Re: Drilling Gazing Balls (and installing thread inserts) (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: List moderator <mod1@xxxxxxxxxx>



---------- Forwarded message ----------
Date: Tue, 12 Dec 2006 23:31:35 -0800
From: huil888 <huil888@xxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: Drilling Gazing Balls (and installing thread inserts)

Actually, drilling hard stainless steel gazing balls and installing threaded 
inserts is quite straightforward and nearly foolproof if you have the right 
tools and follow the appropriate procedure.

All the stainless steel gazing balls I have seen were fabricated from a pair 
of spun hemispheres, welded at the equator. Because these are "spun" 
(cold-worked) from flat stainless-steel stock, they have undergone 
considerable work-hardening. They weld bead itself is extremely hard and 
cannot usually be drilled. If you orient the ball so the weld is at the 
"equator", the least work-hardened areas will be at the "poles". You will 
want to drill your hole at one of the poles. DO NOT use a center-punch to 
locate the hole; this will only work-harden the material further, and make 
it very difficult or impossible to get the drill bit to start cutting.

Here are my suggestions for drilling the hole:
1. Place a small strip of masking tape across the point where you want the 
hole, and mark a dot with a fine-point felt pen. The masking tape will help 
prevent the drill bit from slipping.
2. Use a dead-sharp (i.e. brand new) lathe center drill (see 
mcmaster-carr.com,  Machining and Clamping > Drill Bits > Combined Drills 
and Countersinks [page 2356]) to make the initial hole. The large shank 
diameter of a center drill allows a lot of pressure to be applied initially 
without any chance of bending or breaking the drill bit.
3. Use a variable speed drill that can be operated at very low speed 
initially.
4. Clamp the ball firmly so it cannot move. A large parallel-jaw 
wood-working clamp (Jorgensen style) can be used for balls up to 10" or so 
in diameter.
5. Keep the drill bit at 90 degrees to the surface of the ball to minimize 
the chance of slippage.
6. Position the tip of the center drill on the ball, press firmly, and start 
the drill at low speed. Maintain a fairly heavy pressure so the bit begins 
cutting immediately. If you allow the bit to "spin" against the ball without 
cutting, or try to use a dull drill bit, it will only work-harden the 
material and make drilling impossible.
7. Once the small pilot-diameter of the center drill has penetrated, use 
progressively larger drill bits (or a tapered reamer) to enlarge the hole to 
the desired diameter. Be careful NOT to drill the hole any larger than 
required to accept the threaded insert.
8. Carefully deburr both sides of the hole. For the backside of the hole, a 
narrow pen knife blade can be used, but A Rout-A-Burr type deburring tool 
specifically designed for deburring holes in sheet metal works best (see: 
mcmaster-carr.com, search "sheet metal deburring tools").

Many different types of threaded inserts are available for creating female 
threads in thin sheet metal. Most of these are similar to a pop-rivet, where 
a flanged insert is placed into a hole, and the shank of the insert on the 
back side of the hole is "upset" or deformed to lock the insert in place 
(see: mcmaster-carr.com > Fastening and Sealing > Rivet Nuts for 
illustrations of nine different types). Most of these require some type of 
special tool to "set" them, but the Screwdriver Installed types need only a 
screw and screwdriving tool.  The "special" installation tools run from $22 
to over $100, depending on manual or power, etc. The Slotted Body type of 
Rivet Nut creates a large backside flange diameter but does require an 
installation tool.

Installing the insert:
1. Make sure that the hole is completely deburred, that the insert enters 
the hole completely and the flange on the insert sits squarely on the 
surface of the ball.
2. If using a screwdriver-installed insert, use a socket-head cap screw 
instead of a Phillips or slotted head screw. This makes it much less likely 
for the driver to slip out of the screw. It does take a fair amount of 
torque to deform the insert. Lubricate the screw threads with a drop of oil 
before installing in the insert.
3. If using a tool-installed insert, make sure that you understand how to 
use the tool. Make sure that the tool is kept 90 degrees to the ball surface 
while upsetting the insert.

Regards,
Scott Hanson