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Ed's Tech Tip: Thread Cutting Screw Problems |
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The last topic discussed was the types of self tapping screws that are available. Next, a discussion of common problems associated with these products. One problem that we frequently hear about from our customers are associated with type "F, D," or "G" thread cutting screws: " The screws stripping before they are installed completely or the screw actually breaks when partially installed." When I first started out in fastener sales and there was a problem associated with these types of screws, I always blamed the fastener. I often thought the screws were made improperly. Were the heat treatments incorrect and making the screw too brittle? Maybe the screw itself was oversized or the threads did not have the correct angle. Could the thread cutting slots be manufactured incorrectly? I rarely looked at the application or installation technique. I am sure I drove the owner of our company mad. I would see our customers with these problems and the only thing I would recommend is replacing the screws with different ones. After the endless paperwork associated with a return from a salesperson, I discovered that I could have shipped in screws from twelve different manufacturers and they all would have failed. It was not the fasteners, it was the mating part!!! |
The most common problem and the easiest to solve will relate to the preparation of the hole for the part being fastened. If the hole is too small, the threads will bind and the part may break before complete installation. If the hole is too large the part will install easily but it will not have any clamping force. This fastened joint would be considered worthless. In the I.F.I Fastener Standards book there is a lengthy discussion regarding hole preparation. Yes, even the way the pilot hole is formed can have a great influence on the installation. The preformed holes may be cored, drilled, punched, pierced, or extruded. Each of these has different characteristics. The drilled holes have straight side walls, cored holes are slightly tapered, punched holes usually have a breakout at the far side, and extrusion elongates the hole and thickens the section to be tapped. I have found this is where the problems begin. Quite often the hole is smaller than recommended. Possibly the drill bit is worn or the die punching the hole is in need of replacement. Something quite common now is to find that there is a large amount of paint or undercoating obstructing the hole. This makes the hole smaller than required. The part may then fail upon installation. |
To follow are two charts from the I.F.I Fastener Standards Book. These charts should help you specify the correct pilot hole size. If you are using these types of screws, and having problems, please refer to the chart. You will find what size hole should be maufactured for the fastener you are using. If you find the pilot hole being used is indeed smaller than recommended, this could be where your problem is starting. |
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Table 1: Suggested Hole Sizes for Thread Rolling Tapping Screws and Types D, F, G and T Thread Cutting Tapping Screws |
Table 2: Suggested Hole Sizes for Types AB, B, and BP Thread Forming Tapping Screws and Types BF and BT Thread Cutting Tapping Screws |
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Note: All material for this article has been taken from the I.F.I Fastener Standards Book, Sixth edition. If you would like to read more on this subject, please refer to pages H-1 to H-78 in the Fastener Standards Book. |
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Formula: Mean Hole Dia. = 1.015 x basic thread pitch diameter (Table 1, pg. A-30 and Table 2, page A-31). Tolerance is plus and minus 3 percent of mean hole dia. No tolerances or drill sizes are given for tapping screws with fine threads except for the No. 10 size. Use of fine thread tapping screws is not encouraged. |
Formula: Mean Hole Dia. = 0.98 x mean of thread major and minor diameters (Table 6, page H-21). Tolerance is plus and minus 7 percent of mean hole dia. |
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Of course this is only one of the simpler areas that we are looking at. There can be many problems associated with installation torque and materials being used. I have written these articles to be brief and informative. Every single application must be reviewed individually and tested accordingly. If you are having problems associated with thread cutting fasteners, I would invite you to call or e-mail. I am sure that we could be of some assistance to you. |
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Good Day!!
Here is a frequently asked technical question we hear from our customers. What do the designations mean when referring to the different types of "tapping screws"? Please see the chart below. (This information can be found in the I.F.I sixth edition Fasteners Handbook)
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Pictorial Representation |
ANSI/ASME Designation |
Manufacturer's Designation |
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AB |
AB |
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B |
B |
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BP |
BP |
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BF |
BF |
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BT |
25 |
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C |
C |
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D |
1 |
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F |
F |
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G |
G |
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T |
23 |
Type AB screws: These screws have spaced threads which are the same as a type "B." The primary use of these screws is for use in thin metal, resin impregnated plywood, and asbestos compositions. Because of the versatility of application these screws are recommended over type "A" screws.
Type B screws: These screws have spaced threads and a blunt point with incomplete entering threads. They are intended for use in materials such as thin sheet metal, nonferrous castings, plastics, resin impregnated plywood and asbestos compositions. We also find many of our customers using these screws in electrical assemblies. They have components that need to be fastened but they do not want the wires to be in any danger of being cut. With the blunt point there is less risk.
Type BP screws: These screws have spaced threads the same as type "B" screws but have a conical point extending beyond the incomplete entering threads. These screws are intended for piercing fabrics or in assemblies where holes may be misaligned.
Type BF, BT screws: These screws have spaced threads with a blunt point and tapered entering threads, such as the type "B". They have one or more cutting edges and chip cavities. These screws are intended for use in plastics, asbestos, and other similar compositions.
Type D,F,G &T: These screws have machine screw threads with a blunt point. They have tapered entering threads having one or more cutting edges and chip cavities. These screws are intended for use in materials such as aluminum, zinc and lead die castings, steel sheets, cast iron brass and plastics.
General Discussion: Please keep in mind that these are very general descriptions. There are many companies that manufacture tapping screws. They make very specialized parts intended for specific applications. I wanted to give you a brief overview of some of the basic types available. If you have a special application or you are having problems with any of these screws listed I invite you to give us a call or Email us. We have solved many problems both assembly and application related. Note: All of this information is from the I.F.I. Fastener Standards Book, Sixth Edition.
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Flat Washers:
Did you know that there is a correct and a wrong side to a flat
washer? Well there is. To understand this we first must visualize how
a flat washer is made.
A sheet of metal is rolled flat. A die, with many heads is then pressed-punched into this sheet of metal to produce the desired diameter of the washer. During this press-punch process, a chamfered I.D. is created where the die enters the sheet of metal, much like a cookie cutter being pressed into dough.
The opposite side of the washer will have a raw or sharp edge on the I.D. A bolt or cap screw has a radiused fillet under the head where the head and shaft meet. This radiused fillet is designed to fit into the chamfered side of the flat washer. Conversely, if you were to put the raw-sharp edge of the flat washer against the fillet, this would cause scratching or cutting or the fillet leading to premature failure.
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