Thread cutting help

[SLK001]

It's not the QCTP that matters. It's the alignment of the cutting tool with the work that matters. Do you have a thread gauge (AKA a fishtail)?

 

[Craig1514]

Yeah, I have thought of that. I guess I should buy a newer one. Mine is dated 1959 and doesn't have any metric stuff in it.

Tim

Try the Phillip’s machinist app, the calculator has metric thread calculations and it’s free. If you look around you can probably find Machinery’s Handbook in PDF format. I picked up the large print edition, it’s really big but I can read it without squinting. I used to just cut and test with a bolt, trying to use wires now. I know it’s expensive, but man, that’s a lot of info

 

[mikey]

Here are some photos of my setup, I did find that the tool post holder was not square to the chuck...... I thought I had checked that.

View attachment 400864View attachment 400865View attachment 400866

Thanks for the pics. Looks like you're using the threading kit from PM. Not sure about the quality of the inserts but I would consider getting some from Carmex; they aren't that expensive and the inserts last forever.

Okay, alignment. To align the tool, there is an area just behind the pointed tip of the insert on the left side. If you align that flat area with the face of the chuck then the tool will be perpendicular to the work. You also need to get the tip of the tool at the exact center height of your lathe. If you haven't already made a height gauge to get all your tools at center height, you should because this is important.


Prep you need to consider:

• Know what the major diameter should be. It will be a range and you need to fall inside that range.
• Whenever possible, cut a thread relief. The depth should be at least 0.003-0.005" deeper than the minor diameter and it should be at least 2-3 threads wide when possible. If you do this, you should be able to disengage your half nuts without any issues. When you disengage the half nuts the carriage stops INSTANTLY so practice stopping the carriage as soon as the tool enters the thread relief. Over time, this becomes second nature and stress-free.
• Whenever possible, use a live center for support.
• Always cut a 30-45 deg chamfer on the end of your work.
• Know how your insert likes to cut. Carmex inserts can take a 0.001"deep cut repeatedly. Not sure about your import inserts. I prefer to take a 0.001" deep scratch pass initially, and check the pattern with a thread gauge. If it's good then my next pass will be at 0.009" deep. Then I take a 0.005" pass. If this second pass goes okay, I may take a second 0.005" deep pass, then I'll step down to 0.003, then 0.002, then 0.001" where I stay until I hit final depth.
• To estimate thread depth, one formula is 0.750/TPI. It works, too. If you use any formula to estimate thread depth, start checking the pitch diameter well before you hit the calculated depth. It is easy to cut too deep. You can use thread wires but eventually you will learn to despise them and switch to thread mics ... I bet!
• Cutting threads is not a race; it's actually about accuracy. In the beginning, slow down the speed until the mechanics become comfortable for you. Learn to use your thread dial indicator.
• At some point, learn to grind a good HSS threading tool. A good tool will cut threads with great accuracy, fewer burrs and will take finer cuts than carbide. Yeah, I know I'll get flack for this but I own both kinds of tools and have been using them for a very long time and am comfortable with my opinion. Try it yourself and see.

Learning to cut threads is not difficult if you pay attention to the basics. Turn the work to the right diameter, support it with a live center, chamfer it properly, set up your threading tool properly and then go through the steps slowly and methodically. In time, it will become second nature to you. I cut class 2a thread without even thinking about it now and have to pay attention only with class 3 threads because the stop range is really narrow. Give this a try and you'll see that it isn't difficult to do at all.

 

[Jim F]

Here are some photos of my setup, I did find that the tool post holder was not square to the chuck...... I thought I had checked that.

View attachment 400864View attachment 400865View attachment 400866

your compound angle is wrong.

 

[Liljoebrshooter]

your compound angle is wrong.

No, it is correct. This is how my lathe is numbered also.
It is 30 degrees off of 90.
Joe

 

[Just for fun]

Thanks for all the pro tips.

I'm not sure I'll get a chance to do anything on the lathe today. But I'll follow up with some photos when I do.

Tim

 

[mikey]

Thanks for all the pro tips.

I'm not sure I'll get a chance to do anything on the lathe today. But I'll follow up with some photos when I do.

