Cutting issues

jetpilot67 · May 22, 2018

I am relatively new on the lathe, but while turning this piece of steel it went from cutting perfectly to a rough finish. I tried a new carbide insert, re-centering the cutting height and can not get it to produce a nice finish again. Right when I was down to the .001" from the tolerance I needed. Can anyone tell me why it suddenly produced a rough finish? Oddly enough, it started out that way but early on it disappeared and then it came back.
SB1 Story by jetpilot67 | Photobucket

SLK001 · May 22, 2018

Please post the pictures HERE. What type of steel? How fast? What is your DOC? My guess is that your cut parameters are not optimal for carbide inserts. Switch to HSS and your cut may improve.

You also appear to be cutting dry.

MetalCarnage · May 22, 2018

From your picture it would appear that the cutter is below the center line slightly. When cutting the smaller diameter that would really change the angle of incidence with the cutting tool as well as the cutting speed. Depending on your responses to the previous question there may be more issues.

-Ron

jetpilot67 · May 22, 2018

I tried to add pictures directly to the thread, but after selecting the photo there is no "OK" or "Cancel" buttons in the dialog like there is on the "add link" tab. Maybe I'm too new and not allowed?

Steel was from scrap yard, it was some sort of axle. I don't have a tach, but seemed to be at a medium speed and slow feed rate. I can check my setup and get a better estimate when I get home again.

It was dry as I wiped it down before taking the pics.

jetpilot67 · May 22, 2018

Hi Ron, it might be the camera angle. I did check the centering and it seemed to be right there. I took off about .5" off of a 1.7" bar when it started doing that so I went back to a larger diameter section where the SFM would be same as when I started and it still did the same thing. When it started happening, I was only cutting about .001 to .003" off. I also tried cuts up to .01" on the larger diameter section and still does the same. Could I possible have a worn lathe part like a bearing that is causing this, or is it a setup issue?

kitno455 · May 22, 2018

Did you change your depth of cut during this pass? It can be hard to sneak up on a size by taking progressively finer DOC, especially with carbide. Sometimes, you really need to be taking metal off for it to cut cleanly. Keen sharp inserts intended for aluminum can help, even on steel.

allan

10KPete · May 22, 2018

Carbide is crap for these small lathes. Use HSS.

Now that that's cleared up, what likely happened is called chip welding. No lube, wrong lube would be the culprit normally and is a must with what you're using there. Even then......

Bars, especially heat treated stuff like an axle, can routinely have a tough outside that cuts well and a center like bubble gum. CRS is a good example.

Carbide doesn't cut like HSS it pushes metal out of the way and the only way to not get a chip weld is to get the speed way up to 200 ft/min or higher and a feed rate of .010" or more. Kinda tough on a lathe like an old SB.

There are inserts out there designed for aluminum that have finely finished, sharp (for carbide) high rake edges and many folks have had good success with these on steel. But the speed and feed rate need to be at the max. Someone here may know where to get those inserts.

Hope this sheds some light....

Pete

thomasutley · May 22, 2018

Watch this video and you’ll see what Pete’s talking about re: chip welding.

Link: Iscar Chip Formation.wmv - YouTube

texasgeartrain · May 23, 2018

I have not done particularly well with carbide inserts on a south bend, i think in part because inserts are designed for higher speeds and feeds. Not sure your lathe, but mine tops out at just over 1000 rpm spindle speed, which i almost never run that fast.

Im not that experienced myself, but i generally run mid to slow speeds, in the area of 160 to 325 rpm. I generally do do well with hss or brazed carbide that i would shape like hss.

In your particular case i think the material your cutting is a factor. Id suggest a hand held tach. You put a small piece of reflecive tape on chuck, then shoot it like a lazer pointer with spindle turning. You could run all possible speeds, and document it for future reference. Hand held tachs run maybe $50.

I also cut a piece of flat stock, that i use as a gauge for cutting tool height. I stand it up on end on compound, and match tip of cutting tool to it. My compound is square and flat on top though.

tobnpr · May 23, 2018

I exclusively use carbide on my 9A with excellent results, usually working fairly hard 4140 chrome moly. It's all about finding the optimal doc, feed rate and rpm- and it takes some work sometimes to find it. Guys that are unable to run modern carbide formulations on these smaller lathes just haven't worked at it enough...

Nose radius, and type of carbide are other factors. For roughing, heavy depth of cut, high rpm (double or triple what I'd use for HSS), and slow feed rate works for me. I push roughing inserts hard- but have no trouble using finishing inserts (DCMT) to sneak out the last few thou.

Cole2534 · May 24, 2018

10KPete said:
Carbide is crap for these small lathes. Use HSS.

No, Pete, it's not.

Crap carbide is crap, of course, but a positive rake insert with good geometry works very well on even an SB9. Lathe inserts .com will sort this out for you.

OP- you peeled a good chunk off that bar. I'd recommend you read about the 3 pass method and practice with it, aiming for DOC's of no less than 0.015". Everything is a spring (IE- it all deflects) and this methodology standardizes your last 3 passes so that you may get predictable results from each.

10KPete · May 27, 2018

Cole2534 said:
No, Pete, it's not.

Crap carbide is crap, of course, but a positive rake insert with good geometry works very well on even an SB9. Lathe inserts .com will sort this out for you.

