[jheers]

I need some advice on a part I'm about to run. The part is an extruded aluminum tube, ~1.75" OD ~1.6" ID. The tube has a .065" thick rib (not sure if that's the best term for it, but that's what we've always called it here) running down the center of the tube, so the cross section is two equal semicircles. I need to bore the rib out of one end of the tube, to a depth of 8 inches. I'm doing this on a lathe, since the part is too long to stand up in one of our mills. Historically, we have used a 1" x 8" 4 flute end mill to bore it out. It works, usually, but it is sloppy. I tried drilling it out with a spade drill, but the spade drill just ended up bending the rib and tearing the part up. I have a 1.25" twist drill that I can try starting with, then finishing it out with a boring bar, but I'm worried that the heavily interrupted cut will have adverse effects on the boring bar/insert at an 8" length. Anyone have any thoughts??

[jheers]

Update: The twist drill was much worse than expected. It was very flimsy and walked off center right away. I retried the spade drill and it seems to be working quite well. Now I just need some input on finishing out the ID. The spade drill is 1-9/32 so I will need to bore out the remaining ~0.16" per side.

[Packers]

i dont know man what you are doing sounds like craziness. I would just bore it out. aluminum isnt that difficult to cut anyway, just find a boring bar thats the right length throw in an insert with a positive cutting cutting edge and a tough carbide and just call it a day man

[jheers]

The problem I'm facing with the boring bar is that the only one we have that is long enough has a 0.625" diameter. Even being a carbide bar, it chatters like crazy. And any tool I use to predrill the part before running the boring bar ends up bending the rib, so even if I could solve the chatter problem, I'd still have to figure out how to predrill the part.

[Packers]

What about a boring head, they sell some roughing heads that are not dead nuts accurate but they could do the job, I think they also come with unlimited extensions

[Packers]

Sounds like you may need to go all out on fixturing. Or you could fill the pipe with a resin and machine it all away, messy but it would make the part rigid, stop the chatter

[rowbare]

Maybe an annular cutter would do the job. They are stiff and you can make your own arbor for it.

bob

[jheers]

Sounds like you may need to go all out on fixturing. Or you could fill the pipe with a resin and machine it all away, messy but it would make the part rigid, stop the chatter

Where would I find resin? I have seen the low melting point metals, but that seems to be a bit expensive for this application. We would be losing whatever material we used, so that would add up quick.

I'm also not entirely sure what to do with fixturing. I have a feeling a collet chuck would make the vast majority of my current headaches go away, but that is not a cost I can currently justify to the owner. We have a process that turns out parts that are good enough to pass with a relatively low scrap rate, so trying to sell a big ticket item for this job is a challenge.

Maybe an annular cutter would do the job. They are stiff and you can make your own arbor for it.

The only annular cutters I can find have a maximum cut depth of 3 inches. I need to go 8.

[jheers]

I suppose it may be possible to recover any Fixturing Alloy we would use, by just gathering up the chips and heating them, then recovering the melted alloy... but that seems a little labor intensive, and again, labor is money, and money is not easily spent.

[packrat]

How about a "forstner bit"? Even though their made for wood, the good ones are HSS. If your lucky maybe get one a little over size and grind the O.D. to the size you need, and do away with boring all together.

[jheers]

How about a "forstner bit"?

I see one that is the right size on MSC Direct's website. This is a new idea to me. The tool is only 3-1/2 inches long, but it looks like I can use some sort of an extension. This certainly is worth some research.

[wizard]

It is worth a try but I don't think it will work out well. They are designed to cut wood and this have a small shank in comparison to cutting OD. Beyond that the cutting surface would bang up against the aluminum web twise per revolution and make a hell of a lot of vibration.

This idea however does lead to another suggestion. Consider the way gun drilling works. You have advantages here in that you could use hydraulics to force the swarf out the back of the tube thus avoiding the need to blow the swarf back through the cut. The end of the bit would effectively act as a guide to keep the tracking true. If you can manage a cutting edge that is not radially aligned with the center line you should be able to keep the cutting edge in engagement with the web for more that two brief periods of the revolution. To pul this off in a standard lather you will most likely need to put a guide cap on the end of the tubing to get the bit started. You may be able to get away with a standard coolant pump if you can get the chips to wash out the back of the tube easily.

