Compound Thread Drawbar

[Stan]

I may have posted this before but the thread on MT collets in a mill brought it to mind.

If you put the standard coarse thread on the collet end of the drawbar and a fine thread at the top end, you can achieve high pull in power with less energy expended and when is is time to remove the collet, just turn the drawbar the opposite way and it pushes the collet out. No hammering and beating the bearings is necessary.

My collets use 3/8" X 16 TPI drawbar and I was able to thread the inside of the spindle with 7/16" X 20 TPI. If the ID is not compatible for this, you may be able to add an auxiliary nut to the top of the spindle with a threaded ID.

I put abput 2" of fine thread at the top of the drawbar and screw it into the spindle about 1". I then screw the collet onto the other end, finger tight, and then tighten the drawbar Do not over tighten because the pull in force is a function of the difference between the thread pitches. To remove the collet, unscrew the drawbar until the collet pops loose and unscrew the collet with your fingers.

 

[deverett]

Stan

My recently purchased mill has a self ejecting drawbar system like you describe and I find it a right pita. My old mill had a captive drawbar by means of a simple nut on the top of the spindle which achieved the same self-ejection. (Undo the drawbar and the head contacted the underside of the nut to eject the tool).

I have to change drawbars from time to time because some tools are threaded M10, others 3/8" BSW and I even have one with a 5/16" thread. With the captive nut system, it was a lot quicker to change drawbars. Eject the tool, spin off the nut, change drawbars, insert the new tool and spin up the drawbar by finger pressure before the final nip up tight by hex key and on with the captive nut.

The present system means I have to use a hex key all the way and the short end is too long to be able to turn it more than about half a turn to undo the drawbar. (Reminder to self: Must cut back the short arm on the key to full turns).

Dave
The Emerald Isle