Grinding Lathe Cutters
Grinding lathe bits is a dying art. The widespread availability of ready-to-use cutters has made it possible for hobbyists and small shops to work without having to acquire this particular skill. However, it behooves every machinist and ‘smith to know the basics because High Speed Steel cutters are often the most appropriate for the slower speeds used on manual machines, and being able to grind your own tools allows you to create specialty cutters that might otherwise be expensive or impossible to get.
This tutorial is meant as a follow-along exercise. You can acquire some ‘book learning’ just by reading, but you’ll never really have a command of the process until you practice. By following along, you will produce a set of cutters that are useful for general work, and hopefully you’ll learn enough to be able to produce cutters of your own design.
A bench grinder with course and fine wheels, and tool rests.
A dressing tool
A sharpening stone
5 pieces of high speed steel tool blank, 1/4″ to 3/8″ square
Just about every shop has a bench grinder. They’re available in a broad range of sizes, but the most common have wheels that are 6″, 8″ or 10″ in diameter. They range from small 1/2HP units found at home improvement and hardware stores, to more powerful industrial units. Virtually all come equipped with a course and a fine aluminum oxide wheel. There are a bewildering array of wheels of various grit and composition available, but for our purposes the standard aluminum oxide wheel willl work fine.
Every operation performed with a grinder, whether dressing the wheel or grinding a cutter involves bits of flying metal and abrasive chips – always wear eye protection. Hearing protection is strongly advised as well.
Dressing The Wheel
Especially if you have an old, worn wheel, it should be trued before trying to do any accurate grinding. Grinding wheels in many shops are horribly abused and may be rounded or concave, or be loaded up with metal from materials that have been previously ground. Dressing the wheel makes the front surface flat, and clears away any material from previous use. (Dressing may also shape a wheel for grinding custom shapes, but that’s a topic for another day). Dressing also sharpens the wheel, by removing dull used particles and exposing sharp, new particles.
The picture above shows the three primary types of wheel dressing tool. From left to right they are:
1. The diamond dresser. This is a steel bar, with an industrial diamond bonded to the tip. The diamond, being harder than the wheel material, will cut into the wheel, removing material. Because it has a relatively fine point, it’s easy to carve grooves in the wheel if you’re not careful. It is best used with a guide (shown), at least until you have the technique down. The guide is a custom-made affair, and should be tailored to your grinder.
2. The abrasive stick dresser. The abrasive stick is basically a stick of grinding wheel – it will erode as you dress the wheel down. The abrasive stick is usually used free hand and has a broader bearing surface, making it easier to use freehand than the diamond dresser.
3. The ‘star wheel’ dresser. The star wheel dresser is probably the most common type found in most shops. It uses hardened steel star wheels, that rotate and break off the surface of a grinding wheel. The steel handle normally has hooks on the front that can be placed against a properly adjusted tool rest and used as a guide to get a straight surface, parallel to the grinder’s tool rest. By placing the hooks on the tool rest, the handle is lifted (gently) until the wheels make contact, and the dresser is moved side to side.
To dress a wheel, simply bring the business end of the dressing tool into contact with the face of the wheel and move side-to-side until the surface of the wheel is clean and flat. At that point the corners of the wheel should be fairly sharp as well – if you need a sharp corner continue dressing until you have one. If there’s still metal embedded in the wheel, repeat the process until it is gone.
Using a diamond dresser, slowly guide the tool across the face of the wheel. Diamond is so agressive that a single pass is often all that is required.
With an abrasive stick, uniform pressure and a repeated side-to-side motion are all that is required. Most abrasive sticks are encased in a plastic sheath – just let it grind off. The plastics used are some that should not smoke or melt – the sheath keeps the brittle stick from crumbling or shattering in use.
