From: bennett@adobe.com(Bennett Leeds)
Subject: Re: Trouble Ripping Hardwoods
Reply-To: bennett@adobe.com
Organization: Adobe Systems Incorporated
Distribution: na
Date: Sat, 25 Jul 1992 00:04:51 GMT

24224-guthrie writes
> In article <92205.164804DMEJJ@CUNYVM.BITNET> DMEJJ@CUNYVM.BITNET writes:
> >I was wondering if anyone else has this problem.  Whenever I rip hardwoods
> >against the rip fence, the wood gets burn marks along the cut...
> >I know that the rip fence is paralel to the blade.

> My first guess is that the fence isn't *really* parallel.

This is possible. If the rear of the fence is a thou or two closer to the blade 
than the front, the rear teeth will rub against the wood generating heat, 
unless the teeth are sharp enough to cut off that thou or two cleanly. 

What's less obvious is that if the fence is significantly off you won't get 
burning either - you'll get a cove shaped cut. The reason you don't get burning 
on a cove cut is that the body of the blade doesn't contact the work, the chips 
and dust just fall down out of the way, and the teeth aren't rubbing against 
the wood since the kerf is so wide.


> Assuming that the distance *is* the same, then you may be extending the
> blade too much. It's not a good idea to extend the saw blade much past the
> surface of the wood. 

Wrong. The exact opposite is true. By not extending the blade past the work, 
the teeth stay in the cut longer, causing the teeth to get hot from rubbing 
against the wood more. In addition, sawdust can't escape and is rubbed into the 
work by the teeth, increasing friction and heat.

The old adage about not extending the blade above the work was a safety guide. 
You're better off raising the blade at least an inch above the work and using a 
good blade guard. This lets the chips and dust escape, and gives the teeth a 
breather before coming down and cutting. It also results in having a more 
vertical cutting path, which is shorter than a more horizontal cutting path.


> If you extend the blade too much, the rear teeth may burn the wood a little.

If lowering the blade is reducing burning, then the rear of your rip fence is  
almost definitely closer to the blade. Let's say the fence is one degree out of  
kilter. Think of two lines crossing each other at a one degree angle. As you  
move further from the intersection the distance between the two lines becomes  
greater. What lowering the blade does is decrease the distance of the rear  
teeth from the front teeth, thus reducing the offset distance, which is what  
causes rubbing. And rubbing causes heat. Heat causes burning. Another thing
lowering the blade does is reduce the chance of having the blade's body
contact the wood.

The proper fix is to align the rip fence exactly, or at least make sure the 
back of the rear is slightly further away from the blade, and then raise the  
blade.

Forrest Manufacturing, a blade maker, also recommends raising the blade for
ripping.


> Take it slow with a 60. 

Wrong again. The slower you go the longer the teeth rub in the kerf and so the 
hotter they get. Take an extreme case where you actually stop pushing the wood 
part way through the cut. If you've been having a tendency to burn, this will 
produce a definite burn mark. As brisk a pace as you can safely perform is
the ticket. 


> A 60 tooth planer blade (for example) would not be the best choice.

Right. Most of the blades with high numbers of teeth are made are made for 
crosscutting, where burning isn't as much of a problem. On some Freud crosscut 
blades, for instance, the tolerance between the teeth and blade body is so 
close that they've coated the blade with teflon to try to reduce friction and 
heat from the blade's body.

- Bennett Leeds
bennett@adobe.com