Boring bar speed?

[erikmannie]

Just thinking out loud here, going from 1” to 5” with passes removing, say, .020” of aluminum from the radius in each pass:

Increase radius from .500” to 2.500” which is an increase of 2.000” divided by .020” which would be 100 passes.

You said that there are 4 cylinders total, so 400 passes, and deep passes at that.

 

[MRogersII]

Just thinking out loud here, going from 1” to 5” with passes removing, say, .020” of aluminum from the radius in each pass:

Increase radius from .500” to 2.500” which is an increase of 2.000” divided by .020” which would be 100 passes.

You said that there are 4 cylinders total, so 400 passes, and deep passes at that.

Agreed. Ignorance is not bliss. Not even briefly.

 

[Flyinfool]

Is this 5 inch hole going to be 14, 10 or 6.5 inches deep. That could change how I would attack it. Fingers crossed for 6.5.

I would go with your plan of drilling a ring of holes to remove the bulk of the material, BUt I would size the ring of holds to only leave about 1/8 inch at the most for cleanup. To leave an inch of Dia to bore is still a LOT of boring. Especially considering the very low rpm and slow progress as you get to the bigger diameters.

 

[MRogersII]

Good point. The bored holes are through the 6.5" section. My thinking was this, I have, thus far, manufactured less than 10 (finished) items on the milling machine. Closer to 5. And each item has brought with it its own level of very steep learning curve. One of those was an idler roller support for a Porter Cable band saw that cracked. I had to make 2 of those before I got it right. The original was cast aluminum and I did what I thought was a pretty solid job making a replacement. I also got to do some flat file fitting work which was also a first. However, I failed to take into account that the original cast piece had expanded some prior to cracking. So the first one didnt fit.

The 2nd project is a double rifle that I am in the process of building. Too much there to even start a discussion.

Given the limited experience, and the fact that the aluminum blocks were fairly expensive, I elected to be more conservative on the ring diameter for the 1/2" penetrations/perforations. Once I get one behind me perhaps I can be more aggressive with the 2nd one. I still have some concern that, once the ring of 1/2" circles is complete (and it nearly is), and the center mass removed, i still will have to use the boring bar to take the 20 or so connected half circles and cut them back to a single continuous circle. Probably not explaining this very well. Essentially, the boring bar will make contact at the midpoint between the radii of holes 1 and 2, and then pass to the point of the radii between holes 2 and 3, and so on. Assuming the ring of circles is perfect, and it is not, the boring bar has to work through this stop-and-go cutting for the first half inch before it gets down to continuous contact. Im not sure what that looks like on the boring bar tip. Seems like a lot of miniature impacts. Im guessing it will sound even worse. I could probably take a 3/4" end mill and manually work my way around the ID to "soften the peaks but, I'm not sure if that is best either. Also, even if the end mill idea works, it will only reach so far down into the 6.5" deep bored hole.

As is always the case, I very much welcome constructive guidance.

Michael

 

[mattthemuppet2]

that's a challenging entry level project

if you have a standard 2" boring head, then you'll need at some point to bore using a cross bar rather than a bar sticking out the bottom of the boring head. If you're trying to bore from the top, you'll need a 7" boring bar to reach the bottom AND that much quill travel (unless you're using the knee or head to bore). I'm with flyinfool - chain drill much closer to finished diameter so you can use a cross bar for the boring. Make sure you have a LH bar or are able to run your boring head in reverse without it unscrewing off the arbor. What mill are you using to tackle the project?

As to the original questions, easiest way I do it is to use RPM = (4 x surface speed)/ diameter. SFM of 100 and 200 for carbide in steel and alu is reasonable, maybe half that for steel. You can always go up if you have the head room.

eg. 3" boring bar pass in alu. RPM = (4x200)/3 = 260rpm or so. That's pretty much what I did a little while ago and it worked just fine.

 

[Boswell]

What you describe above is what is called an "Interrupted Cut" Rigidity of your Mill, the Size of your boring Bar and the type of cutting edge are all part of what will effect an interrupted Cut.

 

[Flyinfool]

When you start with the boring bar that is called an interrupted cut, as it is bouncing from peak to peak.
As long as you keep the cuts light till things are smoothed out some, it will not be a problem. The worst part will be that you will have a pretty long stick out from the boring head, This may lead to chatter depending on speed, feed, depth of cut, tool sharpness,lubrication and alloy of AL.

The other thing, At 5 inch dia motor horse power starts to become more od a factor, You may end up having to run at well below optimum speed and or very light cuts to have enough power.

 

[MRogersII]

My mill is a Matthews PM-30. My boring head is a Criterion and accepts 1/2" boring bars. I have several boring bars of different lengths that will work with the Criterion. I also bought an Accusize indexable 7" boring bar that, up to this point, I have had a heck of a time making clean cuts with. I thought that I would maybe treat it as two 3" cuts (one from the top and one from the bottom) until I get closer to the final dimension at which point I would switch over to the longer indexable bar. As for quill travel, my unit only has 3" so, at some point I have to unlock the z, lower the unit and start over again.

 

[MRogersII]

I probably should have mentioned that, there is a machine shop that builds engines and will be doing the final boring and installation of the steel sleeves into the aluminum bores. The issue is that their engine boring bar starts at 3.5". Because of this my Dad had taken the blocks to a different machine shop to have the bolt holes bored and the initial cylinder cuts made. The blocks had been at this other shop for 3 months with no progress so I offered to do it for him. Thats the rest of the background story.

 

[mattthemuppet2]

If you can't use the quill, I'd extend it to where you'll get sufficient clearance to get the boring head to the bottom of the hole, lock the quill, snug up the Z gibs a touch and bore using the head. Pay careful attention to diameter at the top and bottom of the bore, also double check tram on your mill, don't want an accidental "V4" at the end of it

What inserts does the Accusize indexable boring bar take? I'm guessing that you have a yellow coloured molded insert (TCMT or CCMT) in there - those kinda suck in aluminum. You need a nice sharp ground insert, like a TCGT or CCGT, for that job. If I have one of the right size, I'll pop one in the post. You'll be amazed at the difference.