Jumping on the bandwagon of 5 axis router builds, this is a build log for my second cnc wood router or possibly a build log of rebuilding my first router. I've been thinking about these issues for at least 6 months now and this first post summarizes a lot of this thinking. I imagine this process will be a long one, and I'll continue to update this thread as I make progress.
For some background, I designed and built a 4'x4'x6" router based on 80/20 extrusions and Ahren's router parts (cncrouterparts.com) about a year ago, finishing in early 2009. I learned a lot of stuff and used it to make quite a few parts but eventually I got tired of several issues with it:
- Adjustability of the z axis bearings - they're a pain to adjust, and they come loose more than I'd like.
- Dirt build up on the flat steel rails - I did not incorporate any dust shielding in my design. Since the rails are flat, the bearings tend to pack down dust that rests on the rails. On my z axis, this eventually led to the stepper motor occasionally stalling. The dust sticks to the steel rails even when vertical (and even using dust collection). I also put the x axis rails below the work surface which leads to them accumulating more dust than they otherwise probably would.
- The plastic anti-backlash nut on the z axis wearing out (although I've since learned I could probably grease this and it would work better).
- The router (Porter Cable 892) being a piece of junk. I took it in for repair, and they found nothing wrong. However it bogs down cutting with sharp bits in MDF at any kind of reasonable depth of cut and draws enough current to trip 15A circuit breakers. I eventually settled on running at 0.125" DOC at 120ipm. I swapped in a 15 year old Dewalt 1.5 HP with the same cutter and it performed vastly better. I think I got a dud router from Porter Cable, but there's not much I can do about it. So it's getting replaced.
- Limited speed due to motor and screw choice. I used 495oz*in motors from Keling with 1/2-10 5 start precision acme screws and a Gecko G540 with a 48V power supply. These motors really need to be geared down more than what is provided by these screws to give a good force vs speed curve, but to do that I would have to add timing belt reductions on the motors. This would be possible, but I figured I'd go for bigger and better at the same time. This makes everything run slow (which is not the worst thing in the world for me as I'm half hobby, half production usage but nothing on a full time basis), but a bigger issue is that I can't get good chip loads for my cutters. This leads to burning of the material and dulling of the cutter on some cuts.
- Lack of closed loop control. I've had several cases where parts were ruined due to the steppers missing steps and the controller (Mach3) not knowing about it. I believe I solved all these issues since then (bad parallel port in my computer and binding issues on the z axis as previously mentioned), but in the future I'd like to have a better chance to save the parts should something go wrong.
In the middle of 2009, I also bought a large number of VFD's (variable frequency drives for running 3 phase motors) and THK linear bearing carriages for almost no money at an industrial auction. I've also recently been learning how to weld.
From all of the above, I decided to upgrade / rebuild my router with the following changes:
- THK linear bearings for all axes
- Covers for all axes (as much as possible)
- X rails above the work surface if a gantry to reduce the moment on the bearings
- High speed 3 phase spindle with VFD
- Possibly dual Z axis slides, one to carry a low speed drilling / milling head and one to carry the high speed spindle
- Ball screws instead of acme screws
- Rack and pinion drive on the X and Y axes to eliminate screw whipping at high speed and provide a better gear ratio
- Servo motors with appropriate gear reduction to give high speed, high acceleration performance (target of 700ipm cutting speed capability and 1000+ ipm rapids with around 1g acceleration)
I also did some analysis of what slows my construction process down the most on the products I have been making. I determined it was sanding out uneven spots on my finished parts (I cut out slices from sheet goods like plywood or MDF and glue them together to form the finished product). There were many other smaller issues that this machine design would attempt to eliminate, but by far the biggest issue was the sanding. One way to reduce that would be to surface the exterior of the cabinet in one operation after it was assembled. This would require a 5 axis machine. Initially I decided not to build a 5 axis machine due to the cost of 5 axis cam software (~$13k for Mastercam for example). My extremely part time side business currently won't support a purchase like that. However, I eventually decided that even some manual programming to rotate the head and machine in different planes would make the 5 axis machine worthwhile, and eventually it would probably make enough money to pay for itself and a 5 axis cam program. So I decided to design and build a 5 axis machine knowing that I won't fully utilize it until some time in the future. Plus it's just cool.
