Why can't I get a smooth finish when turning steel rod?

[alexccmeister]

Hi all,

I am using carbide tip to turn down a 10mm diameter steel rod for practice run. But I can't seem to get a smooth enough surface . Seems to have grooves after each turn. I am cutting probably about 0.1mm or so. Can't be sure as its a manual lathe. And plus the tip of the carbide gets eaten up real fast.

What do I need to do? Thanks.

[alexccmeister]

Oh dear, I may have posted in the wrong section. Can the Moderator move this to the right section? Thanks.

[sp1nm0nkey]

RPM? What's the depth of cut? And what do you mean by the tip of the cutter gets eaten up? Do you mean literally that your cutter is chipping away? Either your cutter was damaged to begin with or something else is going really wrong.

[svenakela]

Originally Posted by alexccmeister Oh dear, I may have posted in the wrong section. Can the Moderator move this to the right section? Thanks.

Done.

[LeeWay]

I am no pro at turning, but have to do it every week to produce some small parts. Usually just in brass. I used to buy just the brazed straight carbide cutters from Mcmaster Carr. They have two types of carbide. C2 and C5. They do a pretty good job there explaining what each works best on.
Even the bras I turn will eventually wear out the carbide tips so they get dull. If I let a little chatter go on, the tips are likely to chip and break. Once that starts, they either need to be thrown out or possibly sharpened, depending on how bad the chip was.
It is absolutely critical to have carbide aligned properly to the center of the work and that there be as little play as possible in the carriage and tool holder.
If you have any slop at all there or it is difficult to adjust to perfect center, then HSS will be the best choice. You have to turn slower with it, but it can easily put a very nice finish on steels.
If you are using hardened steel, then you would be better off grinding it first or heating it up where you will be turning. Hardened stuff will certainly chew up some cutters of any type.

[alexccmeister]

RPM? What's the depth of cut? And what do you mean by the tip of the cutter gets eaten up? Do you mean literally that your cutter is chipping away? Either your cutter was damaged to begin with or something else is going really wrong.

I am turning about 500 rpm and cutting close to 01, 0.2mm. If you look at the tip of the carbide, its curved in where the tip was cutting the steel.

Done.

Thanks.

I am no pro at turning, but have to do it every week to produce some small parts. Usually just in brass. I used to buy just the brazed straight carbide cutters from Mcmaster Carr. They have two types of carbide. C2 and C5. They do a pretty good job there explaining what each works best on. Even the bras I turn will eventually wear out the carbide tips so they get dull. If I let a little chatter go on, the tips are likely to chip and break. Once that starts, they either need to be thrown out or possibly sharpened, depending on how bad the chip was. It is absolutely critical to have carbide aligned properly to the center of the work and that there be as little play as possible in the carriage and tool holder. If you have any slop at all there or it is difficult to adjust to perfect center, then HSS will be the best choice. You have to turn slower with it, but it can easily put a very nice finish on steels. If you are using hardened steel, then you would be better off grinding it first or heating it up where you will be turning. Hardened stuff will certainly chew up some cutters of any type.

There was some play when I was doing the trial cut. The picture below is the set I got off ebay. Not sure what type of carbide it is.

[LeeWay]

That little bit of play is enough to give poor results and short tip life. My mini lathe doesn't have to be all that tight when I am turning brass, but when I did some rolled screws, I had to tighten all the jibs until I could barely turn the handles. That was the only way chatter wouldn't damage the bits.

If your machine has play anywhere, this moves the tip off center or off angle for your cuts. That is even when aligned just right. I used to have to shim my tooling with feeler gauges in the tool post to get them centered. I bought a quick change tool post and my turning is much better and a even faster. You can chuck up a dead center to help align to center initially. The zero it in with some facing cuts on some soft stock.

You may also be turning too fast. Look at the chip color. Blue is bad. You want caramel or straw color.
I did buy a few inserts off Ebay, and threw them out after using the second one. The insert holder was okay, but to tips seemed to brittle. They might work well in brass, but you don't really need carbide in brass.

[HuFlungDung]

Carbide does not work any magic. It is called for whenever high turning speeds, or hard materials that would cause HSS to quickly overheat and melt its way to dullness.

