Boring Head-Bar Set-up

[brasssmanget]

I did some work with a boring bar a few months ago and have been wondering ever since if I had done it and set up correctly. I managed to muddle through it, but there were times I thought I was going to break something. These pics help - thanks.

 

[bjmh46]

Scrapmetal and barrydc1,

I hope Mikey's pics cleared that up for you. No contradiction in my statements--I was referring specifically to the common far east carbide tipped bars, and describing the two possible setup conditions in a boring head. With the brazed tip bars, as purchased, proper setup in a boring head is not possible. You CAN achieve centerline setup in the lathe, because tool centerline setup is independent of spindle centerline. That is not so with a boring head. Every boring head I've seen has the tool hole on a radial centerline of the spindle when installed.

Take a new Chinese carbide tipped bar, say 1/2" shank, and lay it on a flat surface. Now take a 1/4"parallel, and lay it on the surface just in front of the business end of the tool. Now roll the tool til the top of the carbide is parallel to the parallel, and you will see how far above center the cutting edge will be if installed in the boring head where you've eyeballed the cutting edge parallel to the boring head slide motion. You can also roll the tool so that the cutting point is at the same height as the 1/4" parallel, and you will see how much negative radial rake the tool will effectively have if installed like this.

I've always gotten my best boring head results using tools with neutral radial rake, and positive to very positive axial rake, depending on the material. Sharpen to this geometry and a sharp point, then hone a very small corner radius on the tool. As far as DOC goes, you just have to figure this out for yourself. Every setup and tool/workpiece interaction, sharpness, feedrate (hand or power) is different. I will say that a 2" long 1/2" bar will require a very light touch in steel!

Sorry if I confused anyone with my previous post.

Regards
Bob

 

[mikey]

Bob, my experience with Chinese boring bar sets is limited to the single $10.00 set I own (the only Chinese tool I own) but these bars have brazed inserts that are ground to the centerline of their shanks and can indeed be set up correctly in a boring head. They are not as accurate as my Borite or Micro100 bars but I suspect that is due to the cheap steel and less than accurate tempering of the bars but they can rough with the best of them.

Randy, if you want to use the DOC adjuster of your boring head as intended, and if your boring bars have zero axial rake like most bars do, then the flat face of the cutter must be oriented along the centerline of the bore. If you purposely turn the bar to introduce some rake then the DOC adjustment will be off, which must then be accommodated. Normally, I only do this in small bores in an attempt to reduce tangential forces so I don't run into clearance issues; it works. Most of the time, I just try to make sure the spindle is centered on the bore and then orient the face of the cutter parallel to the travel of the head.

My boring head is a Criterion square head. The sliding part of the head is milled fairly square and I use a combination square with the base pressed against the head and the end of the sliding blade is brought up against the cutting face of the boring bar to get it aligned with the head. If I have a deep bore then I use a ground parallel held against the sliding part of the head to get the combination square down enough so the blade can orient the face of the cutter. Does that make sense?

While aligning the cutter as above is not totally accurate it is close enough for my purposes. I do check to be sure that when I dial in a 0.020" adjustment the bore increases by 0.020". Most of the time this is exactly what happens so it works for me.

One tip that may come in handy someday is to remember that cutting speed can be used to fine tune the bore. As the RPM increases the bar will cut deeper due to centrifugal forces at the end of the cutter. If you are close but just under the desired bore size, try increasing speed a little instead of adjusting the DOC and make another pass. Quite often, just a 25-50 RPM increase can give you that tiny bit you need to come in on size.

 

[ScrapMetal]

Scrapmetal and barrydc1,

I hope Mikey's pics cleared that up for you. No contradiction in my statements--I was referring specifically to the common far east carbide tipped bars, and describing the two possible setup conditions in a boring head. With the brazed tip bars, as purchased, proper setup in a boring head is not possible. You CAN achieve centerline setup in the lathe, because tool centerline setup is independent of spindle centerline. That is not so with a boring head. Every boring head I've seen has the tool hole on a radial centerline of the spindle when installed.