OP- you peeled a good chunk off that bar. I'd recommend you read about the 3 pass method and practice with it, aiming for DOC's of no less than 0.015". Everything is a spring (IE- it all deflects) and this methodology standardizes your last 3 passes so that you may get predictable results from each.

OK, you're making me ( and rightly so ) quantify my statements. :scratchchin:

9" SB won't reach the sfm needed for carbide until you get around 3/4" diameter at which point the issue is horse power to get the depth and feed rate up to what carbide needs.

I did mention the "aluminum" configured inserts which, by all reports, will do a good job on ferrous metals at lower speeds as they are sharp with a high rake.

I don't particularly enjoy making blue hot chips with a small lathe so the speeds/feeds I usually run are well within the range of HSS. That and most of the work I seem to do involves cuts that carbide insert shapes don't lend themselves to. The amount of time that I would use carbide seems to be a small part of the cuts I make so what tiny amount of time I might save using carbide is cancelled out by the time it would take to change tools. Even with a QCTP.

I also see a lot of folks who try carbide for all their work, usually because they don't know how to grind a tool. And I think that tool grinding is a vital part of being a machinist. :codger:

Have fun,

Pete

DanMc77 · May 27, 2018

That video is fantastic!

ng19delta · May 29, 2018

10KPete said:
OK, you're making me ( and rightly so ) quantify my statements.

9" SB won't reach the sfm needed for carbide until you get around 3/4" diameter at which point the issue is horse power to get the depth and feed rate up to what carbide needs.

I did mention the "aluminum" configured inserts which, by all reports, will do a good job on ferrous metals at lower speeds as they are sharp with a high rake.

I don't particularly enjoy making blue hot chips with a small lathe so the speeds/feeds I usually run are well within the range of HSS. That and most of the work I seem to do involves cuts that carbide insert shapes don't lend themselves to. The amount of time that I would use carbide seems to be a small part of the cuts I make so what tiny amount of time I might save using carbide is cancelled out by the time it would take to change tools. Even with a QCTP.

I also see a lot of folks who try carbide for all their work, usually because they don't know how to grind a tool. And I think that tool grinding is a vital part of being a machinist.

Have fun,

Pete

Learning to grind properly is something I have been working on! I have one of those fancy holders, but usually end up using it to simply hold the cutter while I grind it, without the outer piece!

Scott

Conrad Hoffman · May 29, 2018

CRS can be a pain. You can use a healthy rake on steel- look at some of the tool grinds in the old books like How to Run a Lathe. Use a good oil to prevent welding and don't let the part go dry for even a second. If all else fails, learn to grind an HSS shear tool. It will let you sneak up on a tight tolerance with a finish that looks like it came off a grinder. (I use both HSS and carbide on my relatively flexible Logan with no problem. IMO, you want to avoid high force negative rake tooling with small lathes, but the tool in the photos looks OK. Check the tip under magnification, as we've found that style easy to damage.)

michiganbuck · May 29, 2018

I have always found a back rake and side rake help CRS and most any mild mild stock...and a radius at less then the *intersect angles.
Carbide would be fine/Ok with having the right shape..perhaps a carbide bit for aluminum would be good/better.
Back in the day i made cutting tools of carbide for steel and aluminum... grade and shape made them work well.
Carbide is very good (often best) for Cast iron and harder steel..yes stout machine for higher cutting forces..
an OK reference for angles (I don't agree with the cast iron angles, I like a a honed negative for most)
https://littlemachineshop.com/images/gallery/instructions/GrindingToolBits.pdf

Carboloy made a grade 210 for aluminum and it worked very well for steel but had to be set at a rake attitude.
Don't follow the grades today...likely some aer very good. *Every lathe hand should know tool bit angles if only for the special shape not in any catalog.

*Note when the side and front clearance come together the nose often has way too much clearance and so a nose radius might be bumped at perhaps 7 to 12* or so.

and finger nail shaving sharp is good for mild steel.

steve-l · May 29, 2018

Cole2534 said:
No, Pete, it's not.

Crap carbide is crap, of course, but a positive rake insert with good geometry works very well on even an SB9. Lathe inserts .com will sort this out for you.

OP- you peeled a good chunk off that bar. I'd recommend you read about the 3 pass method and practice with it, aiming for DOC's of no less than 0.015". Everything is a spring (IE- it all deflects) and this methodology standardizes your last 3 passes so that you may get predictable results from each.

Yes, Pete is correct. Carbide sucks on the small SB machines. Why? Because these machines are not powerful or stiff enough and they are slow. . . . simple. Now, can you hobble around and make some carbide work? Sure. Why? The machines were designed for HSS.

iwananew10K · May 29, 2018

EVERY new guy should buy one of the "hobby" kits from latheinserts.com

Not only are they seriously good carbide insert tools but for the guy with no experience gives you something to shoot for with regard to grindkng HSS....if you can beat the insert tool then you got the grinding pretty well mastered.

The latheinsert.com tools are that good...and work just as well at low speed and light cuts as THAT is what they are designed for.

I imagine the same arguments happened when HSS started replacing forged tooling....oh my!

Personally I think the really good HSS should be treated like gold and reserved for when you need custom shapes and forms....it's getting hard to find really good stuff and the price is starting to rival carbide anyway.

Cutting issues

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