My ancient version of Sprutcam (7) software contains the first version of what the vendor called a 'Rotary Machining' operation. There are some restrictions, but this particular operation was designed to produce g-code with blended moves of three linear and one rotary axis to produce a continuous tool path for machining camshafts and crankshafts. I have respect for any software that can generate 3-D tool paths, but when a rotary axis is included I'm truly humbled. I've been playing with this particular operation on and off for the past several weeks and hoping I could apply it to the complex web peripheries of my Merlin's crankshaft. Most of my problems with this particular operation have been related to the fact that being a first release, it wasn't hardened against a newbie's attempts to use it. The manual for this software has never been one of its strong points, and so I typically learn new features by trial and error. Once I learned where the dark corners were, I stayed out of them, and eventually I was able to get some useable simulation results.
Even though this operation is categorized as a finishing operation, and unlike the other finishing operations in this package, a workpiece can be included in the simulation. In fact, I was able to import the simulation results of my roughing operation into this finishing program for use as a starting workpiece.
The software is capable of working with a variety of end mill types, but this particular operation seems to prefer a ball mill. Because the webs have relatively wide flat surfaces, I spent a lot of time trying to make a 3/8" cylindrical end mill with .060" radius'd corners work. The radius'd corners were helpful in reducing the machining marks left on my test part during some of the complex reverse rotary moves.
Unfortunately, and this may be a common issue with other four-axis packages, a truly flat bottom is assumed for a cylindrical cutter even though nearly all endmills have bottom relief ground into them. I tried using a flat bottom counterbore on a test part, but the chatter was excessive and the surface finish was poor probably because this tool wasn't designed for side cutting. I've read that Sandvik makes a special cutter for this application, but I was unable to locate it. It sure would be useful when I start working on the camshafts.
The cutter relief in a stepped-over rotary operation will create a series of ridges around the machined part instead of a nice flat surface. The simulator isn't much help in tuning out these imperfections since it doesn't understand the cutter relief.
The software allows two choices for orienting the cutter: 1) normal to the cutting surface or (2) through the axis of rotation. My best cylindrical cutter results with acceptable machining times were obtained using a normal orientation and a spiraling .025" step-over. These parameters were finally determined by cutting actual profiles on a test part. The ridges left behind were high enough, though, that they would have to be manually ground or filed away.
I eventually migrated to a 1/2" ball cutter normally oriented to the part's surface with a lead angle of 10 degrees. This lead angle moved the zero SFM center point of the ball cutter off the tool path for a better surface finish. I was initially surprised to discover the ball cutter produced a nicer finish than the radius'd cylindrical cutter for the same step-over. The simulator also properly handled the material removed from the workpiece with the ball cutter. According to my calculations the theoretical scallop size was only .0002", and the machining marks easily polished out with a Scotchbrite pad.
I originally intended to machine the webs in pairs instead of trying to do all of them in a single operation. I ended up, though, machining them one at a time because of the way in which the software handles the rotary roll-over. Each rotary machining operation starts out assuming the rotary is positioned at the part's zero reference position. When the first operation is completed, the rotary has to unwind its accumulated revolutions and return to zero before the second operation can begin. I found it quicker to re-indicate the rotary's starting position myself rather than wait for it to return to its starting point. This also gave me an opportunity to vary some of the operation's parameters so I could learn more about fine tuning it.
The three hour rotary machining went extremely well considering it was my first serious attempt at using it, but I've learned to leave a maximum stock of only .010" or so in the future for it to finish. The operation makes some quick and unexpected moves off its spiral trajectory whenever it sees certain types of nearby cutting opportunities. It will try to clean out deep lateral ditches in a single pass using the feed/speed parameters that were originally selected for a more modest .025" step-over, and the result can be unnerving.
I manually polished out the web machining marks, and then I returned the crankshaft to the lathe where I measured the run-outs of the bearings to get an idea of any mis-alignments created by my new button axes. The run-outs of the main bearings were less than .002" while the run-outs of the bearings on the three crankpin axes ranged from .003" to .004". I was happy with these results given the trauma I had put the workpiece through when I replaced all my original center-drilled end references. I next turned all the bearings to semi-finished dimensions since I was still experimenting with bifurcated turning tools.
Before starting to grind a version of George's HSS tool for turning the bearings, I thought I would give carbide inserts one last try. The insert I previously used with some success was a low-rake no-name import intended for heavy cutting in a big lathe. Since I had just received a 40% off coupon from MSC I went through their catalog looking for an insert designed for 'light finishing' since it would likely have more rake and have less tendency to chatter in my lathe. Because of the long stick-out required to turn the crankpin bearings, I also needed a wide blade holder to minimize flex during side-to-side cutting.
I found a .199" wide Kennametal KC5025 A2 insert (MSC #80757750) and a matching A2 blade (MSC #03266133) that seemed like my best option. I also had to purchase a blade holder (MSC #51018497) for the blade. The blade holder and one of my Aloris-type tool-holders had to be heavily modified to get the insert at the correct height for my lathe. I cut a raked Britnell notch into the insert with a Dremel diamond cut-off wheel. The stock insert came with .010" radius corners which was half what I really wanted, but I decided to live with them rather than try to modify the insert any further. For the bifurcated cutter to work properly, its cutting edge must be perfectly parallel to the lathe's axis. I tried using the broadside of the blade as an indicating surface but found it wasn't sufficiently accurate because of a very slight tilt in the toolholder. Instead, I used George's method of indicating the tips of the insert itself.
I was thrilled with the results. This combination cut with no chatter, and the surface finish appeared polished. This insert cut smoother with hand-applied moly-based cutting oil rather than the synthetic coolant I normally spray from my Micro-drop dispenser. The only issue is with the minimum depth of cut that I can take. I can easily take .010" (dia.) cuts, but the minimum depth of cut is somewhere around .001"-.002" which is limited by the insert's rake. As a result, when I finally finish the bearings I will likely have to polish more material that I had hoped in order to get them all to identical diameters.
The next step is to bring the web thicknesses to their final dimensions so the bearing counterbores can be completed. These two operations must be completed before turning the chamfers on the counterweights. - Terry[ame]https://www.youtube.com/watch?v=8vc_jYkt2eg[/ame]
