I’ve thought a lot about what would be the subject of my next post…I was in between a technical subject or one about the industry. I decided today to pick the first kind.
I really want to see CAMZone.org as an attractive source of ideas and knowledge about CAM… in fact, although what I write here may potentially reach thousands of people (We got 2200 views in the last four days – Thanks!), I essentially think in the posts pleasing myself – Don’t take me wrong – I’m very exigent with myself.
In the recent years, I was blessed with many opportunities to work with the latest technologies in machine tools and systems, and as I started to deal with more complex scenarios, I started to notice that certain things in the CAD/CAM world are confined and treated as “secrets”, and as such, they are not being shared. A pity.
I think in regards what I said above, the cause is more about a generalized lack of time and technical people with machining background working on CAD/CAM marketing than anything else. I don’t think it’s because these technologies are related to trade secrets as many may think… just one man’s opinion.
Our post today is about a recent possibility in the machining world – B Axis turning
To make sure we are in the same page, please check on the video below what I refer as “Continuous” or “Live” B axis Turning:
Cool isn’t it?
Let’s try to understand what is behind this looking from the mechanical / physical standpoint:
These are by nature multiaxis (5 axis) machines, and as such, they can track certain control points based on certain things that are taken for granted. Modern multiaxis machines use a functionality called TCPM, what stands for Tool Center Point Management. You may find this under the names TCP, RTCP,RPCP… they are pretty much the same thing…but I think it’s worth to explain each in details (That’s why I started CAMZone.org right? )
I’d like to deviate a bit to explain the TCPM functions in depth. This will help you to understand how these machines track the tool tip location and serve as the basis of future posts about multiaxis machining… so please bear with me… it’s worth…
TCP – It is a generic term like TCPM – It’s TCPM short version…
RTCP – Stands for Rotary Tool Control Point – Some may call it Rotary Tool Center Point too.
RTCP it’s mostly used when in the kinematic model of the multiaxis machine the rotary axes are on the tool end… that is, head-head machines, robots, wrist type 5 axis configurations…
Because the tool moves around and the part remains static, they say that the tool tip is being tracked, and therefore, RTCP is used. This is useful for machines designed to handle big and heavy parts, where it turns out to be impracticable to move a heavy / big part… it’s smarter to move the tool instead…
A good explanation of RTCP applied in a head-head 5 axis machine is shown in this post from Sescoi CAM blog. I took the liberty to share their video below as it is a very good visual example of how RTCP works. I recommend you to read the full article to get some tips about how to check the accuracy and the calibration/tuning of your 5 axis machine. Thanks JJAJE for the great material!
RPCP – Stands for Rotary Part Control Point – Some may call it Rotary Part Center Point too.
Although very similar to RTCP, RPCP is used when the part performs the rotational moves and the tool moves along linear axes such as X,Y and Z…
In this video you can see the work of the RPCP rapidly tracking the tool tip location in real-time as the table moves… this demand a huge processing power from the CNC… usually, modern CNCs have dedicated processors to handle this… this computation is called kinematic transformation and it’s studied by a field of computational physics named Inverse Kinematics
Now that we know all these things, we can say that TCPM is a complex set of actions that allows the CNC control to track the location of the tool and the work shifts in real-time and compensate the rotary / linear axes accordingly so that the part / tool tip location / orientation is always kept as computed by the CAM system. It’s a software/firmware thing on the control side.
Thanks for your patience to get here – Now I can get back to the B axis turning subject because we are in the same page about what is TCPM.
Below you can see the picture of a typical B axis unit found on a MillTurn machine – These units are also called by the machine vendors as “Turning Boring Milling Units” or simply “TBM Units”.
When the machine is assembled and adjusted, the machine builder measures it with special equipment and enters in the control parameters representing the distances from the B axis pivot point location to the machine reference point (G53) , the distance from the pivot point to the spindle gage point… by knowing these (And many others) parameters, the control can then track the tool tip location for a given position of the B axis… however, in order to be able to deal with heavy cuts, as any mechanical system, the B axis needs to be clamped – And to do so usually they use Hirth couplings.
