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Posted: 8/17/2012 5:21:40 AM EDT
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I finally got my lathe set up and enough tooling in to start causing trouble.
I have several rifles that I kept around for threading practice (a Savage 64 in this case) so I wouldnt be out a pile of cash if'n I screwed it up. I got it done and and AR flash hider fits quite well. But I have some questions. I had some trouble with finish on my first attempts on some bar stock, but I got that figured out (less speed, shallower cuts). What I need to know is what size and shape tools yall use for 1/2-28 threads. The indexable I used worked ok for a 1/2-20 part I needed, but the shape of the threads on the 1/2-28 seems poor due to the insert profile. 
So what tool is typically used? What kind of profile do I need for the tip? Im new to the single point threading and have no idea how the tools are identified. I thought a E type insert holder would be right, and it would if only the threads were coarser. Thanks, and sorry for the noob question. 
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It actually looks to me as if your tool was not square with the Z axis of your lathe. I don't believe a tool ground for a 20 pitch would not make a 28 pitch thread look like you're showing if properly set up. I could be wrong, and the fix is to a)purchase an insert for a finer pitch or b)grind the point to a sharper apex (maintaining your 60 degree angles). However, if your tool post wasn't perpendicular to the spindle, the thread will look like it has weak crests and wide roots because too much material is being removed by the trailing edge of the tool.
A quick and easy way to square your tool is to loosen the toolpost slightly, and butt the tool up to the face plate or the face of a 3 jaw chuck, then tighten the toolpost down. Also, I'm curious about your entire setup. What RPM, depth of cut, did you advance with the cross slide or the compound rest at 29.5 degrees? How did the tip of the threading tool look when you were finished? One of the hardest things to do on a manual lathe, is to get a good surface finish on threads, especially with carbide tooling. The indexable tools are meant for higher surface speeds than you can keep up with by manually engaging and disengaging your half nut. There are some tricks, such as honing your tool with a diamond lap for a sharper edge, but I still know a lot of people who turn threads within .005-.010" of the desired pitch diameter and then finish their thread with a die and handle. That's not necessary, but it is an option. Could you post a photo of your tool? Is it an insert that has three edges, and rotates in the same plane as your spindle, or is it one with 3 edges, that rotates in the opposite plane? |
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Quoted:
This? http://mallardtool.com/catalog2/images/16LTIN.jpg Or this? http://www.usacarbide.com/catalog/images/TNMCNVTIN.jpg Uh, neither?
I just have a really small triangle insert holder (1/4" bar I think, really small insert). I think I may know what my problem is. They said the E style arrangement was for threading dangit! 
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Quoted:
It actually looks to me as if your tool was not square with the Z axis of your lathe. I don't believe a tool ground for a 20 pitch would not make a 28 pitch thread look like you're showing if properly set up. I could be wrong, and the fix is to a)purchase an insert for a finer pitch or b)grind the point to a sharper apex (maintaining your 60 degree angles). However, if your tool post wasn't perpendicular to the spindle, the thread will look like it has weak crests and wide roots because too much material is being removed by the trailing edge of the tool. A quick and easy way to square your tool is to loosen the toolpost slightly, and butt the tool up to the face plate or the face of a 3 jaw chuck, then tighten the toolpost down. Also, I'm curious about your entire setup. What RPM, depth of cut, did you advance with the cross slide or the compound rest at 29.5 degrees? How did the tip of the threading tool look when you were finished? One of the hardest things to do on a manual lathe, is to get a good surface finish on threads, especially with carbide tooling. The indexable tools are meant for higher surface speeds than you can keep up with by manually engaging and disengaging your half nut. There are some tricks, such as honing your tool with a diamond lap for a sharper edge, but I still know a lot of people who turn threads within .005-.010" of the desired pitch diameter and then finish their thread with a die and handle. That's not necessary, but it is an option. Could you post a photo of your tool? Is it an insert that has three edges, and rotates in the same plane as your spindle, or is it one with 3 edges, that rotates in the opposite plane? RPM was about 70 RPM. DOC was about 0.002 per pass. I was using the compound at 29.5, I only used the cross slide to get the tool set just shy of the surface and the compound from there. Ill buy the tool not being aligned. Ill check that too. Looks like I may be using the wrong cutter though, so I have even more shopping to do!
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When threading with a carbide insert it is very easy to chip the insert. This is particularly true with smaller lathes.
Buy this tool holder and the HSS steel inserts. This will give you the best of both HSS and index tooling at an affordable price. 
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