# Mill Boring bar cut a taper... how did this happen?



## ErichKeane (Feb 21, 2020)

So, I had a problem last night that I ended up solving another way (just ran an endmill down it), but I found myself very surprised by the result!  I had a piece of brass that I was boring out with a boring head.  I got my top dimension about right (~.800), however when I pulled it out of the mill, the bottom was 50 thou smaller(~.750)!

I cannot for the life of me figure out how this could have happened!  I was cutting fairly small passes, and in brass, and it seemed to be cutting the whole way.  I wasn't particularly careful about placing the material (it needed to be somewhat straight, but even that was subjective), but I would think that would just result in a tilted hole.

I can only think the boring bar could deflect, but I don't see a reason why it would deflect further as the spindle came down (I used spindle feed to cut it).  Any mis-alignment or problem with my mill I would think would just 'tilt' the hole instead of cutting a taper, so I've got no idea!

Thoughts?


----------



## 4ssss (Feb 21, 2020)

The boring bars tend to bend the deeper the hole goes, so you should always run the boring head thru a few times on the same setting, especially on a finish cut.


----------



## Tozguy (Feb 21, 2020)

That much taper makes me wonder if the boring head setting moved during the cut. When the head was retracted out of the work did you notice how much clearance there was at the top?


----------



## ErichKeane (Feb 21, 2020)

I'd run it backwards up through the hole while running, and it didn't cut on the way back up, so I didn't think about that being the problem.



Tozguy said:


> That much taper makes me wonder if the boring head setting moved during the cut. When the head was retracted out of the work did you notice how much clearance there was at the top?



Hmm... thats a good thought, I hadn't though of that. I was taking multiple passes and it didn't seem like it was cutting less than it should on each successive one, but perhaps?


----------



## Mini Cooper S (Feb 21, 2020)

What condition is the quill in? If it is worn the cutting action will push the quill away. The further out that the quill is, the farther it will push away resulting in a tapered hole. This is one reason to take several "free cuts" as 4ssss suggested.


----------



## ErichKeane (Feb 21, 2020)

Its  bit of a run-out mill, an old burke millrite that saw high school duty.  I replaced the bearings a few years ago, and the spindle itself seems to fit nice (and I've never had issues with it extended in the past...), but perhaps that is it?  I was getting a surprising amount of vibration in my cut, so maybe something worked its way lose...


----------



## ericc (Feb 21, 2020)

Sweep the bore with a dial test indicator attached to the spindle.  That will tell the difference right away.


----------



## ErichKeane (Feb 21, 2020)

ericc said:


> Sweep the bore with a dial test indicator attached to the spindle.  That will tell the difference right away.


I'm having trouble visualizing what you mean.  Of the thing I cut?  It is long out of the vice


----------



## John O (Feb 21, 2020)

I used boring bars like these, they cut a taper. Switched to indexable and taper was gone


----------



## ErichKeane (Feb 21, 2020)

John O said:


> I used boring bars like these, they cut a taper. Switched to indexable and taper was gone
> View attachment 314301


Thats exactly what I'm using.  Glad to see it wasn't just me then.  I just can't figure out the mechanism, since the 'arm' of the cutter doesn't change.


----------



## projectnut (Feb 21, 2020)

How old is your boring head, and how is the bar secured?  Heads do wear with age.  Often times if the tool isn't secured tight enough (usually 2 set screws) either the tool will move in the head, or will cause enough vibration that the position of the slide will actually change.

Try making a few more cuts paying close attention to the scale when starting a pass and look again before retracting it for a second pass.  If the scale hasn't moved you could have a problem with the bar moving in the head, or actually flexing.  I know larger heads can withstand cuts as deep as .040" per pass.  However I believe that's way too much for a 2" head.  The maximum depth of cut I would make is .020", (.010" is more reasonable with cheap Chinese made sets) with several passes on the finish cut at a maximum depth of .005".

I would also use the shortest, thickest bar available to minimize the chance of deflection.

As for the mill, if there is runout it will be magnified the farther the quill is extended.  To minimize the runout I would retract the quill as far as possible, and raise the table to bore the hole.  It will take a little practice to get a mirror finish, but it can be done.