Take a new Chinese carbide tipped bar, say 1/2" shank, and lay it on a flat surface. Now take a 1/4"parallel, and lay it on the surface just in front of the business end of the tool. Now roll the tool til the top of the carbide is parallel to the parallel, and you will see how far above center the cutting edge will be if installed in the boring head where you've eyeballed the cutting edge parallel to the boring head slide motion. You can also roll the tool so that the cutting point is at the same height as the 1/4" parallel, and you will see how much negative radial rake the tool will effectively have if installed like this.

I've always gotten my best boring head results using tools with neutral radial rake, and positive to very positive axial rake, depending on the material. Sharpen to this geometry and a sharp point, then hone a very small corner radius on the tool. As far as DOC goes, you just have to figure this out for yourself. Every setup and tool/workpiece interaction, sharpness, feedrate (hand or power) is different. I will say that a 2" long 1/2" bar will require a very light touch in steel!

Sorry if I confused anyone with my previous post.

Regards
Bob

That cleared it up very well. (FWIW, I already had a real good handle on how to set up a boring bar but I'm always willing to listen to those who most likely know more than me. :biggrin

Thanks much Bob. :thumbzup:

-Ron

 

[RandyM]

Bob, my experience with Chinese boring bar sets is limited to the single $10.00 set I own (the only Chinese tool I own) but these bars have brazed inserts that are ground to the centerline of their shanks and can indeed be set up correctly in a boring head. They are not as accurate as my Borite or Micro100 bars but I suspect that is due to the cheap steel and less than accurate tempering of the bars but they can rough with the best of them.

Randy, if you want to use the DOC adjuster of your boring head as intended, and if your boring bars have zero axial rake like most bars do, then the flat face of the cutter must be oriented along the centerline of the bore. If you purposely turn the bar to introduce some rake then the DOC adjustment will be off, which must then be accommodated. Normally, I only do this in small bores in an attempt to reduce tangential forces so I don't run into clearance issues; it works. Most of the time, I just try to make sure the spindle is centered on the bore and then orient the face of the cutter parallel to the travel of the head.

My boring head is a Criterion square head. The sliding part of the head is milled fairly square and I use a combination square with the base pressed against the head and the end of the sliding blade is brought up against the cutting face of the boring bar to get it aligned with the head. If I have a deep bore then I use a ground parallel held against the sliding part of the head to get the combination square down enough so the blade can orient the face of the cutter. Does that make sense?

While aligning the cutter as above is not totally accurate it is close enough for my purposes. I do check to be sure that when I dial in a 0.020" adjustment the bore increases by 0.020". Most of the time this is exactly what happens so it works for me.

One tip that may come in handy someday is to remember that cutting speed can be used to fine tune the bore. As the RPM increases the bar will cut deeper due to centrifugal forces at the end of the cutter. If you are close but just under the desired bore size, try increasing speed a little instead of adjusting the DOC and make another pass. Quite often, just a 25-50 RPM increase can give you that tiny bit you need to come in on size.

WOW! Thanks Bob and Mikey.

Mikey this is exactly what I was looking for. Very nicely done. THANK YOU!

Doesn't look like many use it to do OD's though.

 

[barrydc1]

I'm with the others who say this is a great forum. Those explanations are exactly what I needed as well. Thanks not only for the clarifications, but the tips and total technique. I am untrained and self-taught, and so much of the things I do are done from watching a video or pictures that didn't have a lot of clarification. If I get some time I may try to do this with a video or pictures and then have some of you check my work.

One more question. If I start out boring a hole on the mill by centerdrilling, drilling in steps to 1/2 inch or whatever, and then put the boring head in the spindle, can I assume (if my mill is in good shape and a proper tool used) that the cutting tool axial center will be line with the the bore centerline? Then shouldn't the only issue be rotating the tool in the boring head to get the cutting face perpendicular as described above? Am I understanding this right? If I'm not, I'm going to need pictures.

 

[mikey]

You are correct - you'll be exactly on the bore centerline after drilling as long as you do not move the X-Y position after changing to the boring head. This is what most folks do, I suspect, when it is possible. It can be a bit more work to get centered when boring a hole you didn't drill; here, you must indicate it in.

