BentTooner wrote:I can see that the cutting tip of the tool (which touches the part) must be wider than the rest of the tool. Otherwise, the kerf will not be wide enough to let the rest of the tool move into the groove being created in the part. That seems to be happening to me.
If I have that right, I think I can see why the wider, tapered, square-tipped tool I have works so well. That wider tool is straight out of the box, too, but it is 0.140" wide at the cutting tip, tapering to 0.125" at the back. Is this the concept of " relief " ?
It is, but with clarification. What's important is that the taper begins at the cut, not back of the cut. While any relief is better than no relief, if the tool remains the same width for any distance, and degeneration of the cutting edge results in a narrowing of the cut, which now yields the same problem. The less area in contact, the better.
The parting blade I have, on the other hand, is just straight all the way and is only 0.058" wide with NO TAPER AT ALL. You can measure it anywhere along its length and it's the same 0.058".
So, how do you get a skinny parting tool like this to work? Do you leave the front 1/2" as is and grind back the rest of the tool's length to 10/1000's less than the tip? That seems odd... and tricky.
Yes, it is tricky, but not impossible. It's tougher on a tall blade like a commercial parting blade, because there's so much area to grind, but a large diameter grinding wheel and a steady hand can yield surprisingly good results. Also, and this method is not commonly accepted, is that you can use the side of the wheel. That's common practice when grinding tools, anyway---you just have to be mindful to not horse the grind. Wheels typically do not have great strength when pressure is appied that way, but one doesn't use pressure when properly grinding the tool, so it's perfectly acceptable. The amount of taper need not be excessive---a couple thou can work, and, as you alluded, you'd grind it somewhat farther back on the tool than the depth of cut you expect to take. That can limit the tool's usefulness to nothing larger than that diameter, however, so give some thought to relieving a tool. Once you've damaged the cutting edge, you restore the tool by removing the ground portion.
When you grind such a relief, you must duplicate the grind on both sides of the tool, with the narrowest portion of the tool being the back of the grind, at the bottom. The top, moving straight up, would be narrower than the blade by a few thou, and would then taper towards the cutting edge on both sides until the tip is the widest portion of the tool.
Maybe I'm still missing something but it's hard to see how this parting blade I have would ever work straight out of the box. And, dressing it nicely along the entire length (4") would be tricky. Maybe I could mill it down...
Ok, Torch has already enlightened you about milling cutting tools. You now know they must be ground, not milled.
To use such a tool with reasonable success, it's important that it be set up such that its movement is dead parallel to the sides of the tool. It is equally important that the cutting edge by extremely sharp, and that the top of the tool for the depth of cut is not wider than the tip. We're back to the same issue, where you need clearance. Some tools don't have dead sharp corners, so by the time you've relieved the top enough to remove the traces of corner radius, the cutting edge is narrower than the top of the tool. Another consideration is that the tool must be perfectly erect. If it leans to either side, it won't perform, and is likely to stall the spindle (or break the tool).
You can use such a tool with modest success, but it will always be troublesome. That's the reason I don't use them, choosing to hand grind my parting tools from blanks instead. They're slow business to grind, but I can tailor the tool to the job and get the kind of performance I like. For the record, were I running my machines for gain today, I'd probably migrate to insert tooling, which has improved immeasurably since I closed the doors on my commercial shop (1983).
I encourage you to pursue the art of hand grinding lathe tools. My favorite saying is learning to do so will set you free. That will make sense to you the first time you need a tool that may not be available, and you grind it yourself.