Using tip geometry
Here is a table that presents the table angles we need to set when we grind standard lathe tools. Note that the headings at the top of each column eliminate the word “angle” after each heading but otherwise matches the diagram above. The front relief (angle) in the table is the same as the end relief angle in the diagram above. The angles here are very important starting points for modifying tools. If you simply wish to grind standard tools then grind with the angles listed here.
The values in the table are the number of degrees to which you must angle the grinder table for the particular feature you are grinding. For example, when grinding a tool for aluminum you will angle the table of your grinder to 12 degrees and then grind the side; doing so will create a side relief angle of 12 degrees (your table setting) and the side cutting edge angle (dictated by the angle you hold the tool at as you grind the side) at the same time. You then reset the table to 8 degrees and as you grind the end the table angle creates the 8 degree end relief angle as you shape the end cutting edge angle at the same time. You then reset the table to 16 degrees and as you hold the tool at a 35 degree angle (to the belt) you feed the tool straight into the grinder to cut the top face; doing so cuts the side rake at 16 degrees while creating the desired 35 degree back rake at the same time.
In order to alter the tip geometry of our tools to lower cutting forces: Use the largest angle listed in the table as a baseline, and then add about 25-40%. This works out to about 2-5 degrees higher than the largest recommended values and your tool will work better on your little lathe without endangering the tip excessively. While these changes may seem very small and insignificant the reduction in cutting forces they produce are anything but. The tool used for our cutting force demonstration earlier was ground with these conservative changes and it works well.
Keep in mind that none of the tip angles works in isolation.
- Increasing the relief angles without changing the side rake gives you a sharper interface at the edge so cutting forces decrease and finishes improve but you also limit chip clearance and lose some edge strength because the edge is not supported as well.
- Increasing side rake without altering the relief angles reduces cutting forces even more while improving chip clearance but finishing potential drops off a bit. Because the edge is better supported if relief angles are not altered increasing side rake does not sacrifice edge strength as much.
- The effect of increasing back rake is not nearly as noticeable as when altering side rake. However, increasing back rake will tend to focus the cutting forces at the tip of the tool while helping side rake to flow the chip out of the cut.
- If you increase both the relief and side rake angles then cutting forces plummet but the tip and edge strength is reduced a lot more so be aware of it when the tool will experience high cutting loads. This is less of a concern for finishing tools.
- The shape of the tool has a significant impact on strength. The more mass you have in the tip the more latitude you have with angle changes.
- Side rake for brass and bronze is not zero in the table but a flat, un-ground top works well for these materials. Profiling tools also have zero top rake. A flat rake keeps the tool from digging into soft, grabby materials or when there is a lot of edge contact between the tool and the work. You don’t have to grind the top but you should hone it.
- Side rake is the angle that varies the most with different materials. In fact, side rake is the key variable at that tip. If you look at the relief angles in the table you will see that they don’t vary much between materials because clearance on a round part is still clearance and once you have it then, well, you have it. When you vary the side rake the included angle of the side and end suddenly becomes more, or less, acute. It is this alteration in included angle, also known as the angle of keenness, which changes to accommodate various materials. Seemingly small changes to side rake can have a major effect on cutting forces and how the tool cuts. Pay attention to side rake and use it to your advantage – it’s a major player.
- Side rake for Stainless and Back Rake for Aluminum are very aggressive. That’s a lot of steel to grind off. You may want to look into grinding chipbreakers when grinding tools for these specific materials. Chipbreakers break the long, stringy and dangerous chips into smaller pieces and are much easier to grind than the whole top of the tool. I won’t go into chipbreakers here but you should know about them.
For optimum performance you can grind a set of tools (rougher, facing and finishing) for each material you commonly work with. They will work far better than general purpose tools and are worth the time it takes to make them. If you grind them as you need them you will have a complete collection in no time. If you do make sets I suggest grinding the same face of each tool before changing your table angle to save setup time.
On the other hand, you can actually get away with less than optimal geometry. For example, you can grind a general purpose tool that will work on mild steel, aluminum, stainless steel, plastics, tool steels, semi-hardened steels and brass. The tool we will grind as an example below is one of these general purpose tools and the tool angles used are typical for a tool that has to cut multiple materials under varying conditions. These tool angles are a compromise, just as the shape of a general purpose tool is a compromise but they work fairly well.
General purpose tools allow you to minimize the number of tools you need to grind. For example, if you had to minimize the number of tools in the drawer you could have a general purpose right hand and left hand tool, a RH and LH general purpose knife tool for facing (see the end of this discussion), a zero-rake 60 degree threading tool and a zero-rake round nose tool with a 1/64”-1/32” nose radius (for between-shoulders work and general cutting of brass). Add a HSS blade-type cut off tool with a 7 degree face used in a rear-mounted tool holder and you can handle most general jobs in a hobby shop.
An important angle you may see that isn’t in any table is called the Lead Angle (LA).
The LA is not ground into the tool; it is set by positioning the side cutting edge relative to the work piece. In general, a tool is used with the shank positioned perpendicular to the work; in this case, the LA is equal to the side cutting edge angle. However, increasing or decreasing the LA of the tool to suit your purpose works better.
Increasing LA can significantly improve finishes; for this reason MHB recommends using the maximum LA possible as long as there is no chatter. Remember that increasing LA also increases cutting forces because more of the side cutting edge comes into contact with the work. The effect of this is most readily seen on thin, flexible work pieces in the form of chatter. On larger pieces that don’t flex much a large LA can really improve finishes.
If you develop chatter at any time, especially on thin pieces or on hard materials try reducing the LA (move the side cutting edge more perpendicular to the work) and things will improve. In some cases going to a negative LA really helps, especially on thin work or an ambitious roughing cut. The effect of reducing LA is equivalent to reducing depth of cut and increasing feed, which has a positive effect on reducing chatter. Try playing with LA to further modulate cutting forces on the lathe – it can be useful.
Another term you may see is the Angle of Keenness. This is the included angle formed by the side and top of the tool as you look at it from the tip end of the tool. For harder materials this angle is less acute and for softer materials it is more acute. Back in the day, before angle tables became widespread, machinists ground their tools with this angle: less acute for hard stuff, more acute for soft stuff. Since the hardness of the material being cut is accounted for by the angles found in the angle table I ignore this angle but mention it here for completeness. If you were paying attention, this is the angle that changes as you alter your relief and side rake angles. Those old guys knew what they were doing.
Lathe tools come in many shapes, each suited to a particular task.
Continue reading Grinding Lathe Tools on a Belt Sander – Part 2