# diy boring bar with carbide inserts rake angle question



## Cipher (Feb 3, 2018)

Hello,
i would like to make myself a few boring bars for using them with my chinesium boring head in the mill but I'm not sure about the rake angle.
Here is what I have so far:
Inserts I have and want to use: CCMT060204 and TCMT110204 (both have 7° clearance angle)
Material for the bars: 1.2210 silver steel or drill rod, 12 mm diameter h8.

They will be used for very light work (600 watt Sieg mill) with 20xx class aluminum. I will use them only very few times during the next years, therefor I can't justify buying one or two. I did not find any information about insert holders for boring heads, usually I just copy my carbide holders from the ISO nomenclature depending on what I want to use them for. For external lathe tool holders, they came out good enough for what I want them to do.

I would be thankful if someone could point me in the right direction.
Short and simple: What is the commonly used rake angle on a general purpose boring bar to be used in a boring head?


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## cathead (Feb 3, 2018)

The rake angle on the cutter doesn't matter much because one can turn the tool in the boring head to set the rake angle to your liking. 
Aluminum needs more rake(positive) generally than other metals.   At times, a little experimentation is helpful.


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## Bob Korves (Feb 3, 2018)

The real issue is with the diameter of the bore and the relief angle below the cutting edge.  If the angle is not enough, the tool will rub instead of cut.  It is possible to roll the boring bar downward to a more negative rake to increase the clearance angle.  It works!


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## Tozguy (Feb 4, 2018)

What is commonly used for rake angle on general purpose boring bar would likely be what is commonly offered by tool suppliers. I do not see what would be different when used in a boring head versus a tool post. My commercial boring bars all basically have neutral rake but can be rolled to negative if necessary to avoid rubbing as Bob mentioned. 
I have on occasion ground some inserts and holders for more clearance or rake. For cutting aluminum I would consider inserts with chip breakers.


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## middle.road (Feb 4, 2018)

I personally would go with an 11° rake. TPxx series insert, Especially if you're boring less than 1" DIA. holes.
And a chip breaker as Toz mentions would be of benefit.


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## 4GSR (Feb 4, 2018)

Here's drawings to a couple of boring bars I drew up many years ago.  Never did build them, good reference information.


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## Cipher (Feb 5, 2018)

Thank you! My inserts have chip breakers, they work fine with neutral rake. I try neutral and 11° first. Maybe the setscrew on the boring head can stop the rod from rotating without having to grind a flat spot onto the rod.
I will try and report back with some pictures.


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## mikey (Feb 5, 2018)

Cipher said:


> What is the commonly used rake angle on a general purpose boring bar to be used in a boring head?



Cipher, both inserts require neutral radial and axial rake.

There are only two kinds of rake on a boring bar - axial rake and radial rake:




Both rake angles are machined into the end of the bar and are precise. CCMT and TCMT inserts require zero radial rake and zero axial rake.


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## Cipher (Feb 6, 2018)

Hello mikey,
that is the kind of information i was after, thank you. (I got your full answer via mail, seems you withdrew some paragraphs...)
For me it is not ovious how these boring bars sould look. I dont have one for the boring head and have newer seen one. My lathe boring bars have different negative radial rake angles, that's why i am more confused about than i should be. ; )


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## mikey (Feb 6, 2018)

If I may suggest an option, why not go with HSS or cobalt bars? Boring on the mill with inserted carbide bars is a common practice but requires higher speeds, and that leads to higher centripetal forces and more vibration. When you use carbide inserts at low speeds then you have to really pay attention to depths of cut because too light a cut (as in finishing cuts when you're trying to come in on size) results in greater deflection and chatter. Inserts work but you have to know how they like to cut and you have to really plan your cuts with more care.

I really like inserted carbide boring bars on the lathe but on the mill, I use cobalt bars 99% of the time. They are so much easier to use and will take a very fine cut without chattering so, for me, they are much more accurate. In addition, they are simple to sharpen. My 3/8" Borite set is now over 20 years old, have been sharpened at need and perform the same as new.


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## mikey (Feb 6, 2018)

Cipher said:


> My lathe boring bars have different negative radial rake angles, that's why i am more confused about than i should be. ; )



I intended to comment on this and forgot. If you are using CCMT inserts in your lathe boring bars then the SCLCR bar and insert configuration has positive rake geometry. It is intended to be used with zero radial rake when positioned in your boring bar holder. Inducing negative rake with these bars defeats the purpose of the geometry. 

Of course, they are your bars and you can use them as you choose but I just wanted to make sure you were aware that while inducing more positive radial rake is a common practice with solid boring bars, it is not a recommended practice with inserted bars. This is mainly because the cutting edge will then be out of position when you do this but also because you reduce the clearance angle under the insert and that leads to rubbing and consequently, chatter.


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## Cipher (Feb 19, 2018)

I see my error now. I was confused about adding up the angles of the insertholder and the insert itself. My inserts have positiv rake, my holders negativ, that will result in somewhat neutral...
HSS is not really a option for me, since i'm not allowed to make too much noise where i live (not heavy grinding). However since it would be a very simple way to start, i will go for a 3mm round HSS toolbit and a simple holder, just like Stefan Gotteswinter showed on youtube. These bits should be simple and small enough to be ground with a dremel tool. And if I mess up dirlling a few holes and tapping one of them, oh well...


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## Brento (Feb 19, 2018)

4gsr said:


> Here's drawings to a couple of boring bars I drew up many years ago.  Never did build them, good reference information.


Whats with the tilted part in the drawing? Im having trouble reading that.


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