Turning a Small Diameter

[Lew Hartswick]

Other point is what lubrication and tooling are you using ? For the best finish (IMHO) HSS tooling and I use lard as a cutting lubricant.

Crisco (registered trademark) etc. is just as good as Lard and wont go rancid if it sets
around (like it will in a shop). Of course bacon grease smells better.
...lew...

 

[moconnor]

Hello Antman,

In an earlier post in this thread you mentioned that you didn't understand why a tool nose radius was necessary on your lathe tools. It is a very important part of the tool and greatly influences the surface finish on the work as well as the strength of the tool point and the life of the cutting edges. It is also one of those things that once learned and put into practice will positively influence your work in the future.

First, consider on a screwcutting lathe that the feed for turning is most often driven by the spindle and the tool advances in inches or mm per revolution. There are lathes with independently driven feed mechanisms like the Hardinge tool room lathe or Firebird's adaptation to his Myford that are fed in inches or mm per minute, but most are like the former and regardless, the same principles apply to the tool. When the lathe is set-up for single point threading and the tool is fed along the work, we refer to this relationship as the lead of the thread or the pitch. The pitch of the thread is the distance between corresponding points on adjacent threads. The lead of the thread is the distance that a nut would advance along the axis of a thread in one revolution. For a single lead thread, the pitch equals the lead. The formula for Pitch (P)= 1/N, where N is the number of threads per inch. An example: for 20 Threads Per Inch (TPI), the Pitch would be 1/20= .050". So for every revolution of the lathe's spindle the tool will advance .050" or .050" per revolution. The tool bit's profile, ground to the angle of the thread required, forms the shape of the helical groove or thread as the tool advances. In the case of our example, the pitch of the groove is .050". I only mention the thread cutting operation here to illustrate the shape of the helical groove generated. When feeding the tool along the work during a turning operation we are essentially cutting a similar pattern into the surface of our work piece with the pitch of the helical groove equal to the feed in inches per revolution if we use a sharp pointed tool.

Because the mechanics of what is happening at the chip/ tool interface with regard to surface roughness is measured and compared in microns, it is helpful to magnify this relationship in an enlarged diagram. Consider Figure 1 below. If we have a sharp pointed tool and set the lathe for a feedrate of .005"/rev. for a finish cut, what we would end up with would be a 'thread' or helical groove along our workpiece with the profile of our tool and a pitch of .005"/rev.. You would be able to see and feel the groove or lines in the surface of the work. This tool bit would also have a relatively short life because the point is very fragile and would likely break down rapidly if not chip off completely. Sorry for the long winded build-up, but the relationship to threading was important to illustrate this point.

Now consider Figure 2. This tool bit has a Tool Nose Radius (TNR) that is greater than the feed rate selected. The net effect of this is that all of the peaks and valleys that were left by our sharp pointed tool are now smoothed over, leaving a much improved surface finish. In addition, our tool point is much stronger and will wear much, much slower. The TNR, once honed on the tool, can usually be maintained by a touch-up with a stone for some time before the tool develops wear lands that must be removed by grinding. It is critical that the TNR be tangent to both the end and front cutting edge angles and that the TNR have the same relief below it as these cutting edges. It is THE principal cutting edge and must have relief to work properly.

So there is a General Rule that can be used here. The Tool Nose Radius must be Greater Than the Feed Rate or TNR > Feed Rate

Now there are some things to consider beyond the general rule. On our relatively lightly constructed lathes, there are limits to the size of the TNR that can be used. In general, I use an .008"/ .015" TNR for finishing tools and a maximum of .030" TNR for roughing tools. When you start using a larger TNR, the contact area increases which can cause chatter on our small lathes. You will find this out the first time that you try to use a form tool. Another benefit that a TNR provides is it allows a greater feed rate to be used for finish turning, which gets the job done faster. BTW, I own a Myford S7 and a 9" South Bend.

For our smaller lathes the best tool material IMO, is High Speed Steel (HSS). Yes, off-hand grinding is another skill set to develop, but once mastered it is a powerful tool to have for the home workshop. The tool bits can be optimized for the job at hand, they are relatively inexpensive and they can be ground and honed to a very keen edge. When the work material allows (ductile materials), it is advantageous to know that positive rake angles increase machineabilty or the speed and ease with which a material can be cut. Positive rake also reduces the cutting forces required, which is very important for small diameter turning. With regard to carbide inserts, they are available with all sorts of TNR's and positive rake, but if you examine the inexpensive inserts that are often used in the home shop, their cutting edges are rather blunt. The less expensive inserts used as molded can be frustrating to the newcomer who had hoped they would be the answer to their poor finish problems. They have their place in the home shop, but finishing small diameters isn't one of their strong suits.

