If after an hour of drilling and reaming holes you’re frustrated when the dowel pins slip right in, you might need help understanding why it doesn’t work. It’s all about the tight tolerances, friction and elasticity of a interference fit. By pressing a harder steel pin into a slightly smaller hole made specifically for the pin, the metal stretch elastically and holds the parts together. But it’s not as easy as it seems. You have to know how it works to understand this guide’s chart which shows the interference range based off pin size and type of material. Even then, you must do it correctly.
The other mistake many folks make is skipping the reaming process. There is no way to simply drill a hole all the way up to final dimension then hope for a good press fit. Because drills can’t hold true, they won’t produce a hole with nice round and smooth walls. Also, drilling a hole might cause it to open up just a hair too much. As the chart details, these holes are actualy smaller than the pins by thousandths of inches. Missing by half a thou will ruin the interference before you get started.
How to Install Dowel Pins Correctly
To fix this problem, chucking reamers used at low speeds and high feeds produces a clean cut of only three to five ten-thousandths of an inch. But it has to be smooth so the pin seats evenly without buckling or galling.
The amount of interference applied depend on the material used. Alloy steel can be pressed against another piece of alloy steel using quite high pressure and still remain intact, even though some surface material deforms. This is because steel is good at resisting deformation (pressure). When this hardened pin are pressed in place, the two pieces will retain their position over decades subject to shear load.
Brittle cast iron, however, will fail different than other metals. If you apply excessive pressure, the cast-iron surrounding the pin will crack. Based on the data, aluminum and cast iron has lower limits of interference compared to steel. They either fail by yielding or fracturing before reaching the same level of retention strength that steel can tolerate.
Also note that aluminum expands faster then steel when heated. What might be a snug fit at room temperature could be loose after assembly has been thermally cycled. The surrounding environment where the part operates should also be part of your consideration, not just the bench environment where the part was assembled.
The size of the hole isn’t everything; how you install it matters too. Using a hammer to drive in a dowel pin ruins all that high-tech precision. When a shock load hits the pin, it buckle or mushrooms out. It also distorts the hole’s entrance. Always use an arbor press or hydraulic press. Go slow and apply controlled pressure.
If you’re dealing with tight tolerances or big pins, thermal installation would of been your friend. The pin contracts in diameter when cooled with liquid nitrogen. When brought back to room temp, the pin easily slides into position without the need for huge amounts of pressure. When warmed, the pin goes back to normal size while gripping the walls of the hole tightly. It is a simple application of physics working in your favor.
Not an afterthought: You need to use go and no-go pin gauges to verify that your reaming hit target zone. Go goes in easily. No go does not go in whatsoever. If both go through, the hole is too large. Neither going through = hole is too small or off target. Check it first before sliding the pin into the hole. This ensures it works right the first time during final assembly.
Dowel pins provide repeatability. By taking your jig apart and then putting it back together, you know it is going to line-up exactly like it did before. That’s because of the dowel pins. To make this work, you have to get the holes sized correctly. It’s not about banging things together. It’s about building a mechanical memory that works every time. If you follow the reaming sequence and understand the tolerance stackup, the parts will fit tightely together. And the tight fit holds them in place.
