Groovin’

[PAC1]

You know that rule that says NEVER cut a groove as deep or deeper than the diameter of your router cutter in one cut? Well it is there for a reason. This evening I was making a small box which had 6mm ply bottom and so it got a 6mm deep groove. I looked at it and decided it would be fine. It was only a small box made of pine. Anyway I got 95% of it cut when the tone dropped a few octaves. Do I stop? Of course not nearly there and probably just a knot. Finished and would swear I saw the cutter wobble. Hit the emergency stop and ran. On my return the last 70mm of groove went from 6mm to nearly 15mm.
So to reiterate. When grooving NEVER cut a groove as deep or deeper than the cutter diameter in one cut and if the tone drops hit the emergency stop. Or go back to cutting grooves with a grooving blade on a half inch arbour. Or use that small box makers plow plane you bought for such small projects.

 

[Woodbloke]

Basic router procedure; small cuts little n'often. Simples - Rob

 

[Pete Maddex]

Get a carbide slot drill they are indestructible in wood.

Pete

 

[SamQ aka Ah! Q!]

Carbide bits Pete? No, they just self-fracture and turn into unguided missiles when you 'horse' them like PAC1 said.

 

[PAC1]

Get a carbide slot drill they are indestructible in wood.

Pete

I have bought a 6mm winged grooving cutter from Wealden. If I had had this cutter in the first place rather than a 1/4" version I would have been fine. I have now learnt that undersized cutters for undersized plywood are now essential.

 

[AJB Temple]

We've all had to learn this lesson. In my case long before YT was a thing and a I had a small Bosch P52? router.

Biggest lesson for me was buy good quality bits - ie Wealdon. Not Trend. Generally I feel safer for some reason with half inch shank bits too. Very rarely do I use the small routers now, except for trim chamfering.

 

[PAC1]

I keep concluding that I must like taking risks. I had learnt the lesson by 1980. So why 45 years later do I still break the rules. I especially break the rules with routers. It must be familiarity breading contempt.
Adrian, I had a P52 it was a sad day when it died.

 

[Trevanion]

The problem with the smaller cutters is that they don't spin anywhere near fast enough to be effective in wood so you have to reduce the cut and feed rate significantly because they are under a lot of pressure. A 6mm cutter spinning at 32000RPM is only reaching speeds of 22MPH at the rim, where the ideal speed is somewhere between 70-100mph at the rim. A 6mm grooving cutter that's 40mm in diameter manages to get to 80MPH at 18000RPM, so it is much more effective at cutting grooves.

 

[SamQ aka Ah! Q!]

where the ideal speed is somewhere between 70-100mph at the rim. A 6mm grooving cutter that's 40mm in diameter

My gaster has been freshly flabbered; never knew that. Thank you Dan; every day, a learning day.

Like most self-taught ejits, I had thought: "small diameter bit = small cutting volume, coupled to shallow cut = easy progress." Of course, reality (with dipping router note and speed on even 2-4mm cuts) never quite sat happily with 'theory'. Now I know why. Good call, That Man.

 

[Trevanion]

Like most self-taught ejits, I had thought: "small diameter bit = small cutting volume, coupled to shallow cut = easy progress." Of course, reality (with dipping router note and speed on even 2-4mm cuts) never quite sat happily with 'theory'. Now I know why. Good call, That Man.

There's also a case for the amount of cuts the bit is producing too, because the bit is turning at 32000RPM with its two cutting edges, it's engaging with the work 1066 times per second, whereas a 40mm grooving disc with two cutting edges spinning at 18000RPM is engaging with the work 600 times per second. That's where a single flute cutter can be beneficial, especially at the smaller sizes because you're reducing the amount of cuts per second and also the force required to push it through the cut, whilst still keeping up the rim speed.

 

[PAC1]

The Wealden cutting discs have 4 cutters. I understood this to be beneficial because the speed of the cutter meant more cuts per second as the material moved forward.

 

[Trevanion]

The Wealden cutting discs have 4 cutters. I understood this to be beneficial because the speed of the cutter meant more cuts per second as the material moved forward.

They also do discs with two cutters.

Over a certain amount of cut marks to the inch for a given feed rate, where you can see each undulation of the cutter for a given inch on the timber, you have diminishing returns where you're engaging with the work more and thus dulling your tooling faster and generating far more friction which also reduces tool life as well as scorching of the work surface.

On bigger machines such as a spindle moulder, 25 cut marks to the inch was considered about the right amount before you had diminishing returns on your tooling, but I do think 50 is superior for a better finish these days with carbide tooling; however this is practically impossible to achieve with a router due to the high RPM to achieve effective cutting speed. Even with the two cutter disc engaging with the work 600 times per second, you would need a feed rate of one foot per second to achieve that 50 cut marks to the inch goal. If you work to a reasonable feed rate, such as 20 feet per minute, you will have 150 cut marks to the inch.

 

[Woodbloke]

My gaster has been freshly flabbered; never knew that. Thank you Dan; every day, a learning day.

Like most self-taught ejits, I had thought: "small diameter bit = small cutting volume, coupled to shallow cut = easy progress." Of course, reality (with dipping router note and speed on even 2-4mm cuts) never quite sat happily with 'theory'. Now I know why. Good call, That Man.

I believe it's the same sort of thing if you consider a single toof on a TCT saw blade; the periphery speed is actually about 100mph and being sharp and nasty, it's not a clever idea to get in the way of one! - Rob