View Full Version : dowel pins & corresponding drill sizes to get a tight fit....

04-07-2013, 10:44 AM
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?)

04-07-2013, 11:53 AM
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?

04-07-2013, 01:01 PM
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.

04-07-2013, 01:35 PM
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!)

04-07-2013, 02:37 PM
Just use an end mill saller than 1.5mm and cut a circle - any dia you want!!! G.

04-07-2013, 03:26 PM
Just use an end mill saller than 1.5mm and cut a circle - any dia you want!!! G.

Top tip...I have some 1.2mm end mills & will give this idea a try!

04-07-2013, 06:55 PM
A 1/16" drill is a tad smaller than 1.6mm. Using a knurling tool on pins, shafts, axles etc is great for making them grow a bit - almost the proverbial 'putting on' tool. A straight knurl is good in some dry applications whereas a diamond knurl may help hold the glue as a shaft is pushed into a hole.

The usual way to get a hole bang on size is to drill undersize and then ream it.


04-07-2013, 08:50 PM
Some drills will cut under size, it usually depends on how they are ground. Cheap drill bits are often slightly under. With plastic, if you drill it at a slowish speed and high feed (thus avoiding the drill from spinning in the hole for too long) the plastic will usually "stretch" slightly and produce a hole slightly smaller than the drill bit dia. The other option is to drill something like a 1.5mm hole, heat the pin to around 150C with a paint stripper and push it into the hole. Experiment with offcuts first to get a feel for the best technique!

Seeing as you already have "snug" fitting holes, you could also give the end of the pin a light tap with a small ball pein hammer to make them slightly oval so they fit tighter.