![]() There is also relief below the tool end, needed to avoid rubbing on the work(6° to 8°, as usual). This is called the end cutting edge angle. SCE joins the end cutting edge (sometimes via a radius) and then the tool end angles away from the work. The SCE may be perpendicular to the work (leftmost tool in picture) or angled to the right this is called the SCE angle. This is called the Side Cutting Edge (SCE). In soft materials a larger relief angle can be used to allow faster cutting.Ī common lathe bit approaches the work from the right so the left edge must be relieved as well as the front of the bit. The optimum relief angle for most materials is 6° to 8° this provides good support for the cutting edge to prevent it crumbling from the force exerted by the chip. The relief angle is the angle below each cutting edge without a relief angle the tool would simply rub on the work rather than cut. ![]() If you'd like more info on the why's and wherefore's, the book is an excellent source and very readable (but difficult to find and expensive to buy). The following is a brief synopsis of terminology and concepts from the book to help understand the reasoning behind St. I've found I'm using these bits more, especially for roughing, as time goes on. ![]() This simple jig allows new users to grind bits that work well without spending a lot of time understanding the various angles. Surprisingly little grinding is required for bits produced with this jig so it doesn't take long to make them initially and even less time to resharpen - I ground a bit from a blank in under 3 minutes. The jig described produces bits which cut while moving to the left but it is straightforward to build a similar jig for bits which cut going the other way. The key to easy resharpening is making it easy to reproduce the angles initially ground into the bit, exactly what the jig accomplishes. The bits produced should be oriented exactly perpendicular to the work so a QCTP that accomplishes this is helpful a QCTP clamping to a cylinder will require careful adjustment for each tool change so cylindrical QCTPs need extra attention. This jig is useful for common external roughing and finishing bits, not form tools or boring bars. These are bits for quickly removing material as well as bits for improving the finish after roughing. This jig is easy to build and makes it quick and easy for beginners and experts to grind bits per St. brass may self feed and hog in or chatter if the tool angles for steel are used - as is well known in home shops.Īfter reading this book (twice) I made a very simple grinding jig to produce bits with angles appropriate for cutting steel. What I gleaned from the book is that most tool parameters need not be optimum to work reasonably well although some angles may need to be selected appropriately depending on the material, e.g. Tool life, in the form of infrequent sharpening, is important but ease of resharpening and initially grinding the tool are also important. In the home shop "best" tool performance is hard to quantify because we generally don't repeat an operation long enough to wear a tool out, whereas tool life was a major concern for St. Clair tools - makes them work predictably, much like an insert. Ease of resharpening using a jig is a major factor for both the tangential and St. Clair tools are the next most used lathe tools in my shop, especially for roughing. While the tangential remains my most used tool, the St. Clair grind more and more, mainly because I understand it better and adjusting the angles to 4 degrees for side relief and 10+ degrees side rake has made the tools work well on my limber lathes. Update: As time goes on I find I'm using the St. Clair's material provides some useful insight on how and why cutting angles are chosen. I expect these tool designs work particularly well on large, rigid machines in good condition (smaller side relief on my limber machines helps me avoid the chatter regime). Chatter was a problem on both machines during deep cuts with high feed rate until I limited the feed rate by reducing the side relief (see below) and reduced the width of the cutting section. This foray into tool grinding theory was educational but the resulting bits cut too well, so well they chattered in the 7x12 and the Rockwell 10". The basic concepts for both materials are the same, where the cutting bit material's strength and toughness change the design details required for best performance. Clair worked in industry optimizing design and use of lathe bits during the transition from HSS to carbide so he covers both areas well. ![]() Clair and was impressed by his clear description of cutting angles and what they do in single point cutting tools. I read " Design and Use of Cutting Tools" by Leo St.
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