Metric machine thread question

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Old 03-22-18, 05:09 PM
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Metric machine thread question

Greetings. In my job, the most common fastener, by far, is the #12-24 under-cut flat head machine screw, usually made for #3 Phillips driver. Often, due to either poor manufacturing tolerances, and/or the use of poorly made "universal" type (12-24 near the head transitioning to wood thread near the tip) screws, the sheet metal framework that these screws are screwed into, become over-large (wallowed-out, we would say) and tend to strip when tightened. My solution has always been to tap for the next larger size (1/4-20).

Is there a Metric machine size that would be slightly bigger than a #12 but smaller than 1/4" and have a pitch similar to 24TPI?
Since the thickness of the tapped hole is rarely more than 3/32", the metric pitch would only have to be close to 24TPI, I would think. This way, I wouldn't need to tap for 1/4-20 and end up with an oversized head that doesn't counter-sink properly.
 
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Old 03-22-18, 05:36 PM
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As for a size larger metric you can find that easy enough by looking online or your fastener suppliers catalog. Why is such a large diameter fastener being used in thin material? Are the holes tapped? When you say the screws have a wood thread at the tip it almost sounds like they are self tapping. When needed, why are you going to a 1/4" coarse thread instead of a fine thread?
 
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Old 03-22-18, 06:18 PM
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Hello Pilot....answering in order presented: 1). I don't know. What I see is what I get. Though the material is thin, when it's properly tapped it holds tight. 2). Where the material is steel, they are usually (but not always) pre-tapped. Where aluminum is used, it is usually not pre-tapped. Non-tapped aluminum holes will accept self-tapping 12-24 screws, as well as course-threaded "TEK" screws and "universal" screws. 3). Hardware manufacturers often supply "universal" screws so as to be used in wood or metal. The problem is that, again, if not well made, they tend to chew up the material, esp. aluminum, as they're inserted. When you back them out, (to replace the hardware) you end up with a wallowed out hole. There are several ways to make a self tapping machine screw, but often, in our industry, the method used is by placing a number of "chisel" indentations near the tip. This type tends to chew up the material it's being screwed into. 4). I just happen to have a 1/4-20 tap handy and course thread screws are readily available.
 
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Old 03-22-18, 07:26 PM
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These should be pretty darn close, although tolerances vary by manufacturer, etc., but the diameter of a #12 screw should be about .21, the next up metric is an M5.5-.90 at .2165, then an M6-.75 or M6-1.00 at .2362, which is getting close enough to your 1/4 that you may as well stick with it if you're going up that far. I don't know off hand which pitch is closest to 20 tpi, but can take a look at my thread gauges in the morning if you're still interested and someone hasn't already come up with it.
 
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Old 03-23-18, 03:49 AM
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Well, this is only worth what it's worth, but looks like #12 screws run closer to .216 than .21, so 5.5 mm's probably aren't much good since they're real close to the same diameter. In an M6, the 1.00 pitch is closest to your 24 tpi, but obviously not a perfect match since it's a different form of measurement. And of course where you buy them will probably matter simply based on who uses which standard(s).
 
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Old 03-26-18, 08:43 AM
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Thanks Pedro, an M6 looks like the half-way point between 12-24 and 1/4-20. The 1mm pitch is so close it won't be a factor.
Thanks for the help!!
 
 

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