2200 RADIUS CUTTING ATTACHMENT (1 LB)

The radius cutting attachment gives you the capability of putting a radius—either convex or concave—on the end of a part when using the lathe. Most commonly they are used to put a ball end on a part or make a complete ball in a two-step operation.

About the radius cutting attachment...

The radius cutting attachment swings a tool through an arc to put a radius on the end of a part. The pivot points and tool position are adjustable, so it is capable of cutting either a concave or convex shape in a large range of sizes.

This tool came about as a result of studying the method used to shape grinding wheels. Conventional thinking says that lathe tools should move parallel to the tool’s longitudinal axis or horizontally across it. Once it was realized that the tool could also be pivoted vertically on this axis, the design of this tool became much easier to conceive. Believe it or not, a technician with a need to cut an accurate radius to make prosthetic eyeballs got us started on putting this tool into production. Now anyone with a need to put a nice looking radius on the end of a part can benefit from the way this accessory makes it easy. It may not be something that a lot of people need, but it is typical of our commitment to serve all of our customers. If you need to (or just think it would be fun to) put a radius onto or into the end of a part, this accessory makes it easy and repeatable. It can also cut a complete ball in a two-step operation covered in the instructions.

Use of the radius cutting attachment

The radius cutting attachment allows you to put an accurate convex or concave shape on a part with a Sherline lathe. Unlike conventional radius cutting attachments that swing the cutting tool parallel to the lathe table, the Sherline tool moves from the center of the part to the top. The idea came from the method toolmakers use to dress (shape) a radius on a grinding wheel with a diamond tool. A ball can also be cut using a two-part process described later in these instructions. Diameters up to 1-1/2" can be cut. A handle is provided that can be used on operations where the tool faces the long end of the "U" shaped cutter body. On operations where the tool faces the other way the handle is not used.

Center height reference numbers

You will notice numbers engraved on the side of each support. These represent the actual distance from the table to the center of the pivot pin for purposes of calculating the center of your radius. The theoretical distance should be .940", but because there is minor variation in that distance due to production tolerances, so each one is measured and the exact distance for that unit is engraved on the side.

Using the stop screw to avoid cutting past center

It is important to note that the cutting edge of the tool is set to cut with the lathe turning in a particular direction depending on which half you are cutting (top or bottom). As the tool passes the centerline the tool cutting edge still faces the same way, but now the work is rotating in the opposite direction. This causes the tool to "drag". To keep this from happening, a stop screw is provided that will work in most setups. By adjusting the stop screw to hit the table when the radius cutter is at the center position, the tool can be prevented from accidentally dragging on the work. By the same token, always note the direction of rotation and make sure the cutting tool is facing in the proper direction to make your desired cut.

Setting up the tool to cut a convex radius or ball end

The most common application of a radius cutter is to put a radius or ball end on the end of a part. The points of the radius cutter are accurately located on the centerline of the part. (See paragraph above on engraved center height reference numbers.) This makes setting the cutting tool to the proper radius a simple process. Here's how it is done:

FIGURE 1-Setting the tool depth to cut a full radius on a part already at finished diameter. Adjust the tool until it just touches the top of the part. (Pivot supports not shown.)

1. Mount the two uprights in the table T-slot closest to the spindle. Pick out the center holes that will give you the best location for the cutting tip of your cutting tool. This is dependent on the final diameter of the ball. The center holes are accurately drilled on .250" centers and only a rough location is needed at this time.

2. The quickest way to set a cutting tool is by first turning a finial diameter (twice the radius) on the part you are going to work on with a standard lathe tool before mounting the radius attachment. If the part is already turned to the finished diameter and you wish a full radius on the end, simply raise the yoke to the vertical position and lower the tool until it just touches the top of the part. (See Figure 1.) This will establish the proper radius. It would be safest to set the tool to a slightly larger radius just to be safe and then "sneak up" to the final dimension once most of the material is removed and you can see how close the final cuts are coming to your desired radius. Move the radius cutter away from the end of he part with the leadscrew. Rotate the tool (assembly) and move the saddle towards the part until the tool is in a position to take a light cut (approximately 0.020") on the top corner. The first series of cuts are accomplished by rotating the tool up and back. Then move the saddle and radius attachment about 0.020" closer. As you get down to the final cuts you will be able to see which way you are off and make final adjustments with the crosslide handwheel to finish up exactly on center to cut the full radius.

3. Remember that the tool will cut the full amount it has been advanced at the center but will not reduce the diameter at the top and bottom of the part. (See Figure 2). If you move the tool past the top dead center before the tool is cutting at its final position you will undercut the opposite side of the ball. This makes it wise to stay back 10° or so from top dead center until the tool has reached its final position. This creates a "damned if you do and damned if you don't" problem. If you don't cut over the top you can't accurately check the diameter of the ball, and if you do you may scrap out your part. The easiest way out of this situation is to set up on a scrap piece and get the tool set. The radius remains set even if the attachment is removed and replaced on the lathe as long as the tool isn't moved in the radius cutter body.

4. Another way to set the tool would be to accurately cut or mill a gage block to a dimension that is the center height of the part over the table (.940" or the amount engraved on the attachment) PLUS the desired radius of the part. Set this gage block on the table and move your tool down to just touch the top of it with the attachment in the vertical position. (See Figure 3.)

The tool can also be set for cutting a small concave shape using this method only the radius would be SUBTRACTED from the 0.940" dimension rather than added for obvious reasons. (See Figure 4.).

To cut a large concave radius, a height gage can be used to set the tool height. The height gage is set to .940" PLUS the desired radius as seen in Figure 5.

FIGURE 2-The area in black illustrates why you don't want to cut past top dead center until you know you are at final size.

FIGURE 3-Using a gage block to set the tool height for a convex radius. You can use this method when the material has not been turned to the final size of the ball end.

FIGURE 4-Setting up a gage block for a small concave radius. In this example, a .75" radius is cut.

FIGURE 5-Using a height gage to set up a large concave radius. In this example, a 1.5" radius is cut.