BP Drawbar Shear Pin Question

In eighteen years of teaching high school freshmen and sophomores I have never had a student break a BP drawbar in that fashion. Perhaps you are putting WAY too much oomph into your spindle tightening.

Errol Groff

Instructor, Manufacturing Technology H.H. Ellis Technical High School

613 Upper Maple Street Danielson, CT 06239

New England Model Engineering Society

I don't know, I'm not really putting that much torque on it, especially not the last time. The hollow roll pin I put in there definitely doesn't take much to shear.

My guess is that it is NOT a shear pin, ie. designed to break first, before something else does. It most likely is just the easiest way to make it. I don't think my drawbar (aftermarket) has a shear pin. I'd use a taper pin.

Jon

When I made up the 3 drawbars for the Gorton (uses any 30 taper), I welded the hex on, after shearing the first pin.

Gunner

"Pax Americana is a philosophy. Hardly an empire. Making sure other people play nice and dont kill each other (and us) off in job lots is hardly empire building, particularly when you give them self determination under "play nice" rules.

Think of it as having your older brother knock the shit out of you for torturing the cat." Gunner

That is not a shear pin. It is an assembly pin. I have made many drawbars and I have tried turning them from solid bar. This usually fails unless the steel bar has been totally normalized (annealed). Even with a follower rest, the bar bends as its diameter is reduced. So, it is much cheaper to pin a Hex on a piece of hotroll bar and thread the other end. Normally a solid straight pin around .150 is used. Steve

Thanks.

Thanks, taper pin and mabe some loctite 609 it is.

FWIW I replaced the pin on my old Mill Drill with a .187 (3/16) piece of stainless, never had any problems after that.

--.- Dave

According to Steve Lusardi :

Hmm ... you might be interested in the drawbar for my Nichols horizontal mill's vertical attachment. It consists of two parts:

1) The bar, with two different threads on the two ends -- one to mach either of the common 40-taper holders threads. 2) A cylindrical piece, about an inch long, with different threads in each end, and with a pair of opposed flats milled in the middle.

So -- you swap ends on both pieces, and it serves as a drawbar for the other style of thread.

You could mill three pairs of flats to give a final hex if you so desired.

Obviously, if you are talking R8 collets, this is not a problem, but someone somewhere up-thread mentioned 30-taper collets -- similar enough to 40-taper so the problem might exist there as well.

Enjoy, DoN.