Jim Jakosh wrote:I have been cutting thread for 45 years and learned at Western Electric. I have always cut threads by stopping the cross slide on a zero and advancing the compound until the pitch diameter is reached. I never got a tapered thread and the threads are clean and usable. If you use the little plate with the the v grooves in it to orient the bit to the part at set up, your thread form will be accurate. And I believe I learned the right way like you did!!
I hadn't done much if any thread cutting since my apprenticeship in the 80s. When I got my lathe (a 12 x 36) set up in the shop, I jumped right in to refresh my memory. My attempts @ cutting a few different sizes on steel & alum' turned out fine. When I spoke to my brother who has a lot more machining experience than me he suggested the following tip:
When zeroing the cross slide & compound, adjust both so that the handle of the cross feed is positioned @ 10 or 11 o'clock. When I asked him why, he said that in that position when you turn the wheel "down" @ the end of the cut, the cross slide is pulled away from the workpiece.
Harold_V wrote:In parting, I'd suggest to you that if you have not tried this method, do not form an opinion, assuming you have a reasonable lathe. If you keep a sharp tool and enjoy threads that rival threads that come from a thread grinder, you may be pleasantly surprised to discover this method works exceedingly well. Give it a go with aluminum, or even stainless. You'll be amazed at the quality of the final product.
Weekend before last I had to make a very small (tiny for me) replacement adapter to convert a proprietary fitting stack into a much smaller one-piece package needed for space in a specific installation. I'm not overly fond of stainless, particularly threads, but given this would be exposed on the underside of a truck chassis and couldn't take paint, I made it from some 300 series 1/2" stock on hand. Just to make it more fun it was a tiny thread with short run and no run-out space before a shoulder, on a 17x60 lathe. I was using just the described method and got threads that were visually quite superior (and better fit installed) to the high dollar pattern part I was replacing.
Digressing a bit... No amount of cleaning up the last 0.005" with the cross-slide or the compound is going to get proper form if your perfect tool is not oriented properly.
A hand ground tool may be 60* (or not) but may not be symmetrically ground. (more angle on one side and less the other).
I found the use of a fish tail impractical and ridden with potential error sources.
It has to be held by hands in a confined space between the toolpost and the TS ram.
It has to be horizontal in both planes and on spindle center on the ram
Then you have to rotate the tool post to align the tip while the entire set moves laterally slightly.
My solution: I cut a 60* groove in a 1.5" round stock, the angle were determined by the compound, set by Indicating an angle block in the 4 jaws and using a sharp V tool (less than 60*). The tip of the V groove was relieved wit a 0.015 grooving tool (ground hacksaw blade).
A partial "pie slice" was milled to expose a radial section.
The resulting "gauge" is permanently fixed to a Morse taper to fit the tail stock.
The treading tool is already on center on the tool holder, manipulating carriage and cross-feed the tool tip is made to nest in the gauge V groove and I know is at the most precise orientation I can master.
Mauro,
The (proper) use of a fishtail is adequate for the task involved. They've been used successfully for many lustrums--certainly all of my life and beyond. How the angles relate to the shank make little difference, so long as the included angle is proper. In fact, in some circumstances you may benefit by offset grinding a threading tool. An example might be for internal threading. All that really matters is that it is oriented properly, which is what the fishtail will accomplish, assuming it's applied correctly.
Here's a little tip that may help. Trusting one's eyesight by looking down on the gauge with light from above should not be considered acceptable. You can't really see if the tool is in intimate contact, or not. Below, I'll detail how I set a threading tool.
I typically center the tool before attempting to orient it correctly (with the 60° angle split equally, side to side). Once the center height has been established, I then place the fishtail against the object to be threaded, assuming it is long enough to accommodate the fishtail. If no center is involved, you can often allow the fishtail to extend beyond the end of the part, if necessary.
With the fishtail in place, I move the threading tool in to place, in one of the slots in the fishtail (the larger, the better) with just enough pressure from the cross slide to hold the fishtail in place. The threading tool is then loosened enough (in the holder) so it can be successfully tapped, side to side, for proper orientation. The next part is critical to success. A piece of white paper is placed BELOW the threading tool and work piece, with a bright light projected on the paper. You can now see, in fine detail, where the tool makes contact with the fishtail when observed from above. It helps to shade the top of the tool, so there is no light projected on top of the fishtail. Tap the threading tool until it fills the profile on both flanks. Tighten the tool. You are ready to chase threads, with a tool that is properly oriented.
My threading tools tend to have the trailing edge considerably longer than the leading edge (so one can thread to shoulders). The long trailing edge provides a rather substantial surface, to ensure proper orientation. If the tool is properly ground, it can be oriented properly, even with a very short leading edge.
This method was commonplace where I was trained and accepted by QC as being adequate to ensure proper thread profiles.
Harold
Wise people talk because they have something to say. Fools talk because they have to say something.
Harold, as usual you have given a fool proof method.
I have my tool with longer trailing edge as well, i found out the advantage of reflecting light from the bottom, light from the top is obviously useless for this operation.
My tool post has no indexing, like yours, so is easier to rotate it, than moving the bit.
I am still struggling with capturing the fishtail between tool and part (or ram), at the slightest movement the darn thing falls in the chip pan. Most of the parts I make are small, model engines and such.
tornitore45 wrote:I am still struggling with capturing the fishtail between tool and part (or ram), at the slightest movement the darn thing falls in the chip pan. Most of the parts I make are small, model engines and such.
I read that you rotate the entire post to orient the tool. That wouldn't be satisfactory for me, as that alters the orientation of all of the tools in my post. It is for that reason I suggested that you loosen the clamping screws enough to tap the tool side to side. In reality, the rear screw should hold the tool stationary, so the only movement occurs as a pivot at that screw, as the tool moves at the tip. Because it is rigid enough to restrict the tool from sliding, the fishtail usually remains in place. As I generally use the fishtail from the moment the tool is installed in the tool holder, the amount of movement to correct orientation is quite small.
I specialized in small when I operated my shop for gain. One of the smaller parts I had to thread required an 80 pitch thread. They were generated in tungsten---balance weights for gyroscopes in an aircraft guidance system. The tool for these weights was hand ground. I was much younger those days and could easily see to grind the tool.
Harold
Wise people talk because they have something to say. Fools talk because they have to say something.
