Notes on cutting large holes with a round column mill

[Shotgun]

This may be the same as for cutting on any other mill, but I have a WrongFu, round-column clone, so that is all I can talk about.
The task is to cut 3in holes in eight 3/4" plate. All plates are identical, and have been machined to size on the outside.

1. Set the plate in the vice on parallels, using a vice stop to indicate the hole position.
2. Drill a 1/4" to 3/8" hole on each side of where the hole is going, just inside the cut, and spot drill for the center mark. Do one hole across all eight before moving to the next hole. This is where the vice stop is invaluable.
3. Select a hole saw that is at least 1/8" or so SMALLER than the target hole size. This will result in the kerf running over the holes drilled in step 2, so that most of the removed material will drop through them. The hole saws leave a TERRIBLE finish, so we want to undersize it to leave material for clean up. Make sure to use the center bit on the hole saw. It helps to locate and steady the saw, especially as you start the cut.
4. Spin the hole saw at a slow speed (200rpm for around 3")
5. Feed the saw using the fine feed instead of the hand crank. This feels like drilling, but it really isn't. The hand crank doesn't give you control, resulting in the saw constantly digging in and stalling the mill. When it digs, it raises burs inside the kerf slot that then try to tear the teeth off the saw when it starts back up. The fine feed allows for a constant cutting pressure that you can definitely feel as you feed it in. You can even see when the mill starts to bog down and just pause for a second to let it catch up, and then continue to feed (but slower).
6. Once all the holes are cut, switch over to a boring head and bore to size. If it takes multiple passes, adjust the head and pass all eight parts through before adjusting for a larger pass.

This is the process I've come to after doing several features requiring large holes. There is a high probability that I'm doing it wrong. What would you do differently?

 

[davek181]

I have a PM mill, but I agree that the process should be the same as I don't see column design being a factor.

If I have read and understood your process properly I only have a couple comments to make. For locating the hole locations if super accuracy is not required you could use a vice stop and drill each plate in turn. If accuracy is important I would edge find and locate the holes individually one by one.

Once to the boring stage make as many passes as you need to get finished diameter then move on to the next plate and do the same. Do not unmount the part till finished as it is impossible to get it back to the same location.

 

[Aukai]

I use annular cutters whenever possible, but 3" may be pricy for a 1 time project. I hate hole saws on the mill for heavy work.

 

[davek181]

If I make a cylinder head pressure tester plate I use the plasma cutter or torch to rough the holes and grind worst of the cut away and bore the rest of the way if I want it to look good. 3/4 thick would be quite a chore with a hole saw.

If you have a DRO with a bolt circle function you could drill a lot of holes just inside the circle you want to cut so you would have less material to cut through.

 

[Shotgun]

If I make a cylinder head pressure tester plate I use the plasma cutter or torch to rough the holes and grind worst of the cut away and bore the rest of the way if I want it to look good. 3/4 thick would be quite a chore with a hole saw.

If you have a DRO with a bolt circle function you could drill a lot of holes just inside the circle you want to cut so you would have less material to cut through.

I'm cutting A36. The hole saw really hasn't been that bad. My fine feed travels about 100 thou per turn, and it was somewhere between five and ten seconds per turn. I agree with @Aukai that an annular cutter would be better. . . but, dang!!. . .those people are proud of their cutters. And, an annular cutter is a much wider kerf, so I don't know that my mill could handle making so much material into chips.

So, I settle for the hole saw. The worst part is the squall that it puts up at some points. Need ear protection just for that.

I do have one truly critical dimension, and that is the location of this hole to another hole. I'm gonna have to go measure to make sure that is correct. If the vice stop was not sufficient for indicating, them I'm screwed. :-(

 

[Watchwatch]

Rotary table might get you there in a couple of passes. No boring.

Annular cutter is probably the “best” option. They don’t require much horsepower. But I can only imagine the cost on a 3in cutter…


Sent from my iPhone using Tapatalk

 

[slow-poke]

Consider adding CNC capability to your machine. Making large circles with a CNC machine is so easy. I converted my round column to CNC and use the free version of Mach CNC software, I have made countless perfect large circular holes. The advantages are endless, I’m often working on something else while the machine is dutifully machining one of the other parts for the project of the day.

 

[Canuck75]

Consider adding CNC capability to your machine. Making large circles with a CNC machine is so easy. I converted my round column to CNC and use the free version of Mach CNC software, I have made countless perfect large circular holes. The advantages are endless, I’m often working on something else while the machine is dutifully machining one of the other parts for the project of the day.

Slow-poke,-
Hope Shotgun will forgive me for piggy backing on his thread but how does your CNC cope with the Z backlash? Can't see that you have spring loaded the quill in any way and you are using the native gearing to position it?

Canuck75

 

[minsk]

Consider adding CNC capability to your machine. Making large circles with a CNC machine is so easy. I converted my round column to CNC and use the free version of Mach CNC software, I have made countless perfect large circular holes. The advantages are endless, I’m often working on something else while the machine is dutifully machining one of the other parts for the project of the day.

do you have any videos of your cnc build? im picking up a round column mill tomorrow...plan on cncing it..i have made other cnc machines in the past...l

 

[mattthemuppet2]

Other than the drilling of chip holes around the edge of the hole as you wrote above, I've also found drilling a pilot hole and using a solid pilot of that size in the hole saw to make a big difference. Less wander and squealing.