Your router could be spinning the bit at 20,000 rpm. Ignoring the possibility of damaging your work piece, the personal risk is extremely high. A slower feed rate is required with down cut bits as this allows more time to evacuate chips and for them to be cut into smaller pieces that fall from the slot. This does mean that you need to be careful the bit does not overheat and consequently break. This can be due to the build-up of chips combined with a deep cut. Electric Pressure Washers Petrol & Diesel Driven Pressure Washers Hot Pressure Washers Pressure Washer Accessories Detergents, Chemicals and Algicides Pressure Washer Spares Hoses & Accessories As this router also has a 3/8" collet, you could use an adapter to get down to a 6mm shank. Now, adapters aren't great, as they're one more thing that can go wrong, but if you were desperate, you can find a 3/8" > 6mm adapter here. The following chip loads in the chart are applicable for single flute, 2 flute and 3 flute solid carbide router bits that are either up cut OR down cut. These recommended chip loads are NOT applicable to compression bits

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My Makita 3709 has only one collet and the instructions booklet say that it's valid both for 6mm AND 1/4" bits.

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An up cut bit is typically used to remove chips from the hole that it is cutting. Deep grooves need to be fashioned out using continual light cuts. Up cut bits are ideal for this as the upward direction of the flutes inhibits the build-up of debris which can either break or bind the bit. As the flute removes chips it can simultaneously fray the wood fibres, causing damage. You must be mindful of this when using up cut bits and be careful to avoid implementing damage to your project. Electric Heater Accessories Gas Cylinder Storage Gas Heater Accessories Propane/Butane Hoses and Regulators I've seen people do it, too, and they so far got away without accident or even physical injury. (Which makes the whole matter worse because that "proves" that it works.)

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As previously mentioned, up cut router bits are often applied in the removal of chips from the hole being cut. With up cut bits, deep grooves are created via continual light cuts. This is because the upward direction of the flutes prevents debris from collecting, which can result in the binding or breaking of the bit. While the chips are being removed, the bit can also fray the wood fibres at the top of the cut, which leads to damage. All users should bear this in mind. Solid carbide 2 flute up cut router bits will give a good clean finish at the bottom of a cut. Straight Router Bits

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Always start out at the lower feed rate and work your way up from there until optimal cutting conditions and speed is acquired