Achieving fine finish on turned parts

I avoid CRS. O-1 drill rod and Grade 5 bolt shanks usually give me a good finish with honed HSS bits, cutting oil and fine feed. If I want a polished finish I use a single-cut file and then sandpaper.

1. Get the speed and feed right.
2. Get the cutting geometry right.
3. Choose a material that's highly machinable.

I got a head start on these when I took a night class at a local technical college. I've followed up with lots of reading of old books and just plain experimentation.

Pete

Don't forget rough/finish cuts, and that sometimes in some situations, it's possible to take *too fine* of a finish cut. Sounds weird, but I've experienced this.

Sandpaper, emory (which comes incredibly fine, 1200 grit I've seen), and various grades of Scotchbrite--or kitchen scouring pads/steel wool.

Also a steady tailstock. A trick I use is to put the tailstock crank at 3 o'clock, and load it with heavy rings, keeping pressure on the work. I find this to work better than the lock on the tailstock. Also, a positive stop at the chuck some kind of way is helpful.

Ahm certainly no 'spert, just what I do to get thru the day...

Of course, don't forget the erl. I use both a flux brush and dropper.

Scotch-Brite pads also work well. You can try some from the grocery store to start and if these work for you, these come in different grits from mill supply stores. For example

1: get some free-machining steel stock, e.g. 12L14. Some "junk" steel, as found in water pipe and imported tricycles, is about impossible to machine to a nice finish. 2: Carbide tools have their place, but HSS toolbits work very well on mild steel in a home shop setting where production is not an issue. How the bit is ground and honed can make a huge difference. Experiment!

For other steels, another useful trick is 1" wide emery paper -- it comes in rolls. Applied in the lathe, it can clean up a rough surface in seconds.

Tool geometry is very important and it is different for every material. The rule is to use as much edge support as possible without inducing material drag on the tool. Some material is tougher and stickier than others. This defines relief. Second, there must be enough surface speed to induce the plastic temperature of the material you are cutting at the cutting edge., no more, no less. This defines the durability of the tool. Thirdly, some materials work harden when being cut, like some types of stainless, chrome molybdenum alloys and cast iron. With these, your feed must be fast enough so the cutting edge is in front of the hardened surface. The most important consideration in machining on any machine and a lathe is no different is chip control. If you watch the tool closely when your finish is poor, you will see the chip is getting caught behind the cutting edge between the tool and the work just cut. It is these chips that are scraping the work and marking the finish. Tool geometry and back relief solves that problem. Another rule is to observe is the width of the cutting tool nose must equal

1.5 times the feed rate. This bridges the spacing caused by the tool advance across the work. Be careful with this because it is easy to overwhelm a small machine with too great a load. When it comes to which tools to use, there is a lot of junk out there, but in my experience cobalt HSS alloys are the best for the hobbyist. These have an enormous advantage over carbide and indexed carbide tools on the small machines, because you can control machine load with geometry changes and that is an issue with commercial production tools, as these are expected to be used on heavy, high horsepower production equipment. Cobalt alloys are very durable like M35(5%) and M42(8%) and will cut all but the very hardest material. As a hobbyist, production speed is not an issue, so cooling is not normally used and these tolerate high tool temperatures much better than normal HSS. Steve

So, Steve, if I understood you right, you can use M42 and M35 bits without coolant, as long as your speeds are within a certain range. Right? I would prefer not to use coolant for obvious reasons.

i

12L14 is wonderful, cuts like aluminum, but it rusts very easily.

The water pipe I got at Lowe's recently turns cleanly except at the weld. I was only removing the zinc to weld it and not trying for finish but it came out smooth and shiny. It was also within a few thousandths of round. I mention this because it's so unusual.

Ohh, this topic goes round and round (so to speak!)

First, some metals turn like mirrors, and others turn like concrete, tearing and spalling off chunks. It is a characteristic of the metal. High speed, a little cutting oil, and just the right feed rate and depth of cut are important, too. Go too slow on the feed and the work hardens ahead of the cutter. Go too shallow and the tool can ride up on the work, intermittently. The cutting tool can develop a "built-up edge" of workpiece material. As this varies, the diameter changes, leaving rings. You're in over your head, now, and well on the way to becoming a "metal head".

Long parts need to be supported at both ends, or possibly with a follow rest, otherwise the work deflects and causes rings.

Jon

For hobby-level machining, you never need coolant, like flood-style. But, brushing on some thread cutting oil from the local hardware store can make a huge difference. It prevents the chip from welding to the cutting edge, ie. "built-up" edge.

