> From: John Figie [mailto:zephyr9...@gmail.com] > > John Dammyer said > " The tables found online suggest that torque for a 7/16-20 thread is in > the range of 23 ft-lb or more if the material is harder. So assuming the > drawbar isn't the hardest steel in the world then 23 ft-lb is used to > tighten the drawbar onto an R8 collet holding a 7/8" diameter tool." > > Here is my experience as a hobby machinist. > > Now I have a manual bridgeport with an R8 spindle and got to thinking about > 23 ft-lbs. That does not seem like a lot to me, but I have a pretty short > wrench - 9" long that I use to tighten the drawbar with. I usually make it > pretty snug with the 9" wrench. Then I took a torque wrench and measured > what I took to loosen the drawbar and I was just over 20 ft-lbs. I played > around tightening and loosening with the torque wrench and without to get a > feel for how tight I usually make the draw bar and I think it is between 20 > and 30 ft-lbs. I think drawbars are not mild steel so that seems like a > reasonable torque. I have never broken my drawbar. > > So my Bridgeport does not have the little set screw that keeps the collet > from spinning in the spindle. It was missing when I got the machine and > have never got around to putting one back in. Although not proper, I think > it is quite common for this screw to go missing. Most often I use end mills > that are 3/8" or smaller. I have used drills up to 1 1/16" in diameter but > always in a pilot hole. I have used a 2" dia indexable end mill but again > with light cuts for facing. And also a boring head - light cuts again. > With all of this stuff to my knowledge I have never had a problem with tool > pullout or tool slipping. > > Regards, > > John Figie Hi John, That matches my torque tests too. Except I do not have a spindle lock on mine. Instead I use a WUT under the draw bar nut. A long time ago Rick Sparber wrote an article in Home Shop Machinist on creating a WUT (Washer Nut).
Tightening/Loosening the drawbar is now a two handed operation but I can tighten up to at least 20 ft-lb. I also find that my V-Belt to the motor slips if I have the AC Servo holding position and I try and untwist. The motor starts to turn and then the belts brake free. So I'll guess that LCNC will default out due to the closed loop spindle control if a tool (or belt) starts to slip. John Dammeyer _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
