Paradise ponders: exploding saws, hydraulic ignorance, and wooden trigonometry edition... Yesterday morning I started on the project of building the cedar shed kit that arrived the day before. The shed is going onto an existing concrete slab, but there's a small challenge there: the slab is sloped (3.5" in 10'), and I'd kind of like my shed to stand up straight. The kit includes a plywood floor built on cedar 2x4 “joists”. After examining that floor's design, I figured out that I could support the floor on four pieces of pressure-treated lumber, nominally 4" thick. The height of each piece would be different, varying from 2" to 5.5", to correct the slab's slope – I actually got to use some trigonometry to figure that out. I'd also cut each piece of wood using a small angle (1.6°) so that the tops of the pieces would all be in one plane and level. Sounds easy! So off I went to Home Depot to buy my four pieces of lumber.
When I returned, I set up my table saw for the first piece, put on my hearing protectors, lit off the dust collection system and the saw, and started to rip the first pressure-treated 4x4. Then ... BOOM! The saw blade stopped and dropped down below the table top. Scared the hell out of me! It took me a minute to figure out what had happened:
the safety system in my SawStop saw had triggered. That system stops the blade by driving an aluminum pawl into it, and that's what you see at right after I raised the blade back up. That experience showed me just how fast that blade really does stop – right freakin'
now! After I regained a bit of my equanimity, I pulled the blade and brake cartridge out of my saw (they had become one); that's the first photo below. I hammered the brake cartridge off the blade (second photo) to see what the blade did to the pawl. The last photo shows what the brake did to my blade (now in the trash). It's interesting to see that the blade stopped within a very short distance – once that pawl engaged a tooth, it
couldn't move very far. SawStop says that engagement time is 5 milliseconds. At 4,000 RPM (unloaded) and a 10" diameter blade, that works out to 10" of blade travel, about 1/3 of a rotation. I don't much like to think about my finger or hand enduring that much cutting action – but it beats the pants off the blade not stopping at all...
So why
did the saw's safety system trigger? My finger never touched the blade. However, the pressure-treated lumber I was trying to saw was quite wet, as evidenced by its weight. Lumber that's sufficiently wet will conduct electricity, and the saw detects human contact with the blade electrically. Because the lumber was conductive, it fooled the saw into thinking that it was in contact with
me, and it triggered. The saw has a “bypass” mode that will let me run it without the safety mechanism. That's what I'll have to do to saw this lumber – after I replace the brake cartridge and install my new saw blade. I
could let the lumber dry out, but then I wouldn't be able to build my shed until about August!
When I ran up to Logan yesterday to pick up a new saw blade, I stopped by an irrigation company referred by my sprinkler contractor, to talk with them about constant pressure pumps. I'm not going to name the company, as I haven't actually done any business with them. They had me talk with their pump expert. It was kind of appalling, really. I am far from a pump expert, but I do know a little bit about them. I understand, for instance, the relationship between pressure and flow rate; I know what friction loss and head height is. I know how to read a pump curve. That dangerously small amount of knowledge made me a certifiable rocket scientist by comparison with their “pump expert”. He was also very fast with the (outrageously) expensive solutions. For instance, he proposed two $1,400 motor-operated valves when I told him I wanted an automatic bypass for the pump. When I showed him how to accomplish the same thing with a $50 check valve, he had trouble comprehending how it would work and was clearly put off by my interference with his $2,800 sale. I don't think I'll be buying my booster pump from those folks! :)
