Originally Posted by
rwskinner
There are several factors that people over look when they state 20 ft.lbs of torque on a 7/16" draw bar will cause XXX clamping force.
One, almost all the folks use the dry thread clamping force calculations, and rarely are drawbars assembled dry. Antiseize, grease, oil on the threads will increase clamping force much higher than when dry while using the same torque.
The next thing that really affects the clamping force at a given torque is Drawbar material, and drawbar length. Unless these are taken into account you are pissing in the wind. You have to take into account how much the draw bar will stretch at a given torque.
Most people use a calculator design for a short cap screw with dry threads, and not a stud that is 18 or 24" long. Big mistake.
Now with that all out of the way, seems most folks with 700 to 800 lb springs do fine while others claim to need 2000-3000# spring pressure. This has a lot to do with tool length, feed rates, DOC, material and so on. Lots of variables. Much to do with drawbar, and collet prep.
1000# springs with a 1200# pressure from a 4" cylinder is generally ample for small machines like bench tops. If you want more, you can increase the springs then change the cylinder arrangement such as multi-stage cylinders, or use lever ratios to increase the force.
For Example: I use a 1.500 cylinder @ 100 psi that puts out right around 175#. I push a Class 2 type lever that increases the force by 6:1. This yeilds 1060# on the drawbar which over comes approx 650# of springs.
The above was the math for Hoss's drawbar, I just did all the math.
Angle of the lever can greatly reduce the amount of force applied to the pressure point or draw bar so be sure to do the math on that as well.
On mine, on a SX3, using Tormach Tooling, I have not had a cutter pull out yet, however rarely do I do DOC more than .100 per pass with coolant.