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- Dec 20, 2012
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If I'm reading all of this correctly, you have to bore two work pieces, each with a 5" diameter bore that is over 6" deep using a mill with 3" of quill travel and a Criterion boring head that takes 1/2" tooling. Is this right? And you're thinking of using 7" long steel inserted carbide boring bars for this job, right?
The largest Criterion head that I'm aware of that uses 1/2" bars is a DBL-202B (please correct me if I am wrong; I don't know every single head out there). That head has a max capacity of 3" using the outboard hole and 6.687" using the horizontal hole. Given that you're roughing out the hole with drilled holes, I assume you are making that hole large enough to clear the head of the bar that is being held horizontally because that is what you'll need. You have to bore this hole from both sides of the work piece because you only have 3" of quill travel.
Now on to your questions about vibration. Vibration with a boring head is caused primarily by Centripetal Forces. The formula for CF = mv2/r, [EDIT: this should read mv squared/r] where m = mass of the bar, v = speed and r = radius of the bore. You can see that mass is fixed for any given bar so it is the velocity or speed the bar is moving at that has the most influence on CF. Even a little bit of speed has a huge impact on CF so when boring big holes the question is always ... how slow can your mill go while maintaining torque? Note also that as the radius increases, CF goes down so going big is not a major factor. It is the speed that you can maintain and the mass of the mill that will determine how big you can really go.
In real life terms, if the mill is light and not too powerful and cannot achieve a low enough speed then the mill will vibrate. This translates into poor accuracy and finishes and in the worst case the bar may not cut at all.
Now, in order to reduce the effects of CF given the mill you have the best option is obviously to reduce mass to the extent possible and find a speed that will allow the bar you use to cut as effectively as possible. You have to keep in mind that inserted carbide needs speed to cut well and at larger diameters like this, speed is going to cause problems for you due to CF. Not only that but using a steel bar will cause more deflection, especially given the interrupted you will be making.
If this were my project I would find a really good solid carbide LH boring bar from Micro 100 that is long enough to get the job done using the travel of the head. I cannot be sure it will survive the impact but the carbide from Micro 100 is a lot tougher than most and I have yet to break one of these bars, even with interrupted cuts in steel. The geometry is essentially the same as a HSS bar but with the stiffness of carbide so it will cut clean and finish well while still being tolerant of a wide range of speeds.
As I don't know which head you have, I will leave it to you to choose a bar but I would personally not be using an inserted carbide bar.
As for flipping the work piece to bore from the other side, solid clamps in X and Y to reference the work piece should allow you to flip it accurately enough.
Hope this provides some food for thought.
The largest Criterion head that I'm aware of that uses 1/2" bars is a DBL-202B (please correct me if I am wrong; I don't know every single head out there). That head has a max capacity of 3" using the outboard hole and 6.687" using the horizontal hole. Given that you're roughing out the hole with drilled holes, I assume you are making that hole large enough to clear the head of the bar that is being held horizontally because that is what you'll need. You have to bore this hole from both sides of the work piece because you only have 3" of quill travel.
Now on to your questions about vibration. Vibration with a boring head is caused primarily by Centripetal Forces. The formula for CF = mv2/r, [EDIT: this should read mv squared/r] where m = mass of the bar, v = speed and r = radius of the bore. You can see that mass is fixed for any given bar so it is the velocity or speed the bar is moving at that has the most influence on CF. Even a little bit of speed has a huge impact on CF so when boring big holes the question is always ... how slow can your mill go while maintaining torque? Note also that as the radius increases, CF goes down so going big is not a major factor. It is the speed that you can maintain and the mass of the mill that will determine how big you can really go.
In real life terms, if the mill is light and not too powerful and cannot achieve a low enough speed then the mill will vibrate. This translates into poor accuracy and finishes and in the worst case the bar may not cut at all.
Now, in order to reduce the effects of CF given the mill you have the best option is obviously to reduce mass to the extent possible and find a speed that will allow the bar you use to cut as effectively as possible. You have to keep in mind that inserted carbide needs speed to cut well and at larger diameters like this, speed is going to cause problems for you due to CF. Not only that but using a steel bar will cause more deflection, especially given the interrupted you will be making.
If this were my project I would find a really good solid carbide LH boring bar from Micro 100 that is long enough to get the job done using the travel of the head. I cannot be sure it will survive the impact but the carbide from Micro 100 is a lot tougher than most and I have yet to break one of these bars, even with interrupted cuts in steel. The geometry is essentially the same as a HSS bar but with the stiffness of carbide so it will cut clean and finish well while still being tolerant of a wide range of speeds.
As I don't know which head you have, I will leave it to you to choose a bar but I would personally not be using an inserted carbide bar.
As for flipping the work piece to bore from the other side, solid clamps in X and Y to reference the work piece should allow you to flip it accurately enough.
Hope this provides some food for thought.
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