I totally forgot to address the DOC question you had. As far as I am aware, there is no formula for determining DOC for a boring cut. Every mill differs in its power and rigidity capabilities, and some boring heads and boring bars are better than others. My suggestion is to practice with your particular set up before doing an important project.

Every bar likes to cut a certain way, even within a set of bars, so you need to know how much that bar will take off for a given setting. I usually do a trial cut with, say a 0.020" cut and see how the cut goes. If it chatters a lot I may reduce the DOC until it cuts freely. Each material has different requirements and I have notes on what speed/feed/doc works for all common materials with a given boring bar set on my mill. I got that by practicing as above. Note that roughing and finishing DOC differs in that as the DOC falls below the size of the nose radius, tangential and radial forces increase so what you dial in is not what you get. You need to do trial runs until you find out how your bars like to cut.

I'll give you an example. One of my best Borite cobalt bars likes to rough aluminum at 0.015" DOC; here, it runs clean and smooth and can do it accurately all day. I know that this bar will also produce a 0.004" increase in ID if I dial in 0.0045" on a finish pass so I rough until I get to a point where I can dial in 1-3 finish passes to come in on size. This is how I do it and it works for me. I'm sure others will differ but the bottom line is that you need to find out how your mill/head/bars like to cut and make note of it.

One more point: be sure you can reliably measure a bore before you try tuning your set up. Trust me; even a seasoned guy can mess up with measurement and blow the bore because of it. Buy a XXX ring gage or two and learn how to use your tools so you can trust your measurements.

With all that said, don't worry, Barry. Most of us hobby guys have no formal training. We learn by standing on the shoulders of all those that went before us. That, and screwing up enough until we figure it out!

 

[mikey]

Randy, I have not used the boring head to cut a boss - sorry to be useless here. However, I suspect that the set up would be fairly simple. If I were going to try it I would orient the cutter face so the cutting point is toward the center and then slowly feed in. I should think this would work okay. The only thing that concerns me is the backlash in the Z-axis, which might just be enough to crash into the shoulder of the work. Granted, a boring bar doesn't self-feed like an end mill does so this may not be a problem but it would worry me some.

 

[bjmh46]

Boss turning should be approached with great caution if you need to run the spindle in reverse with a threaded shank boring head. With a left-hand tool, and running in the normal direction, no problem.

Regards
Bob

 

[barrydc1]

I totally forgot to address the DOC question you had. As far as I am aware, there is no formula for determining DOC for a boring cut. Every mill differs in its power and rigidity capabilities, and some boring heads and boring bars are better than others. My suggestion is to practice with your particular set up before doing an important project.

Every bar likes to cut a certain way, even within a set of bars, so you need to know how much that bar will take off for a given setting. I usually do a trial cut with, say a 0.020" cut and see how the cut goes. If it chatters a lot I may reduce the DOC until it cuts freely. Each material has different requirements and I have notes on what speed/feed/doc works for all common materials with a given boring bar set on my mill. I got that by practicing as above. Note that roughing and finishing DOC differs in that as the DOC falls below the size of the nose radius, tangential and radial forces increase so what you dial in is not what you get. You need to do trial runs until you find out how your bars like to cut.

With all that said, don't worry, Barry. Most of us hobby guys have no formal training. We learn by standing on the shoulders of all those that went before us. That, and screwing up enough until we figure it out!

Thanks for answering those questions, that is about how I understand it as well. I do have the means to indicate in a hole that I didn't make, so that is also as I suspected. I have been trial and erroring my boring a bit. Unfortunately, I was trying to bore a hole the other day and after 3 attempts still kept getting it too large by about. 001 or 2, but it was round. This was in mild steel with my chinese carbide, but it may well have been due to the issue you mention above and radiuses. The hole was supposed to be 1.002 +/- .0005. Anyway I gave up after 3 holes and need to get back to it with this new info.

Also you mentioned measuring. I am using a telescoping bore gage and calipers to measure that unless I need finer measurement and then I use the micrometer. What is the XXX ring gage you mention?