Screw size for Parallax robots [SOLVED]

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  • localroger wrote: »
    By the way, the designation #4-40 refers to the hole diameter and the thread pitch, so this would be a #4 hole (that the fastener passes through, not a tap) and 40 threads per inch.
    Be careful. When referring to imperial hole sizes, a "#4 hole" would be probably be interpreted as a hole drilled with a #4 drill (Imperial has fractional, number and letter drill standards). A #4 drill is 0.209" in diameter, which is WAY larger than a 4-40 screw.

    Even crazier, when the number size for a screw is increasing, the size of the screw is increasing (#10 is larger than #4), but when the number size of a drill is increasing, the drill diameter is decreasing (#10 is smaller than #4). Can we keep the two standards separate so we don't get confused? Of course not. For a #4-40 screw, the standard tap drill (75% thread depth) is a #43, and a tap drill for a #10-32 screw is a #21. Make sense to you? Me, neither.

    Watch out for "rules of thumb" about hole sizes, since things like thread clearance depend on the material you're threading.

  • jones wrote: »
    localroger wrote: »
    By the way, the designation #4-40 refers to the hole diameter and the thread pitch, so this would be a #4 hole (that the fastener passes through, not a tap) and 40 threads per inch.
    Be careful. When referring to imperial hole sizes, a "#4 hole" would be probably be interpreted as a hole drilled with a #4 drill (Imperial has fractional, number and letter drill standards). A #4 drill is 0.209" in diameter, which is WAY larger than a 4-40 screw.

    Even crazier, when the number size for a screw is increasing, the size of the screw is increasing (#10 is larger than #4), but when the number size of a drill is increasing, the drill diameter is decreasing (#10 is smaller than #4). Can we keep the two standards separate so we don't get confused? Of course not. For a #4-40 screw, the standard tap drill (75% thread depth) is a #43, and a tap drill for a #10-32 screw is a #21. Make sense to you? Me, neither.

    Watch out for "rules of thumb" about hole sizes, since things like thread clearance depend on the material you're threading.

    That and more is why I wish we had switched to metric years ago when standards were being settled on. Passed up a chance to dramatically reduce the total number of fasteners and have a simpler more logical system of specifying them.
  • jonesjones Posts: 281
    edited 2020-01-14 - 16:34:35
    kwinn wrote: »
    That and more is why I wish we had switched to metric years ago when standards were being settled on. Passed up a chance to dramatically reduce the total number of fasteners and have a simpler more logical system of specifying them.
    In principle, yes. In practice, not so much. I'm only a "machinist of necessity" (e.g. not trained as a machinist) but even in the work I've done it's become apparent that in the real world things just aren't that simple. I agree that a metric standard eliminates the nutty "gauges" so common in Imperial, but there are times when you need all those "in-between" sizes. Even something as seemingly standard as a tap can be had in undersized or oversized versions, because in some circumstances standard threads just don't work well enough. One simple example that I had to deal with is that nylon hardware, whether it's metric or imperial, needs extra thread clearance. The screws are made to the same standard as metal hardware, but they just run tight. Supposedly that's due to moisture absorption, but I'm in a dry climate and they still ran tight. I had to use a lot of nylon hardware on things I built for customers doing radiation research (e.g. using accelerators), so I had to have oversized taps. In fact, I had two "over" sizes; so-called H-7 (0.003" over) and H-11 (0.005" over).

    Another common example, again bedeviling both metric and imperial, is when something needs to be plated after threading. You have to compensate for the thickness of the plating on the threads or the screws won't fit.

    When you run into things like that, and there are many such pitfalls waiting, the simplicity of metric falls apart just as fast as it does for Imperial. In the end, you need the size that works, and the real difference between the two standards is just what's stamped on the tool or printed on the box. The problems are the same.

  • Well, @jones, you are right that both systems need to adjust for things like plating, but the simplicity (and beauty) of the metric system is that you have - say - a 3 mm screw and will need a 3 mm drill bit. Easy. Figuring out that you need a #43 for a #4 screw is just insane.

    As you and I mentioned the 'gauge' unit is one of the most silly ideas the imperial measurement offers.