The star-wheel dresser can be used free-hand in much the same way as an abrasive stick, but the easiest way to use it is to adjust the tool rest on the grinder to leave a gap big enough that the hooks at the front of the dressing tool can be place on the front edge of the rest as shown in the picture. Lifting the handle up, brings the star wheels into contact with the grinding wheel, and again the dressing tool is moved side-to-side across the wheel until it is properly dressed.
You shoud re-dress your wheel when any of the following occur:
1. You can see metal embedded in the surface. I don’t mean tiny sparkles, but as soon as there’s noticable build-up. Material that has built up in a grinding wheel smears against anything you are trying to grind, marring it and creating excessive heat. Since it’s just rubbing, it dramatically slows down metal removal as well.
2. The wheel loses its desired shape. If the face of the wheel is rounded or concave, it’s hard to grind a flat surface.
3. The wheel doesn’t cut as well as expected. This is tough to judge when you’re starting out, but you’ll get a feel for how fast material should come off. If it’s coming off very slowly, it may be that you’re using dull abrasive. This is especially true if you’re using a wheel that is used (or misused) by others.
4. You’re worried about contamination. Infrequently you may want to grind something that needs to be kept free of other types of metal. When that occurs the ideal situation is to use a dedicated wheel, but if you can’t do that, the next best thing is to dress the wheel you have.
Grinding wheels are meant to grind relatively hard metals. Other cutting techniques are better suited to softer materials. If you find yourself wanting to grind the edges off aluminum pieces, the right answer is a belt sander, or a special abrasive-impregnated-rubber wheel. Grinding aluminum or other soft metals on a regular grinding wheel will quickly load it up with material. Done excessively this is actually a safety hazard.
You need a quench cup. Most people use a small (~12 oz) can of water. Your grinder may have even come with some manner of cup. Fill it about two-thirds full. Place the cup as close to the wheel you are using as is practical – bear in mind that vibration will make it ‘walk’ if it’s not confined in position. You’ll be quenching frequently, so having a conveniently located cup is important. Some of the provided cup holders are most *inconvenient*.
As you grind, you will dip and swirl the tool in the water each time it begins to feel hot to the touch. Give it a full 2-5 seconds in the water to get the heat out, a hasty dip won’t cool the part enough and you’ll be quenching again almost before you’ve removed anymore metal. Slower quenches mean faster grinding!
Apply only enough pressure to hold the tool against the wheel. Heavy pressure will not speed up the grinding significantly, but it will cause you to burn your fingers, dull the wheel more quickly and load it up with metal. Worst of all, you will lose your ‘feel’ and are more likely to created faceted surfaces than clean, even ones. Proper grinding requires some patience. If you burn yourself, or discolor the steel, you’re pressing too hard. Relax, get comfortable, and take the time that the job takes. You shouldn’t rush your cutter any more than you’d rush the pieces you’re going to cut with it.
Grinding Lathe Cutters
Any machinists reference (Machinery’s handbook, Audel’s Machinists and Toolmakers Handybook, etc.) will provide a list of angles appropriate for use with a given material. In a production environment, where tool life and maximum metal removal are critical, those angles are critical, but for use on manual machines while creating prototypes they can vary quite a bit with very little noticeable impact. The cutters I describe here will work for most lathe-cut materials, and I’ll detail how you might modify them for special jobs. For now, we’ll use about 10* for everything – As long as you’re close to that, don’t worry too much about the exact angle.
I’ve generalized the procedure for grinding lathe cutters into five steps:
1. Grind the cutting edge with clearance
2. Grind the opposite edge with the same clearance
3. Shape the tip as required
4. Grind the rake. Rake is the angle of the top, and it slopes in two directions at once – toward the back of the cutter, and away from the cutting edge. This compound angle is ground in a single step.
5. Hone the business edge(s).
Grinding a standard right-angle cutting tool
We’ll start by grinding a standard right-hand cutting tool. This tool is useful for facing cuts and for cutting from the tailstock toward the headstock. (i.e. conventional turning)
Note: I have no idea why this is called a right-hand tool – but it is. I always want to get this backwards. My best guess is because it cuts from the right even though it cuts to the left (toward the headstock).