A good general purpose HSS tool bit configuration that I use for finish turning and facing is attached in Figure 3. It has a 35 degree End Cutting Edge Angle and is set with the side cutting edge of the tool at a -5 degree angle relative to the face of the work piece. This allows the tool to turn, face and get right in close to the center because the end cutting edge angle is parallel to the point of the center. This tool will also provide a beautiful finish on the face of the work when feeding away from the centerline. Because of the -5 degree angle, it is like having a very large TNR as you feed outwards as the increased contact of the cutting edge smooths over the feed lines and provides a great finish with a fairly fast feedrate. The tool can also be used to undercut a shoulder because of the acute angle of the point if the TNR is too large or a sharp corner is required. Try it, I think you will like it a lot if you haven't done this before.

Kind regards,
Mike



View attachment Fig.1 TNR Less Than Feed Rate.pdf

View attachment Fig.2 TNR Greater Than Feed Rate.pdf

View attachment Fig.3 TNR Turning & Facing Tool Bit.pdf

 

[zeeprogrammer]

Excellent post Mike. This really helps me a lot to understand.
I had noticed this effect before but hadn't taken the time to figure it out.
Seems obvious now

 

[Mosey]

The valuable lesson learned today...small diameter turning tool!
Is this appropriate for making small poppet valves, like for a small IC engine? (To avoid tapering of the stem)

 

[moconnor]

Hello Mosey,

The tool that I posted in Figure 3 above would work well if you used a small TNR (.008"). For small diameter turning a part like a valve stem or any similar type of part where deflection is an issue, set the left side of the tool bit or its side cutting edge angle at right angles to the lathe's axis so that most of the cutting force is parallel to the part's axis. The -5 degree set-up angle would work for light finishing cuts, but if you were trying to turn the diameter in one step like say .250" dia. down to .125" diameter, anything that will minimize tool deflection will help. Use positive side and back rake to reduce cutting forces as well.

Regards,
Mike

 

[picclock]

Hi Antman

Glad to hear you sorted it. Last cut depth should really be about 5 thou or less, and 20mm overhang is quite a lot, depending on the size of HSS you are using. To get those really good finishes everything needs to be really tight and spot on, because any movement of the tool due to load or differing metal hardness is going to affect the finish. This is also the reason for the shallow final cut depth, to reduce the load on the tool so that any flexing is minimised. You can overdo that though if the tool ends up rubbing instead of cutting.

If you get a good finish, polishing is quite fast and rewarding as it really makes things look professional.

Best Regards

Picclock

@ Lew - I don't think we have Crisco on this side of the pond (UK side), although I have never had lard go off. I just have a piece in an old tin on top of the radiator to keep it soft, and top it up with a chunk from the fridge time to time. I wonder if lard is the same in the US ?.

 

[Antman]

Hey Zee good to have you back. Did you also learn something from Mike’s discourse. Mike, thanks, I see why I should keep plugging away at radiused tools. Difficult to judge what the radius looks like. I have some music wire and some stainless fishing trace somewhere, I’ll measure it and maybe some of it will be that small. Those diagrams made a lot of sense ... back to the grindstone. I have a half tub of nice commercial cutting grease, but in the lathe I prefer to use coolant, that I dilute a little less than when I started out with it. I'll give the grease a try. Thanks guys for your replies.
Ant

 

[Mosey]

Lard over here, is of course pig fat. You can buy it in the market and those of us who wish to die young of arteriosclerosis use it for cooking, for example put it under the skin of the turkey or capon you are roasting. The rest of us buy Crisco, which is hydrogenated or artificially thickened oil. Just as effective at clogging your arteries, and tastes slightly less wonderful. Takes a few years longer to die, but tastes almost as good. No one I know of uses lard for cutting here, we buy expensive artificial cutting fluids, that are less effective but come in a can. Since I use carbide inserts, there is not much need for cutting oil.
Anyway, the comments about tip radius are really good. Must pay attention to that.

 

[tomol409]

I do not use a roundnose tool for a fine finish, as the small work I do requires sharp shoulders. I have always used the method of reducing the plan trail angle of the tool to a bare minimum, ( that is the angle between th front edge of a knife tool and the workpiece, as seen looking down) Too much and chatter will result, but set right, with the correct cutting speed for the metal , (and this a constant related to diameter and speed). with as agreed by all, sharply honed edges. The first time I did this many moons ago on a piece of En1A free cutting mild steel, I could not believe the quality of the surface finish, and have mostly used this method ever since. Half the battle is to use free cutting metals as much as possible.
To move to another hoary old point, I have never used tools with no top rake on brass, and I always use the tools ground for steel. I have never had any problems with this, and I feel it gives a better finish anyway. It saves grinding two sets of tools anyway. Recently I acquired a Pultra lathe on which I make diesel model engine parts, and I mostly use a tiny parting tool for both turning and parting. The tiny flat edge of the parting tool cannot create ridges or screw effect and the finish is usually superb. Also the 3000 rpm speed
helps. Of course with a bigger set up chatter could set in.

 

[Antman]

Tomol I get where you are coming from except the speeds. Maybe I'm trying too fine of a feed which is why I can't get my surface speeds up. Thanks,
Ant