The Blade Guard Part 1
There are so many ideas on the internet for table saw blade guards…Some are good, some are not so good. The main thing is to determine the need and purpose. Well, the need and purpose is easy: Something to cover the blade and provide some way to deter the user from feeding their hand through the blade or making mistakes like trying to pick up a 1/4" cutoff that is sitting next to a spinning flesh shredder! Dust collection is secondary. Here are some issues that I have found with dust collection with a overhead shroud:
1. They sit too far above the table to be effective or they have too many openings compared to the size of the duct used. The velocity of air decreases with the inverse of distance squared. Change the size of the crossectional area that the air is moving through and it compounds the issue. The biggest problem here is when the small harmful dust gets beyond the influence of the suction from the guard, there is little hope of catching it without moving a large volume of air. Most people use a shop vac. This setup can provide some collection with the right setup. A 3" hose is better. However, without the right guard, collection will be mediocre and you will be breathing in everything that escapes the shroud.
2. The mechanics aren't right. Often times, when a board passes under the blade guard, it pivots or raises and exposes a large area under the guard. If this area is substantially larger than the diameter of the duct, air velocity through the open area will suffer. You want both air volume and velocity.
3. It would be nice if you could use the guard in a variety of configurations: cutting dadoes, cross cutting, and ripping. Ripping with a rail in a vertical tenoning jig could be a challenge…But I think I have it figured out…the dust collection that is, and it's simple! Ripping small strips will be doable also.
I'm sure there are more points and I could keep going on but I want to get to the important stuff now: The blade guard. To address points 1 and 2 I had some ideas. However I still wanted to search the internet high and low for alternative solutions and I found this:
I'm not sure if he made up the shroud or copied it but it's a good idea and addresses several problems with 1 and 2 above. The guard has independent flaps on the front and back and each side moves parallel to the table top independently. If you pass a board under the guard to the left, only the left side moves upwards, etc. With this independent setup, it restricts the direction the air is coming from and also dynamically changes the cross sectional area through which the air can move. It looks to be a good solution. With the right overhead mount, offsetting the blade guard and opening one side can provide decent collection and enable operations with a tenoning jig, ripping small strips etc.
However, there are some minor issues with the design…I think. The front and back flaps may open the guard too much which will decrease the air speed in the opening. A bristle door sweep for an industrial garage door may work better on the front and back of the guard. With the length of the sides and possibility of opening too much area for air to flow beyond the already open areas in the guard, a 2.5" or 3" will be ineffective…So here is my plan: I will build the guard similar. It will have independent sides. I will experiment with the front/back flap and see if bristles work better. I will use 4" duct to connect this to my collector system. I will close off the blast gate on the table saw cabinet to an effective 5" opening. This will give me a total cross sectional area of about 32 sq inches which is less than my 7" duct but I may be able to play with numbers….So here is what I have completed so far: