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Another boring bar question

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DiscoDan

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#1
I did a search but didn't find exactly what I want I want to know. I will be boring some bearing bronze. The starting hole is 3/4" and I need to bore about 3". Most of the boring bars I see are either carbide tipped or indexible carbide. I believe C2 carbide would be used for bronze. What is the preference between carbide tipped and indexible? I kn I will there are some HSS boring bars as well but not as prevalent.
 

Bob Korves

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#2
Both will work. The brazed ones will probably need some work to get the angles correct and to make them sharp -- unless you buy higher end tooling. Ideally you would get a carbide shank boring bar, but that costs considerably more. Rigidity is a function of the elastic modulus of the material it is made of. All steels have essentially the same elastic modulus, whether hardened or not. Carbide shanks are MUCH more rigid than steel shanks. Diameter of the shank, and the material it is made of, makes the difference in rigidity with boring bars. Use the biggest diameter boring bar that will do the job and not get clogged with chips.
 

mikey

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#3
I did a search but didn't find exactly what I want I want to know. I will be boring some bearing bronze. The starting hole is 3/4" and I need to bore about 3". Most of the boring bars I see are either carbide tipped or indexible carbide. I believe C2 carbide would be used for bronze. What is the preference between carbide tipped and indexible? I kn I will there are some HSS boring bars as well but not as prevalent.
Dan, there are a number of considerations here. I'll start and the other guys can correct me:
  • Bronze is not difficult to bore so a special grade isn't necessary.
  • A 3/4" ID X 3" bore is a fairly deep hole, especially if it has to be accurate all the way down the bore. This has to do with both bar diameter and bar material. A boring bar is subject to deflection and we are encouraged to use the biggest, stiffest bar we can fit in the hole and still clear chips. For your hole, and assuming it has to be consistently accurate all the way through, you are looking at a 1/2" or possibly 5/8" diameter carbide boring bar. The reason a carbide bar is called for is because deflection is greatly influenced by the modulus of elasticity of the bar material. This means that a steel bar can be extended roughly 4 times its diameter past the holder before deflection becomes a problem, whereas a carbide bar can go 8-10 times its diameter. So, a 1/2" steel bar can bore 2" deep, while a 1/2" carbide bar can go about 4-5" deep. These are rough guidelines but it is wise to heed them because they have a significant impact on accuracy.
  • Boring bars are available with steel shanks and brazed carbide or inserted carbide or HSS tips, solid carbide and inserted carbide or tungsten shanks with inserted tips and many types of shop-made bars. The deflection guidelines apply to all of them.
  • Of all your choices, the best would be an inserted tip carbide shanked bar. It will give you the best chance at accuracy and consistency. A 1/2" bar will work; a 5/8" bar might also fit but will cost considerably more. A steel bar might also work but you really have to know what you're doing and your tolerance levels will need to be a bit looser.
Choosing a bar for a rough hole is no biggie. Choosing one that has to bore an accurate hole to tight tolerances requires you to pay attention to everything and you sort of need to know what you're doing.
 

mksj

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#4
It is unclear as to the depth and diameter, from the description it sounds like you want to start at 3/4" with a final diameter of 3" or is that the depth? As others have mentioned, you are limited by the boring bar diameter and material as to the depth of a hole that you can bore. There are some variants of steel, that can bore slightly deeper than standard steel boring bar, heavy metal and carbide boring bars. I prefer indexible type as you can change the insert specific to the material. The boring bar insert seat can be negative, neutral or positive rake. I typically use CCMT /CCGT neutral or CPMT/CPGT positive rake insert boring bars. You get less chatter, and I tend to use a sharper ground edge insert for softer materials with a bigger nose profile. Keep you boring bar clamped in the holder as short as possible, least amount hang out.

If you desired to bore to a larger diameter, then I suggest you look into at least 2 boring bars, a 5/8" and a larger 1". Both can take the same insert, so something like a CCGT 32.52 insert. Using a 1" boring bar, you should be able to bore a hole to a depth of 3-4", carbide probably 7". Carbide is much more expensive then a good steel boring bar, like 8-10X. They are also brittle and can snap easily. Really need an assortment of sizes, and maybe 2-3 types of inserts. I routinely turned/bored bearing bronze, it can be prone to chatter. I choke up on the boring bar, and recently purchased a used 1" carbide boring bar which works well. Also a larger nose profile will give a smoother surface, but need you need to take a deep enough cut so it shaves the material as opposed to pushes the material. So one last approach to boring smaller holes with a lathe is to drill out most of the material, and then do the final sizing with a 3 or 4 flute end mill. I use an ER40 holder so can accommodate up to a 1" end mill.
 

benmychree

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#5
I do most of my boring with TPG inserts, but also like HSS boring tools, made by Bokum Tool Co., they are form relieved and sharpened only on top, they are made in lead angle, 90 degree, and threading styles and also grooving and chamfering styles. Sizes are available from tiny to quite large. I look for them on E Bay mostly. Comet tool company also made a copy of them years ago, now defunct.
 

markba633csi

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#6
Keep deflection at a minimum by taking very light final cuts
M
 

john.k

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#7
If you get bronze chips in your washing,you will know about it...............stories of bronze chips are legendary.
 

rwm

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#8
If you get bronze chips in your washing,you will know about it...............stories of bronze chips are legendary.
Enlighten me?
R
 
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