. In this document, care-recommendations are provided which warn against both the loss of these Roomba parts, and the cross-mixing hassle that can occur, if screw sizes get mixed prior to going back into Roomba.

While out-of-scope for this document, we note that three out of five motors use pairs of 'machine-screws' in their mountings, and one machine-screw is used to secure the edge-brush to its shaft, thus only seven and are used in Roomba.

Proper Cataloging of Screws, As Removed

Various differences between Thread-Forming Screws, (TFSs), as used in Roomba, will be emphasized as a basis for avoiding crossed applications. TFS that are put in the wrong hole, or set of like holes, can result in cross-threaded, hence weakened, plastic, female screw threads. Here are some general statements about Disco's TFSs:

  1. Almost all of Roomba's TFSs have Phillips-drive, pan-heads, but the 82-degree, flat-head (or counter-sunk screw-head) style will be found in use on the Dirt-Detector transducers; and special round-head-types having co-formed flat-washers will be seen at spring-ends and, string-ends.
  2. TFS-materials are either corrosion resistant steel, (CRES), or zinc-plated steel. The latter are strongly magnetic, while CRES-TFSs are weakly magnetic.
  3. TFSs which use the #2-Phillips drive size, typically have a nominal major diameter of 3-mm, but measured diameters show groups with both 3.0-mm and 3.3-mm major diameters. Then, adding more threat to crossed-threads in the plastic parts, three pitch values have been found in these diameter groups, viz., 1.1-mm, 1.6-mm, and 1.8-mm. This paragraph simply says: Avoid exchanging self- threading screw positions.
  4. TFS which use the #1-Phillips drive size, typically have a nominal major diameter of 2.6-mm, or 2.7-mm. Screw-pitch is constant at 1.4-mm, except for the easily recognized screw (used for spring-eyes, or string-termination) having a small, rounded head, and with a flat washer formed as part of the head. Those screws have a 0.9-mm pitch.

Roomba TFSs are Irreplaceable

Roomba's TFSs appear to be ordinary TFSs, however, if any are lost via mishandling, they cannot be replaced by simply visiting a hardware-supply store. The reason is, Roomba's TFSs are manufactured to some unknown (probably 'Oriental', probably Chinese National) standard; it is a metric-screw standard which matches no other standard found to this day.

Certainly, a missing screw can be replaced by a procurable TFS which has characteristics that are as close as possible to the missing screw; but finding such a replacement appears not to be easy. If a TFS, which has been manufactures to a US or Imperial Standard, must be substituted for a Roomba-TFS, that new screw must be dedicated to that same location (i.e., marked in some manner) to guarantee it stays with the same hole.

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