What am I doing wrong with the feed rate?

[Shotgun]

I've been experimenting with cutting the feed rate on my 12" Atlas lathe. With the slide gear out, I get .0084"/turn. With it in, it is .0042"/turn. I added a step down gear to bring it down to .0021"/turn.

I've attached a picture of the wrist pin from a Corvair piston that I was experimenting with yesterday. It is .865" diameter. I'm turning at 725rpm, using a carbide insert to take off .005".




I got the best picture I could with it under my magnifying glass, but it is still a bit blurry. The tool marks are very clear in first section. It's a very fine thread, as it is often described. With the slide gear out, that is cleared up for the most part, but it still isn't completely smooth. Once I instal the step down gear, it is the smoothest I've ever turned anything, but there are these rings at random intervals that I can't explain. I recut this several times, and it's always the same.

What is going on here? And, how do I make it stop?

 

[Cadillac]

Your problem is probably not the lathe. Your problem is you decide to use a wrist pin for test cutting. A wrist pin is hard as hard! Carbide might cut it but you'd be looking for more like diamond,ceramic, or pvd. I really don't know but I do know wrist pins are ground not cut because of the hardness and finish needed. Change out your material and all previous experiences will change guaranteed.

 

[Papa Charlie]

What type of insert are you using? What is the radius of the cutting tip? From your comment regarding a threaded look, it may be too fine of a radius.

Also, as @Cadillac pointed out, wrist pins are generally hardened unless you have tempered the metal.

 

[Shotgun]

Shoot, I don't know what the diameter of the tip is. I got it as a part of a Chinesium toolholder set. Granted, the wrist pin is hard, but cut just fine. . . just occasionally throwing a tiny spark. But, why would it cut fine, then finer and then start throwing in random lines?

Aaah! Is the material so hard that the bit can't bite into the metal, and just get pushed off instead?

 

[Papa Charlie]

That is what I would assume. Also, I have heard that many have trouble with the Chinese inserts. I have never used them so no personal experience. Some say they don't others yes. But add to that a hardened piece of stock and it can't be good.

Edit: I know that I can see a marked difference between a 1/64 and 1/32 radius on my inserts just by looking at them. The 1/64 looks like a sharp point in comparison.

 

[Lo-Fi]

Is the material so hard that the bit can't bite into the metal, and just get pushed off instead?

Essentially, yes. Especially on cheap inserts which aren't ground - five thou is probably too shallow a cut on hard material. You need to load it up to stop the insert skating on the surface. Get that nose radius fully engaged in the material in a rigid setup and you'll get some spectacular finishes when hard turning.

 

[pacifica]

Shoot, I don't know what the diameter of the tip is. I got it as a part of a Chinesium toolholder set. Granted, the wrist pin is hard, but cut just fine. . . just occasionally throwing a tiny spark. But, why would it cut fine, then finer and then start throwing in random lines?

Aaah! Is the material so hard that the bit can't bite into the metal, and just get pushed off instead?

If using inserts it make a big difference knowing about positive and negative rake angle, nose radius, chipbreaker ,ground or molded tip and insert
composition. Check the chips generated also.
When I grind HSS cutting tools adding a chip breaker improves the surface quality by 50%.
Then there is cutting depth, speeds and feeds.
The surface may look great when removing tenths of an inch and then not so good removing 1 to 3 thousandths.

 

[mikey]

I was just going to post the same thing as Lo-Fi posted. I don't know what the nose radius of your insert is but unless it is a 0.008" nose radius, a 0.005" depth of cut will result in excessive radial forces and the insert will just skate over a hard surface. I am going to guess your insert has at least a 0.016 nose radius so go with at least a 0.008" depth of cut. Even better would be a 0.020" depth of cut and experiment with the feed to get the best finish.

Most of us feed to slow. That results in overlapping cuts by the nose radius; this produces very fine lines that are visible. Ideally, you want the feed fast enough to eliminate the overlap.

 

[Shotgun]

I didn't even know to ask the right question. But, now I know that I'm using a DCMT070204 bit. The final "04" means that it has a .4mm (.0157) radius.

I'm going to switch out to the CCMT060204 bits ('cause I got a cheap 10-pack, so I can sacrifice a few), and see how deep of a cut I can take, and what it will look like.

 

[FOMOGO]

Bingo! they are very hard, taking a deeper cut should help considerably. I have a large number of wrist pins around, and use them for checking run out from time to time. Just out of curiosity I think I will chuck one up and see what it cuts like. Mike

Aaah! Is the material so hard that the bit can't bite into the metal, and just get pushed off instead?