Tool Nose Radius

[Mike OConnor]

Hello All,

There have been some questions in a few other threads about surface finish and the relationship to the tool nose radius when turning, boring and facing on the lathe. 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. This is how I would explain this to my students.

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 work piece 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



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 10K South Bend Lathe.

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. And home workshop lathes usually do not have the higher speed range to use them efficiently, especially the older lathes which were designed for HSS and plain carbon steel tool materials and seldom have a top spindle speed over 1000 RPM.

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 smoothes over the feed lines and provides a great finish with a fairly fast feed rate. 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

P.S. I tried to place these diagrams in the text, but couldn't figure out how to do it. Lost my post twice before giving up and attaching the PDFs. 

[Chris J]

Thanks for that, I'll try and absorb it slowly.

[Don1966]

Mike as a self taught machinist, I really appreciate you taking the time to explain this. I never knew are even taught about it. I only knew from reading that a round nose tool would give a better finish, but never understood why. Thanks to your documented explanation I total understand it. We could use a lot more tips and tricks just like these to help us to better understand the how and why about cutting metal.
Thanks so much for taking the time to do this.

Don

[steamer]

Another very important factor with the cutting tool itself, get yourself a fine Arkansas stone, and stone the edge so that the cutting faces are polished mirror smooth.    A polished tool will cut a polished surface...tool radius and feed rate aside.

Dave

[Mike OConnor]

Excellent point Dave.

Stoning the cutting edge angles (flanks) and the TNR, as well as the top of the tool will prolong the life of the tool and greatly improve surface finish. Stoning the top of the tool will reduce the friction from the chip sliding over the surface and help to control the built-up edge (BUE) when machining ductile materials like low carbon steel and aluminum.

When your tool bit is prepared on the bench grinder or belt sander as some folks prefer, the surface left by grinding may look very smooth. However, if you could magnify it so that you could see the surface left by the grinding, you would see many jagged peaks and valleys. The peaks are fragile and break down quickly and the valleys collect the workpiece material from the chip sliding over the surface. When you stone the tool, you are knocking down those peaks which reduces friction, strengthens the cutting edges and increases the life of the tool. It takes some extra time and effort to prepare the tool bit, but it is far easier to maintain the cutting edges with some touch-up honing many times before the wear lands require removal by grinding.

Chris J & Don, glad you found something useful.

Kind regards,
Mike

[Rayanth]

Mike, thanks for the post, it helped me understand the why's and wherefore's much better. I made a tool to your specs, and run a few cuts with it, but I'm still new to all this so my results weren't exactly mirror-smooth, but much better than I'd seen from other attempts. Practice will make perfect, time will tell, etc etc.

-- Rayanth

[Walsheng]

Mike:  Great explanation. Exactly how I was taught 40+ years ago by a very fine Yankee toolmaker although his language was considerably saltier! As a matter of fact, English was his second language.  His primary language was profanity, and he was a master of that.
We always used India stones to hone the cutting edges and I still do. As long as you wind up with polished cutting edges.

John

[Arbalest]

I sharpen most of my tools on a belt sander and have a used 240 grit belt that puts a nice polish on stuff.
They don't actually make such a fine belt for my machine so I split wider belts of the correct length down!

[zeeprogrammer]

Nice thread here. It helped a lot.
Thanks everyone for your contribution.

[Chris J]

Can I ask a stupid question at this point to see if I am understanding this properly.
For normal turning and facing work are you saying that the feed rate (auto or manual), and the speed of the work against the cutter have an impact on the finish ?

[Arbalest]

It's by no means a stupid question Chris. Yes, the higher the speed and slower the feed the better the finish should be.

[kevin45]

One thing to remember about a radiused nose on a lathe tool......it can push your material some. So if you have stock extended out past the jaws a considerable amount, the chances of the tool pushing the stock away from the tool is greater.

Also if one uses carbide insert, make sure you have a small diamond stone or hone to radius the tip slightly. There is an believable difference between a zero radius nose and just hitting the tool with a diamond hone and adding a radius as small as .002. Not only does it leave a better finish, it won't dig in, then jump. therefore standing the chance of breaking the insert, and also ruining the part you are turning.