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Table saw blade height #1: Setting up for through cut

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Blog entry by momiji posted 10-19-2010 12:15 AM 3008 reads 0 times favorited 7 comments Add to Favorites Watch
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Maybe there is something I don’t understand. I can’t figure out how setting blade height to barely clear the gullet of the blade above the piece of wood being cut is appropriate.

I can, however, figure out two good reasons not to set the blade anywhere below its maximum height: First, considering that heat dulls the carbide tips and that heats builds up with friction and friction is proportional to the lenght of travel of the teeth in the material being cut, setting your blade low, let’s say with an average angle of fifteen degrees from horizontal with the piece of wood being cut , increases the length that the carbide tips are in contact with the wood to nearly 4 times compared to an average angle of seventy-five degrees! Do the math: cut a piece of one inch thick at an average angle of 75° and the blade travels 1.0353” through the wood. At 15°, that travel becomes 3.8637”. If you want the carbide to cool off, give it air, not wood…

Moreover, the gullet of the blade which carries the hot shavings away will do so for a longer period (ok, it’s about 1 millisecond for a 10” blade, but the principle ramains) thus transfering more heat to the blade, increasing blade wobbling, resulting in increased friction, etc.

Secondly, a low angle of cut, combined with negative hook angles of some blades, results in an attack angle that forces the piece of wood upward, towards you face, where a high angle of cut results in a mostly downward pressure from the cutting tips.

Something I miss?

Care to share on that?

-- Ah! The loving smell of heated Acer Saccharum...



7 comments so far

View interpim's profile

interpim

1132 posts in 2110 days


#1 posted 10-19-2010 12:46 AM

I think the main reason I was taught to put the gullets just over the wood height was more for safety reasons, as in the less blade above the wood the less likely your hand is going to wander near it. Then again, I learned on an old saw that didn’t have a blade guard, and riving knives weren’t even an option.

-- San Diego, CA

View A10GAC's profile

A10GAC

189 posts in 1730 days


#2 posted 10-19-2010 01:21 AM

When I was just starting; I was told that the difference between just above the work and maximum height was the difference between cutting your hand or losing a part of it completely. I guess the thought behind it is if you get pulled into a low blade you might just get lucky and only get cut.

Honestly, I never gave heat buildup much thought before; the biggest cuts I make are in sheet goods. It takes me a bit to setup between cuts; so, I guess the blade would be cooling during setup changes. I’ll have to see what kind of heat builds up in the blade next time I use the saw.

-- Men have become the tools of their tools. - Henry David Thoreau

View Derek Lyons's profile

Derek Lyons

584 posts in 2220 days


#3 posted 10-19-2010 02:49 AM

Yes, you’ve missed the safety implications. The more blade exposed, the higher a chance of intersecting it with your tender flesh. Also, if your wood pinches the trailing edge, a low blade will launch it horizontally while a high blade will launch it vertically – towards your face.

I suspect if you formally measured the temperature, you’d find there is virtually no difference between a high setting and low setting – because if the blade isn’t exposed (to shed dust and to cool) above the workpiece, then it’s so exposed below the workpiece. (I.E. the circumference of the blade is fixed, the amount in contact with the wood virtually so.)

-- Derek, Bremerton WA --

View charlie49's profile

charlie49

54 posts in 1486 days


#4 posted 10-19-2010 03:35 AM

momiji;

Please read this article from Router Forums and realize that the gentleman only had his blade a little above his work.

Charlie

Belton, Mo

http://www.routerforums.com/shop-safety/16865-one-post-i-never-wanted-make.html

View WannaBBetter's profile

WannaBBetter

62 posts in 1454 days


#5 posted 10-19-2010 05:37 AM

I have scars to remind me to raise the blade to only just above the work piece

-- I cut it three times and it's still too short

View JimDaddyO's profile

JimDaddyO

287 posts in 1731 days


#6 posted 10-19-2010 02:39 PM

took in a table saw seminar, and Dave from the Sawshop recommended having the blade at full height. His reasoning was that the blade runs cooler, and that the blade is not pushing the wood back at you so much. He also recommends a slow feed for a nice cut and had examples to show. I tried it, and I think it might be OK on wider cuts, but on a cut that is a bit closer to the fence, I find my hand a bit too close for comfort to the blade, even using the push stick. He mentioned that kickback is a more common hazard than cutting yourself, I can see his logic, but I think I will try to keep the shiney spinning thing farther from my fingers.

-- I still have all my fingers

View momiji's profile

momiji

11 posts in 1614 days


#7 posted 10-21-2010 04:26 AM

Thanks to all for the input on this subject because it made me deepen my thoughts on the matter which are:
- I didn’t mention anything about safety since I presumed everyone was using the recommended safety apparatus on all through cuts which I admit is sometimes cumbersome, especially with thin ripping or cutting small pieces.
- As far as measuring temperature, I guess I could set up an experiment with an infrared thermometer to see if the increased cooling time really makes a difference. I could also measure amperage because it will be directly proportional to the amount of work the blade does which I suspect will also be proportional to the temperature build-up. And I could do a Rockwell test on some teeth just after the cut because hardness is inversely proportional to temperature. But my shop isn’t a lab and I don’t feel like doing it!
- As for kick back which mostly occurs when there is pinching at the trailing edge, it is true that it will push the stock up. Then again, a riving knife or splitter helps reduce the amount of pinching and a blade guard and/or anti-kickback paws will eliminate kickback altogether.
- I also had forgotten resistance from the stock being cut: a high blade will force it down and the friction with the table top could be problematic. With a low blade, resistance will push against you hand and if one slips… For the moment, that resistance is the most accurate way for me to know that the blade has dulled.
- There is the issue of tearing I had not addressed. The angle of attack, the relief angle of the teeth and the gullets are also cause for concern just as their number and size. Just looking at the blades catalog of any major manufacturer makes you realize that you should almost have one blade for each material that you cut and the direction in which you cut it! That’s why scoring blades exist: to prevent tear out. And I do try to avoid slow feeding in dense material to avoid burning.
- I’m not a metallurgist but if I remember correctly, heat absorption is proportional to mass. Would that mean that thin kerf blades don’t stay sharp as long as regular ones?

-- Ah! The loving smell of heated Acer Saccharum...

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