Time to read: 11 min
Have you recently assembled Ikea furniture? If so, I apologize. I send my condolences for the frustration you experienced when the holes did not line up with the pins you put in other holes or when you realized you were missing a fastener. The good news is that you did gain something from the experience (other than a piece of Swedish furniture that you may or may not be able to ever move) – you gained excellent experience with slip fit geometry and even interference fit geometry (depending on the machined size of the hole diameter for each dowel pin fit – ha)! Yay!
Pro-tip: If you are looking for a bit of comedic relief to add to your workday, head over to this video where you can see lots and lots of dowel pin slip and interference fits in action with an Ikea furniture assembly that is sure to bring tears of laughter to your eyes!
We’ve previously explored interference fits (press fits), using a dowel pin as an example, to explore interference fit design limitations. Using the example of dowel pins, there is another important application: slip fits (aka push fits or clearance 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 about slip fit geometry and slip fit tolerances? Read on!
Slip Fit Basics
To make a slip fit work, the parts must be able to slip into place, meaning there must be minimal assembly friction. The goal of a slip fit is easy alignment, requiring enough space around the dowel that alignment is easier: no excessively tight fits – this concept is commonly known as slip fit clearance. How much clearance is excessive? More on that in a moment, but first, the ideal uses of slip fits.
Ideal Use of Slip Fit Geometry
A rule of design for assembly (DFA) is to avoid tight tolerances, where possible. If slip fits are only for aligning tight geometry, should we redesign the parts for looser tolerances?
Yes and no.
Sometimes, especially in machinery design, you can’t have the loose tolerances which would be ideal; because, for example, a head bolting onto an engine block needs to be exactly positioned. Dowel pins are perfect for this type of fit: they hold the two parts in correct alignment, allowing relaxed tolerances around bolt location and hole diameter.
Another handy application for dowel pins is when 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. Another type of jig is a true position fixture, which utilizes a slip fit to determine CNC machining accuracy.
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 taught tolerance 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.
That extra 0.004 seems small, but remember, we’re using dowel pins because the parts need close alignment. So, what is the solution to our square problem?
Access our Tolerance Analysis Calculator
Keyword: Pre-terminated Cable Assemblies