I think you understand that as any mechanical component, when the axis is clamped the values may differ from the values gotten when the axis is loosen… and the machine PLC tells the kinematic transformation / tip compensation cycles in which situation the B axis is and the cycle then uses the proper parameter into account in the transformation equations… yes… there’s one value to be used as the gage distance when B is clamped and another when it is not… freak stuff…
Why I’m talking about this: Because you need to know that the B axis units are very powerful resources in a MillTurn machine once you know their limitations and how they work. It’s not about workarounding anything, but instead, applying the correct technique for the given task.
Pitfalls of B axis turning in the current CAM systems:
Many CAM vendors will tell you they can do it (Live B axis Turning). But what many will offer you is a 5 axis milling sequence to machine a curve with a ball mill. This tool will have to have the zero point set in the center of the ball tip, and they will machine this curve (Which is drawn at the Y0 – XZ Plane) using a 5 axis milling toolpath… then with heavy post-processor mods it will mimic a turning operation. To do this, you will have to set the control point of you turning tool to be in the center of the turning insert too… as shown below:
This is usualy called control point #9. Just a few CAM systems are dealing with this now when it comes to live B axis turning and the usage of control points other than #9 in B axis units. This is because the CAM system (And not the machines) CAN’T track the location of the tool tip when the B axis angle is other than 0. If you do not use position #9 with these systems, your CAM will generate wrong coordinates in the tape file (NC Program) that will give you nothing but scrap and tears.
Drawbacks of this approach:
- Poor surface finish – It’s a point to point toolpath right? – A multiaxis one… large toolpath tolerances on the CAM system won’t help either…
- You cannot use cutter compensation (G41/G42) because of the reason above.
- Huge programs – Impossible to edit or change without a CAM system.
- Huge processor usage on the CNC side – If you have an anti-collision system on the control it can turn into a non-smooth cut because the control is using too much memory during the operation due to the multiaxis toolpath… the machine may become very sloooow…
- The post-processor mods can cost you an arm and a leg.
- Poor control over the feedrates and the execution on the machine – Single block mode will make it even worse, trust me.
- If you are expecting to hold a tight tolerance (0.05mm), you can forget it.
- Premature tool wear due to the reasons listed above…
Keep away from this crap as much as you can. This approach is not real B axis turning – It’s in fact a nasty workaround suggested by CAM vendors that are not offering a true solution yet. By the way: Don’t buy the small talk about using “Compressors” (Cycles that change the Jerk/Acceleration settings of the control) on the program. They don’t make miracles.
Frankly, currently, you can forget about holding tight tolerances (< 0.02mm) with live B axis turning even with the correct approach. It’s a mechanical limitation – Not a CAM fault. I think the machines will evolve in this direction though.
The ideal CAD/CAM system to program the modern MillTurns is one that among many things, allows you to work with common control points like #3 in the B axis unit. I would say that 98% (Where are these numbers taken from? ) of the current systems in the market don’t do this now. We need more R&D in CAM guys… We need to push the vendors to get it…
The only ones I’m aware of that currently do this efficiently are Esprit and TopSolid CAM and Delcam Feature CAM. Gibbscam released this functionality (Live B axis turning) in their latest release, but I don’t know if it can work with control points other than #9. It probably does it – I like to think that Bill Gibbs is a very smart dude
If you could reach this article before buying a MillTurn machine, then it will be great. Most CAM sytems will force you to work with control point #9 and this will create you some issues that are hard to revert later depending on how many tools sets you have created for your machine. If you can afford to get a system capable of using all control points, preferably with “Live” B axis turning, then is better.
Note: Although Unigraphics NX is not able to use “live” B axis turning yet, they included support for tilted (Static) B axis turning in NX 7.5. This means they do support control points other than #9 now. This is a good start and better than nothing. At least you won’t have to turn your tool tip measurement methods up side down – You can preserve them.
