One of my Introduction to Metalwork students asked me to expand on drilling speeds:
” A quick question – Drill speed. You touched on it briefly in one of the sessions, but is there an optimum speed for different metals? If so, how is it determined? “
At the simplest level (its more technical in a manufacturing environment), it has to do with the metal itself, so its a “material property”.
There are “recommended” or “optimum” cutting rates measured in “surface feet per minute” or in “metres per minute”.
Therefore if you need to a drill a 10mm hole, the speed for drilling depends on the material.
Its a super juicy question, so I just built this simple calculator in a Google Sheet. Its totally unlocked, so you don’t need a Google account to view it. Of course anyone can “edit” it, so I’d prefer you “Make a copy” if you already have a Google account:
No Google account? Prefer to use Excel? Download the Excel version.
- Go to the front tab, “Surface Cutting Speeds”
- Scroll to the black table in rows 11 to 12
- Click on cell B12, and use the drop down to select the material you want to calculate a drilling speed to suit.
- Click on cell D12, and use the drop down to select the drill bit size you want to calculate a drilling speed to suit.
- The green cell E12 will display the calculated drilling spindle speed in revolutions per minute (RPM)
- Fit this drill bit to the chuck of your drill.
- Secure the metal part you want to drill, either in a vice or clamp it down.
- If using a drilling machine (drill press), change the belts or gearbox settings to set a speed as close as possible to the calculated value. I typically err on the slow side (I’ll set the speed to the slower value if it falls right between two values)
- If using a powered hand drill, you’re really guessing. Look on the side of the drill, it should tell you the maximum speed. Squeeze the trigger all the way in. Thats the maximum speed. You can only really guess what speed you have when you press, for example, half way.
- Aluminium can pretty much be drilled as fast as you want.
- Steel normally gives you some smoke signals….literally….if you’ve used any lube/cutting fluid and its smoking and the chips are blue, then its too fast (and generating too much heat). Slow down before you destroy the drill bit!
The other tabs in the calculator are just informational.
The “What Diameter?” tab shows you the calculated drill sizes for a given spindle speed. For example, at 1000rpm, you could drill a 24mm hole in aluminium, but only a 3mm hole in stainless steel.
The “What Spindle Speed?” tab shows you the calculated spindle speed for a given drill bit size. For example, if I was drilling a 10mm hole, I’d need to set the spindle for 2426 rpm for aluminium, or 291 rpm for stainless steel.
The chart in the last tab is just a visual depiction of the formula. You could pick a material and a hole size, and estimate the spindle speed by reading it off the graph. The calculator in the first tab does this very accurately.
Hope that helps! Any questions or comments, drop them down below!