I've just purchased a Trend sunken hinge cutter for kitchen cupboards I'm making in MDF and cut two test holes each inside edge of the holes are burnt black. What causes this?
A: The fact it's man made Fibre
B: Router running too fast [set at max]
C: The bit is blunt [even though its new bought yesterday]

At that price am I expecting too much for cutters ablility to cut further hinge holes after the 16 I've got to cut? if not then that makes each hinge hole expensive to cut [just over 87p a hole] and will have to have a re-think on types of hinges, maybe cranked type for surface fitting doors.

Thanks in anticipation.

Rob, March 2008

Well I never JD you learn something every day as I always thought those cutters where used in a router as they were sold on the router bit shelving in B&Q, a bit like saying an orange found in amongst apples on a fruit stall makes it an apple, pays to read up first don't it.
So out comes my home made board support stand and try test cuts on my ped drill to get to the right speed.

As for jig saws I still try to get to grips with cutting square edges, straight or curved, suppose it's my being impatient when the cut ends up at an angle resulting in much sanding to return the edge to 'square' getting there though cut one yesterday and it's perfect by cutting with less forward pressure. One I did I covered up the cock up by turning it in to a 'Bullnose edge worked a treat, friends comments were 'I like the bullnose edge it makes a change from square'. I think the problem is too much pressure to try to get the cutting over quicker. As they say 'don't blame to tools blame the operator'. And the other 'You get what you pay for' if the tool is not working as expected.
Many thanks to both you and John
Have a nice week
Rob

Rob, March 2008

Rob - an after thought:
Sounds like you have an interest in the production economics of your hinge sinking operation! It might or might not interest you to know therefore, that there is a very simple equation called the Taylor Tool Life equation. It relates the life of the tool (T - in minutes or seconds usually) to V (the cutting speed in m/s).
V multiplied by T to the power of n equals a constant. (n depends on the material being cut/tool material).
That means that the higher the cutting speed, the shorter the tool life (sounds pretty obvious really!).

Any way, if the hole is blackened on the inside, it's not a big deal - no-one is going to see it after the hinge is fitted!

JD, March 2008

Apologies for not reading your question in sufficient detail.
Cutting speed is definitely your problem!
Routers rotate at speeds 10 times higher than drills! The no-load speed of a router is typically in the range 8,000 to 25,000 rpm whereas a drill will only run at perhaps 2,500 rpm.
You have to remember as well that it is velocity at the tip of the cutter which counts - not just the rotational speed. The larger the diameter of the cutter at a given rotational speed, the higher the linear velocity at the tip - and that's where the burning happens.
I dug out an old router bit catalogue (Trend actually!) and they recommend that for hinge sinking cutters over 25 mm in diameter, that you run it in th erange 2,000 -12,000 rpm to avoid burning. It also recommends: USE WITH PILLAR DRILLS ONLY - which is what I suggested before!
Good luck and don't forget to wear your goggles!!!!

JD, March 2008

Sorry JD but B: in my question did refer to a Router. Perhaps from both yours and Johns suggestions I should try more test cuts reducing the speed of the router a notch at a time to see what speed produces a clean cut. I always thought Trend cutters were reasonably good as I use Trend router bits in my DIY projects.

Rob, March 2008

The blackening of the material being cut is due to excessive friction/rubbing between the cutting tool and the workpiece. I would guess that you are using this in a hand held portable drill. It is less likely to happen if the drill is mounted on a drill stand or the cutter is in the chuck of a pedestal drill - i.e. the cutter will drill straight down with less side to side movement.
Dwell will cause it to blacken the bottom of the hole.
The same thing happens with a jigsaw if you try to cut a very tight radius (i.e. relative to the width of the blade). The black is carbon due to burning the material - slower speed will also help as it will reduce the amount of work being converted into heat in friction and reduce the temperature!
As the other chap said, MDF is very abrasive anyway.

JD, March 2008

Thanks John but it wasn't the cutter that burnt it was the side of the hinge hole in the MDF with slight burning on the bottom of the hole. I assume try the same slower speed a

Rob, March 2008

The cutter has turned black due to overheating, which in turn takes the edge off the blade. Try running the tool slower if you can, or reduce pressure on it. MDF is an abrasive material anyway, due to the volume of adhesive used in its manufacture. Maybe a TCT tipped bit is the way to go - around £13.

John, March 2008