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Internal boring - clearance?

Terry Keeley

Titanium
Joined
Oct 18, 2005
Location
Toronto, Canada eh!
Got to try my new Vargus "Mini Pro" boring bar today and was a little disappointed. I got the C06-24W405, pg. 53 here:

https://www.vargususa.com/portals/0/Docs/catalogs/eng/VARGUS-MiniPro Catalog inch.pdf

Says min bore is 0.230". I was trying to open up a 1/2" hole and no matter what I did the bottom of the insert rubbed with the cutting edge exactly on center. I just eyeballed the top of the insert level and am sure it was very close (bar is round with no flat). The only way I got it to not rub was to move it above the C/L.

Is this normal? What about twisting the bar to a negative rake angle? What am I missing here?
 
I had the same issue with a little Circle brand 3/16” insert boring bar. Getting it positioned correctly ie: rotation and on c/l was a bit of a chore. If you have those little bars hanging out too far or try taking too much at one time the tip ends up below c/l causing your heel clearance issue. Try a little above center and be certain of you doc.

Murf


Sent from my iPhone using Tapatalk
 
The OD of the bar itself should be on center.
Rotate the bar until the insert cutting tip is on center.
The insert top face will now be negative and the heel clear should be correct.

Where you went wrong was setting the insert top face flat since you had no orient flat on the bar and flat to the world makes the most logical sense.
You can get the same working geometry as designed by setting the insert flat and going quite a ways above center but your cross slide calibration will be off.
Also with this setup the top working geometry will change and become less negative if you go from a 1/4 inch hole to a 2 inch hole. Sometimes this is a good thing.

Take the catalog picture and rotate it until the insert is flat, now the insert is above center.

If this confusing give me the bar and insert numbers and I can post a couple of drawings showing how it works.
Bob
 
Easy way to check tool height is to put a solid round in the chuck and run the tool across the face to see if it leaves a clean surface. If it leaves a tit in the center you need to raise/rotate the tool to o eliminate it. If it runs into the tit, you need to lower the tool
 
The OD of the bar itself should be on center.
Rotate the bar until the insert cutting tip is on center.
The insert top face will now be negative and the heel clear should be correct.

Where you went wrong was setting the insert top face flat since you had no orient flat on the bar and flat to the world makes the most logical sense.
You can get the same working geometry as designed by setting the insert flat and going quite a ways above center but your cross slide calibration will be off.
Also with this setup the top working geometry will change and become less negative if you go from a 1/4 inch hole to a 2 inch hole. Sometimes this is a good thing.

Take the catalog picture and rotate it until the insert is flat, now the insert is above center.

If this confusing give me the bar and insert numbers and I can post a couple of drawings showing how it works.
Bob


So put the bottom O/D of the bar on the C/L and rotate the insert negative until the cutting edge is on center?

How will putting the O/D of a 3/16" bar on center allow me to bore into a 0.230" hole? Guess I'm not "seeing" it.

Inserts are WC0W4213 on pg. 42, as mentioned the bar is C06-24W405, pg. 53:

https://www.vargususa.com/portals/0/Docs/catalogs/eng/VARGUS-MiniPro Catalog inch.pdf
 
Hi Terry:
You've misunderstood CarbideBob:
He's saying the centerline of the bar OD should be exactly coincident with the spindle centerline of the lathe.
Now if you rotate the bar and insert until its tip is coincident with the horizontal plane through the lathe spindle centerline, the insert will be tilted slightly down toward the tip and the front clearance and top rake will be as intended by the bar manufacturer.
All these relationships get far more important as the bore you're cutting gets to the small end of the range at which the bar can work.

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining
 
So put the bottom O/D of the bar on the C/L and rotate the insert negative until the cutting edge is on center?

How will putting the O/D of a 3/16" bar on center allow me to bore into a 0.230" hole? Guess I'm not "seeing" it.
No, no.
Center of the bar on center with the chuck.
Put a mag base and DTI on the chuck and indicate the back or mount section of the bar. Should be zero and at this point one cares nothing about where the insert is pointed. Straight up, back or anywhere is fine.
Bottom of a 3/16 bar would be .1875 below and the top would be .1875 above. Then rotate the outside tool tip onto center.
I'll try to do some drawings tonight.

