Work Holding: drilling and taping a ball bearing

Hi,

I need to drill and tap a 1.25" 316 SS ball bearing. The thread needs to be 5/8-11 UNF. I'd like to drill to at least 1/2" depth. This is for a tool I am building.

I have available 3jaw, 4 jaw, and if I need to I could buy a 5C step collet for the job. But the contact area is very small and I am not currently satisfied with the level of grip. I have made a simple back stop in the spindle to stop the ball simply being pushed backwards. ideally I don't want to mark the ball bearing too badly and if I put a burr or scratches on it I'll want to clean it up.

So how best to hold the ball bearing?

If I can't hold it I'll abandon the plan and turn a ball from stock that is qlready drilled and tapped.

Thanks,

Steve

Is this a real ball bearing? It will be mighty hard. I think a way to do this is to make an ali ring which fits over the ball, saw a slot in the periphery, and use this as a "collet" in the 3-jaw so that as the vice clamps the ring deforms to give a greater contact area and protect the ball.

Good luck!

If it's a proper ball bearing, get it red hot and let it cool slowly. this should anneal it enough to let you drill and tap

Roy

Make a split collet, like this:

The collet was bored to fit the small and large balls, with a smaller through hole to act as a stop on the smaller ball. I was able to machine the third ball using the same speeds and feeds as the others using the split collet.

I don't think 316 can be hardened. However, it does work harden. I speak from bitter experience having just badgered a number of drills when drilling and tapping it in both the lathe and mill. Hesitation is fatal. Just drill continously with a good feedrate and don't even think about peck drilling. For stainless I drill for a 50% thread engagement and use my newest, and sharpest taps. Tapping goo also helps.

Andrew

John,

Yes it is a real ball bearing but of far eastern origin so I do not yet know the hardness. The idea to use the hard ball bearing is that the contact point of the ball to the surface it sits on needs to be a point load (or as near as practical), nice and smooth, and not wear to a flat over time. I am making a kingway tool. Google "kingway alignment tool" and you'll see some pictures.

Your suggestion reminded me of the "pot chuck" that GHT or was it Prof Chaddick wrote about. I don't have any quorn castings gathering dust but I do have a copy of the book so I'll dust that off and give it a read.

thanks,

Steve

Andrew,

Thanks. I am not sure from the picture but have you bored out the inside of those split collets to give 2 rings of contact to grip the balls either side of the widest point?

Thanks,

Steve

I have done it with pot chucks like andrew shows with plain sides, the softer aluminium grips the ball better than steel would. Collets with backstops work well but you would need a large one for that size ball.

correction for an error in the first post (I can no longer edit it) the planned thread is 5/8-18 UNF. Not 5/8-11 which is UNC.

How much of the surface of the ball is actually used? I ask this because it looks to me like you could just turn a spherical surface on the end of the rod to achieve the same result. Use a suitably hardenable steel and polish afterwards.

Martin C

If I recall correctly the embryo collet was drilled thru about 10mm. Then it was drilled 3/4" part way through. That locates the smaller ball and provides axial location. Finally a portion was bored 7/8" diameter to hold the larger ball. So correct, it does grip on two rings of contact. The collet is split diametrically almost all the way through axially. And then another diametric split 90° from the first, but from the other side. A bit like an ER collet. Here is the collet in action:

The six finger headstock collet probably helps with closing force and keeping the split collet more or less concentric.

Andrew

Hi Martin,

Based on the photos on the net and the original US patent available via google the design calls for on occasion the use of a ring which is placed underneath the ball. I am thinking this is to average out the height changes when moving along a scraped bearing surface or to cope with localised damage/worn area. So the ball needs to be a sufficient diameter and be accurately made. I have guessed at Ø1.25" but have purchased a Ø1" ball bearing as well to try. (I am thinking that when using the tool over a ground surface the ring would not be needed.) So with that in mind a good quality ball bearing was my initial thinking of how best to get a good enough spherical surface.

Thanks,

Steve

EDIT: Also to clarify  I went with stainless to keep the dreaded rust at bay. No other reason. I am building most of it from 303 bar stock plus some cast iron, which will be painted and some  brass.

Edited By SteveI on 11/04/2017 14:46:35

That's basically right. There's little (316) or essentially no (316L) carbon in it, so heating and quenching does nothing much for you. So the only option is surface hardening which is a diffusion process. It is possible to get hardenable stainless steels (used in ball bearings etc) but they are more exotic.

The simple methods diffuse either nitrogen or carbon (or both) into the surface, either in gaseous or salt form. The downside of those methods is that the chromium reacts with the nitrogen or carbon to form chromium nitride or carbide. The result is that the stainless steel is no longer quite so stainless, as it's the chromium that gives the corrosion resistance.

There are proprietary processes that can diffuse carbon and nitrogen into the surface without the formation of carbides and nitrides but they are beyond the capability of normal home or even most industrial workshops. You need near vacuum and high (by our standards) temps, plus time (from several hours to several days), with a plasma activation. The carbon and nitrogen are typically supplied in the form of gases like acetylene, ammonia etc although the process details are closely guarded. There may also be a secondary oxidisation process to improve corrosion resistance.

Examples are Bodycote "Kolsterising", Arcor and Sulzer Oerlikon "Balitherm IONIT OX". There are others...

Murray

Steve, having looked up what it's for, an alternative approach would be to machine a cone into the end of the bar to which you want to fit your ball, then clean the cone and ball very carefully, then araldite the ball into the cone using proper slow setting epoxy, under pressure and putting it somewhere warm so the glue is less viscous. You want the ball to be in contact with the bar to make sure it is aligned. Leave it 24 hours or so to cure, and I think it would be perfectly strong enough for the application. This way you would start with a nice pristine, stainless, hard as blazes bearing ball and it wouldn't get marred by gripping or trying to anneal it.

You can soft solder 316/316L using a flux specifically for stainless.

There was a recent post here from CuP Alloys recommending a flux for SS. From their website it's called Stayclean, can't be sent by post, and all residue has to be removed after soldering. This could be difficult if the ball is soldered into a locating recess on the end of the bar. I like the idea of soldering for strength, but in this application that shouldn't be so important so adhesive might be preferred.

.

For the proposed application, that sounds very encouraging ...

First grind a flat spot on the ball, which will locally remove the surface hardening; then drill as Andrew suggests.

MichaelG.

Hi,

Well I had a spare 15 minutes today and after finding no suitable bits of aluminium and thinking hard about machining a cone in the end of the rod, I decided to give it a go holding it in a 1.25" 5C step collet which due to the step stopped the ball bearing being pushed backward. I tightened it up as hard as I could. I had planned to follow Andrew's advice and drill for a 50% thread engagement but unfortunately I did not have a suitably sized drill. I had a 37/64" which I think is~65%. I started with a solid carbide 6mm spotting drill which after about 5mm promptly snapped. I can only assume that the initial hole was off center. I managed to get the tip out and continued with HSS until the 37/64". First part of the job done with golden straw coloured swarf. Does that point to it being 316?

I then screw cut the 5/8"-18 internal thread as I did not have a suitable tap to hand. I'd never screw cut such a short blind hole before so it was an "interesting" job.

In retrospect I think an aluminium pot chuck would have been better. The collet has left a visual mark where it gripped the ball bearing but that won't affect my application. I also think the Chinese ball bearing was not as hard as it should have been. I am hoping that it will be hard enough.

Many thanks for all the advice.

Steve