I’ll describe this first process in pain-staking detail, after which the other cutters should seem like more of the same.
Start by setting the tool rest on the grinder between 5* and 10*. You can measure this angle with a protractor, angle gage, or just eyeball it. The good news is for the vast majority of your work these angles do not have to be exact. For hobby use, err on the side of too much angle. The cutter will still work, it will just be less strong than a cutter with a shallower angle.
Any time you are grinding with a rest the rest should be as close to the wheel as possible without touching it. Any gap is an invitation for something (clothing, a finger) to get caught between the wheel and the rest and get sucked in.
Step 1: Grind the first (long) side. This side is ground at roughly 30* from the side – again, don’t get too hung up on the angle. Look at the pictures and drawing and make sure you start on the correct side, or you will end up with a tool for cutting away from the headstock – useful, but not what we’re after. The angle of the tool rest imparts side clearance as we grind. This step is the slowest as it involves the most metal removal. Take your time, quench often, don’t press too hard.
After each quench, bring the tool back to the wheel and align the top by eye. You align the face by feel. I like to push (lightly) on the cutter, with my thumb directly opposite the wheel, this helps push the established surface against the wheel and prevents creation of a new surface.
Take a minute and compare your first surface with the pictures and drawings. You should have side clearance as shown in the end view and an edge that is ~1.5 times as long as your bit is wide. The ground surface should be a single surface rather than a collection of facets. Correct any problem areas before continuing.
Step 2: Grind the opposing side. The angle here is sharper, roughly 45*. It now becomes extra important that you not press too hard. As the material forms a point that area is more susceptible to heat damage than the thick sections. Quench often and don’t let the piece discolor. (If it does, it’s not the end of the world – it’ll probably still work fine as long as it wasn’t bad, but strive for perfection).
Now we have the basic shape. There are a number of cutters we could make with just those two steps – threading tools come to mind – we’ll cover them shortly.
Step 3: Put a small (~1/32”) radius on the tip. Continue using the tool rest for this step to get a nice looking radius all the way down the front edge. Just touch the tip very lightly to the grinder and rotate the bit to ‘touch in’ a small radius. This needs to be done smoothly and in one shot – if you try to touch it up too much you’ll end up with a large radius. This radius is what will enable a smooth finish. If we left it sharp, the cut would look like threads up close. The combination of some radius and a slow feed are what give a smooth appearance and texture to the work.
Step 4: Grind the top and side rake. The rake is put in free hand. Look at your cutter. We want a slope that goes from the first edge we ground back to the square (unground) edge. At the same time, we want a slope that goes from the tip back toward the shank. That means we need to start grinding not at the tip and the long edge, but at the back and the ‘far’ edge – where the grind will be deepest. You’ll have to hold those angles while pressing (lightly) against the wheel.
Here’s the general position you’ll use:
The bit is angled a bit up from parallel, such that the leading edge is parallel to the wheel. It is also tipped *forward* so that grinding begins *away* from the edge and proceeds toward it. Here’s the started surface.
As you grind, hold the long edge horizontal to the cutting wheel. When you get close to the edge, switch to your fine wheel and continue grinding until the grind reaches the long edge. That’s it – you should now have a nicely formed cutter.
You may use the cutter as-is, or hone the sides for fine work. (When honing be careful not to round your cutting edges!)
Take it to the lathe and give it a try. You should be able to face and turn (toward the headstock) and with a slow feed you should be able to achieve a nice finish in aluminum or steel.
Grinding that one cutter should have given you the process and technique to grind three more useful cutters to follow. None of them introduce new concepts, so let’s describe them and grind them.
Now for practice you’ll grind the next tool (almost) on your own. (Referring to the directions above as necessary)
The right hand cornering tool
The right hand cornering tool is almost the same as the right hand turning tool, with two differences
1.The tip is kept sharp, allowing you to cut a sharp shoulder.
