Here is a question for a machinery design engineer or bright lathe operator.
Hopefully you are familiar with a typical lathe. I believe my Vectrax
15 x 50 lathe is typical. I have been told it is one of the better import brands. Either way, I have had it for 8-10 years with no problems - till now.It broke when I failed to disengage the half nuts and the tool post base ran into the work. I was threading a 1-3/4-4 ACME "nut". I had successfully done about 29 pieces and was about to finish my last one when I got lazy and didnt get the threading half nuts disengaged in time and BAM. My lathe has a shear pin located in the coupler that drives the acme threaded "threading" rod. The rod I am talking about runs the full length of the lathe. The half nuts engage this rod and drive the carriage along to do threading operations.
Enough background. I am questioning the selection of the shear pin material the mfg used. The threading rod has 4 TPI. I built an exact replica of the coupler to test various materials to use in the replacement shear pins. The coupler on the machine is basically two collars 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 to axis I mean the center of the shear pin is located .794" out from the center of the threading rod. They used a single 5MM (.197") roll pin as a shear pin. It is supposed to break when an unexpected load is felt. Well, it didnt break. In fact, the threading rod pushed so hard that it broke the casting that holds the threading rod to the lathe frame. This casting is located on the far right end (away from the chuck) of the bed frame.
I built a shear pin testing tool that has the same dimensions used in their coupler, only in mine I am able to use a torque wrench to measure the torque at which the pin will break. I also used a slightly smaller hole for the pin. I used a 3/16 diamter pin in my design instead of the 5MM (.197") pin.
Based on my tests using various pin materials heres what I got:
Alum sheared at 70 ft/lbs
360 brass sheared at 90 ft/lbs 1018 CR sheared at 120 ft/lbs Roll Pin sheared at 160 ft/lbsUsing an alum pin and 70 ft lbs, I calculate that the threading rod would push on the casting (and the carriage) with 21112 lbs of force before the pin would fail. Using a Roll Pin, there would be a whopping
48255 lbs of force. Seems to me they have screwed up in their design of the shear pin. This casting is only held in place with two 6MM bolts and two 3/8 dowel pins. The only reason the carriage stopped advancing is because the casting broke and allowed the threading rod to move about half an inch to the right, which allowed the coupler to seperate enough to pull the "shear pin" out of the hole.I am thinking of at least going with an alum shear pin instead of spring steel Roll PIn. What do you all think? Is there anything else I could do to design a more crash proof design.
Any comments would be greatly apperciated here and to my email address.
thanks Steve Bales