I’m a believer. As a new guy I totally sucked at grinding lathe tools. It’s almost painful to admit how many stubby, misshapen, multi-faceted, overheated and just plain ugly lathe tools I made back then. The amazing thing is that some of those tools actually worked as well as the pre-ground tools that came with my lathe. I started to believe those guys that tell you, “just get it close and it will work.” Of course, I was too embarrassed to call myself a hobby machinist with those Franken-tools so I bought an expensive set of inserted tip carbide tools that I thought would make a major difference but was disappointed. They couldn’t rough as deep or finish as well as my ugly high speed steel (HSS) tools, at least not on my lathe, so then I believed those guys that tell you, “stick with HSS on a hobby lathe.” Hey, desperation can make you mighty receptive, you know.
Fast forward 15 years and now I believe, after having ground many experimental tools, that the best lathe tool for a hobby-class lathe is a HSS tool with its tip geometry modified to reduce the cutting forces it produces, ground on a belt sander.
Since standard tools are intended for use on industrial lathes their geometry can produce cutting forces that are excessive at times, especially when roughing but this can also affect sizing and finishing cuts. To be clear, cutting force is that force produced by the tip geometry of the tool that must be overcome to make a cut. We can look at it as a continuous resistance generated by the shape of the tool as it is pushed through the material during the cut. A standard HSS tool has a broad, wedge shape and creates a lot of resistance (carbide is even worse), so we must dial down our control inputs (depths of cut, feeds and speeds) to use them. If we alter the tool so it has a narrower included angle it cuts with less resistance, thus lowering the rigidity and power needed to make a given cut.We still have to manage our control inputs but since the cutting forces are lower we would expect to be able to cut deeper, achieve better accuracy in sizing and finishing cuts, and finish better with greater ease before running into the rigidity and power limits of the lathe. And this is exactly what happens.
To demonstrate this I took a 0.050″ deep cut in 12L14 mild steel on my admittedly older hobby-class manual mini-lathe using a very good quality inserted tip carbide roughing tool with a new insert, a sharp freshly ground high speed steel (HSS) roughing tool with standard tip geometry ground for steel, and the general purpose tool with modified tip geometry (not optimized for steel) that was ground for this discussion.
As you can see the modified tool performs favorably compared to the standard HSS and carbide tools, and I assure you they size and finish better as well. The trade off for this enhanced performance is a reduction in tip strength but most small lathes are not rigid or powerful enough to break a tip off, even with pretty aggressive cuts so it’s an acceptable trade off.
These modifications are not some deep dark secret. They are simply an alteration to the standard angles in ways that are already known to reduce cutting forces. You just need to know what to change, when and how to change it, and by how much. Grinding these changes into the tool then becomes very simple and will allow you to tailor the tool to your specific lathe and needs.
If this interests you then follow along as we go over basic tip geometry, at least enough to grind a tool with, and discuss how these modifications can be made. I’ll talk you through the thought process used to alter the tool we grind so you get a feel for what you’ll be doing in your shop. Then we’ll grind that tool on a belt sander, pretty much step-by-step, to solidify the concepts. We’ll finish off by showing you the Knife Tool, a facing tool I can highly recommend. Due to the length of this discussion I will break it up into three parts.
Please understand that tip geometry and cutting forces are extremely complex subjects. Entire books are written about this stuff by folks far more knowledgeable than me. I am not an expert on this subject; my goal is to give you only enough information to get you started and I consider this discussion to be a supplement to your reading. Therefore, any opinions, inaccuracies or errors are my own. The angle modifications I am sharing here were derived from experimentation in my shop and work for me and my lathe; your results will vary. These changes are NOT necessary; you can grind standard tools just as easily.
Finally, I am not a technical writer, engineer or pro machinist — just a guy like you — so I will write this in a style and language that I am comfortable with.
Experienced guy warning: Having been a new guy myself I recall the early grinding days well enough to know where the confusion points are. Accordingly, I am going to repeat and re-emphasize some points frequently, in several different ways, so that it clicks for the new guy. This will also be very detailed and necessarily longer because new guys need detail in order to succeed. For you more experienced guys, my apologies for the nausea this creates. Please ignore me and move on.
As always, working with machines and sharp stuff can result in injury or worse. Please be careful and proceed at your own risk.
First, the grinder
The bench grinder is the most commonly recommended tool for grinding lathe tools. This machine tends to cut slow, cut hot and cut facets. It may work in more experienced hands but for the new guy – maybe not the best tool to learn on.
A far better option for grinding lathe tools is a simple belt sander, preferably one that uses 2″ wide belts. These machines are cheap, widely available and surprisingly capable for home shop use. The smaller machines with 1″ wide belts don’t have enough width to grind a tool evenly, the platen is too flexible, the belts wear too quickly, and coarser belts are harder to find.
As a tool grinder the belt sander is almost ideal:
- Belt sanders cut very fast and very cool when you use the right belt and a wax stick lubricant. Average grinding time for a 3/8″ HSS tool is under 4 minutes and less than 2 minutes for a ¼” HSS tool.
- With their wide flat platens they allow for simple tool alignment so facets are easy to avoid. This is far, far, far better than trying to realign a tool to a narrow round wheel that requires frequent dressing.
- Belts are widely available, cheap, and do not require dressing, balancing, or conditioning. They also have a very good service life if used with wax lube.
- Changing grits takes seconds and going from shaping to a mirror finish by stepping up through the grits takes a few minutes. Honing to a final polish is less than a minute away.