The correct approach:
Currently I do not work with any system that is able to program “Live B axis Turning” toolpaths. Like many, I’m workarounding the limitations of my current system and writing code by hand. So I can’t post any pics of a system here – I also kindly ask you to do not ask for more examples of code for obvious reasons.
I can share, though, an example of the ideal code – It is like the one shown below – WFL MillTurns with Sinumerik controls offers this clean and elegant solution:
N30 G96 M4 S50 ;SET SPEEDS
N40 CYCLE_UNCLAMP_B ;MACHINE BUILDER CYCLE TO UNCLAMP B TO ALLOW CONTOURING
N50 G00 X500 Y0 Z1600;MOVE TO START POSITION
N60 CYCLE_TURN_B_ON ;MACHINE BUILDER CYCLE TO ACTIVATE KINEMATIC TRANSFORMATION (TCPM) IN B AXIS TURNING
N70 G1 G42 X100 Z1000 B0 F20 ;ACTIVATES CUTTER COMPENSATION AS A NORMAL TURNING OPERATION – STARTS AT B0
N80 G1 Z950 B-45 ;MOVES Z TO 950 AND INTERPOLATES THE MOVE SO THAT AT Z950 THE B AXIS IS AT -45 – IT STARTED AT B0
N90 G1 X200 B-90 ;SAME HERE
N100 G1 Z900 B45 ;SAME HERE
N105 G3 X150 Z875 I25 K50 B30 ;AN EXAMPLE OF THE SAME USING CIRCULAR INTERPOLATION – AT THE FINAL ARC COORDINATES IT WILL BE AT B30 – IT STARTED AT B45
N110 G1 X100 B22.5 ;SAME HERE
N120 G1 Z850 B0 ;SAME HERE
N130 G0 G40 X200 Z800 ;CANCELS CUTTER COMPENSATION ON MOVE OFF THE PART
N140 CYCLE_TURN_B_OFF ;DEACTIVATES TCPM – YOU HAVE A NORMAL LATHE NOW
Drawbacks of this approach:
- It can’t hold tight tolerances (<0.02mm) – Mechanical limitations related to simultaneous multiaxis machining… every machine has its limits while on the multiaxis mode… specially big ones. This is normal. Don’t curse your machine vendor. Learn the right techniques to do the job and apply them where appropriate.
All other problems from the first example are not present in this method. It works very well with the minimal code possible and the best condition for the CNC control and the anti-collision system, if available.
A good solution is: during the B axis contouring, you can disengage the tool from the part, activate the B axis clamping mechanism, finish the feature with the tight tolerance, disengage again, unclamp the b axis, activate the TCPM mode and conclude the profiling of the not so tolerated features with live B axis turning again. The air cut is minimal. Be reasonable about this – Some years ago it was not even possible.
It’s not only about the mechanics of the equipment. Even the roundness of the cutting insert plays a key role here. Carbide inserts are often obtained by the shrinking process, and although their accuracy has improved dramatically along the last decades, only special inserts are grinded to have a radius with a perfect accuracy. If the radius of the insert is not perfect, the diameter also won’t be perfect along the entire B axis move because the control takes into account the current position of the tip considering a perfect theoretical radius informed on the tool register table. So part of the problem is the tool and part the machine.
Needless to say that due to the mentioned above you should not use inserts with Wiper geometry in B axis turning, otherwise the problem will be greatly magnified.
Well, my final thoughts on this post is that I think this is becoming a very popular technology and it’s up to the companies that are under a maintenance contract with their CAM vendors to push to get this technology in their platforms. CAM companies are driven by pressure and if you do not fight for your hard-earned maintenance dollar, your CAM vendor won’t do it for you.
I hope this post can help to elucidate some doubts and myths about B axis turning. If you liked it, pass it along! I invite you to subscribe to CAMZone.org to be informed about new posts.
Thanks for getting here and to Phil Collins that helped me in this post.
Best of luck!