Here we go again with grinding tools.
Ovservation one: 16 degrees of back rake is a lot. Depending on what you are cutting, probably way too much. Back rake values in the range of +/- 10 degrees are probably the normal range. The greater the backrake, the more likely the tool will break or wear rapidly. The less the backrake, the stronger the tool edge is. Negative values are considered less and produce the strongest edges.
Observation two: until you are more experienced, I would suggest you start with rake angles in the range of 4 to 10 degrees for HHS tools. You can get some experience with your lathe and your materials and then start to modify the angle as needed.
Observation three: the main thing you need when cutting metal is rigidity. This is an advantage of a turret style holder. It is solid, sturdy. You really don't want to throw this advantage away by adding any extra devices or adapters between the turret holder and the tool. Simple is best.
Now, how to get the proper geometry with your tool holder. Many, but not necessairly all, turret tool holders will hold the tools in a horizontal position. This is not necessairly good or bad, but it is a neutral starting position for the angles so whatever angle you want, that is the angle you must grind. I suspect from your comment "Just clamping them in gives no backrake." that your holder is of this horizontal type. So, you MUST GRIND whatever angle you need to cut with into the tool: both rake and clearance. This means that you should grind the backrake angle on the top of the tool. And grind the clearance angle on the front and side edges as needed.
Another consideration is how you will bring the tip of the tool to the center line of the lathe. This is important because if it is above or below the center line, then the rake and clearance angles will change with the diamter of the work as you cut it. In order to actually use the angles you ground the tool to, you must be on the center line. If you are above it, the rake angle will be greater and the clearance angle will be less. This can become extreme and the clearance angle could become negative. A negative clearance angle would mean that the tool is rubbing on the OD a bit below the cutting edge and the cutting edge is not even in contact with the work. It can not cut under these circumstances. You must have clearance in order to cut. This is true for any cutting edge. On the other hand, if you are below the center line, then the tool may have a lower rake angle and cutting can be harder in a smaller machine. If taken to extremes, the tool can even be drawn under the rotating work with distasterous results. Oh, and if you are facing a part, you must be on the centerline to cut to the center of the part. Too high and you get a bit of a mess. Too low and you leave a button.
Your turret holder has been designed with a particular sized tool in mind. Most such holders do not incorporate any means for adjusting the height of the tool. So you must establish this height to put the cutting edge on the center line by several parameters. First, the tool size. If the tool is too small, it can not reach the centerline without shimming. If too large, it will have to be ground down (on the top side) to the proper height. So it is best to use the size that the holder was intended for. Before standarizing on the 3/8" size, be sure this is the intended size for your holder. Second, the tool must be ground properly. A larger tool can be ground down to the correct height. Third, you can use shims under the tool to bring it up if it is below the center line. A bunch of sheet metal shims in several thicknesses can be very useful. If you use shims, be sure they extend to the front edge of the holder or the tool will be unsupported for whatever amount it is short of it: another loss of rigidity.
Final observation: a turret holder is great for a production situation where several tools can be ground and mounted properly and then hundreds or thousands of identical parts can be made by just rotationg the turret. However, it is not very good in a small or home shop environment where parts tend to be made in singles or very small quantities. A far better tool holding system there is the quick change style holders. These allow a wide range of tools to be mounted and adjusted for the proper centerline height and then removed and remounted in the exact same position in seconds. You are not limited to the four or six tools that a turret will hold and tools can be removed for sharpening and returned to the same position (minus what was ground off) to continue the job with minimal adjustments. And if you must remove a tool from a turret holder, you instantly loose it's position. In a quick change holder, you can remount it and continue working on the same or an identical part with the same dial readings.
Last edited by Paul Alciatore; 11-28-2010 at 07:49 PM.
Make it fit.
You can't win and there is a penalty for trying!