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Custom made 3/4" boring bar up-scaled from a 3/8" boring bar that can't hold it's pants up.

rons

Diamond
Joined
Mar 5, 2009
Location
California, USA
I need to strengthen a 7" boring bar from 3/8" to 3/4" size. I didn't expect that this 3/8" boring bar would start doing poorly at 3" extension.
Just cutting aluminum with .004 depth. Tried two different inserts. The finish looked like I was using a Holtzapffel lathe.
I don't know why the 3/8" bar has flats. It would be stronger if they left the exposed side round and the other three flats nawrrower.

The 3/8" bar has a 10 degree down tilt into the page. The front edge has a 5 degree bevel.
The down-step on the 3/4" bar (right side) had to be done because the Hardinge holder has a 3/4" hole on one side and a 5/8" hole on the other side.
The max length extension of the 3/4" bar will be 5.5".

DSC_1558.JPG
So is this a good design to scale up or is there a better design for a 3/4" size?
The 3/8" bar uses CCMT-32.51 inserts. Should I get larger inserts for the 3/4" bar?
Since I started this I looked at how cheap boring bars are at McMaster. Oh well, might as well finish it.
 
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I didn't expect that this 3/8" boring bar would start doing poorly at 3" extension.
The rule of thumb is 4:1 max length to diameter for steel bars and 10:1 for carbide. Tool nose radius and tool edge sharpness also have a big bearing on what is achievable. You'll always have better luck pushing the overhang with smaller TNR. Using .008 or less will really help.
 
Sunk cost fallacy.

Here you go, less than $44: https://www.maritool.com/Indexable-...STEEL-BORING-BAR-.750-shank/product_info.html

The angles you're referring to are better known as "relief" and "rake".
I did have a side angle on this project. The metal bar came from a discarded recycle cart axle that nobody was picking up.
So I picked it up after 2 weeks. I used it to line up my tail stock in the whittling down process. Now this bar has sentimental meaning .. :cloud9:
 
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If you are gonna try and hold the 3/4 bar on the 5/8 step to the right that's also a chatter recipe.
Looking from the tip of the proposed bar. Travel to the right until the tool holder. 3/4" bar in a 3/4" hole for 1". The next 1" is 5/8.
The Hardinge L23 holder was made like that. I thought about enlarging the hole to 3/4" all the way through. But I might ruin it.
 
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You want a bar that uses CPMT inserts. High positive, small nose radius, and honed. Buy the one that is made for fine finish on aluminum if you want a fine finish and/or just taking tiny cuts. A SCLP is the correct bar for those inserts. All those attributes add up to less cutting force and less deflection.

Here is a chart that will help. http://www.carbidedepot.com/formulas-insert-d.htm
 
I'm not up on all the technical aspects...but my Scooby senses that I use every time I machine anything tell me that much stick out is gonna chatter. I don't like the bigger boring bar anchored in a 5C either but it would work, I guess.
 
I'm not up on all the technical aspects...but my Scooby senses that I use every time I machine anything tell me that much stick out is gonna chatter. I don't like the bigger boring bar anchored in a 5C either but it would work, I guess.
The 5C is in a four side holder for milling a square or flats on a round bar. Post #8 explains about the L23 holder for this bar.
More contact area than a 5C.

If you don't have one of these sets you are being deprived. This is a better picture than all the amazon links.
 
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Is it the insert or the angle it's being held at?
IMHO ;- Angle it's held, set with a 5 deg lead and clearance, it seems to drag on the trail side of clearance, if you incress that clearance you can't get a square bottom to the bore (should that be required)

I was having problems, and IIRC I think it came up in one of Ox's posts.

YMMV - as it has with some others on this thread :giggle:
 
I repeat C type inserts are NOT GOOD for boring - it's been covered many times on PM
I'm looking at my boring bar which takes parallelogram (C) inserts.
The relief angle on the insert is 7 degrees.
The rake of the holder is fixed because of machined out flats. The angle is 10 degrees.
That makes 17 degrees total.

A H.B. Rouse square shank tool takes triangle (T) inserts which are at 11 degrees.

Assuming the sharp edge of the insert is on center there is a 6 degree difference.
The angle of 17 degrees is in the direction of shear scraping.
 








 
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