Is this the set? https://littlemachineshop.com/products/product_view.php?ProductID=2682&category=-1134493617 What sre the limitations? I thought from posts above that it would be an improvement.
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Mill Boring bar cut a taper... how did this happen?
- Thread starter ErichKeane
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- Dec 20, 2012
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Yeah, that's the set I saw.
Inserted carbide needs speed to finish well. At small diameters, no problem. At larger diameters, centripetal forces really come into play. Say you need a relatively large bore but need a good finish for a bearing or a precision slip fit. In those cases, finish becomes really important so you need to get your speed up, right? Since centripetal forces vary with the square of the velocity, the faster you have to go to get the finish you need, the greater the centripetal force becomes and then you're chasing your mill down the driveway as it vibrates away.
Not only that but you must also contend with the nose radius on inserted tools so it gets tricky when you have to hold tight tolerances. You have to know how to account for the impact of radial forces with inserted tools. You can do it but it requires some skill. This is much less of an issue with cobalt bars; you will find that as long as you do not change speeds, what you dial in is pretty much what you can expect.
Boring, either on the lathe or mill, is not the simple thing most hobbyists think it is. There is a lot more to it than meets the eye. Make a hole, yeah, no problem. Make a hole to actually fit something ... maybe not so easy. Boring on the mill produces the same three cutting forces all boring bars produce but now we have centripetal forces to contend with, too, and that is a whole other ball of wax. Anything you can do to make the cut more predictable is a good thing and I've found that cobalt bars go a long way to help with that.
Inserted carbide needs speed to finish well. At small diameters, no problem. At larger diameters, centripetal forces really come into play. Say you need a relatively large bore but need a good finish for a bearing or a precision slip fit. In those cases, finish becomes really important so you need to get your speed up, right? Since centripetal forces vary with the square of the velocity, the faster you have to go to get the finish you need, the greater the centripetal force becomes and then you're chasing your mill down the driveway as it vibrates away.
Not only that but you must also contend with the nose radius on inserted tools so it gets tricky when you have to hold tight tolerances. You have to know how to account for the impact of radial forces with inserted tools. You can do it but it requires some skill. This is much less of an issue with cobalt bars; you will find that as long as you do not change speeds, what you dial in is pretty much what you can expect.
Boring, either on the lathe or mill, is not the simple thing most hobbyists think it is. There is a lot more to it than meets the eye. Make a hole, yeah, no problem. Make a hole to actually fit something ... maybe not so easy. Boring on the mill produces the same three cutting forces all boring bars produce but now we have centripetal forces to contend with, too, and that is a whole other ball of wax. Anything you can do to make the cut more predictable is a good thing and I've found that cobalt bars go a long way to help with that.
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I agree to check for lack of clearance or the bar just plain rubbing as you enter the hole. Cover the bar with Dykem (layout dye) and run the bar though with a very light cut. Check the bar and look for signs of contact. You should also probably just be able to feel a tapered step in your bore with your finger.
Ted
Mikey, that makes sense. I do know that carbide needs speed and DOC to work well. I am thinking along the lines of getting both. Use the indexable to replace brazed carbide on the small holes. like less than 1-1/2" and get the cobalt for when I need accuracy or larger holes. Most of the work jigs I make, if I'm in around .002, I am fine. But there is going to be some stuff like threading and slip fits, that I would like a better more uniform hole.
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On a boring head, most feed dials are in 0.001" increments, right? As you know, the depth of cut an insert can take depends on the nose radius and, at minimum, that will be about 1/2 the nose radius. So, just like on the lathe, you must know what the minimum effective cut your insert requires and know that it will reliably cut it at the speed you happen to be running at. I've seen it said that you can take a 0.001" depth of cut with an inserted tool in a boring head. Well, you have to show me that before I'll swallow it because the behavior of an insert won't allow that. In contrast, a sharp cobalt bar will so yes, maybe get both and run tests so you know how they must be used. Then you'll see why I prefer cobalt the vast majority of the time.
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Most import brazed carbide bars don't work well right out of the box. It is necessary to grind proper relief on the side and end before they cut properly. I suspect that was the cause of your tapered hole.