Which Boring Bar?

[Mitch Alsup]

The trick with boring bars is that they are never SMALL enough, big enough is rarely a problem.
I have a number of insert boring bars (bottom feeders from china:: 6mm, 7mm, 8mm, 10mm, 12mm) that are perfectly capable of 0.31 and larger
But I have needs to bore holes of 0.247,5 as races for 1/4" ball bearings pressed in with thumb pressure.
measuring that small ID accurately enough is also a big problem.

For big stuff, I will drill 1/2, 3/4 and 7/8 before starting to bore with a 16mm boring bar.
I generally use CNMG inserts but will substitute CCGT when doing aluminum or brass.

 

[Janderso]

Mitch,
You said it. I have a pretty good selection of boring bars but they are usually too big.
I found I use this one quite often.

 

[Janderso]

This is what I meant about the geometry is off with the cheap brazed carbide boring bars.
Criterion on the left, cheap brazed carbide on the right.

 

[DavidR8]

The one on the right looks like a tooth off an excavator bucket!


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[Janderso]

The one on the right looks like a tooth off an excavator bucket!


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I should expand my thoughts on these two examples.
You can always get your tool height correct and adjust the cutting relief etc. on the lathe but if you are going to bore with these on the mill, that excavator bucket isn't going to cut it's going to be a problem!

 

[pontiac428]

Cheap brazed carbide does pretty good for the money once you re-grind the working end to proper angles... at least up to the quality of the carbide material itself.

 

[benmychree]

I have one that I bought from KBC not long ago -- 5/8" diameter I think and about 6" long. The holes are square and take 3/16" square blanks. That being said, if you're making your own a round hole works just fine too. There was a post here just the other day (maybe the other forum?) where someone made one with a round hole for broaching. Once the set screw is in place and tightened I think it holds the blank quite rigidly. The KBC is "ok" but not stellar in the fits for the blanks. It works though.

You can also grind them from square HSS -- I have one I ground both ends on like that and it's my favourite for being able to control the cutting angles. It's also quite stiff (HSS vs carbon steel). I'll post a photo maybe when I get home later. It's lasted me for a number of years already.

-frank

There is literally no difference in stiffness between HSS and carbon steel, hardened or unhardened, it is explained in the book "Tool Steel Simplified", which unfortunately, I cannot find just now, it can be proved by taking samples of HSS or any other hardened steel and comparing them by fastening them to a stationary object, and some distance away, attaching equal weights to each at a given distance from the attached end, and noting the deflection; until the tensile strength of the soft sample is reached, they will deflect equally, then the soft sample will take a permanent set or bend.
The only way to get a stiffer boring bar is to substitute carbide for steel.

 

[mikey]

I'm in the process of researching tooling for this little lathe, and would like to know what you'd recommend for a boring bar.
Do I buy the bottom-of-the-barrel china stuff.....the kit with brazed on carbide tips? Or, get a quality bar with carbide or HSS inserts?

Spokerider, you are going to find that this is one of the most difficult questions to answer because boring is a somewhat complex process and there are a lot of "it depends" thrown into the mix. If I asked you what you plan to bore with it, my guess would be that you're not sure but you just want to have it on hand for when you need it. At least that was my thought when I was a new guy with a new lathe. Little did I know how little I knew.

I put some thought into this and, knowing what I know now and considering the size of your lathe, I would recommend you buy a set of cobalt boring bars from LMS. They will allow you to bore a reasonable range of holes to respectable depths and should be capable of pretty good accuracy if you do your part. They also have shanks to fit the tool holder you have. I strongly recommend that you NOT buy inserted HSS or carbide tools at this point unless you understand what you're buying and how to use it.

I'm all for shop made tools and grinding your own so I don't want to take anything away from the input you've gotten but new guys need decent tools to learn with and they are just not capable of grinding them yet. I would rather see you learn to use the lathe at this point, and that includes boring. Later on, when you have some ideas about tool geometry and some grinding equipment available, you can give it a go. I, and all the other guys, will help where we can.

 

[francist]

no difference in stiffness between HSS and carbon steel

That is correct. I did find a slight difference in Young's modulus between the two but for all intents and purposes they are even. The stiffness appears to be concentrated between my ears, for the most part.

 

[benmychree]

Yes, that is correct, modulus of elasticity is what I was unable to remember earlier --- I first became aware of it from the Tool Steel Simplified book back in the 1960s during my apprenticeship; very few machinists knew or believed that it could be so ----