- Belts will snap at the splice occasionally, especially if they are old. This will scare the crap out of you the first time it happens but I find that preferable to an exploding wheel. Know also that touching a finger to the edge of a running belt is very, very bad for you.
Most common belt sanders are not perfect in stock form. The OEM table is typically inadequate and needs to be upgraded to a solid table that can be quickly set to precise angles. The table should be made of steel to avoid sharp edges from catching and dragging as you move the tool across the belt.
The stock mild steel platen on these machines typically won’t last long with any serious grinding done on it. I suggest buying a ceramic glass liner you can epoxy onto a flat (preferably ground steel) platen to greatly resist wear from grinding operations. Better known as Pyroceram, these liners are readily available from knife making suppliers for about $20.00 as of this writing. The key thing is that the platen remains flat and does not flex at all. Once the table and platen are handled a belt sander makes a fine tool grinder and will also handle most other grinding jobs in the typical home shop.
Getting a handle on the angles
Take a look at this diagram from Machinery’s Handbook (MHB):
|Fresh off the belt sander, all the angles in the diagram above have been ground into this roughing tool for steel. Despite the complexity of the diagram those angles are actually simple to grind, as you will see.
Lathe tools cut at a single point of contact so they are commonly called single point tools. This point is the interface between three surfaces: the side, the end and the top. Each of these surfaces is usually angled in two planes.
- The words relief angle in the diagram above refers only to the vertical angle of the side and end faces of the tool tip. The words rake angle refers only to the horizontal angles of the top surface of the tool. The words edge angle have to do with how the tool is shaped; due to the wide range of shapes a tool can take these edge angles are not found in the typical angle table.
- Relief angles are primarily clearance angles that allow the tool to cut at the tip and upper edges of the tool without the area immediately below rubbing against the work when the tip is set to center height. They are critical angles in that they form one half of the main cutting edge, the other half being the top rake angles. Relief angles affect finish, tool life, cutting forces and cutting temperatures. For a small lathe we want to use the largest relief angles we can get away with without weakening the tool. This is especially true with finishing tools where larger relief angles prevent rubbing and greatly improve finishes. Larger angles also enhance penetration of the tool into the work for all cuts, a good thing for a small lathe.
- Rake angles direct chips away from the working area of the tool. Note that there are two kinds of rake: side rake and back rake. Of the two, side rake is far more important for a turning tool, while back rake angles are less critical (except on parting tools where back rake assumes the importance of side rake on a turning tool). Different materials require different amounts of side rake for efficient chip clearance. In general, cutting forces will run perpendicular to the side cutting edge and as the tool cuts the chips will follow the path of least resistance. Rake, particularly side rake, provides this path. Much of the heat in a cutting operation is carried off by the chip so clearing those chips efficiently will reduce cutting temperatures. Being the other critical half of the cutting edge side rake, and to a lesser degree back rake, has much to do with reducing cutting forces. Cutting forces and cutting temperatures decrease, and tool life increases as side rake and back rake become more positive up to an optimal point (MHB). Therefore, for a given set of rake values in a cutting tool table you want to go for the higher value for use on a small lathe to improve chip clearance and reduce cutting forces.
- Edge angles define the tool’s shape as seen from the top of the tool. They vary with the purpose of the tool and also by how much strength is needed at the tip. Tools meant for heavy cuts, like a rougher, will have more mass at the tip to handle higher cutting loads, while a finishing tool will have a more delicate tip appropriate to the lighter cuts it is meant to take. These angles are less critical than the other parameters and are really dictated by your needs for tip strength, finishing potential and access to corners.
- The nose radius at the tip of the tool varies with the general purpose of the tool. Roughing tools will typically have a smaller nose radius, while finishing tools generally have a larger nose radius. Note that a large nose radius greatly increases cutting forces because it is constantly being pushed out of the cut. For tools up to 3/8″ square it is best to limit the nose radius to a maximum of 1/64″. You can go up to a 1/32″ radius on a finishing tool but lighter cuts are necessary. When forming or grinding it you do not need to measure the nose radius with a gage – just estimate it.
Honing a tool after it is ground is a controversial thing. Many experienced machinists say they don’t bother. However, any edge defects that occur during the grinding process will transfer to the work; we may not care too much about it for a rougher but for a finishing tool this is not a good thing. Since honing also extends tool life and reduces the need to re-grind a tool I can’t see a reason not to hone a tool.
Honing is best done with a fine or extra-fine diamond stone. I prefer the solid surface stones on a steel base, not the plastic base with dots. If I need to do a lot of stoning I use a 2″ X 6″ stone . If I am honing after grinding a tool on the belt sander then I use the credit card size; they are cheaper and easier to handle. I use the fine stone to get off most grind marks and the extra-fine to obtain a nice homogeneous surface. Maintaining your tools after this will take only a few strokes per face.
Hold the stone in one hand and the tool in the other. Focus your pressure over the center of the face and stroke in one direction lightly, preferably under a running stream of water. Keep your wrists locked to avoid changing angles and creating facets. I suggest honing the side and end first, leaving the top of the tool for last; this removes any burrs and leaves a very clean edge. Your goal is to remove all evidence of grinding marks. Be sure to catch the nose radius when honing. If you need the finest finish possible for your work hone the tool, then polish it on a translucent Arkansas stone and your tool will cut like a razor.
Now that we have a better idea of what all those confusing angles are and what they do, we need to see how they are used to make a lathe tool. Part 2 will look at the Angle Table, how we can modify the angles to reduce cutting forces, and we’ll discuss the different shapes lathe tools can take.
In Part 2, we’ll put it all together and grind a tool on a belt sander.