One can do a .230 hole with a bar that is 1 inch in diameter on the mount if the end of the bar is reduced. Certainly hole depth is limited.
I do know it is confusing a tad as our "world" is sort of anchored to machine slides and a zero geometry to it.
Bob
 
Hi Terry:
You've misunderstood CarbideBob:
He's saying the centerline of the bar OD should be exactly coincident with the spindle centerline of the lathe.
Now if you rotate the bar and insert until its tip is coincident with the horizontal plane through the lathe spindle centerline, the insert will be tilted slightly down toward the tip and the front clearance and top rake will be as intended by the bar manufacturer.
All these relationships get far more important as the bore you're cutting gets to the small end of the range at which the bar can work.

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining


Ahhhh, now I get it. Had another look at the bar, the top of the insert is set about 1/3rd from the top of the bar if that makes sense, so with it level it's above the C/L of the bar.

I guess the geometry of the top of the insert allows the whole thing to be at a negative angle but the cutting tip is actually neutral or slightly positive.

I'll give it a try.

Thanks!
 
Best way to understand boring is to draw the hole and the tool at 10x scale and look at what you see..Very often the clearance needs to be three differnt angles under the cutting edge...you can put your square head protractor on tha drawing and see what clearances you need,

Next best and common method is to hand hold the boring bar cutter to a hole the same size as the rough cut bore and just look at where the edge hits and that way you can see clearance below the cutting edge...or where it will/may rub.

https://www.amazon.com/General-Tool...hvlocphy=&hvtargid=pla-4583657821786696&psc=1

QT:[Weird seeing the insert set at that negative angle but it works, thanks.]
it may work for under the cutting edge ..but not the best for boring a steel part...a lot more cutting forces and strain on the boring bar having a negative top rake in a steel boring operation...
 
Here is the drill;
Draw a 1/4 " circle @ 10X
So draw a 2 1/2" diameter circle with a horizontal center line.
Now draw a .093 (draw .930") insert with its top lip at centerline of the 2 1/2 circle and left edge touching the circle OD from the inside ofthe circle
Just draw it, the insert as square edge rectangle
Now take your protractor and strike a line from its (insert) top left edge that misses touching at the heal of the
insert by 15 thousandth (at 10x so .150") and look to see what angle that is.

Surprising that it is so much when only about 7* would be enough on an OD cutter..
But at 7* on the circle inside and the heal likely would rub so perhaps a double clearance angle might be needed. Perhaps a 10* primary and an 18* secondary would work. Perhaps 3 clearance angle would be best.

Take the time to do this drill one time and you will understand a boring cutter.
Yes this is high school shop class drill.

*Every machinist/lathe hand should do this drill one time.
 
....
But at 7* the heal likely would rub so perhaps a double clearancr angle might be needed..perhaps a 7* primary and 1 15* secondary would work.
.

Not unheard of but very rare to have a primary and a secondary on an catalog standard carbide insert.
You are thinking round tool grind and not indexables. It is just different worlds with the same thing to get done.
Insert top to side rake is fixed along with thickness, bar has to be designed so the standard insert clears on the heel with some room to spare for trapped chips in that nasty triangle out there on the side.
Without a doubt a primary and second works better but one has to design around a fixed set or family of inserts.
People want $5 inserts not $50 ones not to mention the pocketing problem on a dual rake insert at this thickness.
Bob
 
I am just trying to show how a inside the bore cutter looks inside of the bore. If it is not cleared it will not work, break the boring bar, use poor tool cutter geometry with negative top rake, have so much pressuer It chatters and gives a poor finish, dosent cut at all. No wonder many have problems with boring if they never did the drill.

Wow, just look at the cost of not knowing how the cutter will look in the hole. Break a few of these and better to send all the work out.
McMaster-Carr
McMaster-Carr
McMaster-Carr
McMaster-Carr
 








 
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