I’m countersinking holes in some softwood. I’ve got a brand new bit which seems to get good reviews. Yet every hole comes out hexagonal. The bit just keeps bouncing or chattering. I’ve tried my old bit and get nice countersunk holes.

What am I doing something wrong?

Yes, you’re not using your old bit! 😉

I’ve had this too. Not sure what the cause is but my guess is that it’s happening with the new bit because it’s sharp. So it digs in and bounces out. Probably OK on hardwood?

Incorrect cutting speed.

Chatter. Try going very slowly.

almost certainly cutting speed / feed rate, your old bit will be a bit blunt and not grab the wood so much to induce chatter, try running the drill faster or slow down the feed rate or drill a run into harder wood to blunt the tool a bit!. now as a caveat I would say my experience is mostly in composites and metal drilling!

Its pentagonal if you look closely.

If its chattering it won’t be round. Try a lower speed, or if your drill is not variable speed pulse it. Also lean on it a bit more. If all that doesn’t work, use your old bit.

I’m not a fan of those styles of countersinks, quite prone to being messy – wandering or chattering

these style

give a much cleaner cut and support themselves in the pilot hole better, as not all the surface is cutting edge, so they don’t chatter and stay centred

Thanks for all the tips. I found slowing the speed right down helped a lot.

maccruiskeen, where do you get those from? I’ve never seen anything like them.

The “Snail” type shown above are great but work best in a drill press as they need a lot of pressure to work. The conventional type work best in holes smaller than 5mm or in much wider ones (for some reason).
Hafele and Axminster do them.

you are pressing too hard, light weight on the bit is how to avoid chatter.

maccruiskeen, where do you get those from? I’ve never seen anything like them.

i got mine from screwfix but not sure if they do them anymore. I didn’t find they required a lot of pressure, only ever used a drill driver They don’t work well blind though, you want to to have a hole there already that you’re countersinking, you can’t go straight into the wood with the bit. They also hit a point where they don’t want to drill any further though, a sort of natural depth stop.

In the end through I switched to one of these festool doofers

costs more than some people would want to spend on a drill driver. But works perfectly, partly in the sense that it stays sharp, doesn’t snap and you cans set the depth it drills and countersinks to independently, but also its quite big and chunky – so after 5 years I still haven’t lost it and can find it in my bag when I’m looking for it, which is the downside with the snail bit – they can be at you’re fingertips and you can still spend ages looking for them.

I find too slow causes the problem. Running the drill flat out and pressing very gently works for me