When I first got into airsoft and started working on replica AEGs I noticed that we were using bushings instead of bearings. I asked the local airsoft gunsmith and his response was that 6mm bearings just couldn't take any kind of load or speed without coming apart. At that time there was a very limited number of 7mm boxes available and they were very expensive. Years later and 7mm mechboxes are still rare and sometimes expensive. So I embarked on a quest to find if I could step up my el cheapo ACM mechboxes to 7mm and install bearings. After multiple attempts here is what I found worked for me.

Safety First!

1. Wear safety glasses.

2. Always unplug your drill press when setting up or making changeovers of any kind.

3. Do not wear loose fitting clothing.

It is highly suggested you practice on old mechboxes before doing your target mechbox.

Compared to using an old-fashioned brace and auger bit, drilling holes with a modern drill press and step bit is almost as effortless as punching out dough with a cookie cutter. But since we want all our holes to be the exact size, round, straight and at the proper angle, it pays to know a few good drilling techniques.

 

Maintain your drill and bits

It is essential to keep the bit as sharp as possible or there is a chance it will overheat. Overheating a drill bit will damage it beyond repair. Lubrication is a good idea. I used a few drops of superlube oil placed on the lower part of the bit and around the 6mm and countersink of the hole.

Clamp and secure what you are working on, but not to firm. The part should have give but be stable. I found the need to place small bits of wood under the mechbox at key points to get it balanced and flat.

Be careful when drilling holes into the thin metal mechboxes. Drill bits can quickly snag on the jagged edges of the hole grabbing and throwing the workpiece. Being that we are working on a drill press, clamp the mechbox down to the press table on a piece of backer wood, under the metal. As the drill bit breaks through the underside ease up on the pressure to help prevent it grabbing.

Place the drill in the center of the hole.

The existing 6mm hole will provide a center for the specialized step drill bit, preventing it from slipping or skating, and ensuring accuracy. Here you should clamp the part in place before plugging the drill back in and turning it on. There may be an issue with the mechbox laying flat and perpendicular to the drill bit. I have found small scraps of wood placed strategically under the box to level it works best.

Feed the bit slowly and slowly increase speed. Vary the pressure and don’t let the drill bind.

Be sure to drill with a slow but firm pressure. As you approach the size you require be careful not to go too far, resulting in an oversized hole.

Don’t force the drill, let the machine do the work.

Speed is another key factor for drilling metal. In this case we will be drilling out an ACM mechbox which is made of soft metals. The recommended speed for our setup is listed at about 2000 to 2500 rpm or slower.

Withdraw the bit often while drilling to remove chips.

Also be aware of any sharp edges or burrs that the holes may have. Burrs can easily be removed using a bit of sand paper.

Take your time. To rush is to be slow, but smooth is fast and final.

Last note on this, I used a precision 7mm finishing bit (seen in the drill press pic) to finish my holes. The result was the bearing fit quite tight. 

And I will say it again...It is highly suggested you practice on old mechboxs before doing your target mechbox. 

 

About the metric step drill bit

A step bit, step drill is a bit with a stair-step profile. Due to their design, a single bit can be used for drilling a wide range of hole sizes. In our particular case from 6 to 7mm and even 8mm. This bit is used for hole enlarging.

Understand that by the nature of this particular bit, metals tend to cause premature drill bit wear and dulling.

The straight flute design is poor at chip ejection, and can cause a burr to be formed on the exit side of the hole, more so than a spiral twist drill turning at high speed.

The step bit was invented by Harry C. Oakes of Wyoming, New York in 1971.

Look for the future article... Lab testing of Bushings vs Bearings... Noise & ROF comparisons.

NOTES:
Everything is In Our Opinion Only - We lay no claim to any of these ideas, suggestions, tunes, etc. We learned, in person and online, from people who are a lot smarter than us and we merely use and try to improve upon what we consider to be useful.

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