It occurred to me last night that knowing how the tool and tool angles work is all well and good but it doesn't help if the new guy doesn't even know how to use the tool itself. Aaron pointed this out and I flippantly said that figuring it out is part of the experience. While that's true, it is also arrogant of me.
It might help to talk about evaluating a cut to figure out what is going on. To be honest I'm just guessing when a part isn't turning out how I hoped. I'll adjust RPM, try adding some lubricant and adjust the depth of cut, sometimes that works sometimes I finish the piece with a fine and sand paper.
Aaron is only one of many new guys who are just starting out. What sets him apart is that he has the courage to put himself out there to ask what might be perceived as a "stupid" newbie question when, in fact, it is a really good question. We can learn to grind a good tool and even learn about tool geometry but if we cannot use the tool effectively then all of that knowledge does absolutely squat. Thanks for the epiphany, Aaron.
I decided to go off on a short tangent and quickly discuss how I set up and take a cut, then evaluate or adapt to the cut. Once that is out of the way, I'll get back to tool modification. I hope this is okay. I also invite the other more experienced guys to jump in and correct, add to or otherwise contribute to this basic approach to turning.
Setting up
Let us assume we are using a 3 jaw chuck that most of us own. The 3 jaw holds most round stuff quite well and as long as we don't remove it from the chuck, whatever we turn in it will be on the exact centerline of the lathe. Before chucking a piece up, wipe the jaws clean of debris and deburr one end of the work piece and stick it in the chuck. Be aware that if the piece sticks out more than 2 times the diameter of the piece then you should use a live center to reduce deflection. A Sherline chuck tightens with Tommy bars so you just snug it down onto the work piece but most chucks have a square key that tightens a pinion that rotates the scroll to tighten down on the work piece. On a quality chuck, one pinion will be marked by the factory. If your chuck is marked (dot, hash mark, whatever) then the accuracy of the chuck is obtained only when tightened by that one pinion. If it is not marked then you should take a ground rod and determine which pinion results in the best accuracy and mark it yourself. Your lathe chuck is not a drill chuck; tighten from one pinion only and do so firmly. You do not need to put a thousand pounds of torque on it.
With the work piece in the chuck, mount your lathe tool in the post and get the tip of the tool on the center height of your lathe. It is very educational to see what happens when the tip is above, below and on center and you should try it. Keep tool stickout to a minimum and lock the tool holder firmly in the post. Many turning issues are the result of a tool that is not on center or is not solidly anchored; pay attention to the basics.
Material and your job
Every material has a recommended cutting speed (in SFM = surface feet per minute) and it differs depending on which tool you use - HSS or carbide. Here is a useful chart:
https://littlemachineshop.com/reference/cuttingspeeds.php. Find the cutting speed for your material and tooling and calculate your lathe speed:
RPM = SFM X 3.82 / diameter of the work piece. The calculated RPM is not written in stone; you will adjust to the cut as you go. For example, if you are taking a roughing cut and encounter
chatter, you can do three things:
reduce the lead angle of your tool (turn the tip slightly toward the chuck), reduce speed or increase feed.
If you use the LMS speed chart, the cutting speeds assume a feed of 0.004”/rev. You need to adjust this as needed to give you a decent result. If you have chatter, feed faster. If you have surface defects (lines or whatever), try feeding slower. If the finish is good then, well, its good. For short pieces with a HSS tool, I often feed manually instead of engaging power feed and tend to feed slower for a good finish. Carbide tools require more consistent feeds but I won’t go into that here. Feeding manually allows you to feel how the tool is cutting and I do this whenever I cut something new to me. I am feeling for a slight resistance to the feed as the tool cuts; this gives me a good idea of what the feed rate should be for that speed in that material.
Okay, we’re almost ready to cut. Pick a cutting fluid if you’re going to use it. Brass, most plastics and cast iron do not require cutting fluids. You don't need a flood; brush it on.
Face the part
Almost all lathe work starts with facing. Turn the tool so that the side cutting edge just behind the nose radius contacts the work and lock it down. Turn the chuck by hand so the tool contacts the highest spot and back the carriage out. Feed in a few thou in the longitudinal direction and turn the cross slide in to face the part. Repeat as needed to obtain a clean face. If you have a nib on the face after facing then your tool is not on center height; if the nib is round, the tool is too low. If the nib is cone shaped, the tool is too high. Now you can center drill it if needed or continue on to turn the OD.
Turn the part
Realign the tool so the shank is perpendicular to the work piece. A square tool will rough well in this position because the lead angle (the side cutting edge angle) is built into the design. If the tool chatters, turn it slightly toward the chuck and the chatter will usually resolve. This is because reducing the lead angle is the equivalent of reducing speed and increasing feed, just by turning the tool toward the chuck.
Pick a depth of cut. On a small lathe and especially in materials that you haven’t turned before, a roughing depth of cut should be conservative. Forget the Testosterone thing; big hairy cuts are for YouTube guys. Start small, maybe 0.0250” deep and make a cut. You are trying to see if the cut goes well with the speed, feed, tool, lathe and cutting fluid you have. If the lathe cuts without strain and the finish is acceptable, make another pass at 0.040” deep. If the lathe handles it and the finish is still good, go to 0.050” and so on. You are learning how heavy a roughing cut you can make with your set up for that material. When you are satisfied with the results, take note of how the tool is positioned and what your speeds and feeds and depth of cut was and write it down. Over time, you will know this from experience but for now a record is invaluable.
Now realign the tool so the tip is pointing about 10-15° toward the tail stock. You are going to take a sizing cut. Sizing cuts are cuts that you do after a roughing cut but before a finish cut. The purpose of a sizing cut is to smooth out the rougher surface produced by the roughing cut; this gives you an accurate surface you can measure so you know how deep a finish cut you need to come in on size. Sizing cuts are not deep cuts; typically, I use 0.010”, just enough to take off any surface irregularities. Once the surface is clean, measure it and see how much more you need to take off to hit your final size. I normally take additional sizing cuts until I am about 0.010” from final size, then I reset the tool an additional 5-10° toward the tail stock; this is my finishing position.
After touching off on the part, I dial in half the remaining depth of cut and make a pass. Then I measure the OD carefully and dial in the required depth to come in on size. A square tool will usually cut what I dial in and I have come to rely on its accuracy. It also produces a very fine finish when the tool is turned as above because it is shearing with the nose radius and the side cutting edge.
Now you can part off and you’re done.
I encourage you to practice, practice, and practice. If I could pick one material to learn from it would be 12L14. It will tell you when you’re doing it right and when you’re doing it wrong.
Use your senses when running a machine. Listen to her and feel the lathe as it tightens up under the strain of the cut; you will know when she is cutting well and when she isn’t. I can tell when my depth of cut is too deep or if my tool needs honing just from the way my lathe feels in a cut, even if I cannot see this on the work surface. In time, you will, too.
Hope this helps. If I missed something, and I’m sure I did, then ask and we’ll sort it out.