I also use the bars pictured in John O's post.  Often times they come from the factory poorly profiled.  I have seen them have little or no cutting edge, tapered in the wrong direction with the cutting edge on the top, and on one set the steel backing bar was extending beyond the carbide.  There are some US and German made sets that are much higher quality.  They do however cost considerably more.


----------



## John O (Feb 21, 2020)

It was a few years ago. I found the reason some where on the internet but can't find it back now.


----------



## ErichKeane (Feb 21, 2020)

projectnut said:


> How old is your boring head, and how is the bar secured?  Heads do wear with age.  Often times if the tool isn't secured tight enough (usually 2 set screws) either the tool will move in the head, or will cause enough vibration that the position of the slide will actually change.
> 
> Try making a few more cuts paying close attention to the scale when starting a pass and look again before retracting it for a second pass.  If the scale hasn't moved you could have a problem with the bar moving in the head, or actually flexing.  I know larger heads can withstand cuts as deep as .040" per pass.  However I believe that's way too much for a 2" head.  The maximum depth of cut I would make is .020", (.010" is more reasonable with cheap Chinese made sets) with several passes on the finish cut at a maximum depth of .005".
> 
> ...



Its pretty old?  Its a Criterion that I bought off a retiring machinist.  I also got the bars off of him, they seem like high quality (not chinesium). I will admit I left a smaller bar in than I could have used, so maybe that is it.


----------



## ericc (Feb 21, 2020)

John O said:


> I used boring bars like these, they cut a taper. Switched to indexable and taper was gone



If the indexable tool cuts parallel, you can probably skip the dti test.  The feed axis is perpendicular.  The problem is deflection of the tool.  Maybe not bending of the shank, but deflection of the entire tool.  This is often caused by incorrect sharpening causing the tool to cut more on the side than the front.  Compare the grinds on the brazed bars with the indexable tool.  There might be a clearance issue on the front of the tool.


----------



## rgray (Feb 21, 2020)

ErichKeane said:


> I was getting a surprising amount of vibration in my cut, so maybe something worked its way lose...



So that is the first problem to address. 
Machine problem? Boring bar dull? Using cutting oil?(cut brass dry or it may chatter) Boring head gib loose?etc.


----------



## f350ca (Feb 21, 2020)

The brazed boring bars are notorious for being poorly ground. I'll almost bet there wasn't sufficient clearance and the bar was rubbing, that will create a taper.

Greg


----------



## mikey (Feb 21, 2020)

I'm going to guess that you might have excessive spindle wear. It sometimes shows up as bell mouthing of a bore. The deeper you go, the more constrained the bar is and the taper is accordingly reduced at the bottom. You won't detect this when measuring static run out because its a dynamic thing but you might be able to see it with some careful measuring and comparing that with the specs of the spindle bore. 

It can also happen if you had a chip or debris in the spindle taper when you inserted the shank of the boring head. It might be a good idea to clean the taper well and see if you can reproduce the bell mouthing. If you are sure the spindle is clean and the shank of the tool is clean and you still have bell mouthing then you may have a spindle wear issue.


----------



## higgite (Feb 21, 2020)

At first glance, .050" seems like a lot of taper, but to give us some perspective, how thick was the piece that you bored? As others have pointed out, the deeper the hole, the more the quill must extend, thereby giving the bar more leverage to deflect the spindle.

Tom


----------



## ErichKeane (Feb 21, 2020)

higgite said:


> At first glance, .050" seems like a lot of taper, but to give us some perspective, how thick was the piece that you bored? As others have pointed out, the deeper the hole, the more the quill must extend, thereby giving the bar more leverage to deflect the spindle.
> 
> Tom


Probably about 3/4 "?


----------



## Tozguy (Feb 21, 2020)

ErichKeane said:


> I will admit I left a smaller bar in than I could have used, so maybe that is it.



Would you elaborate on this please.


----------



## ErichKeane (Feb 21, 2020)

Tozguy said:


> Would you elaborate on this please.