2.The tip angle must be less than 90* (75* is ‘ideal’) allowing you to cut into a corner and leave a nice 90* angle using the motion of your lathe apron and cross-slide. Just the type of cut you’d make if you were cutting a thread tenon before chambering a barrel blank.
Give it a try – same steps as before, just don’t put the radius on the tip and watch your side angles just enough to make sure that the tip is less than 90*.
Two more! Now grind the left hand tools. Use the same procedure as before, but reverse the side angles to make tools that are suitable for turning away from the headstock.
At this point you have a nice complement of general turning tools – an excellent start for lathe work.
Threading requires a form tool. Form tools are different than most cutting tools in that they are used to transfer their shape (form) directly to the work. Form tools generally have flat tops (no rake), so it’s easy to see what shape they produce. However, having a flat top means they generate more pressure when making a cut, so cuts need to be lighter to avoid chatter or deflection in the workpiece.
The threading tool is easier to grind because there’s no rake – the top is just flat. In the case of the (standard) threading tool the shape being transferred to the work is the precise angle of the thread (60* for standard threads). It’s important that the shape be perfect, so this time your side angles are critical, but it’s still easy to achieve them.
Grind the first edge at 30* as before. I like to grind this side extra long – it will provide more clearance when threading to a shoulder. Then as you grind the other edge, you’ll want to check your angle frequently and get a perfect 60*. You could do this with a protractor or angle gage, but for 60* cutters every machinist should have a ‘fishtail gage’ made for precisely this task (as well as some other threading-related tasks).
Use the gage to check your angle as you grind the second edge and when it’s perfect (and the tip of the tool is pointed) it’s done. Polish the top on a sharpening stone and you’re ready to cut threads!
There are a huge number of other form tools you could grind, just let your imagination do the work. Note that form cutting involves large forces so you shouldn’t expect to cut large forms on a small lathe. I’ve seen it recommended that a tool for steel never exceed 1/8” in contact with the work at one time.
Here’s another form tool for radiusing the corner of a workpiece – remember your clearance and I’ll bet you can grind one without further instruction.
There’s no such thing as a ‘complete’ set of lathe tools, but if you’ve followed these instructions you now have a set that will serve you for a great many tasks.
The boring tool on the left will be discussed in a future article.
A ‘real’ machinist is quite likely to grind his tools without the aid of a tool rest. Once you have seen the angles necessary, it’s fairly simple to do, but takes practice. I encourage you to work on this aspect of grinding AFTER you have ground the tools shown here, in the manner described. Using the rest, in my opinion, helps teach what it is that we’re after. Learning to grind freehand is a convenience (it’s faster) but a luxury (it’s unnecessary). In the end results are all that matter and you can get results with a tool rest that are in no way inferior to free hand ground cutters. In fact, the greatest complement I can think of for a free hand cutter is “it looks like it was ground in a fixture”.
Not mentioned in this work are ‘chip breakers’ which are essentially any mess you may choose to make on the top of the cutter to encourage chips to break rather than form long curls. I consider chip breakers completely optional in the home shop and leave further discussion to the many books that cover cutter grinding.
Finally, here is a table of angles for use in various metals.
|Material||Top Rake||Side Rake||Front Clearance||Side Clearance|
Don’t get hung up in these angles – they are formulated for maximum tool life at maximum cutting depths and speeds. The ‘generic’ cutters you have just made are adequate for many materials at non-production feeds and speeds – this table is presented only to help you optimize your cutters if you find yourself cutting a lot of a particular material.
Don’t fuss too much with the angles above, instead remember what they’re actually telling you:
Iron: use shallower angles
Stainless, Copper, Aluminum: use sharp angles
Brass and brassy metals: use little to no angle
Finally, I should note that I use sharp angles in steel when I want a perfect, burr-free cut. It’s great, but tool life is reduced.