Correcting for an incomplete radius cut

When cutting a full ball end, if your first attempt measures undersize, you will have to scrap out the part and start over. That is why I suggest you start with what you know will be a slightly oversize cut. Once you are near the final size, here is how to adjust the cutting tool depth to get the exact size:

1. Measure the diameter of the ball you have cut. You can't reset the tool until you know the diameter it is actually cutting.

2. If, for example, the ball is 0.010" oversize, the tool must be moved in the cutter body 0.005" (half the desired distance) closer to the part. To do this, first record the leadscrew handwheel setting with the backlash taken up in the cclockwise direction and the tool touching the end of the center of the part. This should be done right after your last cut was made so you know the tool is just touching the part.* Now use the leadscrew handwheel to back the saddle/radius cutter assembly up more than the correction needed. Turn the handwheel clockwise again with the difference calculated in. The tool should now be .005" from the part. Loosen the set screws holding the tool in the body and move the tool until it just touches the part*. Tighten the set screws and make your final cut.

*NOTE: Whenever you move a tool up to touch a part to set its position, don't push it into the part. Make sure it barely touches. Pushing a tool into a part will cause it to take an extra couple of thousandths off on the next cut, and your part will come out undersize.

Cutting a concave radius

Full convex radii are easy to measure because you can use a caliper or micrometer. A concave radius is more difficult to measure. It is better to spend the time accurately clamping the tool using a height gage than trying to check your radius with a template you can't view accurately. Some things in machining have to be controlled with the setup rather than with an inspection method and this is one of them. Concaves up to about a 3" radius can be cut. (See Figures 4 and 5.) Remember that when the cutting tool is extended a long way from the support of the yoke, it can be more difficult to control. Lighter cuts must be taken to achieve a good finish and accurate size but the tool should be controlled in a positive manner. Don't let the tool set on the part without cutting. Use the various pivot holes to try to keep the point of the tool as close to the yoke as possible to maximize the rigidity of your setup.

When cutting a concave radius you will use the holes nearer the center or short end of the yoke. For smaller radii, the cutting tool points into the "U" of the yoke. For larger radii the tool can be reversed and pointed toward the outside of the "U". In deciding which center hole to use it will help you to know that the center pivot hole is centered on the inside surface of the "U" and that the pivot holes are located on .250" centers.

Here is a simple formula that can also be useful when working with concave shapes:

r = (c2 + 4h2) ÷ 8h

where r = radius, c = diameter of pocket and h = height (depth of pocket)

FIGURE 6-Measure the diameter of your pocket to obtain dimension "c". Then measure the depth of your cut with a depth micrometer or with the depth rod of your caliper against a straight edge to obtain dimension "h". You now have the dimensions you need to accurately calculate the radius you have cut. The radius can be calculated to the same level of accuracy as your measuring technique.

Making a complete ball

Using the radius cutter you can cut past the vertical point to make more than a half of a circle. However, because the cutter body will eventually hit the chuck, steady rest or some other part of your setup, you cannot cut a complete ball. (This is a problem for conventional horizontal-swing radius cutters too.) You can still make a complete ball using this tool, but you will have to do it in two steps.

First, turn half or a little more than half of the ball to the final radius and cut it off leaving enough material to form the opposite side. Make a mandrel with a diameter about 2/3 of the final diameter of the ball. Cut an angle into the face that will allow the completed half to be centered on the mandrel and epoxied to it. After the epoxy has hardened the ball can be completed with light cuts. Once finished, the ball is broken off the mandrel. By measuring the part with the anvil of the micrometer on the previously machined surface and the spindle of the micrometer on the surface you just machined the completed dimension should be equal to the diameter. A ball should always measure the same in any direction.

FIGURE 7-Cutting a complete ball using epoxy to attach the ball to a mandrel to complete the second half of the ball. A) Turn a little more than half the ball. B) Part off the piece leaving enough to complete the ball. C) Epoxy the piece into a mandrel with a tapered depression. D) Complete the ball using light cuts. Measure across the first and second turned portions of the ball to confirm the diameter.

Another method would be to center drill, drill and tap a hole in the end of the half-completed ball. Using a cutoff tool, part off the piece from the stock leaving sufficient material to complete the ball. Make a mandrel with a threaded stud centered on the end and screw your part onto it. Place the mandrel in the chuck and use the radius cutting setup you used to make the first half to complete the rest of the ball.

FIGURE 8-Using a threaded stud in the mandrel to hold the ball for the second operation. A) Center drill the end and tap hole, then turn first half of ball. B) Part off. C) Attach part to threaded stud in mandrel. D) Turn final half of ball using light cuts and measure to confirm diameter.

The radius cutting attachment further extends the capabilities of your Sherline machine shop. With it you can apply a professional touch to your parts that would be difficult or impossible any other way. Though I have shown just a few examples here, I think you will find that, with a little imagination, there are many more ways it can be used.

-Joe Martin, President and Owner
SHERLINE Products

EXPLODED VIEW AND PART NUMBERS

Radius Cutting Attachment Parts List

NO. REQ.	PART NO.	DESCRIPTION
1	11971	1/4" Cutting tool
1	2210	Radius cutter body
1	22110	Radius cutter support (left)
1	22111	Radius cutter support (right)
2	2212	Radius cutter pivot pin
2	30561	10-32 T-Nut
6	3108	10-32 x 3/8" cone point set screw
1	3210	10-32 hex nut
1	3562	10-32 x 7/16" SHCS
1	4069	10-32 x 3/4" SHCS
1	4274	10-32 x 7/8" SHCS
1	4206	Plastic handwheel handle