I had a boring bar in the head already, a fairly small one. I only needed to take a bit of material, so I just used that.

I went back today and used a larger boring bar, and went up/down about 6 times and still got the taper, though only about 5 thou this time.


----------



## ErichKeane (Feb 21, 2020)

See the pic, originally used the one on the left. Just swapped to the one in the head after I pulled off the wax (made in India btw).


----------



## Tozguy (Feb 21, 2020)

That's progress. As with any boring bar the shortest thickest one is best.
With those brazed carbide ones we need to grind them for best results, sharp and with sufficient relief.


----------



## mmcmdl (Feb 21, 2020)

How deep a hole were you boring ? Those grinds on the shown bars don't look good in my opinion . Lack of clearance for one . Sounds like you were rubbing the bar shank for that much taper to occur .


----------



## ErichKeane (Feb 21, 2020)

1" deep it ended up being. Hopefully that is all this is  I've not really used a boring head but once before, but don't remember having trouble with that one.


----------



## mmcmdl (Feb 21, 2020)

1" deep in brass ? It should be straight as an arrow . If your bar was flexing or rubbing , you would have heard it when breaking thru the bottom . A rubbing sound vs. a cutting sound .


----------



## Chewy (Feb 21, 2020)

Since we are on this subject!  How about some recommendations for 1/2" indexable boring bars? I have a good 2-1/2" Enco head and have been using the brazed bars.  I did 5/8" & 3/4" holes in 1018,  1 inch deep. Had about a hair over .002 taper. It was for a slip fit on a vise work stop.  The shaft was turned on 1" square 1018 to 3/4" and had a .001 taper over 6". I liked that.  The bore was OK but I would like to do better. 

I need a small bar to handle around a 3/8" hole and then some different lengths to take over from about a 7/16" hole. Can't afford the expensive ones, so what brands/sets are you guys using? Looking for good quality import tooling that uses a common insert. 

Saw this one.  https://www.ebay.com/itm/4-Pc-1-2-I...485788?hash=item1ec767c61c:g:uBYAAOSwVqlaNFW4
Then just add a smaller dia one.  Ideas? Good? Bad? Recommendations on other sets?
Thanks!!  Charles


----------



## mikey (Feb 22, 2020)

Chewy said:


> How about some recommendations for 1/2" indexable boring bars? I need a small bar to handle around a 3/8" hole and then some different lengths to take over from about a 7/16" hole. Can't afford the expensive ones, so what brands/sets are you guys using? Looking for good quality import tooling that uses a common insert.



I don't think I've ever seen an indexable bar with a 1/2" shank that can go into a 3/8" hole, although it very well may be out there. Even if it were, I would suggest you consider a cobalt HSS set instead. It will be cheaper to buy, last for many years, will cut finer and finish better. LMS sells a set. I own Borite cobalt bars and they are just excellent.


----------



## Chewy (Feb 22, 2020)

Mikey, I saw this one. https://www.ebay.com/itm/SHARS-286-...545429?hash=item4621c93615:g:IxgAAOSw47NdUYg2

I can get by with out it but I was wondering if anybody has any experience with it.   Looking for the cobalt ones now.  Only see the carbide ones at this time.

I am working on trying to improve my accuracy and I was thinking about upgrading from the cheap carbide ones I have now.


----------



## mikey (Feb 22, 2020)

You are going to find that inserted carbide used in a boring head has its limitations but I won't go into that here. I own cobalt bars, brazed carbide bars and inserted bars for use in the mill and 99% of the time I use the cobalt bars.


----------



## Chewy (Feb 22, 2020)

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.


----------



## mikey (Feb 22, 2020)

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.


----------



## Technical Ted (Feb 22, 2020)

mmcmdl said:


> How deep a hole were you boring ? Those grinds on the shown bars don't look good in my opinion . Lack of clearance for one . Sounds like you were rubbing the bar shank for that much taper to occur .



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


----------



## Chewy (Feb 22, 2020)

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.


----------



## mikey (Feb 22, 2020)

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.


----------



## Illinoyance (Feb 23, 2020)

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.


----------

