How to avoid burn marks

[youvee]

Hi All,

I have just started on my wood-working after having purchased the Triton gear which I have found to be very wonderful to work with. I have a few projects in mind for this year - including a coffee table, bookshelf, garden bench and perhaps a shoe-rack.

To start with, I have been trying to rout few grooves in a Jarrah plank I have for my first project and have been plagued with few consistent burn marks. The router bit is a 9.05mm radius hemi-spherical bit (without bearing) and I am using it to rout grooves (almost semi-circular - actual depth is 8mm) all along the length of the 2m long plank.

The initial short trial piece used (just 250mm long) does not show the burn marks at all, but switching to the 2m long piece results in consistent burn marks ~ 0.5 - 1mm deep at periodical intervals - every 150mm or so - along the length of the groove.

Is this because :
- since I handling a 2m long piece on my own, I am pushing the piece not in a smooth & continuous manner but in a push-stop-push sequence while changing grips at intervals,
- router speed is set to the maximum (5) since the bit diameter is not very large : should I drop it down ?
- any other wrong technique that I am using ?

Also, would you have any tips on improving the groove quality. I sure do not want to throw away the timber. Sanding also seems to be a tedious task since the number of grooves is large. However, I can perhaps increase the depth of the groove (8mm now) by an additional 0.5 - 1mm and rout now with an improved technique like maintaining a continuous push and/or with a lower router speed ?

Please help !

Ciao -

Youvee

[rodm]

If you tell us what type of router bit you were using it would help solve this. If you are using one of those cheap router bits like the ones you get in a wooden box then this is most likely your problem. If you are using a good quality bit then is it sharp and are the cutting edges clear of build up?
Are you cutting the grooves to full depth in one pass?
If you get back with this then I am sure one of the experts will be able to answer your problem.
The push stop technique will not be helping so you may want to change your technique. I am asuming that you are using the Triton Router Table so if this is the case then use finger boards to hold the timber against the fence. Support your plank on the infeed and outfeed sides of the router table. Do a couple of practice runs to work out how and where to grip the timber. Use push sticks/blocks if the pank is narrow.
Anyway that is a start and I'm sure you will get advice from some of the more experienced woodworkers.
You should be able to clean your burns out with a shallow pass but wait until the problem is identified.

[DarrylF]

What Rod said basically - if the bit is real cheap & nasty or blunt, or if it's gummed up with junk, or if you stop in one place too long, or move too slow, you're going to get burn marks. You could try lowering the speed a little, but my guess is you're going to solve most of it with technique - try taking a dry run first to get your stance etc sorted out.

All that is compounded if you try to take big bites - so cutting the groove in two passes will also probably help.

To fix that piece sort out the technique so you can take basically one continuous pass, drop the bit down a little and clean it up.

[soundman]

Burning indicates that the bit is not cutting cleanly and or clearing its waste.

reducing the bit speed is unlikely to solve the problem.

the other problem with eny of the face milling cutters is that the edge speed varies across the face of the cutter.

the outside edge is cutting faster than the centre edge making correct feed rate a compromise.

additionaly cutting an enclosed trench may present chip clearance restrictions.

the harder wood will make th problems worse.

practise on pine at first.(cheap & soft)

I would.
make shure that the bit is sharp. (it is a carbide bit?)
Cut in a number of passes
cut a final pass taking about 1/2 a mm of material.

experiment to achieve the optimum feed rate.

Too fast rough cut & poor controll posible danger.

too slow burns.

Feed as fast as the machine feels sweet.

cheers.