In addition, I plan on starting off this project by quantifying cutting forces through some experimentation. This will give me a design target to shoot for instead of just picking a force that I think will be required to cut. However I did get a bit ahead of myself and already have these parts available for this machine:
- 3.7kW (5HP) water cooled high speed spindle from Chai (linearmotionbearings2008 on ebay)
- Mitsubishi 7.5kW VFD to run the spindle from previously mentioned auction using 240V single phase power (derating the VFD by half per Mitsubishi tech support for running off of single phase power)
- parts to make a water cooling setup for the spindle, mostly from www.crazypc.com
- 20mm x 5mm pitch ball screws with anti backlash nuts and 15mm end blocks from Chai
- X2 milling head spindle from Little Machine Shop: http://www.littlemachineshop.com/pro...ory=-269978449
with a set of R8 collets and a drill chuck
- THK HSR25 bearings and rails (bearings from previously mentioned auction, rails from ebay)
- Fanuc 5s AC servo motors from ebay (~0.9kW continuous output - 5.9N*m of torque, 53N*m of torque max, 2000rpm max speed)
- Fanuc 10s AC servo motors from ebay (~1.8kW continuous output - 12N*m of torque, 78N*m of torque max, 2000rpm max speed), some including brakes
- 15kVA transformer from ebay for the power supply
I've also been thinking about the C and A axes drive mechanisms. So far it is looking like a worm gear setup with manually adjustable mesh clearance might work. I'll rotate the head to distribute wear evenly around the gear for longer life. See this thread for more info:
http://www.cnczone.com/forums/showthread.php?p=737050
However I also have a Thomson Accutrue 100:1 planetary gear head and 2 harmonic drive 100:1 gear heads that will support the torques I am interested in (over 300N*m at the output). The harmonic drives should have very low backlash, but I will test them and see. The Accutrue should have more backlash, but it might have an acceptable amount. I'll also have to test that and see how it performs.
I am currently working on writing an Excel sheet with calculations for various parts of the router. I'm also thinking about different form factors than the typical moving gantry. If I want more z travel than the typical 6"-12" or so, I will probably move away from the moving gantry. One possibility would be a fixed gantry moving table design. However some quick calculations show that my part height will still be limited to 2' or less depending on the exact height of the ceiling in my garage due to the requirement to get a long tool above the entire part and the extra vertical space that takes. (I am moving in a few months, so this question along with other experimenting will have to wait until then). An option to get more z travel given a fixed ceiling height is a moving y axis design like the Tarus claymills. This would get me up to a 3'-4' part height depending on several factors. However some quick calculations around the THK HSR25 bearing show that some serious consideration would have to be given to minimizing forces on the bearings if I want to get any kind of reasonable life out of them. It looks like I would need bearings on both sides of the y and z axes assemblies unlike what the Tarus machines look like. A sketch I did today of an idea for this style of machine is attached below. It would use all rack and pinion drives. The spindle is a bit out of proportion to the rest of the machine.
I've also been thinking about construction methods for this machine. I plan on using steel for framing members and using some combination of pins, bolts and welding for joining parts together. My primary concern is how to get a flat, plane surface for mounting the linear rails. I don't want to haul a massive frame to a surface grinder. I also don't relish the thought of long hours with a machinist's level and a scraper or grinder and a surface plate. Because of all this, epoxy surface plates were very interesting to me, but after some calculations I decided they were not suited to the task of supporting linear rails due to issues of static deflection under the weight of the moving components, creep (deflection over time), and differing coefficients of thermal expansion between the epoxy and the steel it would be adhered to. However, it would be a good surface to mount linear rails to and mount a surface grinding wheel to. This wheel could then be moved back and forth over the rail mounting surface and the finished rail surface height set relative to an epoxy surface plate poured around this surface. The epoxy surface plate will connect the different sides of the machine together (in whatever configuration that might end up being) in order to allow two planar rail mounting surfaces to be created.