How to Drill Large Holes With a Mill - and Why It's Better Than Using a Drill Press
Kevin is making a commissioned fountain and needs to cut a 3" hole in the center of the water feature's 3/8" steel base plate. He's going to use his 1970s-era Bridgeport mill, or milling machine, and a bimetal hole saw to cut the hole.
You are probably wondering why he is using his mill to drill the hole through the metal rather than a drill press. He's using the mill because of its low speed. He can turn down the mill to a slow speed - he'll probably run it at 100 RPM. Compare that to his drill press, which runs at about 1300 RPM. That's way too fast to drill through metal with this hole saw. "It'll burn them up," says Kevin. "Once you've lost the sharpness of the teeth, you're just gumming it to death." He also likes the big table to clamp metal to.
He'll clamp this piece of steel to the table and drill his center hole, then mark his four bolt holes that will be used for the fountain's installation. He'll be able to index them and drill them easily.
Kevin takes a moment to address the fact that mills like this are usually 3-phase but this one has a rotary phase converter on the back of it, so it takes single phase 220 and turns it into 3-phase 220. It sounds like a jet motor starting up. The converter is almost like a little generator. When he flips the switch, the phase converter spins up.
On the mill itself, Kevin points ou the on / off switch that makes the machine run forward or backward. On the other side of the mill is the hi / neutral / low range switch, which works with the speed control. He currently has it set at low range.
He fires up the machine - the wrong way. Then he turns it on the right way.
At first, it is running at 50 RPM - you can hear how slowly it is turning. "That's a little too slow," Kevin says. He turns it up to 100 RPM.
This mill does have an autofeed mechanism, but it isn't working right now. Kevin shows how he simply places his hand on the quill feed and lets the drill - which is nice and sharp - do its job. The Bridgeport's handle also has a nice feature that lets it resets its tension.
As the hole saw drills through the metal, Kevin keeps it lubricated every couple of minutes with cutting oil. It gets noisier and noisier - until it stopped being as noisy. He explains how the chips are starting to fill the kerf as it gets deeper and acting like ball bearings, preventing the blade from cutting. He raises the blade and uses his air hose to clear the kerf. "Of course, you should always be wearing safety glasses or a face shield," says Kevin.
The blade cuts through the metal. Kevin points out how nice and clean and pretty the hole is, with straight sides. He'll grind a little around the edge of the circle to get shiny metal, then flip over the metal plate and grind around the hole on that side, beveling it just a little. Then when he sets his tube inside the hole, he'll be able to get a nice clean weld on the top and the bottom.
This is why Kevin likes using the Bridgeport to drill large holes. It's neater, cleaner and more precise. The table is lower than the one on the drill press so you don't have to live the heavy metal plates as high. It's a nice machine.
And don't miss Kevin making a really good question even more interesting at the very end of this how-to video .....
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