Boring Bars

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Steven57

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I want to do some deep boring.
Can I make a boring bar out of mild steel to hold the cutter?
Would I need to harden it or just use thickness for strength?
 
Would need to know depth and diameter of bore job, plus material to be bored.
 
Depth up to 8"
Diameter 3/4" to 1 1/2"
Material Alum, mild steel, stainless 304 or 316
 
I've made boring bars for HSS and insert cutters from regular steel in the 1/2" to 1" range. They worked for me.
 
Steven57 said:
Depth up to 8"
Diameter 3/4" to 1 1/2"
Material Alum, mild steel, stainless 304 or 316
Click to expand...

Given the considerable depth, you cannot use steel - mild or any other kind - you need carbide, and even then you'll be pushing past the limits of a carbide bar at 0.75" bar diameter. You can do it with a 1" diameter carbide bar but you will need a large enough drill to get the bar in there. The acceptable limits for a steel boring bar is about 4 X the diameter of the bar if you want to be able to hold tolerances. For carbide, you are looking at about 8 - 10 bar diameters deep. Beyond this, the chances of holding tolerances and keeping the walls of the bore parallel drop off fast.

Just so you know, the hardness of a steel bar is inconsequential. It is the modulus of elasticity of the bar material that counts and going this deep, carbide is the best option. Even a tungsten bar will not get you deep enough - you need carbide, and that is going to cost you some bucks for a bar this size.

Is a reamer possible for your job? That is, if they even make a reamer in the size you need and you can afford it.
 
Carbide has a Young's modulus of about 90,000,000 psi, steel around 30,000,000 psi. The formula for deflection (under a load that doesn't permanently bend the part or below its yield strength) is:

Change in length = (Load x length) / (cross section area x Young's modulus)

That boils down to a carbide boring bar deflecting 1/3 of what a steel bar would deflect. If you peck and peck away at it, you may get close enough with steel. If it's bored all the way through, you may try a 4-jaw chuck and indicate the work in. Go 4" deep from either side if you want to try steel, otherwise go with the carbide bar suggested above.

Bruce
 
I have a few old school boring bars I used in my younger years when I was learning to cut iron. The few bores I had to machine that were in the 6:1 to 8:1 deep range, were bored with one of these home made bars. Took lots of trial and error on grinding your tool bits to keep chatter to a minimum. Depth of cuts were no deeper than .030-.045" to the side, lots of time less than that. I found plumbers lead from roof flashings worked the best for vibration dampening. Just wrap it around the boring bar as snug a you can get it by hand out by the tool bit. Also, if the bar is long enough, hang weight on the opposite end, like a heavy walled tube or something with a hole in it. This will help too. Even with carbide inserted tooled boring bars, hanging weight on the opposite end of cutting will help.

Over the past few years, I've been able to buy a couple of solid carbide boring bars along with a couple of devibe bars, too. Even they can give you havic at times.

Ken
 
What was posted (OP) isn't really a boring job at that ratio of depth / diameter and would be a deep drilling job with a ream to follow. I'd attack it with step drilling, then follow with nearest sized ream abd then an adjustable reamer to get to pass muster. That's a tough and boring job (NPI). I'd hate it! Even with coolant through the bit and careful peck drilling doodoo occurs very quickly. OTOH with proper indication you can do 4" from both ends and finish with a hand reaming.

Steve
 
Another option would be to order billet with the rough hole size, and machine for the bearings or other critical points. Sometimes you can save money by machining ends and welding pipe to them.

Steve
 
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