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2019 in the workshop
 

Cutting a US thread

05 May, 1919 - Cutting a US thread &nbsp; I'm going to cut a 1/4 X 28 thread and document it so that, once I've forgotten how I did it, I can go back here to find out. First thing is to set the horizontal movement to cut 28 TPI. This requires four gears: 30, 27, 34 & 50 as per the chart. <div style="Font-size:5.0pt" 05 May, 1919 - Cutting a US thread &nbsp; Here are the four gears in place with the gears in motion. These gears drive the leadscrew and the leadscrew drives the apron to which is attached the thread cutting tool. <div style="Font-size:5.0pt" 05 May, 1919 - Cutting a US thread &nbsp; These are the components of my lathe. No. 16 is the leadscrew and No.19 is the apron. No.7 is the Toolpost that contains the thread cutting tool <div style="Font-size:5.0pt" 05 May, 1919 - Cutting a US thread &nbsp; Next, I need to check that the gears are set to 28 TPI. I set up a piece of scrap in the chuck. <div style="Font-size:5.0pt"
05 May, 1919 - Cutting a US thread &nbsp; I set up the dial gauge as a cross-check on the graduations marked on the cross slide. I move the cross slide in until it just touches the work piece and zero both measurements. <div style="Font-size:5.0pt" 05 May, 1919 - Cutting a US thread &nbsp; This is the half-nut lever. It is in the disengaged position. This means the apron is not locked to the lead screw which is the screw visible in the right side of the picture. <div style="Font-size:5.0pt" 05 May, 1919 - Cutting a US thread &nbsp; I intend to check the gears to make sure that they are set to cut 28TPI. I turn the motor on and the lead screw begins to turn. I move the lever down and engage the half-nuts and the apron moves to the left. The thread-chaser dial stops moving. <div style="Font-size:5.0pt" 05 May, 1919 - Cutting a US thread &nbsp; I'm making a scratch cut in the work piece to test if the gears are correctly set to 28TPI. <div style="Font-size:5.0pt"
05 May, 1919 - Cutting a US thread &nbsp; When I have gone far enough, I lift the lever up and disengage the the half nuts. The apron stops moving. The thread-chaser dial starts moving again. If this had been an actual thread cutting project, I would not have been able to re-engage the half nuts in their correct position. Better to have turned the motor off and let the apron coast to a stop. <div style="Font-size:5.0pt" 05 May, 1919 - Cutting a US thread &nbsp; I then use a thread gauge to check that the tool is cutting 28 TPI. There's a "28" inscribed on the thread gauge. All seems well. <div style="Font-size:5.0pt" 05 May, 1919 - Cutting a US thread &nbsp; The next step is to turn the outer diameter of the actual work piece to be threaded to 1/4". I found that about .003" undersize was about right. This is a rubbish bit of metal and I'm not about to waste a good bit. <div style="Font-size:5.0pt" 05 May, 1919 - Cutting a US thread &nbsp; Unfortunately, the picture is blurry. I bring a round nose tool into the work piece so that it just touches the 1/4" outer diameter. <div style="Font-size:5.0pt"
05 May, 1919 - Cutting a US thread &nbsp; This is the finished cut: 0.05 mm from both sides resulting in a cut of 1.0mm. Why 1.0mm? the work piece OD is .250" and the tap drill (which sets the ID) is .213" with a difference of .037 which is close to 1.0mm. Later, I buy a 3.0mm round nose tool that does a better job. <div style="Font-size:5.0pt" 05 May, 1919 - Cutting a US thread &nbsp; Now I put the screw cutting tool in the tool post and align it front to back. This is so that the 60° thread is square with the work piece. <div style="Font-size:5.0pt" 05 May, 1919 - Cutting a US thread &nbsp; Now I set the dial and cross screw indicators to zero with the tool touching the work piece. This so that I can stop thread cutting at a depth of 0.5 mm. <div style="Font-size:5.0pt" 05 May, 1919 - Cutting a US thread &nbsp; I take note of where the thread chaser dial is as I push the lever down. It's at "6". <div style="Font-size:5.0pt"
