unbreakable shear pin on Vectrax Lathe

steve bales · 2004-01-05T03:46:11+00:00

Here is a question for a machinery design engineer or bright latheoperator.Hopefully you are familiar with a typical lathe. I believe my Vectrax15 x 50 lathe is typical. I have been told it is one of the betterimport brands. Either way, I have had it for 8-10 years with noproblems - till now.It broke when I failed to disengage the half nuts and the tool postbase ran into the work. I was threading a 1-3/4-4 ACME "nut". I hadsuccessfully done about 29 pieces and was about to finish my last onewhen I got lazy and didnt get the threading half nuts disengaged intime and BAM. My lathe has a shear pin located in the coupler thatdrives the acme threaded "threading" rod. The rod I am talking aboutruns the full length of the lathe. The half nuts engage this rod anddrive the carriage along to do threading operations.Enough background. I am questioning the selection of the shear pinmaterial the mfg used. The threading rod has 4 TPI. I built an exactreplica of the coupler to test various materials to use in thereplacement shear pins. The coupler on the machine is basically twocollars that are arranged such that they are against one another.There is a single shear pin running parallel to the shaft and about.794" axis to axis, that ties the two collars together. By axis toaxis I mean the center of the shear pin is located .794" out from thecenter of the threading rod. They used a single 5MM (.197") roll pinas a shear pin. It is supposed to break when an unexpected load isfelt. Well, it didnt break. In fact, the threading rod pushed so hardthat it broke the casting that holds the threading rod to the latheframe. This casting is located on the far right end (away from thechuck) of the bed frame.I built a shear pin testing tool that has the same dimensions used intheir coupler, only in mine I am able to use a torque wrench tomeasure the torque at which the pin will break. I also used a slightlysmaller hole for the pin. I used a 3/16 diamter pin in my designinstead of the 5MM (.197")...