Time to read: 6 min
We’ve explored interference fits and all their design limitations, but dowel pins have another application: slip fits. While interference fits create tight assembly tolerances, slip fits are just the opposite—they can easily give you a self-locating assembly, making manufacturing easier when needing close alignment. Want to know more? Read on!
Slip Fit Basics
从“滑动拟合”的名称中可以明显看出：零件必须具有最小的装配摩擦。滑动拟合的目的是容易对齐，需要在销售销售周围足够的空间对齐很容易：没有过度紧身。多余量是多少？稍后更多，但首先是滑移的理想用途。
组装设计规则（DFA）是避免公差紧张。如果拟合仅用于对齐紧密的几何形状，我们是否应该重新设计零件的零件？
Yes and no. Sometimes, especially in machinery design, you can’t have the loose tolerances which would be ideal—a head bolting onto an engine block needs to be exactly there. Dowel pins are perfect for this: They hold the two parts in correct alignment, allowing relaxed tolerances around bolt location and hole diameter.
另一个方便的合销申请是当parts need to be disassembled and reassembled numerous times while retaining alignment. While this is rare in consumer products, it’s common in the tools used for assembling them. Assembly jigs—tools for holding parts in correct alignment during assembly operations—are one of the biggest areas that can benefit from slip fit dowels.
And we are going to get into the question of how tight is too tight, but first, let’s discuss how we’ll be communicating those values.
Let Go of Traditional Tolerancing
Tolerancing is simple, right? Draw a line, mark a dimension, give a reasonable range of acceptability, and bam! You’re done. Right? Wrong.
In school, we learned linear tolerances, and good students even paid attention when the professor taughttolerance analysis。But typical tolerancing has a flaw when tolerances get really tight, as they are when using dowel pins for alignment: the square.
“The square”? For a hole with horizontal and vertical tolerances of +/-0.010 inches, how far off-center can the hole be? Intuition replies just 0.010 inches, but that’s not for the corners—we create a square of acceptable locations, and in the corners, the hole can be 0.014 inches off-center.
额外的0.004似乎很小，但是请记住，我们正在使用销钉销，因为零件需要密切对齐。那么，解决方形问题的解决方案是什么？
实际上，这不是唯一的问题，对吗？如果引脚是居中但不是垂直的，该怎么办？如果该洞是偏离中心，垂直和圆形的洞怎么办？可以多少变化？他们会在宴会上提供冰淇淋吗？（也许这只是我的关注。但这是一个大问题。）
Slip Fits and GD&T
宽容的黄金标准，GD&T(geometric dimensioning and tolerancing), fully resolves these issues, by calling out exactly the acceptable variation in all aspects of design geometry. While they look like a complex symbolic language, GD&T callouts are well worth the time spent learning, and a few basics will illustrate their necessity in slip fit pins.
GD＆T对其所有维度都使用基本耐受性，这意味着它查看几何形状并描述了理想。但是，每个几何特征都没有引入随机误差（如平方公差区域），而是用自己的一组可接受的公差来调用。
例如，让我们看一个带有两个销钉销孔的简单块，这将在平板上对齐。对于此组件，我们关心该块垂直于板，并且精确地位于板上。我们需要将这些设计考虑因素转化为公差。
To begin with, we mark out the ideal geometry—due to manufacturing tolerances, the part will never be ideal, but these are the starting points—and then look at how much variation we can tolerate in each of the features of the block. (“Features,” in this case, means the two holes, but essentially refers to any geometry capable of variation: holes, channels, surfaces, etc.)
Looking at the first hole, we need to specify several things: hole location variation, cylindricity or hole surface variation, and how perpendicular it must be. But variation compared to what? The bottom? The other hole? Some ethereal fixed point in space? Compared to datums.
Datums are your design’s anchors, and they’re the reason why GD&T is so practical. For our first hole, we care that it’s perpendicular to the bottom of the block (which will rest on the plate), so we mark the bottom of the block as datum A and mark the hole perpendicularity tolerance to datum A.
我们的第二个问题是孔的位置（与块的前部和侧面相比，它成为我们的基准B和基准C），它使用真实位置标记（不要与Schwarzenegger惊悚片True混淆，that’s confusing enough all on its own). True position solves our square problem by giving us a radius of acceptable variation for the hole location.
最后，我们关心第一个孔滑过销钉销，因此必须相当圆柱形，并使用圆柱体标记可接受的变化，因此我们的第一个耐受性标注看起来像下面的图片。
Whew! I’m glad we’re done with that. But, wait! What about the second pin? Here’s where GD&T becomes even more useful.
Because the two pins are used for alignment, we know that they will have tight tolerances, so the two pins must align with each other even more than the other features. We can mark the hole for the second pin in reference to the bottom, front, and side surfaces, just as with the first hole.
But once the geometry is marked like this, we also add another callout referencing the first hole. The first hole becomes another datum, D, and we add a second position tolerance, much tighter, which calls out the variation between the holes. And, voila! Without compromising our fit, our manufacturing team has as much tolerance as possible for the geometry.
Further Notes on Geometry
And, finally we can discuss the original topic: How tight should a slip fit be? That’s the easiest part of the whole design process.
To be a true slip fit, the only requirement is that friction not keep the dowel from sliding into the hole. Ideally, that means that the dowel and the hole could be the same diameter. However, in reality, dowel pins are commonly up to 0.0002 inches oversized, which becomes an absolute minimum oversize to ensure a clearance hole (slip fit).
除此之外，您应该允许最大的制造变化。如上所述，这需要查看零件对齐并指定参考拟合和功能的公差。但最低限度，计划超大孔0.0015英寸，以使清除效果拟合。
使用单一销钉进行对齐，这是您的最低限度。对于两个引脚，给自己尽可能多的空间。对于三个引脚……等等，为什么要有三个销钉？三销overconstrainyour design, and should be avoided; use no more than two pins for alignment.
最后，请记住，如果孔和销钉接近相同的尺寸，当您让空气逸出空间时，组装将变得更容易孔的一侧。特别是当碎片要组装时disassembled repeatedly, this makes life easier for the assembly crew.
Slipping a Dowel into Your Design
When bolts are just too sloppy, slip-fit dowels may be your best friend. Keep in mind the required tolerances and applications in manufacturing jigs, and try these out to make your next assembly自定位。And once you’ve finished your tolerance analysis, try out ourCNC machining servicesto create your prototype!
