Problem- Cutting steel on lathe with carbide insert. Not smooth.

[alexccmeister]

Hi all,

I did a trial cut using my new carbide insert tool on a GI rod (in fact its a bolt).I just want to practice turning down the end of the bolt about an inch long. First few light passes was to remove the threads. After getting on solid work, I made a few more light passes to turn it down. All the time, the bolt was centered with a live center. First few passes for turning down the diameter, I am getting ok finish, lots of grooves as expected. I think I am cutting with spindle speed of around 800 to 1000rev. So on the next pass I thought why not use a bit of oil on the bolt. When the cutter was cutting away, I could see that the cut was really nice. Smooth surface. Could this be the oil?

Anyway, I wanted to turn the bolt down further, put a little bit more oil and made a few more passes. I kept stopping and starting my spindle so that changes the speed quite a bit while cutting. Sometimes I cut at 1200rev and sometimes 1000rev. I can't be sure. But the next few passes were a complete disaster. The finish was atrocious. I am not even talking about grooves like the first few passes. I am talking about gray fluff like steel coating the bolt. After cooling it a bit, I felt it with my fingers and its rough. Further cutting with some more oil only produces a fair finish. In fact worse than the finish I get from the first few passes.

Basically I got lucky in getting one pass where the finish was the one I want. But what happened in the next few passes? Is it the speed I am turning? Is it the oil? Is it the carbide tip getting blunt? I have no clue. I will try this exercise again later and see if I can get the same result. So many variables, I can't figure out why.

Anyone?

Thanks.

Alex

[cam1]

Hi
In order to diagnose the problem we'll need to know:
Specifically what kind of "carbide" cutter are you using?
What is the feedrate?
What is the material? (not heard of GI rod).

regards

[alexccmeister]

Hi,

Thanks for the reply. I got the carbide cutter from Littlemachineshop.com. They are the one in the link below. Any good? Should I use a different one?



Feedrate is roughly 1000 rev per min for the spindle. I am not sure whats the gearing ratio between the spindle and the leadscrew.

As for material, I am using both brass and steel. The GI rod is basically a screw which I bought hoping to turn the end down to make a Z axis cnc kit. But never got to doing this and I was looking for some steel rod to do and the GI screw was the next best thing.

Alex

[cam1]

Is the cutter set on center?
These preground cutters do not have chipbreakers (to curl the chips). The nose radius is sharp, which doesn't give a good surface finish.
For mild steel on a "hobby" lathe I'd recommend using High Speed Steel (HSS)cutters (you sharpen them to suite). This will give you really good results on Brass and Steel.
You can find the basic geometry for the releif angles in the Machineries Handbook.

regards

regards

[g-codeguy]

I'm not familiar with that material either. Is it soft? What diameter are you getting the bad finish at? Is the tool on center? Oil will help give a better finish, but there are more important things.

When you said, "Feedrate is roughly 1000 rev per min for the spindle.", it tells me you are pretty new to machining and didn't really understand the question. Provided that I am understanding what you are saying, of course.

1000 rev per min is not a feedrate, but the spindle rpm. Can I assume this is a manual lathe? And that you have a rheostat to adjust the feedrate with?

Diameter and rpm have to work together. Smaller the diameter, the faster the rpm has to be. The softer the material, the faster the rpm has to be.

Soft material at slow rpm causes tearing instead of cutting. Taking a hefty cut helps considerably in this instance. A roughing pass will look better than a finish pass. A little faster feedrate also helps.

Carbide inserts are designed to run within a specific SFM range. Some grades are more forgiving of violating this range than others. Sandvik 4025 is an example of an insert that runs very well outside its specified SFM range.

Are you using an insert designed for steel? I am assuming that you can't reach the necessary rpm for the insert you have. The Ceratip CR7025 grade is designed to run at slow SFM in steel. I really like this insert.

You can also try a ground PVD coated insert. These are designed for stainless and other materials such as Inconel, Hast. C, etc. However, their sharp edges does a better job of cutting the material than a honed insert.

Another option is to use a cermet if you aren't making interrupted cuts. They run at a much wider SFM range than carbides, and give excellent finishes.

How fast can you run the spindle? 500 sfm at .5 diameter is over 3800 rpm. Running dry (with a dab of oil) does allow you to run slower as there is more heat build up. It is my understanding that some coatings need this heat for best results.

EDIT: I see I got beaten by faster fingers...again.

