Need a 6" hole - best way to cut?

Not too big for my lathe. You live near Seattle?

Clark made a generous offer, but if location is a problem, a boring head in your mill could be the solution. Understand that it's not easy to trepan with a boring head, although it can be done with success. You'd be using what would be a parting tool, but clearances are far different from one used for parting a cylinder. Such a tool is prone to breakage and chatter because of the smaller amount of support.

If you use your mill and a boring head, you'd be better off to lay out the hole desired, then remove the bulk of the material using an end mill, walking around your layout, staying a generous amount away from the line. Once you've removed the center, leaving an irregular and undersized hole, you would then use the boring head to open it to the desired size and location. Boring heads that come with a variety of tool holders (boring bars) usually have one or more that will accept HSS tools identical to those used on a lathe. The tools have clearance angles somewhat different from those ground for a lathe because you're working inside instead of outside, so think the same as if you were boring on your lathe when you fashion the tool you desire. Otherwise it's the same as a lathe tool.

Harold

Would it fit on a rotary table under your milling head?

That's always a good tool to have!

Ummm ... yes. It works much better. You could use boring bits from the lathe, but I make my own and get much better results. I made bars that fit into the boring head, drilled a cross hole at the working end of the bar and put HSS bits in there. (If you want, I can show you pics).

I get 0.01mm with mine.

Nick

Start with a hole saw to remove the bulk of the material. A boring head would be the most convenient way to finish the hole, but a +-.010 tolerance would be easy to hit with a homemade boring bar, or even a fly cutter with the tool ground for boring rather than facing.

Ned Simmons

What I forgot: I start the hole with a mill as big as possible to make the opening. Sometimes, I make a hole circle that goes 1/2 a mm to the desired size. Makes working with the boring head much faster.

Nick

Fastest/best way is a rotary table and your mill. If you don't know how to setup just ask.... Rick Renner

He didn't say how long the channel is. If it's 8 feet long, that would be some lathe!

Thanks for all the help. The lenght of the channel is about 15". I do have a rotary table and would like to hear how to use it for this cut! Thanks everyone Steve

Plasma cut or water jet. If you are buying the steel channel from a supplier, they could probably put the hole in for you.

John

Clamp the work piece to the rotary table and position the center of the hole to be cut directly under the center of the miller's spindle. Offset the miller's x or y position (your call) by 3 inches minus half the diameter of the end mill you intend to use for the cut.

Rick Renner

What Rick said, but don't cut the hole to size with one pass. Deflection of the cutter will ruin the hole, and it won't be very pretty even if it's on size. Start undersized, leaving about 1/16" per side for a couple finish passes. Once you've removed the center mass, take a small cut on the diameter by moving either the table or saddle, depending on which one you started with, then measure. Move the appropriate amount for the final pass. Remember----when you move the table or saddle a thou, it takes two thou off the diameter.

Harold

I left out an important first step: the center of the rotary table must be positioned directly inline with the center of the miller's spindle. Then without moving the rotary table's position on the mill table or the table itself, position the work piece such that the center of the hole to be cut is in line with the miller's spindle.

Rick Renner

No, you don't have to do that. If it is quite on center, it is enough. If it is too much off-center, you will get trouble because of non-linearity. That means if you move the head 0.1 (whatever) the hole will increase more than 0.1 * 2. But with more measuring, you can compensate that. With a hole that big, chances are that you don't even notice it.

Still guessing how to clamp a channel 15" long on the RT and milling a hole without getting into lotsa trouble. Clamping and aligning the center. I wouldn't like to do that.

Oh, here is an other approach. Get a mill that is as big as possible, yet smaller than the desired hole. Now if you mill a hole circle, you get a nearly round hole. Sometimes (depending on the ratio of the two diameters and the number of holes) this is good enough. BTDT. Fancy math will reveal the error.

Nick

That's what I sayd. You have to messure more. Means, if 0,5" are missing in diameter, you can't feed 0.25" or you will be oversized.

I sayd, that the center of the (future) hole has to be on center with the RT. _Not_ the milling head. The head can be off center.

Done that many times. A little applied gemetry will show you that it works.

Nick

What Rick says. Good machining practice dictates that the rotab should be dialed true with the spindle, which one accomplishes by sweeping the bore of the rotab with an indicator mounted in the spindle. The part is then mounted with the desired hole central to the C/L if the rotab. By observing that setup procedure, the part will be properly located as it relates to the rotab, and the movement of either the saddle or table will be linear, so diameters need not be calculated.

Harold