Tim

Sorry, Tim. Threw a lot at you. Let me try to make it really simple because it is really simple. Look at this thread; it has the features you should be trying to incorporate.



The red line is pointing to a 30 degree chamfer that allows the threading tool to start smoothly. The blue line is a thread relief that is ~0.005" deeper than the minor diameter and about 2-3 threads wide. This allows a run off area for your tool at the end of the thread; it also gives a clean termination to the thread and gives you space to disengage your half nuts.

You cannot see it but this is a Class 3A thread so the Major Diameter and pitch diameters are precise within a few ten thousandths of an inch. Other than cleaning with a brush and a burst of air, this is as the thread came off the lathe.

All the vital cutting info you need to cut just about any Imperial thread is contained within the attached table. You will see that the major diameter is listed for each thread as a range; you have to turn your work piece within this range to get the thread fit you want (class 2a or 3a). Also note that there is a pitch diameter range; you must cut your threads until your thread wires or thread mic says you've cut deep enough for the class of thread you need.

So, you would mount a work piece in the lathe, face it, center drill it and mount a live center for support. Then turn the OD until it is within the major diameter range for the thread you want to cut. Then cut down the end of the work to a 30-45 deg taper about 1/2 a thread wide. Use your parting tool to cut the thread relief about 0.005" deeper than the minor diameter and make this relief cut wide enough to span 2-3 threads. Adding a thread relief will not weaken the work piece significantly.

Now that your work piece is prepped, double check to be sure your tool is perpendicular to the work and that the cutting tip is on center height.

Go slow. I would go at the slowest speed your lathe can run at for the first few tries, then speed it up as you get more comfortable. Again, this is not a race; it's learning. Don't forget to do a 0.001" deep initial scratch pass to confirm your thread pattern and don't forget cutting oil as you make your threads. Given that this is a low speed/high pressure situation, sulfur-bearing oil with EP additives works well for threading.

You can use a standard nut to check for fits as you learn, and for a lot of the work you'll do in your shop, but eventually you want to be able to cut threads to spec. For that, you need this table and you need either thread wires or a thread micrometer to calculate or read pitch diameters.

This should get you started. Sing out if you get hung up.

 

[Just for fun]

I played around threading a bit today. So, according to my manual when cutting a TPI of 20 it says I can engage the half nut on any number 1-8. Everything was going pretty well; I was just about down to my final cut and instead of engaging the half nut on number 6, I engaged it on number 8 and wiped out the threads.

Once I pick a number am I supposed to stay with that number, or did I do something else wrong?

Another question I have, is it better to move in with the compound or the cross slide. I had made about 4 passes when I remembered that I have seen people using the compound when thread cutting. I was using the cross slide only and continued until I wiped out the threads completely.

 

[Just for fun]

I believe I have it figured out.... and it is all my fault. The half nut engagement is a little bit more of a problem then I thought. This evening I went back to the shop and chucked up a piece of aluminum and without the lathe running I cut some threads by just turning the chuck by hand. I realized doing it that way that I was not engaging the half nut at the right time all the time. What I didn't realize was that even though the half nut was not fully engaged the carriage stills moves and it cuts threads, just in the wrong place.

That all being said I learned that when the half nut is fully engaged there is at least another 1/4 to 1/2" of travel in the lever. It's going to take some practice to hit it every time. But now that I know what it feels like when it is fully engaged things are bound to get better.

Tim

 

[SLK001]

Here's a chart that I use on my lathe (a SB 10L). This chart is correct for my lathe. My lead screw is 8TPI and this chart is dependent on the lead screw pitch and the number of teeth on the thread dial.

It is always safe to just pick a number on the thread dial and engage only on that number, but it is really convenient to be able to use the other engagement points (minimizing the "wait" times for the dial to come around). If you're having problems engaging the half-nuts, perhaps you are running your lathe too fast. Try slowing down to your lowest speed to practice your threading. As the lead screw is probably an ACME thread, it will have an infinite number of start positions if not properly engaged, so reduce the number of start positions to 1 by insuring that it is always fully engaged.