In any event I think the key here is to get the cutting edge to stay engaged with the web for as long as possible. That might require multiple edges set at odd angels. If you are into wood working think about how the blade on a skewed plane looks in comparison to a standard plane. The idea is to shave off a small cross section at a time and to maximize cutter engagement. The End of your cutter would likely end up looking really odd and not be usable for anything else.

If you think about it, your problem right now is that each attempt you have made with the different cutters results in the cutting edges slamming into the web x nmber of times per revolution. It doesn't matter if you are using an end mill, drill bit or something else the impact is the same. Thus you need a solution that doesn't do that. That means a radical tool.

I see one that is the right size on MSC Direct's website. This is a new idea to me. The tool is only 3-1/2 inches long, but it looks like I can use some sort of an extension. This certainly is worth some research.

[rowbare]

My thought with the annular cutter was that you would make an appropriate length arbor for it so cutting length shouldn't really matter. Chip clearance is probably not a big issue given that the the extrusion is mostly hollow.

bob

[jheers]

I've never used an annular cutter before, so I may be missing something, but the issue I am seeing is that the central web (thanks wizard, I knew there had to be a better word) needs to somehow be broken out, since the annular cutter leaves the center of the cut intact, meaning that cutting beyond the length of the cutter wouldn't work.

[wizard]

I could see some form of a custom made annular cutter working to the extent of cutting the periphery but then what do you do with the web left in there? Twist it off? I suspect you would need a custom cutter with fine teeth so that one tooth is always engaged. I just have a very hard time imagining a way to break that web off eight inches inside the tube. If it wasn't for the depth I'd suggest making up a fork to twist the waste off.

I've never used an annular cutter before, so I may be missing something, but the issue I am seeing is that the central web (thanks wizard, I knew there had to be a better word) needs to somehow be broken out, since the annular cutter leaves the center of the cut intact, meaning that cutting beyond the length of the cutter wouldn't work.

You would have to make your own! That isn't all that bad and you probably could get one to work fairly well without much engineering effort. Weld some tool steel onto a piece of tubing and grind in some teeth. You might be able to make the annular cutter cut a fairly thick kerf with some skewed teeth which might simplify shearing off the waste.

The other option is to use a more off the shelf annular cutter and to break the web in a number of cycles in and out. I still see issues trying to do it 8 inches into the tube. However your fork would be a lot stiffer. Remember this is aluminum so a little muscle goes a long ways. You do have the potential for deforming the tube by shearing the web this way. If that is a problem the I don't have any other ideas at the moment.

I'm not sure if you are following me here when I say fork. So imagine a bar to fit the ID of the bored tube with a slot down the enter of the end. You would slip the fork over the central web and then twist it 90 degrees to break off the waste. Obviously the twisting motion has the potential to impact the good part so I'm not sure how this would work out. The obvious thing here is that if you go the annular cutter route you will need a solution for that waste.

[jheers]

As of right now, I have adjusted a little bit of the basic setup. I was using jaws with an oversize diameter. I have posted before on this forum about my chuck jaw woes, the basic idea being that I have a very limited supply and getting new ones is not easy. The outside of the tube is scalloped, so the way the oversize jaws held the part actually introduced some eccentricity. To fight the eccentricity issues, while distributing the clamping force evenly around the tube, which has very thin walls, along with adapting to a different set of oversize jaws, I made a split bushing out of 1018. It has a 2.25 OD and a 1.76 ID and clamps the part very nicely. This allowed me to up the clamping pressure a bit and has helped to combat the chatter I had been facing. I'm still using the end mill for the moment, but some new ideas have come my way during this discussion. I think I may be able to get my hands on a boring bar with a 1" shank, which should have very little chatter compared to the 5/8" boring bar I had tried. I like the annular cutter idea quite a bit, but I don't know if it is practical at this point.