05 May, 1919 - Cutting a US thread &nbsp; Heart in mouth, I turn the motor on and engage the half nuts. Live screw-cutting begins. <div style="Font-size:5.0pt" 05 May, 1919 - Cutting a US thread &nbsp; Because this is really scary stuff, I set the spindle speed to a very slow 120 RPM. This gives me time to react when I get things out of sequence. From here on, operating all of the controls in the correct sequence is critical. <div style="Font-size:5.0pt" 05 May, 1919 - Cutting a US thread &nbsp; A bit out of focus, but I turn the motor off half way along to check progress. I turn the motor on again and put my right hand around the half-nuts lever ready to lift it up once the cutting tool gets into the hollow. It requires a determined pull on the lever, no namby-pamby, half-hearted stuff. <div style="Font-size:5.0pt" 05 May, 1919 - Cutting a US thread &nbsp; The cutting tool reaches the hollow so I yank the lever upwards. <div style="Font-size:5.0pt"
05 May, 1919 - Cutting a US thread &nbsp; Phew, the first pass is successful. Now this is the really tricky bit: I've disengaged the half nuts but they have to be re-engaged in exactly the same position again otherwise the second pass of the tool will not be over the same cut. It's very important to leave the apron exactly where it is right now. <div style="Font-size:5.0pt" 05 May, 1919 - Cutting a US thread &nbsp; I control the forward and backward motion with the motor switch. <div style="Font-size:5.0pt" 05 May, 1919 - Cutting a US thread &nbsp; Also, while the motor is stopped, a very important part of the sequence is to pull the cutting tool clear of the work piece. This so that the tool can go back to the start again for the next (and subsequent) pass. <div style="Font-size:5.0pt" 05 May, 1919 - Cutting a US thread &nbsp; Next part of the sequence is to put the motor into reverse to take the cutting tool back for a second pass. <div style="Font-size:5.0pt"
05 May, 1919 - Cutting a US thread &nbsp; Here's the tricky bit: the motor is now in reverse. When I started, I engaged the half-nuts with the thread chaser dial at 6 (because that's just where it happened to be). There's nothing special about 6 so long as the lever goes down in the same place every time. Because the lead screw is turning slowly, as the 6 returns past the reference mark, I push the lever down and it engages cleanly. <div style="Font-size:5.0pt" 05 May, 1919 - Cutting a US thread &nbsp; I then watch as the cutting tool returns to the beginning. At this point, I don't disengage the half nuts. The apron stays locked to the lead screw. Note: "An imperial leadscrew cannot use the threading dial while making metric threads, and vise/versa. &nbsp; If the threads and the leadscrew do not match the same system, then half nuts must remain engaged, and the lathe run in reverse to get back to the starting point. Since there is a ratio of xx/127 between the two systems, you will not likely re-engage the half nuts anywhere near the right spot." &nbsp; Why xx/127? It is because 1.0"=25.4mm and half of 25.4 is 127 and 127 is a prime number. <div style="Font-size:5.0pt" 05 May, 1919 - Cutting a US thread &nbsp; Now that I'm back at the beginning I turn the motor off and carefully take the cutting tool deeper for its second cut. I found that a graduation of .05 mm per cut was about right. This gave me a cut of 0.1 which means I have to repeat this sequence nine more times to get the correct thread depth of 1.0 mm (the same diameter as the #3 tapping drill). <div style="Font-size:5.0pt" 05 May, 1919 - Cutting a US thread &nbsp; The half nuts remain engaged. Concentrate Jeffrey; don't relax for the next nine cuts. Note that the thread-chasing dial is still at "6". <div style="Font-size:5.0pt"
05 May, 1919 - Cutting a US thread &nbsp; I turn the motor on again for the next 0.1 mm cut. <div style="Font-size:5.0pt" 05 May, 1919 - Cutting a US thread &nbsp; After ten passes, the threads are a bit rough so I clean them up with a 1/4" X 28 die. <div style="Font-size:5.0pt" 05 May, 1919 - Cutting a US thread &nbsp; A prop nut fits very nicely. That's it. I feel confident that I can now cut any metric or US thread. Next time I hope to use a better piece of metal than I used this time. End of this project &nbsp; Click to go to home page <div style="Font-size:5.0pt"  
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