wrote in message
ups.com...
I have a DeWalt DC390 18v cordless circular saw. This takes 165mm
diameter blades on a 20mm arbour. I can't find any blade finer than
30 tooth to fit this saw. I have seen a Trend 48 tooth blade that has
the same size arbour (20mm) but is 160mm diameter (5mm less).

I have 2 questions. The first of course is if I fit the smaller Trend
blade, will it be dangerous? Secondly, will it give me the fine cut
that the blade is designed for?

No. It will not be dangerous except for the fact that the blade guard may
not cover the teeth on the upper side of the saw.

Whilst on this subject, am I right in thinking that the smaller the
blade diameter, the fewer teeth are needed for finer cuts?

No. That is not correct thinking. You can simulate more teeth by feeding
the saw slower. In addition to fewer teeth you will also have a slower
tooth tip speed because of the smaller diameter and this will compound the
problem. Feed slower to help off set this effect. More teeth tend to make
finer cuts because each tooth takes a smaller bite out of the wood. If you
slow down the tip speed the teeth will tend to take bigger bites and
typically will give you a rougher cut.

For example,
the trend described above has 48 teeth around its circumference. This
gives 0.095 teeth per centimetre of circumference. On a 300 mm diameter
blade, to get 0.095 teeth per cm you would need 90 teeth around the
circumference. Which is a "fine" or even "extra fine" cut if
you read the adverts. Is this sort of conversion the right thing to do?