[dwilson]

I tried drilling the 9A landing gear weldments in place through the side skin of the fuselage tonight. In 6 holes I severly dulled 2 bits.

What am I missing here? Do I need really hard bits such as cobalt bits or am I doing something stupid?

Duane Wilson :

 

[Jamie]

Hi Duane:

I would recommend using Beolube or similar. Also, you may wish to try drilling a smaller size and then using progressively larger bits. I used cobalts and I had no problem at all. Also with steel you really have to nail the drill speed, so try different ones. I tried drilling my first hole with my cordless drill (more torque when poking through the back side of steel) but it was too slow and wouldn't cut...had to use the air drill instead.

Best of Luck!

 

[Rick6a]

I tried drilling the 9A landing gear weldments in place through the side skin of the fuselage tonight. In 6 holes I severly dulled 2 bits. What am I missing here? Do I need really hard bits such as cobalt bits or am I doing something stupid?
Duane Wilson :

Could be any number of things, all of them related to technique. Without knowing what you are doing specifically, in general I would suggest the following. Always pilot drill holes first with either a #30 or a #40. Since in this case it is a rather thickish steel weldment, I would personally feel a bit more comfortable using a #30....less chance of a #40 snapping off in the hole. Step up the hole to final size in increments. Use a reamer for the final pass. Use a relatively low RPM....with a moderate feed pressure, using boelube or any oil for that matter as a lubricant on the tip of the drill bit, even candle wax will work,....do not force the hole with heavy pressure and/or the drill motor running wide open. Go slow and easy. Use SHARP drill bits. High speed drill bits will work just fine but do not last as long as cobalt drill bits. Don't be afraid to throw dull drill bits away..... unless you have a bit sharpener. Bottom line...sharp drill bits, pilot drill the hole first, drill hole in stages using a low rpm. For instance to drill a 1/4" bolt hole , I would pilot it first, bring it up to 5/32, then to 15/64, then final ream with a .2495 or .2500 reamer.
Rick Galati RV-6A "Darla"

 

[LettersFromFlyoverCountry]

I find a slow speed drill and Boelub (and patience) works well. Don't try to overwhelm the steel as one is tempted to do with aluminum.

 

[tacchi88]

Drilling

Drill speeds are a factor in drilling, as well as tool sharpness. The steels we drill are usually 4130 normallized, but apply enough heat, such as a dull bit turning too fast will "work" harden the steel you are cutting. Coolants, as well as proper speed, plus a sharp bit work well.. High speed drill bit work well and are less like ly to break. Cobalts are better suited, but are also brittle and can break. The brittleness may also break tiny pieces of the cutting surface rendering it dull, and begin to work harden the steel one is cutting. Most cobalt bits are difficult to sharpen. Cobalts are more critical as to speed. A word of caution about coolants. With 4130, should a dull bit raise enough temperature, and cooled suddely, it may harden it making it almost impossible to drill with common bits. The next step would be a crabide bit, and they ain't cheap.
Rule of thumb, drill speeds are generally slower as the steel is tougher. Cutting pressure should be enough as to see a curly shaped shavings. If the shavings are consistant, and not blued, speed, cutting pressure, and drill bit are in balance.