Question 3 of 3 - Help – Getting burns at start and stop with router groove?

[Benjimin Young]

I am using a round ball ½” router bit which seems sharp but getting burn marks at the start and stop ends when I create a groove. Is it router speed or a bad bit or technique?

[glenn bradley]

A little of both. Generally this can be cured with technique which includes feed rate. Is this a stopped grrove or does the profile pass all the way through each end of the cut? Either way, a bit of hesitation at the beginning and end of the cut can cause this. You will probably all but eliminate this problem with practice but, a reduction in bit speed can help you get there. Slow your bit speed a little, start and end the cut confidently and consistently.

Other methods include making the cut 1/32" shallow, resetting the cut depth and making the last 1/32" pass quickly and confidently. feed rate is a "sweet spot" thing. Too fast and the cut is rough or tears out, too slow and you can burn. the sweet spot is pretty wide though ;-)

[Benjimin Young]

Thanks Glenn. Yes it is a stopped groove. I will try some different router speeds. As to feed rate, I guess that will take practice, now I remember why I keep all that scrap wood around. LOL

[Jim Becker]

Certain woods are also more prone to burning because of the sugars in their structure. Cherry and Maple, for example. It's sometimes difficult to eliminate all burning regardless, but a steady feed rate so that the "very warm" bit isn't staying in one place will keep things to a minimum.

[Jamie Buxton]

Dull bits burn more than sharp ones.

A deep cut causes more heat than a shallow one. Depending on what you're doing, you may be able to hog out most of the cut with deep passes, and then do a final shallow pass mostly to remove the burns left from the deep passes.

[Matt Day]

Generally when you start and stop a cut the router stays stationary for a moment and accordingly that’s where the burn appears.

I’d try the method described above where you hog out the majority and don’t worry about burning, then clean up the last bit without stopping.

[Lee Schierer]

When cutting a stopped groove, you want to start your cut a bit diameter from the end of your groove. Once you have plunged the cutter in, back up to the groove start and then make the full length cut. When you get the the far end, move back slowly from the end of your cut as you turn off the router. When the bit stops lift the bit out of the groove. You should have little if any burn marks.

[Simon Dupay]

Dull bits burn more than sharp ones.

A deep cut causes more heat than a shallow one. Depending on what you're doing, you may be able to hog out most of the cut with deep passes, and then do a final shallow pass mostly to remove the burns left from the deep passes.

Not true it's the other way around a deep cut is removing more wood therefore the wood absorbs more heat than a shallow one

[Al Launier]

It also depends on whether or not one is conventional milling or climb milling which is determined by the direction of cut.

Visualizing the shape of an individual chip (if not powdery) shows that a chip cut conventionally is thinner at the beginning and thicker at the trailing end. Whereas when climb milling it's the opposite with the trailing edge thinner than the beginning of a chip. Climb milling provides for a better finish, but also leaves less material to soak up the heat. However, climb milling requires a more secure set up to hold the work piece as the cutter wants to "grab" as it moves along the cut. So, if cutting on one side conventional milling is less apt to burn the wood. Unfortunately, when cutting a slot both sides of the cutter are engaged with a tendency for the climb cut to burn more than the conventional cut side and at the end of the slot the heat is trapped causing the burn. So, in & out quickly to reduce the burn.