    Since 1972 (I think) the metric system is official adopted by the US of A. Scientifically the Imperial units are now defined by metric units, alas in opposite to say Canada the people living in the US refuse to move away from Imperial for reasons I simply can not understand, yet. But - heck - I am just living 14 years here, maybe one time I see the light.

    Sure there was a time where people cut down coins for change, and yes I see that a 12 based system allows for more even dividers as a 10 based system does, but who still cuts down a dollar coin to pay for say a quarter?

    The imperial system is outdated and even the people originally inventing it switched to metric generations ago.

    So WHY keep it?

    Mike
  • jonesjones Posts: 281
    edited 2020-01-15 - 06:00:22
    Don't misunderstand, Mike, I completely agree that the metric system is easier in many ways, not only in mechanics for tooling and fasteners, but the SI system in general. Working with researchers, I generally used metric units in drawings since that's what they were comfortable with and used metric fasteners in the things I built for them. Sometimes it wasn't practical to do so because I was having to adapt an existing device that was Imperial.

    And while you might reach for a 3mm drill to make a clearance hole for a 3mm machine screw, what reamer would you use for a 3 mm dowel pin? It depends on how you need it to fit. As soon as you start talking about running fits, slip fits, press fits or shrink fits, you'll find you're splitting millimeters and need a whole drawer full of reamers, just like Imperial.

    Don't be irrationally hard on the Imperial system, at least in the context of machining or fasteners. When you grow up with it and are used to it, it works just as well although it does involve using a few tables and such, but for most things there really isn't that much. And as I said, when you start getting past the simple stuff, you find you're looking up things for metric tooling and fasteners too. And yes, the various gauge systems are crazy, but they work just as well if you're used to them. It's just that in my experience, it really didn't make a lot of difference in how much time or effort it took to build something because I used one system or the other. YMMV.

    If it were up to me, the US would've switched completely, but we didn't, so it is what it is. If you don't like Imperial tooling or fasteners, don't use them and you'll be in excellent company. I'll use what makes sense for what I'm working on, just like I always have (when I had a choice!).

    Bob
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 22,571
    edited 2020-01-15 - 06:32:45
    The Imperial system is evolutionary, having developed bottom up, as needs arose. As a consequence, it's messy, but nonetheless useful and workable. The metric system is revolutionary, having developed top down from an idea that things should be nice -- and rational. Although there is merit in both approaches, I tend to favor the former. It's simply more human than ideal.

    For example, look at weights and measures. Volumes are binary in the imperial system: a quart is 1/4 of a gallon; a pint, 1/2 of a quart; a cup, 1/2 of a pint; an ounce, 1/8 of a cup, and so on -- all very simple and approachable for common use. In the metric system, everything is decimal -- a rather unfortunate consequence of humans having ten fingers, instead of 12 or 16. Even the ancient Babylonians recognized the value of fractional number systems when they adopted base 60 as their method of counting. And I would bet that, even in metric system countries, the concept of a dozen pastries still prevails.

    Lengths in the imperial system are approachable on a human scale. Inches and feet are easily envisioned and estimated by eye. But centimeters and millimeters are rather too small for such convenience. What's 100 mm? In my mind, it will always be about four inches. That, I can envision.

    But that's just the way I was raised. Perhaps those brought up with the metric system have the same intuitive grasp. In other words, arguments about which system is better border on the religious, rather than on the pragmatic. Both systems work -- and they work well -- for whomever adopts either.

    -Phil
  • Thanks for both of your answers, I think Phil put it best, it might be even a religious thing but maybe more dependent on what you grew up with, I have no problem to envision a meter, 10 cm or 4 mm, but 8'2'' make my brain curl, its (8 times 12 +2) times 2.56 whatever...

    Mike
  • FWIW, when one is measuring with a tape measure, feet, inches and fractions are, IMO, much more awkward than meters, cm or mm. I can easily visualize the Imperial sizes, but doing mental arithmetic with them is another story. In that I completely agree with Mike.

    It's when you're working to small fractions of your basic unit, like thousandths of an inch or hundredths of a millimeter that I think the differences between the two systems cease to mean much.

    About the "gauges," most started off as some manufacturer's proprietary system that became standard by virtue of wide acceptance. It's really not surprising that when you compare different gauge standards, they seem to make no sense.
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