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View Full Version : Need help w/ early K2 prototype



Phil Thien
07-30-2008, 2:30 PM
In 6/2003, I purchased an early prototype of a desktop K2 CNC machine. I used it a little early on for making a few very simple parts that I suppose I could have made other ways. I was initially surprised that the machine had a tendency to stall (at as little as 30 to 36-IPM), so I haven't used it since.


I FIRST WANT TO SAY THAT THIS IS NO INDICTMENT OF K2CNC. I AM A NEWBIE and for all intents and purposes, I think people should assume that I don't know what the heck I'm doing. FURTHER, this machine was a prototype, and I only paid $700 for it (no controller, but did include steppers).


So I've been reading and decided to drag it out again and investigate ways to fix/improve it.


It initially shipped with white plastic nuts on what I guess are Acme screws. The seller also included brass nuts separately.


In the last couple of days, I've replaced the X and Y-axis nuts with the brass ones. I found that I can now drive faster (no stalling at 36-IPM, can often get away with 48-IPM, and can even go for a little while at 72-IPM before stalling).


The Y-axis had quite a bit of grease on the screw and in the nut. It was kinda hard to turn on the screw. The X-axis had very little grease or lubricant, and the nut was easier to spin but noticeably kinda chattery (if that is a word). I guess both plastic nuts caused some chatter or vibration that the brass ones don't seem to exhibit.


I've learned that geometry is critical. After replacing the Y-axis nut I noticed that it stalled at very low speeds (10-IPM). I loosened up all the machine screws that I had to remove, jogged it around a little, then tightened them up. A HUGE improvement. I think I still have some work to do in that regard.


Perhaps shimming nut "brackets" would be helpful. Anyone ever have to shim?


The nuts don't seem to have any backlash feature to them, and I'm not sure that I'd be able to add anything like that as there isn't much clearance.


The machine uses 1/2-10 screws/nuts. The nuts have a .70 outer diameter, a .98" flange diameter, and are .74" long.


I've included two movies of the unit under power. You can hear that it does make some resonating noises under "normal" operation. You can also hear it stall in the second video.


I've also included a couple of pictures of the nuts/screws.


I'm using a 3-axis Xylotex controller with PACSCI 116oz/in steppers. The unit originally came with steppers that were larger, but I got new steppers w/ the Xylotec unit and I'm afraid one of the things I've done it put smaller steppers on where I need larger ones. Like an idiot, I sold the old steppers on the bay a long time ago. If this is a case where the larger steps would make all my problems go away, I sure am going to feel like an idiot (again).


My reason for posting this is that I'm hoping some experienced users can provide some pointers on things I can do to improve the unit. Any helpful advice would be appreciated.


I'd like to get the speed up to whatever a 10-TPI screw machine should run at with steppers. If possible, I'd like to minimize backlash (although I haven't used the machine enough for backlash to be a problem, I'm trying to foresee the future). Any and all tips/pointers for these or other matters would be helpful.


I plan to use the machine to cut small plastic and wood parts, and perhaps aluminum some day, too.


Many thanks for any and all help.


A link of the unit working "normally":
YouTube - k2_proto_operating (http://www.youtube.com/watch?v=0pb3X8rK-pU)


A link of the unit stalling:
YouTube - k2_proto_stalling (http://www.youtube.com/watch?v=D8BPpZCpVpg)

Rob Wright
07-30-2008, 8:56 PM
Phil - I will preface this with "I am not an expert"- just have two home built's.

You did a good thing in keeping things loose and jog the machine around to find the sweet spot, then tighten everything down. That's the only way that I have been able to align my machines. It takes a little while but worth it. I have also shimmed the lead nuts to get them normal to the lead screw.

Are you running Mach3 for machine operation? There is a backlash feature that can be enabled to help with any problems that you may be having. I look at this as a last resort. If the plastic nuts were"chattery" this was the backlash that you were experiencing. I can't believe that the brass nuts don't exhibit as much as the plastic ones - I would think that they would be pretty tight to have that happen.

Anti-backlash lead nuts can be bought for $25 each from www.dumpstercnc.com (http://www.dumpstercnc.com) . He will most likely be able to help you with getting one that will fit the housing. I run his on mine and are worth every penny. I am sure that you could cut his down to make it work if he can't/won't do it for you. They are made of Delrin - self lubricating - so no lithium grease needed.


116 ox steppers - WOW - kinda small. Yeah, that might be a problem for you. From my limited understanding of steppers, they loose torque pretty fast at higher RPM's depending on how they are wired. I know that on my first system, my 270's would stall at 40ipm pretty regularly with a 16tpi lead screws. I had major alignment problems to say the least.

Just watched the 2nd video - I noticed that it stalled near the end of an axis, near an alignment block for the screw, high rpm's on the stepper, low torque available, and misalignment.

Is your acceleration set too high? I have mine set at 5 and have not had any problems on either machine now that I am there. Also, I have the xylotex controller and I have heard that it is prone to resonance issues at speed. Go over to CNCZone dot com , there is a lot of discussion with that issue.

Hope that helps - PM me if you have any other questions - I am a newbie at it but have had a great time messing with this stuff over the last 2 years!

-Rob

Phil Thien
08-01-2008, 12:29 AM
Thanks for the help Rob. I worked with the machine some more this afternoon and did more reading and I'm coming to the conclusion that I should be happy w/ approx. 50-IPM with 10-TPI threads. And I'm not unhappy with the machine, I just want it to work like it is supposed to.

Anyway, I've been dreaming up a method to reduce the transfer of vibration from the steppers to the machine. I have a few more parts to pickup tomorrow. I'll post some results when I'm all done.

I plan on ordering some new steppers, and I'll look into the anti-backlash nuts.

Thanks again.

Rob Wright
08-01-2008, 2:12 PM
Phil, I don't know what the ends of those acme rods are held with on the K2, but my new machine has 1/2", 5-start, 2 turns per inch lead screws - these are great - could run 5 times faster than what you currently are running. They are available at McMaster-Carr in 3 and 6-ft lengths. Something to think about if the ends of the current ones aren't turned down for bearings or connections to the steppers...

As far as vibration there is a great thread on installation of harmonic balancer's to the shaft of the steppers on CNCZone
- Rob

Phil Thien
08-01-2008, 9:57 PM
Phil, I don't know what the ends of those acme rods are held with on the K2, but my new machine has 1/2", 5-start, 2 turns per inch lead screws - these are great - could run 5 times faster than what you currently are running. They are available at McMaster-Carr in 3 and 6-ft lengths. Something to think about if the ends of the current ones aren't turned down for bearings or connections to the steppers...

As far as vibration there is a great thread on installation of harmonic balancer's to the shaft of the steppers on CNCZone
- Rob

Hi Rob. My current screws are 1/2-10 single-start. The machine didn't come w/ anti-backlash nuts. I have also discovered that the size of some of the components would preclude the use of 1/2" anti-backlash nuts. BUT, I could replace the screws w/ 3/8" and then get anti-backlash nuts that WOULD fit, and I would be killing two birds with one stone (as I could use 3/8-10 two-start and get 100-IPM).

My screws are turned down to fit into bearings. BUT, I live in Milwaukee so it shouldn't be too hard to find someone in town that can adapt new screws to fit.

I found that discussion about vibration on cnczone. My solution was to replace some steel and aluminum spacers on my machine with nylon ones. BOY, did that quiet it down.

Still plugging away. Having a blast. Thanks for all the help.