dowel pins & corresponding drill sizes to get a tight fit....

**HankMcSpank** · 04-07-2013

I need to use some 1.6mm stainless steel dowel pins ...these will fit into some 1.6mm holes drilled into acrylic by my cnc machine.

Problem is, at the moment is that a 1.6mm dowel pin, fits a bit too loosely into a 1.6mm hole (don't get me wrong it's actually a 'neat' fit, but I really seek a solution where I have to gently tap the dowel pin into the hole with a small hammer - at the moment I'm having to superglue the pins to hold them in place in the acrylic (!), but that's not ideal as there's little for the glue to key into & the pins can still work loose)

So what's happening here?

Is this because the 1.6mm drill is out of spec, or perhaps the drill has melted the acrylic a tad, resulting in a slightly oversized hole?

I can actually source 1.65mm pins, but then wonder would these even fit into a 1.6mm hole?!! (or is it normal to buy slightly oversize pins to overcome this problem?)

**irving2008** · 04-07-2013

Someone like JohnS may hve a different view but my understanding is that a drilled hole will always be slightly bigger than the drill size. to get an interference fit you need to drill the hole smaller, but it will depend on the dowel tolerance. Looking up a typical dowel its +0/-.005mm. If I read the tables right you need a .01mm smaller drill e.g. 1.59mm for an interference fit in steel, so maybe smaller in acrylic - 1.58mm maybe?

**martin54** · 04-07-2013

Need to watch what your doing here, acrylic is quite brittle & drilling a smaller hole to tap the dowel into could cause stress fractures to develop over time. There are better adhesives available that might be a better option, penloc is one suitable for acrylic I use a company called Eurobond for adhesives & they are always helpful, don't have a number to hand for them though.

**HankMcSpank** · 04-07-2013

Thanks chaps....re changing the glue, my thoughts here are that the problem isn't likely to be the acrylic (the drilled hole will likely be providing a degree of 'keying' for the glue), but more likely the stainless steel dowel....probably little for the glue to grab onto.

RE stressing & cracking over time....this is of little cause for concern, as this is all for a just a cheap winding jig that I've made & I can make another quickly if the acrylic fractures - I'm more miffed that the pins presently have a tendency to work loose than worrying about the acrylic fracturing over time.

Re the increments irving mentions (ie 0.01mm/0.002mm) ....alas I can only source the dowel pins in increments of 0.05mm (& the cost/sourcing options gets out of hand if I start trying to source drills in sizes like 1.58mm!)...I therefore might take a punt & see if I can coax a 1.65mm dowel pin into a 1.6mm hole! (put the pin in the freezer & warm the acrylic in a low heat oven & then see if it fits!)