Thread: Larger quality router bit for concave hollows?

Hey Bohdan, my reply seems to have gone astray. I was saying that I don't think I have the confidence/expertise to grind off the shaft protrusion, and leave a smooth finish to the centre of the cutter. It is to be mounted in a Triton router table, and the short rectangular workpiece is to be vertically lowered onto the router bit, leaving a SMOOTH circular concave depression approx. half an inch in depth.

Elan, just a courtesy follow-up: I've just got off the phone to Carb-i-tool in Melbourne and a techo there called Carri (I think): i was quite blown away by how helpful he was, and they can directly supply either of those half core box bits (T440 & T448) for $53 & $59 respectively, at about $10 postage. He's also sending an up to date catalogue. So many thanks for your pointers. My next issue is to see whether i can do this in Camphor, or the latest timber they're spruiking up here called Blue (or Silver) Quondong, which is fairly commonplace up here. A group called Quality Timber Traders (QTT) is doing this via my next door neighbour Martin and a growing group of local Northern Rivers landowners, in conjunction with the local uni, Southern Cross University in Lismore.

You definitely need the core box bit as a cutter with the bearing removed will leave a round depression with a burnt dimple in the middle.

Originally Posted by Pat the Rat

the short rectangular workpiece is to be vertically lowered onto the router bit, leaving a SMOOTH circular concave depression approx. half an inch in depth.

Ah, didn't realize you were just plunging without feeding across. In that case, disregard the multiple passes thing and go to full depth in one smooth motion; multiple depths will probably end up worse as bits tend to grab/kick if the whole profile engages at once.

Originally Posted by elanjacobs

Ah, didn't realize you were just plunging without feeding across. In that case, disregard the multiple passes thing and go to full depth in one smooth motion; multiple depths will probably end up worse as bits tend to grab/kick if the whole profile engages at once.

I also thought you were edge routing not face plunging. Was trying to work out why the bearing was going to be a problem, it's all clear now.

I wasn't think edge routing, more like a stopped dado but round.

Ahh, Elan & Alday, it's ALWAYS nice to learn a new expression/piece of jargon: :Face plunging! Precisely, and the phrase that I should have used. Nevertheless, Elan, your updated advice on the lack of need for multiple passes is of great assistance, and your logic makes absolute sense.

I've just come back from another neighbour's place with the unfortunate information from their model of the project that the holes need to be 45mm diameter and 13 - 14mm depth. I'm not sure that I can source a core box bit of that diameter, but the internet was working a little faster earlier this afternoon (it slows down when all the kids get onto the limited satellite bandwidth between 3.30 and 6.30pm...that's NBN and regional Australia for you!). Again, thanks for all of your assistance: this really is a wonderful site!

Originally Posted by Pat the Rat

the holes need to be 45mm diameter and 13 - 14mm depth

A 2" diameter bit will get you bang on those numbers....which is conveniently in the gap that carbitool doesn't cover

That's a mighty big core box bit, make sure you've got everything well clamped down before you plunge. I notice that the biggest diameter Carbitool make is 1-1/2" (38mm), I'm sure they could make one for you, BUT it's liable to be a bit exxy. You can get a 2" (50.8mm) in the Arden here - https://www.aliexpress.com/store/pro...621573066.html - the A0502638.

Whoo!, cancel that, here's a real nice price - https://www.aliexpress.com/item/New-...AbTest=ae803_3

Happy routing!

I have to admit that I'm a little flummoxed here. Elan, you're quite correct in that a 2 inch bit would do the job; however, finding one that is quality and safe to use is rather more problematical. And now Aldav (is that Dave, from a post I vaguely recall some years ago??), apart from locating a couple of options (& thanks for that!!!) has raised a really important issue. It IS a big core box bit, and since I'm intending to simply press the work piece down (that "face plunge" op!) upon the top of the bit, albeit only to a depth of 13 - 14mm, it's not exactly going to be "well clamped down"!

It WILL be hard registered against the router table fence at the front (after it has been fully lowered upon the bit), and against stops (probably a ply jig) to left and rear, and then to right and rear (as I do a second hollow a set distance to the left of the first hollow, if that makes sense). I mean I'll register the edge closest to me against the rear jig edge, and gently lower the piece onto the bit until the front edge is fully down and against the fence. So in the first plunge, the risk is in the work piece being slammed to the right by the bit biting and my being unable to hold it steady. The second plunge SHOULD be OK, since the rotation direction of the bit will simply try to force the work piece further into the right stop.

So.… is the 2 inch (Arden?) bit safe to use in this way??? By the way, the work piece is a simple rectangle, approximately 250 x 125 mm, and say 20 to 22mm thick. Hope all this makes sense. This is starting to make my head spin, and the dingo is fed up with my ignoring her!!

I have to admit that I'm a little flummoxed here. Elan, you're quite correct in that a 2 inch bit would do the job; however, finding one that is quality and safe to use is rather more problematical. And now Aldav (is that Dave, from a post I vaguely recall some years ago??), apart from locating a couple of options (& thanks for that!!!) has raised a really important issue. It IS a big core box bit, and since I'm intending to simply press the work piece down (that "face plunge" op!) upon the top of the bit, albeit only to a depth of 13 - 14mm, it's not exactly going to be "well clamped down"!

It WILL be hard registered against the router table fence at the front (after it has been fully lowered upon the bit), and against stops (probably a ply jig) to left and rear, and then to right and rear (as I do a second hollow a set distance to the left of the first hollow, if that makes sense). I mean I'll register the edge closest to me against the rear jig edge, and gently lower the piece onto the bit until the front edge is fully down and against the fence. So in the first plunge, the risk is in the work piece being slammed to the right by the bit biting and my being unable to hold it steady. The second plunge SHOULD be OK, since the rotation direction of the bit will simply try to force the work piece further into the right stop.

So.… is the 2 inch (Arden?) bit safe to use in this way??? By the way, the work piece is a simple rectangle, approximately 250 x 125 mm, and say 20 to 22mm thick. Hope all this makes sense. This is starting to make my head spin, and the dingo is fed up with my ignoring her!!

Just a thought... How fast can your drill press spin?

Obviously a lot slower than a router, but it might be worth a try and it means you can clamp everything down. The low speed also means that balance (and therefore quality, to some extent) is much less of an issue, so a cheap Chinese bit is probably fine.

That's a point of some note Elan. It's a pretty old cast iron job, possibly English circa 1945-50, and pretty slow. however, it does have a triple size pulley drive set-up, and is on the largest (therefore slowest) setting. I have no idea what the fastest RPM might be, but it's possibly worth a try. Given that each piece has to have two rows of six "holes", i may have to rethink the entire approach....possibly a production run of say 50 for each hole.

Do you think that a slower speed of the router bit AUTOMATICALLY means a lower quality finish to the surface of the finished hollow?? And do you think that the router set-up concept that I described is in fact intrinsically dangerous??

Once you get down to drill press speeds I think it might affect the surface finish, but I don't know how much; there will be a minimum viable speed, below which the bit will just tear the wood rather than cut, but I think the highest drill speed will be fast enough.

I don't think your setup is inherently dangerous, but I would suggest you make a jig with handles on top to hold the work more safely. I was thinking something like this, with a mating wedge that can be tapped in and out to secure the work in the middle

jig.jpg