Popular Mechanics (old) machining projects; reliable?

[mikey]

After looking at Mikey's response, and thinking about this setup a bit, maybe it will work. But he says that cutting forces are directed downward. I cannot see how this could be, since there is an axial and tangential component, and the latter is up. Even after torque is neutralized at the compound slide, the force is still up on that flimsy sheet metal bracket. After looking more closely, however, any deflection in the direction of the force will result in the tool tip following a large circular arc out of the cut, so this might add to the stability of the tool. Still, it is surprising that the author tried it and found it to work.

Hey Eric,
Thought I would try to clarify this a bit. Note that the parting tool is mounted upside down and the spindle is turning in the normal counter-clockwise direction. When the tool contacts the work, the tangential forces at the tip of the tool push it up, right? Axial forces (feed forces directly down the body of the parting tool) compound this and add to the total cutting forces transferred to the tool. This happens with all inverted tools used in the rear of the work. On this project, they use a round bar under the tool holder as a pivot and the tool WILL pivot on that bar, transferring forces from the tip of the tool to the rear of that bracket the tool post sits on. Supporting the far end of the bracket is a flimsy Z-shaped piece that is adjustable for some reason; that weird piece is intended to handle the transferred cutting forces as they attempt to reach the structure of the lathe but I suspect it doesn't work all that well. In any case, as with all rear mounted tools, the cutting forces are transferred down and into the structure of the lathe.

As we know, parting tools are essentially form tools, and form tools create huge cutting forces due to their relatively large contact patches. This is why a rear mounted parting tool must be very rigidly mounted. The tool as shown in the article does not instill confidence with regard to rigidity but the IDEA is sound. What they came up with in the article amounts to what is essentially an extension of the cross slide bed upon which a tool post can be mounted.

I don't own a lathe like the one pictured. My lathes have T-slots running the full length of the cross slide so rear mounting tools back there is simple. BUT if I had an older lathe then I would find a way to create a raised platform above the tail of the cross slide so I could mount tools back there. HOW to do it will take some thought and experimentation but I would imagine that one could anchor the spindle-side of this extension to the compound T-slot like they did in the article and lock the rear of the extension into the tail of the cross slide with two bolts that pass through the extension. The bottom of the extension would need to be contoured to match the existing cross slide but I think I would attempt it. If successful, you would have a solid piece of material, probably aluminum, extending over the rear of the cross slide to which parting tools, knurling tools and perhaps turning tools could be mounted. And this extension would be quick to employ and remove.

Having used rear mounted parting and knurling tools for many years, I am convinced that they allow for much greater rigidity than can be found in a front mounted tool on a small lathe. There is no doubt in my mind that if I owned an older lathe that I would already have found a way to rear mount tools back there.

Good luck with this if you attempt it.