Because carbide is so hard, it is very brittle. The cutting edge is especially prone to crumbling away if it is razor sharp. So most factory inserts are lightly honed, which means the edge of the insert is rounded over so that it will not crumble so easily. This edge hone produces poor cutting action on a light cut, because the chip piles up on the honed edge before it can flow back across the tool face. This phenomena is called 'built up edge' and it will cause uncoated carbides to crater, which is a crumbling erosion of the cutting edge, often leaving a visible crater behind the tip if you stop before it fails completely.

So carbide works best on a manly cut with a manly feed For finishing, it requires a very high cutting speed, and on a lathe, this means running the spindle at a very high speed if your work diameter is small, like 3200rpm for your 10mm as a minimum. If you can get the cutting speed up above 400 surface feet/min, you will begin to enjoy improved chip flow which results in a smoother, shiney surface.

I cannot tell from the photo, but often carbide toolholders will hold the tool in an attitude that tips the top face of the tool down towards the centerline. This is called negative rake, and also works against you when taking light cuts. However, you can often purchase inserts to fit such a holder that has an embossed chipformer in the top which improves the rake angle, moving in the direction of positive rake. This helps improve chipflow.

But ultimately for light work at reasonable speeds on small work on a small lathe, you might as well get some HSS blanks and learn to grind your own tools. You can get a very good finish with a razor sharp HSS, which will not crumble away, so long as you keep the surface speed of the work below 100 feet/minute (in steel). A small amount of cutting fluid will also lubricate the chip and improve the surface finish.

If you are machining C1018 (also called cold rolled shafting), the gummy material is fairly challenging to get smooth. I'd recommend C1045, or C1144 (stressproof) for manual lathework as these materials machine to a nicer finish more easily.

[diecutter]

The indexable toolholders and inserts you show are good for turning small diameter steel rod as long as the inserts have a positive rake (the sides of the insert are angled, not 90 degrees to the cutting edge). Negative rake inserts are mainly for heavy cuts in large, powerful machines. They can be flipped to use the bottom faces, and are more durable for heavy cuts. For small work on a small lathe, a positive rake insert will use less power, produce a finer finish, and cause less workpiece distortion and surface tearing.

[marceccl]

Carbide eaten up sounds like hard steel. 900 r.pm. feed .0015.
At 10mm there should be LESS than 40mm of stock unsupported. (deflection).
Take file to steel. If cuts easily not hard.
High RPM. ( high as you can and keep the machine on the ground), .01-.02 depth of cut, feed .004-.01 per rev. with .032 rad. on tool.

[msomerville]

If you are on center it sounds like you might have too much stock hanging out of the chuck. Make sure that your chuck jaws are ground properly and the stock isn't flopping around.

[SACEM]

To start less asume that your tooling has positive cutting angle, no small lathe can handle the deflection caused by a negative tooling, that you are centered, that you don't have free play on your gibbs and that your chuck jaws are centered, with that OK I would say that your problem may be Turning Speed, at 500 RPM with 10 mm stock diameter you are cutting at:
10 mm x PI (3.14) x 500 RPM / 1000 mm per m = 15.7 m per minute

That speed is half the recomended cutting speed for HSS in standard (AISI1010-1045 quality) steels and less than one sixth the basic cutting speed for Carbide Tooling, besides that your depth of cut is too light (not deep enough) so , as someone else stated before the carbide tip comes with a raidius wich probably will be bigger than your cut, this will cause the tool to be scratching the surface instead of cutting it.

I agree with other posts in that you should stick with HSS tooling for such small diameters, raise your speed to 1,000 rpms make your tip will very small raidius if you are going to take small depth cuts and be sure that the angle of incidence (contact) between the cutting tool and the bar is positive.

If you look at the tool-bar position right before it starts to scratch the surface and you dram an imaginary horizontal line from your bar center to your cutter you should have the tip right dead center or slightly below center, never above 'cause you would be rubbing, and the line that could be extended from your cutting surface in the tool should be going upwards over the center of the bar that you are turning, thats positive rake angle.

Hope to have helped.

Regards