# Correct Router Bit Speed



## KnickKnack (Aug 20, 2008)

I entirely understand why you would want, indeed would need, to run large router bits at slow speeds.

What I don't really understand is why you should run small bits at high speed, as seems to be recommended everywhere. Surely they should just be run at the *right* speed based on your feed rate?
I've just cut some slots with a 4mm bit running at medium speed, with a slow feed rate (I'm an habitually slow feeder), and it all went just dandy. Same speed I've used for 10mm and 20mm bits. I take multiple shallow cuts, of course, if that makes a difference.

Thanks in advance for the gem of knowledge I'm obviously missing.


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## AaronK (Nov 30, 2008)

sounds right to me  they probably say you *should* run it at high speed because you'll tend to get a better quality cut at a given feed rate, making the router bit look like it's performing better.


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## renners (Apr 9, 2010)

Depends what kind of timber you're routing too. I've had glowing embers come out of walnut, (though the bit could have been dull), maple's another one where the speed dial goes down.
There's a very experienced kitchen fitter with a shop full of cheap fitting consumables called "Fitters World" - he sells thru ebay, and I think he opened "Fitters World NA" (North America) and is selling cutters and blades in the states, his advice on even 1/2" straight cutters is to run them at half speed on chipboard, "otherwise you'll burn the cutter out" - he's dead right. I have no affiliation to the above, but he knows what he's talking about (even replaces his Festool routers after every 150 kitchens).


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## Sawkerf (Dec 31, 2009)

I doubt if there are any hard and fast rules for bit speed that would apply to everyone. Cut quality depends on several variables including bit sharpness, wood species, grain direction, etc. IMO, one of the biggest variables is feed rate as well as feed technique which can vary with the individual. For any given situation, a bit speed that works for me may - or may not - work for someone else.


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## richgreer (Dec 25, 2009)

I own several tools, in addition to my routers, that have variable speed capability. I'm thinking of ROSs, jig saws, drills etc. It seems that you just find the speed that feels right and go with it.

I suspect that, relative to others, I run my routers a little faster and drills a little slower.


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## Bluzman (Dec 25, 2011)

What if the other variables are constant or ruled out; moderate feed rate, brand new/sharp high quality bit, cutting a classic ogee in one pass, grain is not going to cause problems on the piece you are working with.

Would it a good rule of thumb be a faster router speed for hard woods and a slower speed for soft woods or vice versa?


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## BurtC (Oct 30, 2009)

The harder the wood, the slower the bit speed should be. Like others have said, it is a trial by error type decision.
I usually run a piece of scap (same wood) thru first and twwek the speed until I get a good smooth cut. On hardwoods, I only take about 1/8" off per pass. Then one last smoothing pass.


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## Waldschrat (Dec 29, 2008)

Well there are formulas for the proper cutting speeds, feedrates and cut quality, all depending what you are looking for.

What you, KnickKnack, are refering to is cutting velocity, or in the industry refered to as "Vc"

Take a calculator and plug in your numbers and you will see what I mean:

Vc = (pi x d x n) / (60×1000) the rusult is in m/sec

pi = 3.14
d = diameter in mm of your cutter
n = revolutions per minute (be it router, shaper, or even table saw blade)

so in your case, if you calculate it, you you will see.

A couple of things to think about the Velocity needed is determined from a number of things already mentioned above, including also the wear from excess heat on the cutter. But for the *quality of the cut *the important thing is to meet or exceed the "fore-splitting" speed of the wood to keep the cut clean, and closer spaced "arches" in the material.

For a large cutter to have the Vc one needs to turn down the "u" in the formula or on the machine. (like on a record player the outside of the vinyl record spins at a faster velocity than the inside, but has the same revolutions per minute).

If you would be interested I could post more math solutions/formulas to determine feed rate or material cut quality.


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## vipond33 (Jul 25, 2011)

What you are also concerned with is chip load per tooth. Smaller = better. Large chips will fracture ahead of the cut line resulting in tear out. Cutters with built in chip breakers (mechanical breakers or those with an oval interior grind) will allow faster, cleaner, more aggressive cutting. Without them, most normal router bits are really "flying wedges".

All things being equal, slower speeds combined with slower feed rates will give you an equivalent clean cut, but for the problem of "dwelling".
Any bit will rub slightly just past the cutting edge (and this is compounded by fibre spring back and dullness), resulting in heat and burning or glazing of the wood. The slower you go, the longer you are in contact with a given area. For example, go slow on cherry at any rpm and there will be hell to pay.

A perfectly sharp tooth combined with a high relief angle (radial is better though uncommon on low cost cutters) working at the highest possible speed (safely) for a given diameter will work best. My dentist tells me his microscopic cutters are spinning at up to 250,000 rpm. 
So run dead sharp bits with a chip load averaging around paper thickness and you will have joy in the shop.


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