[cam1]

You had more to give.....

cheers

[Geof]

I am guessing you are doing this on a manual machine.

Your rpm is okay, stabilizing the material with a center is good but seeing grooves suggests you were feeding the tool along the part a bit fast. On a manula machine you need to develop the trick of very smooth and slow rotation of the hand wheel; you need to use two hands in a hand-over-hand kind of motion to keep up a constant motion.

Carbide tools are easy to chip on a manual machine due to uneven feeding, and/or intermittent cuts like you were doing turning away the thread. The poor finiosh you started to get can be due to minute chips on the tool which do not allo it to cut cleanly.

Until you have developed a smooth technique it may be better to use HSS tools; they are more forgiving. And with a due the respect to the one who gave the suggestion to use cermet....no, not on a manual machine.

[g-codeguy]

Goef,

Why wouldn't you use a cermet on a manual lathe? Admittedly whenever I run a manual lathe, I run whatever insert is already in the holder. Tool room guys always use carbide. I assume you are referring to how easily they break compared to a carbide?

However I know that cermets will do a good job when run dry. When I do use them, I use them for finishing only...although I have roughed 8620 with the XN4 insert from NTK with good results.

I do prefer carbide for roughing. Less chance of breakage. As I stated, I wouldn't want to use cerments to rough on an interrupted cut, but I do use them for finishing interrupted cuts.

Been running an 1-1/8 in. hex 416L SS job for over 3 years with a ground XT3 finishing insert from NTK. Have also used a ground PVD coated carbide with the same results. As a matter of fact we have had 2 machines running this job for the past 2-3 months. One using the cermet, the other using a carbide.

[Geof]

QUOTE: Why wouldn't you use a cermet on a manual lathe? Admittedly whenever I run a manual lathe, I run whatever insert is already in the holder. Tool room guys always use carbide. I assume you are referring to how easily they break compared to a carbide?


Exactly, on a manual machine it is very difficult to get a nice smooth feed, especially for a novice. It is much better, I think, to use a tool that is difficult to chip such as HSS until some proficiency has been acquired.

It is a case of crawl before walking and then run.

[cam1]

Yup:
Good old HSS, a few pleasant minutes on the grinder, hone and add radius with a fine diamond stick, and you're good to go. Obviously not sufficient for todays frantic pace, but certainly suitable for one-offs and reworks.

regards

[alexccmeister]

Hi all,

Thanks for all the comments. I will revert back to HSS for some cutting then.

To be honest, I am very new to lathe. I know about feedrate on mills but on lathe I am abit lost. In terms of using the lathe, I have some ideas after researching on line and doing some test run on the lathe. Other than that, I haven't got much idea about the technical side of lathe turning.

Originally Posted by cam1 Is the cutter set on center? These preground cutters do not have chipbreakers (to curl the chips). The nose radius is sharp, which doesn't give a good surface finish. For mild steel on a "hobby" lathe I'd recommend using High Speed Steel (HSS)cutters (you sharpen them to suite). This will give you really good results on Brass and Steel. You can find the basic geometry for the releif angles in the Machineries Handbook. regards regards

I have to admit the carbide tip is not exactly centered on the work piece. I had a look at it sideway and I notice the bottom of the carbide tip is centered, not the top of the tip. How do I adjust the top of the carbide tip to be centered? I can't move the compound rest any further down. The holder for the cutter seems to thick for this lathe.

For my HSS cutter I have to shim the cutter to get it to the right height.

Alex

[CarbideBob]

More speed, go faster.

The rough dull finish you are seeing is the result of the material tearing and rewelding to the workpiece.

As the part dia gets smaller you need o spin it faster to keep the same surface footage. Think of it this way, if you put tiny wheels on your car the engine would have to spin much faster to move you down the road at 55 MPH. If you put 20 foot tall wheels on it the engine would push you to 55 MPH at idle.

I suspect you may have damaged the cutting edge or have built up material on the top face. Examine the cutting tip for chips, side wear, or buildup with a magnifying glass or microscope.

Yes the oil will help if the speed is too slow but once you weld some material to the top of the tool it's all over and the oil won't help anymore.
Material welding to the top of the tool is know as built up edge (BUE). This is caused by too low of a surface footage and is easily cured by cranking up the speed.

The only thing I use HSS for is bandsaw blades and drill bits, but then again I've got a building full of carbide so my tooling costs are pretty low.

Can you get someone to mill the holder down? If not how about grinding it down on a bench grinder?
Bob