Deep boring

[mackeym]

I need to bore out a large piece of unhardened 4140 round stock. The round stock is 4.25" in diameter and ~16" long. The bore needs to be 3" in diameter and go through the entire length.

My current approach: Initially drill a 1" diameter hole using a lathe with a reduced shank drill bit with a DIY extension. The hole diameter can then be increased with a boring bar (in 2 operations...first bore through half of the length then take the part out, rotate it and do the second half), which leads to my question. The required overhang of the boring bar is 8". Can this be done with a 1" diameter steel boring bar. I've found some carbide boring bars with the required overhang specs, but they're very expensive.

Is it possible to avoid chatter and get a decent finish if a sharp HSS bit with small tip radius is used with a small depth of cut (say 0.003 to 0.004")?

Has anyone had experience with really pushing the limits on boring bar overhang?


Recommended max. overhang rules of thumb are:
Steel boring bar overhange: 4 x boring bar diameter
Carbide boring bar overhange: 6 x boring bar diameter
Carbide with some type of dampening: 10 x boring bar diameter

This boring operation would be 8 x boring bar diameter.

[Geof]

I have done something similar to this by customizing a boring bar holder.

Drill through as you propose.

Use a 1" boring bar with as much overhang as possible and bore almost to size. Probably you will only get to a depth of about 5 inches or so.

Now make a boring bar holder out of 2" dia material with a 1" hole to take your 1" bar. With the 1" bar secured in this hanging out the 5 inches you can now bore to about 10 inches deep but you cannot come out to the full diameter because your 2" dia holder interferes. ACtually if you offset the 1" hole in the 2" holder you can get a larger dia at this step.

But now you have an almost 3" hole 5 inches deep and an almost 2" hole to 10 inches deep.

Now use a 2" dia boring bar to enlarge the almost 2" dia part out to almost 3" to the 10 inches depth.

Turn the part around and repeat the boring sequences so you get almost 3" dia all the way through.

Now if you are lucky you can use a 2-3/4" bar to finally bore to the 3" size.

[stevehuckss396]

Originally Posted by mackeym I need to bore out a large piece of unhardened 4140 round stock. The round stock is 4.25" in diameter and ~16" long. The bore needs to be 3" in diameter and go through the entire length. My current approach: Initially drill a 1" diameter hole using a lathe with a reduced shank drill bit with a DIY extension. The hole diameter can then be increased with a boring bar (in 2 operations...first bore through half of the length then take the part out, rotate it and do the second half), which leads to my question. The required overhang of the boring bar is 8". Can this be done with a 1" diameter steel boring bar. I've found some carbide boring bars with the required overhang specs, but they're very expensive. Is it possible to avoid chatter and get a decent finish if a sharp HSS bit with small tip radius is used with a small depth of cut (say 0.003 to 0.004")? Has anyone had experience with really pushing the limits on boring bar overhang? Recommended max. overhang rules of thumb are: Steel boring bar overhange: 4 x boring bar diameter Carbide boring bar overhange: 6 x boring bar diameter Carbide with some type of dampening: 10 x boring bar diameter This boring operation would be 8 x boring bar diameter.

If you have enough room between centers you could maybe mount the stock to the cross slide and make a long boring bar out of drill rod that spins on center. You would have to make more than one boring bars as the hole size increased but you could cut all the way thrue in one bore. This is how model engines bore the main caps for there crankshafts. Give you a nice straight bore if you have the room. You could do it on a 40".

[RICHARD ZASTROW]

AKA line boring