Drilling Through Steel Spheres

I agree with Gigaconcept. Sinker-type EDM will certainly perform this task. As far as the precision level of the hole (roundness, smoothness, straightness, location) versus the slow speed of a sinker EDM, one might consider using a sinker EDM to rough the hole and then finish it with a travelling-wire EDM. The sinker can certainly produce a round and straight hole if it is given enough time. How cost effective will this be? We cannot determine this matter. It is perhaps not a matter at all in this case.

A possibility if the ball is not harder than say, 45 Rockwell C: use an appropriate fixture and a tungsten carbide drilling bit to rough the hole after initial spotting and then finish with either a sinker or travelling-wire EDM. Of course this will not work well if the steel ball is of the hardness associated with ball bearings or precision gage balls!

Ing. Robert Forbus

Make a fixture with 2 or 3 bolts holding the sphere in place. Have an inside radius cut in the ends of the bolts so that the sphere fits snug inside the radius. For example, a bolt on opposite sides and one on top holding it down, on the bottom (under the sphere) of the fixture put a piece of rubber or another non-sliding material to keep vibration from turning the sphere, this will allow you to nest the sphere so that the bolts do not damage it but the rubber will keep it from spinning. the bolt that comes down over top of the sphere...drill a 1/4 hole through the length of the bolt to guide your drill bit. To maintain precision, drill a pilot hole and then drill it with a size under your 1/4" so that when you make your final pass through the sphere you don't have much material to remove. This should do it. I am not sure what your tolerance is, your accuracy is going to depend on the type and quality of drill bit or machining tool as well as if you are using a hand drill or a cnc machine.

Hi!!

Method suggested of holding on Conical fixture while drilling on EDM would be the most appropriate one,however to secure the Ball in Conical fixture it would be advisable to use some sort of removable glue or plaster of paris.

If more precision is required pre drilled hole on EDM can be further processed on wire cut EDM which already has been suggested.

You have not indicated volume of production. if large quntity is to be proceesed you may consider using CBN as tool material. In this case you would require a drill jig to guide the tool and ball would need to be held between two jaws of self centering vice having V - type anvils

Udayan

If I were going to do this in my own poorly-equipped shop, I'd chuck the ball into my lathe's three-jaw chuck, and use a 3/16" cylindrical grinding bit in a tool post grinder, and simply grind the hole through, using a very slow feed speed. Offset the bit slightly to obtain the final size, and true the grinding bit before the final cut. This would take time, but you could certainly get the hole size to within .0005 of 1/4". Centering would be as good as your three-jaw chuck (and, of course, you could do a little better with a four jaw, if you're careful).

Blink:

I tried your method this past week with brittle silicone balls that I wanted to make into drumstick heads for playing bass steel pan.

Unfortunately, the pressure exerted by the vice allowed the surface of the ball to develop a stress fracture in a line horizontal to the vice jaws.

So I held the next several balls in one hand while successfully drilling them out with the other (which I could do because they were just silicone balls and the only precision I needed was that they fit onto the drumstick). In every case, I had first fairly deeply scored the ball to keep the drill bit from travelling.

What I believe I learned from the experience was that balls don't respond well to drilling when a transverse force is applied across the ball.

And that an encompassing grip with no transverse force allowed the ball to be drilled with no fractures.

This would indicate that a molded hemispheric holder with a friction (silicone, rubber) grip might be the method I would choose if I were doing this in a more formal setting.

Mark

Hi Mark:

If the hardness level Calamari is talking about is anything close to that of a steel ball bearing, then brittleness is not a concern. Ball bearings are often subjected to impact loads (as in vehicle accidents) high enough to dent the races (themselves very hard) and the balls never fracture. I've looked at a thousand or more such balls from the steering head bearings of crashed motorcycles (with dented races), and have never seen a fractured ball.

Given the right feed speed into the ball, and the right rotational speed of the ball in the lathe, the holding force does not need to be very high (about 5% of the 2000# load a small ball bearing sees in an accident impact would be plenty) . The lathe chuck is typically softer than ball bearings. One could pad the jaws with aluminum, but then some of the centering accuracy (within .001 inch on an average three jaw chuck) would be lost. An aluminum padded 4-jaw chuck could counter the aluminum deformation, but set up would take longer.

Calamari didn't specify what he meant by "precision", but if he wants the hole to be within .001 of centered on the ball, and within .0005 of .250, then any compliance in the mount would reduce accuracy and invite chatter. One could experiment with several 1" ball bearings, which are cheap enough to throw away by the handful, before doing the final drilling.

I suspect that the surface hardness of a ball bearing is much greater the center hardness. The process could be sped-up by grinding when near the surface, and cutting with a rotating carbide cutter (also in the tool post grinder) when deeper. The final sizing would be via grinding.

Ken

A really low tech poor man's method might be to use putty to form a well on top of the ball and fill the well with a paste made from silicon carbide and water. Now you can use a steel rod with a flat end to grind your way through. I've heard you can do this to drill holes in glass so it seems like it would work on any hardness of metal.

Being a simple minded fellow, my first thought, depending on the required finish and tools at your disposal. Is to place the "sphere" between two thick washers in a vice on a mill table. Indicate for your location and if you can score both a carbide center and finish drill, you may begin.

Use Blink's way. It is your best bet. Only that I would make a starter hole using a very short , say 1/8x1" bit. changing it later to the final OD. The chuck can be padded to avoid damaging the sphere. Is it a hollow sphere?

Wangito

the sphere is solid.

I had considered embedding the sphere in an alloy called Cerrobend Alloy which I understand is available to assist machining unusually difficult parts. But have never used it, and it contains a bit of base Pb.

You can do that. how many of those you need to drill? maybe the qty will justify building a jig? I would than look for someone who can modify a lathe chuck.to precisely hold the sphere. Embedding is another process, and if you need just drilling a Small amount, maybe it 's not worth it. Another important factor is the tolerance, how precise do you need it to be. Cerrobend is just one, there are several similar alloys. I don't have enough experience to form an opinion. Are you moulding the spheres or are you buying them? If these are ball bearings, you are going to have some fun trying to drill it.

Wangito

http://www.pawsworkholding.com/

milo

Take the balls to someone with a water- jet cnc cutting machine.