Brake on a Shopsmith?

wa2crk · Post by **wa2crk** » Tue Mar 01, 2011 1:53 pm

a saw blade revolving at 3450 rpm and having 60 teeth will have 3450 teeth pass a pencil point per second. If the reaction time of the system is 5 milliseconds that means a minimum of 15 to 17 teeth will have to cut into the hot dog before the system reacts. Depending on how fast the hot dog is moving determines the severity of the "cut".
Many accidents are caused by losing control. When making a cut with a table saw your hands are moving relatively slow. If you slip, your hands are moving considerably faster so you will move farther into the blade before the system reacts.
Also consider the speed of the perimeter of the blade when considering kickbacks
The circumference of a 10" blade is 31.14"
times 3450 RPM = 107433 inches per minute
107433 divided by 60 = 1790 inches per second
1790 IPS divided by 12 = 149 FPS
149 FPS / 88 FPS (60MPH) = 1.7 speed factor
88 FPS = 60 MPH, therefore 60 MPH times 1.7 (speed factor) = 102MPH
Soo a chunk of wood being kicked back by a 10" blade will come off the saw at approx 102 MPH form about three feet away. I don't think that I am fast enough to duck.
Bill

jcraigie · Post by **jcraigie** » Tue Mar 01, 2011 4:38 pm

Math lesson for the day complete. That's pretty darn fast. Sooooo I think I will continue to stand to the side of the blade whenever possible instead of trying to play dodgeem.

foxtrapper · Post by **foxtrapper** » Wed Mar 02, 2011 6:20 am

No real reason one couldn't use a soft-stop power switch. All you do is rig the unit up so that when the switch is in the off position the motor is wired as a generator. That will bring it to a halt quickly. That's what the various hand held circular saws with a brake use.

JPG · Post by **JPG** » Wed Mar 02, 2011 1:52 pm

foxtrapper wrote:No real reason one couldn't use a soft-stop power switch. All you do is rig the unit up so that when the switch is in the off position the motor is wired as a generator. That will bring it to a halt quickly. That's what the various hand held circular saws with a brake use.

They also have 'universal' motors with permanent magnets(maybe)! The universal motors become a generator when 'turned off' since they be rotating in a magnetic field(those without magnets utilize the collapsing field of the 'field windings'). All they need is an electrical load to that generated electricity to cause a 'braking' action.

SS and larger TS motors are a different animal! They do not 'generate' unless externally 'excited' and then not very well!

dusty · Post by **dusty** » Wed Mar 02, 2011 1:59 pm

JPG40504 wrote:They also have 'universal' motors with permanent magnets(maybe)! The universal motors become a generator when 'turned off' since they be rotating in a magnetic field(those without magnets utilize the collapsing field of the 'field windings'). All they need is an electrical load to that generated electricity to cause a 'braking' action.

SS and larger TS motors are a different animal! They do not 'generate' unless externally 'excited' and then not very well!

What voltage would you expect to see in the output of a motor being driven as a generator (without external excitation).

If you unplug the Mark 5/V and put a meter on the a.c. power terminals and then spill the shaft with a drill motor - you will measure a voltage.

JPG · Post by **JPG** » Wed Mar 02, 2011 2:04 pm

wa2crk wrote:a saw blade revolving at 3450 rpm and having 60 teeth will have 3450 teeth pass a pencil point per second. If the reaction time of the system is 5 milliseconds that means a minimum of 15 to 17 teeth will have to cut into the hot dog before the system reacts. Depending on how fast the hot dog is moving determines the severity of the "cut".
Many accidents are caused by losing control. When making a cut with a table saw your hands are moving relatively slow. If you slip, your hands are moving considerably faster so you will move farther into the blade before the system reacts.
Also consider the speed of the perimeter of the blade when considering kickbacks
The circumference of a 10" blade is 31.14"
times 3450 RPM = 107433 inches per minute
107433 divided by 60 = 1790 inches per second
1790 IPS divided by 12 = 149 FPS
149 FPS / 88 FPS (60MPH) = 1.7 speed factor
88 FPS = 60 MPH, therefore 60 MPH times 1.7 (speed factor) = 102MPH
Soo a chunk of wood being kicked back by a 10" blade will come off the saw at approx 102 MPH form about three feet away. I don't think that I am fast enough to duck.
Bill

This is the reason(very well explained BTW) I would like to see Mr. (SMUG) G..... throw himself onto the saw blade of one of his 'super responsive, super safe, the answer to all our unsolicited needs', Made in China saws instead of slowly inching up onto the side of the blade with a 'moistened with salty water' finger!

What he claims and what he demonstrates differ significantly!

END RANT!:D

Oh! I remember from somewhere the speed of the saw stop is 5000 rpm. that increases the tooth count to 25/5ms and workpiece missile speed to 148 mph!

JPG · Post by **JPG** » Wed Mar 02, 2011 2:46 pm

dusty wrote:What voltage would you expect to see in the output of a motor being driven as a generator (without external excitation).

If you unplug the Mark 5/V and put a meter on the a.c. power terminals and then spill the shaft with a drill motor - you will measure a voltage.

Ok! So there will be some residual magnetism retained by the poles in the 'ac induction motor' due to the collapsing field when the motor is turned off. The field strength will vary depending upon the timing of the 'turn off' relative to the currents flowing in the coils surrounding the pole pieces.
That in turn causes the generated energy to also vary.
Since the strength of the residual magnetic fields is small, the amount of energy 'generated' is also small.

Now IF you pass a dc current through the coils, a strong magnetic field can be induced in the poles(not as effectively as a universal motor) and that increases the energy 'generated', but that energy is generated internal to the 'armature' and the internal current flow force opposes the inertia of the coasting armature rotating in the induced field.

Spin that induction motor 'generator', then short out the motor coil wires and see just how much the generated energy decreases the coast down time(brakes).

A lot of oversimplification used here, but the principle be the same!

terrydowning · Post by **terrydowning** » Wed Mar 02, 2011 3:04 pm

I think I'll just keep myself out of the way until the darn thing stops.