I use retired toothbrushes to apply the oil.

Jon

But coolant does more than cool the tool and the work. Coolant or some other type of cutting fluid used properly will prolong tool life and improve surface finish. One way the finish is improved is by lubricating the surface so that if the chip coming off the tool rubs against the work it does less damage to the work. ERS

Roller burnishing!

This matches my (meager) experience.

1. I haven't splurged yet on free-machining stock, but among the bits and pieces of 'mystery metal' that I use some work quite well and some are very hard to get a good finish on. If I ever get to the point of feeling that finish is more important than just getting a part that works I may buy some.
2. For a good finish I use a large radius bit (1/16 or 3/32, if I can get away with it), slow feed, and lots of lubricant. As a counter-example, consider that if you use a really pointy tool and high feed rate you're just making threads...

2a. Make the tool sharp, then hone it by hand on a fine stone. It seems that the finish on the tool transfers to the finish on the part, so make the tool as smooth as a baby's butt (I always thought that was hyperbole, until we had kids...). I do the final sharpening by stroking the round face across the hone, crosswise to the direction of the cut, to knock down any angles. I also touch up the tool every time before I put it in the holder, to make sure it's sharp and smooth.

1. I generally don't use sandpaper or files unless accuracy doesn't matter (I'm not that good!). So if the cut has to be accurate I have to achieve the finish with the tool.

OK. I think that the mystery metal that I tried, was not as good at achieving a good finish. I have some brass stuff that I will try turning down a little and see how good finish I could get with that.

Brass is a useful metal any time your self-esteem is flagging, at least if you use the right alloy. That stuff is just made to machine easily, and it's a nice color to boot.

Good.

Well ... yes a rounded nose helps.

Speed is mostly determined by the diameter and the materials of both your workpiece and your tool. Carbide will work at higher speeds than HSS, and HSS at higher speeds than the old high carbon tool steel.

Also -- some metals are easier to get a good finish on than others. If you can find some 12L14 steel, that is wonderful to turn. The typical hardware store steel produces a terrible finish.

You are turning steel aren't you?

Just to be sure -- most lathes beyond a certain minimum size have both the half nuts which clamp onto the leadscrew and a worm gear which picks up from the keyway in the leadscrew. The feed rate on the pickup is a lot finer than the feed from the half nuts. Not as precise, so not good for cutting threads, but a lot finer so you get better finish. For turning and cross-feed the pickup is the best choice.

If the workpiece is longer than about four times its diameter extending from the chuck it needs either a follower rest (also called a "traveling steady rest") or the end of the workpiece needs to be center drilled and supported by a live center. And if the jaws of the chuck are worn bellmouthed you will get problems with a lot less extension.

You need to minimize play in the dovetails in both the compound and the cross-slide -- perhaps by tightening the gibs a bit.

Don't extend the cross-slide very far.

And what kind of coolant or lubricant are you using where you are cutting? The easiest to find would be a squeeze bottle of Rigid threading lube -- usually stocked in the plumbing area of the hardware store.

There are lots of other coolants which need some kind of distribution and recycling system -- and which spray all over the shop unless you have a shield around the chuck.

Just some thoughts of what may be your problem.

Enjoy, DoN.

I found the brush method a bit awkward, especially when cutting off, as I couldn't get the fluid to where it was needed, down in a deep narrow groove. A simple mister which sprays soluble oil emulsion works well. I have been using Rustlick WS-5050, but there are many alternatives. Emulsion makes less mess than black sulfur oil.

People have also used homebrew gravity drip cans to apply coolant.

In both cases, the trick is to attach the applicator to the lathe carriage, so the coolant is applied at the cutting point no matter what.

Joe Gwinn

All good advice but the nose radius/feedrate ratio relation can be solved a different way.

In pretty well all my fine finishing requirements I feed from right to left so I use an assymetric tool with a plan view that is near square with almost zero corner radius (less than depth of cut).

It is presented to the work with the forward cutting edge about 15 deg off square so that the chip curls nicely away from the work. The straight front finishing edge is a few degrees off parallel to the work so that it bridges the feedrate spiral.

The forward edge provides near ideal cutting conditions. The straight front edge behaves as a large radius nose but with the advantage that the equivalent radius is easily varied by trimming the bridging angle,

Jim

I use a needle oiler to put the oil down in the cutoff groove. Very little oil is needed but the bit will jam within seconds if I stop.

Filing is accurate if you use the tool marks as a guide and stop as they disappear. One light stroke with a fine file only removes about a ten-thousandth so with a little practice and measuring you can calibrate your feel for it.