Help with thread finish on lathe, nothing I have ever seen before

[Bob Korves]

Well, it does take a little bit of skill to grind a good tool, too ...

Yes, Mike, it takes some effort learning and practicing to become proficient at grinding tools that work well. It also gives great satisfaction when nothing you have is working, and you make one that does the job perfectly on the first try. "Wait, I am not a lemming, I am a machinist!"

 

[Bamban]

Bob is right, I am too lazy to grind my own, and on top of that I don't have the skills to do so either. But, for the price of the Chinese carbide inserts, I would rather spend my time running the lathes than running the grinder. If I were to wear out a tip of a CCGT, a DCGT, a VCGT, and a ER16 AG60 doing 3 barrels, I still will not trade the time to grind tools for the cost. I did asked for Jeff's help to grind a tool so I can save set up.

I do use a lot carbides, mostly designed and produced for aluminum. They are real sharp, but they don't last. Remember gents, I am not a machinist, and I am not in the business, I just do barrels for me, for my brother, for the kids, and for a few friends.

 

[projectnut]

Well, it does take a little bit of skill to grind a good tool, too ...

It's a skill some of us had to learn because we got into the profession when carbide inserts were almost non existent and outrageously expensive. Fortunately we had coworkers with the experience and patience to show us how it's supposed to be done. Once the skill has been mastered it takes only minutes to create a new profile, and seconds to renew an existing one to pristine condition. It probably takes more time to search eBay to find vendors and buy the right tooling than it does to create it from inexpensive tool blanks. In the time you spend waiting for the tooling to be delivered the job could probably be finished if you ground your own tooling

90% of the tooling I use today is still HSS. I do have maybe a hundred or so pieces of brazed carbide tooling of different profiles, but I can count on both hands the number of inserts in the cabinets. I only buy them to fit specific form tools that are hard to duplicate in HSS.

To this day most technical college entry level machining curriculums still start with HSS tooling. At the beginning of the first semester the instructor hands out several different size blanks. The students are required to grind the blanks to the profiles needed to complete the semesters projects. Those skills may seldom be used in their professional carriers. However they will be able to recognize a sharp tool from a dull one, and know why and when a tool isn't cutting right.

 

[jbolt]

If he is getting a .003 - .005 cut on a spring pass there is either too much deflection of the tool/stock due to the various possible issues described by others or the 4140 is work hardening on the previous pass(es) causing the tool to cut too deep as it passes through the hardened skin of the steel on the spring cut.

As others have mentioned 4140 is best turned on a manual lathe at higher SFM with sharp tools and heavy cutting oil. HSS is my first choice for 4140 but I have also had great success with the 60 degree On-Edge carbide inserts. I always prefer using inverted threading tools and threading away from the shoulder when ever possible.

What is the machine model?

 

[stioc]

But he said he's having the same issue with aluminum too.

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[JimDawson]

But he said he's having the same issue with aluminum too.

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For aluminum sharp tools, oil, and higher speeds are even more important. HSS is really the best for the application.

 

[qualitymachinetools]

I am going to try to reply to everyone, sorry if I miss anything!

He did use oil on other passes, I guess just not on this one that I have the video of. Maybe a little better, but same type of pattern. I do agree about the speed, when I used a similar type of tool, when I was at 50 to 100 RPM, I could produce the same exact results. I sped it up and took lighter cuts, and was able to get a nice thread.

Also same here MKJS, many times when I take a spring pass, I still get a good chip too. He did have a HSS tool he tried with similar results, but the shape of that tool looked straight, just a 60 degree and flat on top. Not sharp like it needs to be to cut from what I can see. The carbide tool he has says ACCUSIZE0500 on the side, I do not know what that one is.

Guys if he can turn the same material with a good finish using the same tool bit He is using a different bit to turn with the good finish, only using this tool for threading. The machine is only a year old, I dont think he has used it for threading more than a few times so everything is essentially new. No wear, something loose possibly, but no wear. Originally the compound angle was wrong, but that was corrected. I know that look well, 2 different angles on the threads, I get that one all the time.

Jobolt, same exact machine that you have.

Hopefully he decides to take my advice and join and chimes in here soon.

 

[jbolt]

There are two things on his machine I would look at mechanically.

(1) Make sure the t-nut for the tool post is not flush or proud of the top of the compound. I see he has a QCTP so I don't know if he machined the t-nut or if it came to him pre-installed. On my machine the t-nut for the factory toll post was ground with the compound without enough pre-load. When the tool post was tightened it would pull the t-nut a few thousands leading to a less ridged setup.

(2) The location in the cross slide for the access hole for the t-nuts that hold the compound to the cross slide puts the one of the t-nuts partially within the access hole when the compound is set for threading. Not a great design being in the top. T-nut access should be through the bottom of the cross slide.

I found the compound was the weak link as far as rigidity goes and was the reason I went to a solid tool post riser. The compound should be okay for the size of thread he is cutting but it is worth checking.

If the tool post t-nut is okay a good way to check the compound rigidity is to turn it 90 degrees so both t-nuts have a solid perch, run the compound all the way in so there is no overhang and lock the gibb. Try threading this way and see what the finish result is.

Accusize is an import tool brand.

Threading has a much higher feed rate than turning. Couple that with a slower surface speed and your comparing apples to oranges.

Someone earlier mentioned using a center. This is a great way to add extra stiffness to the setup and can help with some materials that can be problematic on our smaller machines.

 

[dpb]

Cracked / defective tool holder?

 

[higgite]

He did have a HSS tool he tried with similar results, but the shape of that tool looked straight, just a 60 degree and flat on top. Not sharp like it needs to be to cut from what I can see.

Have you thought about sending him an HSS threading tool that you know is sharp, properly ground, with all the proper relief angles and radiused nose? Might eliminate one unknown. Possibly even solve the problem.

Tom