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Help with thread finish on lathe, nothing I have ever seen before
- Thread starter qualitymachinetools
- Start date
Looks like too deep, and too slow, and no oil. Using that tool he should be up around 400 RPM or better, but much more than that you can lose control on a manual machine. A sharp HSS tool would be a much better choice for that application.
If the carbide had relief designed in the trailing edge it would cut better like high speed bits . Just because it's ground to 60 degrees don't mean it's sharp and made to cut steel. I've watched lots of the YouTubers cut threads and the tool is being bent down and springing back up. The rigidity of the lathe is why it's doing it. Either the Gibbs on the ways need adjusting or the overhang of the tool , or holder or compound but that's what I've observed on most small LATHES. Hope I explained it correctly.
So a few things to follow up on.
1. Using inserts, the speed is way too slow as mentioned above.
2. He needs to try a different threading tool, preferably HSS. The threading tool he is using seems to have the wrong geometry, my external laydown threading tool the insert is held in a negative rake, the insert tip is positive and very sharp. The height and cutting position are critical to clean threading. If he is using a generic Chinese type, this may be an issue. Maybe have him borrow a threading tool from someone else or send a loaner or have someone grind a HSS blank.
3. 4140 is a difficult material both to turn and thread, it requires much high feed/speed to cut cleanly along with some cutting oil.
4. I almost always have a significant curl when taking a spring pass. Only take ONE spring pass, repeated a spring passes can often cause chatter in the thread.
1. Using inserts, the speed is way too slow as mentioned above.
2. He needs to try a different threading tool, preferably HSS. The threading tool he is using seems to have the wrong geometry, my external laydown threading tool the insert is held in a negative rake, the insert tip is positive and very sharp. The height and cutting position are critical to clean threading. If he is using a generic Chinese type, this may be an issue. Maybe have him borrow a threading tool from someone else or send a loaner or have someone grind a HSS blank.
3. 4140 is a difficult material both to turn and thread, it requires much high feed/speed to cut cleanly along with some cutting oil.
4. I almost always have a significant curl when taking a spring pass. Only take ONE spring pass, repeated a spring passes can often cause chatter in the thread.
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It is difficult to make any judgements from the photo. A real closeup would help. My first impression was torn threads. Try a hand ground HSS tool with a little (5-7*) side rake. Use flank feed and dark cutting oil.
Being a rookie, it is not my place to call out anyone on spindle speed using lay down carbide inserts. My SBL13 lowest speed with the back gear is what I use to thread so I can precisely catch the end of the before hitting the shoulder without any thread relief.
Here is the video I just took to record the spindle speed at the lowest setting.
Watch "SBL13 Lowest Speed Using Back Gear" on YouTube
Granted, this is not 4140, but I do not see any problem with the thread done at the lowest lathe speed.
I have encountered rough threads before, what happened I just opened a fresh box of inserts from China, and proceeded to use one. Thread was awful, grabbed one from the old box, also Chinese made, No issue. Checked the new insert, it was so dull it won't even bite into my fingernail.
The new box being from a different eBay supplier, I ordered 3 boxes of 10 from the old supplier and random checked each box, they are almost razor sharp.
Here is the video I just took to record the spindle speed at the lowest setting.
Watch "SBL13 Lowest Speed Using Back Gear" on YouTube
Granted, this is not 4140, but I do not see any problem with the thread done at the lowest lathe speed.
I have encountered rough threads before, what happened I just opened a fresh box of inserts from China, and proceeded to use one. Thread was awful, grabbed one from the old box, also Chinese made, No issue. Checked the new insert, it was so dull it won't even bite into my fingernail.
The new box being from a different eBay supplier, I ordered 3 boxes of 10 from the old supplier and random checked each box, they are almost razor sharp.
Guys if he can turn the same material with a good finish using the same tool bit then my suspicion would be the leadscrew for threading. Is it possible it's worn or dirty/sticking or what about the thread settings and gears? Another time I've seen this is when the compound angle wasn't right but in this case it 'looks' ok from here.
Carbide tooling is usually duller than HSS. The reason is because it is more brittle than HSS, and if sharpened to a really sharp edge, the edge can crumble easily. So the manufacturers leave it relatively dull. HSS is easy to make really sharp, and if what it is cutting is not too hard, it stays sharp quite well, and is very easy to touch up. Carbide edges can be sharpened pretty easily with a diamond wheel after roughing them in with silicon carbide, and can be honed to a very sharp finish on the cheap by using a diamond file or "credit card." The edges may not hold up well, however. For work in a home/hobby shop, HSS can easily do 90% of the cutting to a professional result. I think we use too much carbide insert tooling for our mix of jobs, and we do so because we are too lazy to grind our own tools to match the work at hand..
