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In the following sketch, I want to bore a 3.500" dia hole in a piece of 1/2" thick aluminum plate. I could do it on a mill or on a lathe, but if I do it on the mill, using a rotary table to cut a disk <3.500 dia with an end mill followed by final dimension using a boring head, I will lose the center when switching from end mill to boring head.
I'm inclined to chuck this up in a 4-jaw on the lathe and boring to size. That way, I won't lose the center. My question is; can I bore a hole starting at the perimeter of the hole, or do I have to start at the center and work outwards? I have two of these to make.
If you use the rotary table just zero the table axes over the rotary table axis, then after you mill out the center just go back to 0,0 and bore it to size.
Or after milling it out with the rotary table use a DTI to center the spindle over the milled hole and then bore it.
Glenn
Operating machines is perfectly safe......until you forget how dangerous it really is!
Starting at the perimeter would be trepanning. You could do that but you would need a properly-ground cutter to do it on the lathe. I agree with GlennW, though. There is no reason why you would lose your center on the mill.
My concern with using a RT is; my boring head may not reach out 1-3/4" radius and I don't have a boring bar. My boring head is a Criterion 2". Also returning to center is uncertain since I don't have a DRO and the backlash may prevent me from accurately re-centering after the part is moved off center.
Just make sure the final movement is in the same direction as original location and you should be able to get back to the same location using dial 0,0.
There exist boring head specific offset bars that address your issue. Basically fits in one or 2 holes of the boring head and provide an offset cutting edge for larger diameters. Or you could space up the work piece (123 blocks or the like) and put a boring bar in the side hole (if your head has one, the 2" Criterion should). Your 2" head will be able to fit easily in that (close to) 3.5" hole to take it to final size.
Not sure what you mean by "don't have a boring bar" as you would need one for either vertical or horizontal boring with the boring head. And even if you don't have one somehow, the ones that take HSS bits are very cheap and easily cut to desired lengths.
Mr Ron wrote: and the backlash may prevent me from accurately re-centering after the part is moved off center.
Backlash is easy to work with if you establish certain work habits. Mine are to use dials in the way they read positive, and I do that routinely. The sole exception is if I'm working in a "window", where I use the dials in both directions.
When you establish center for the rotab, make marks on the machine which indicate the location. The dials are set at 0, 0, each with the handle being turned clockwise (so the dial reads positive). When you wish to return to center, line up the marks on the table and saddle, with the dial turned clockwise. That is center.
Perhaps I don't understand your particular problem, so if I don't, make mention. I've worked with dials all my machining career and don't have any issues with their use.
H
Wise people talk because they have something to say. Fools talk because they have to say something.
You could mill the bore also. Just set rotary table to 0,0 and then off set the hole radius minus 1/2 of end mill diameter. Best to go undersize first and then do a final cut to dimension in climb mill direction on aluminum.
10 Wheeler Rob wrote:You could mill the bore also. Just set rotary table to 0,0 and then off set the hole radius minus 1/2 of end mill diameter. Best to go undersize first and then do a final cut to dimension in climb mill direction on aluminum.
Rob
I thought about doing that, but I'm not sure about the accuracy of my RT. Will the resulting hole be perfectly round? I don't know. The RT was made in Japan, so it's probably not too bad. If I do go with the RT, which direction is best CW or CCW?
Does your mill do anything different when milling a straight slot X-axis left to right vs. right to left (or Y in/out vs. out/in)? It shouldn't.
Do it on the rotary table. Mill through the full thickness except for 3 points around the circumference, leaving those spots shy by about .010", then break the center out and bore to size. Though, I don't think a pinch clamp like that needs to be bored either. A finish pass climb milling should come out more than good enough for that piece.
"Never trust a man who puts a witty quote in his sig line." -Mark Twain
Thank you all for the advice. Looks like it's the RT to be used. The pinch clamp (router bracket) is for a wood router to be used on a CNC router. Perfect circularity is not required, but closer is better.
