Hard Ball vs a Reamer

Hello Ronsetto,

I have used both methods. A reamer is more conventional and in fact easier to set up and to use and very repeatable. In my case using a hard ball was not easy as it tended to wander a little and subsequently a gage pin .0002' below the diameter of the ball was not able to go thru the hole.

Hope this helps.

Broaches and hobs I know. What's the advantage of a ball? What guides it through the hole?

Usually reamer is used for closed tolerance holes. Hard Balls are used when you want to improve the fatigue properties of the hole. That is how we do it in aircraft manufacturing.

Gannet is correct,[/P]

The only reason to use hard balls or so called mushroom pins in order to reshape the hole is for introducing compressive stresses in the surface area in order to enhance the materials fatigue strength.[/P]

As a rule, I would go 1/64" under the final hole size when using a reamer. In other words, for a .5000 hole, I would drill a 31/32" hole. What would be the initial hole size if using a ball?

I do not know what type material you are using but 1/64 on the diameter seems excessive to me. We usually ream the hole to be 0.004 undersized and then we would ballized to final. We were not sizing the hole we were cold working it to improve fatigue properties because of the cyclic shear loading it would be experiencing.

It depends on several parameters as:

- young modulus of the body in which the hole has been drilled (and even better pre reamed)

- wall thickness since if the wall is too thin (all is relative) the ball will deform par of the wall elastically and the resulting hole will be less

- the ball will deform plastically part of the wall but a small spring back is always present.

- the ball has to be as mentioned by other either wolfram carbide or ceramic if you intend to use it on steels, for soft materials and low number of operations you may use ball bearings balls.

- the lubricant is an important parameter since if its viscosity is not the right one the wall will be scratched

- I used the method for hydraulic pump elements d=1/4" and l/d about 5..6 without problems, balls from ball bearings manufacturers very well guided by a bar with a 90° cone angle I had to test several oils till it worked. The balls were used only 1 time. Piston block soft steel the scope was as well the hole calibration as the strengthening of the wall to increase its wear resistance. Results were OK.

Hi,

we did calibrate inner diameter (12.7mm or 0.5") of bronze rings with carbide balls and oil lubrication, length of rings was near 5 mm, inside was pre-machined to 10...12µm.

The rings should perfectly match to the OD of ball-bearings - no play allowed and no radial stress on rings (shape deterioration) wanted.

Necessary besides very good cleanliness:

a. regular inspection of the balls if wear is beginning somewhere.

b. control of oil viscosity and temperature and amount applied

c. sorting the ball-bearings into µm classes

d. using different balls for the different classes

e. gaining experience about the amount of back-spring in the rings and thermal pretreatment to achieve a reproducible elastic/plastic behaviour

Today I would try Si3N4-balls first.

RHABE

I've used ball sizing on holes which could be 1.5m long, not easy to find a reamer that long. Ball sizing can have the advantage of burnishing the hole surface to improve finish but, as mentioned earlier, they can wander if not properly guided.

Forcing the hard ball through the hole is BURNISHING. It is used only to even out the ridges & valleys or peaks and troughs of the surface of the hole. The uneven surface is formed during previous boring or drilling. No matter what process, the surface is always uneven. The degree of unevenness or roughness depends on the process. Drilling is quite rough process (H9 tolerance), reaming (H7 or H8), grinding (H6) and honing (H5). As discussed by others, reamer removes the material. But, in burnishing, there is no removal.

Not trying to be a wise guy, but the elastic modulus of the ball material has as much to do with its effectiveness in hole sizing as its superficial hardness. A low modulus ball will deform as it is pushed through the hole, while a high modulus ball will exert higher force on the hole wall.