Hi Jen,

I have no experience of belt drive so cannot offer any advise here. (Incidentally the Zing will be belt drive and they work OK).

Not sure about cutting with many light passes, I will have to try this and see what happens. From past experience when cutting profiles a second pass had little effect unless you can get the carbon residue out of the bottom of the kerf. The true solution is probably more laser power so it can be vaporised before it has a chance to burn 

Tweakie.

Speaking of the router, i have decided to change the X&Y axis from belt to a ball screw setup. Both axis already have pretty hi precision rails they slide on, it is just that little bit of play in the belts that gets frustrating, especially when there are a lot of quick direction changes. they sort of vibrate, which sometimes causes a couple of thou error in the parts.

Would it be worth exploring some kind of belt vibration dampening before i embark on changing it to ball screw?

 
Jen

Jen,

Slightly off topic here but I'm just wondering what size your drive belting is.  The reason I ask is that I've just built a router that uses 3/8" belts for the xy drives and I'm trying to determine what kind of accuracy I can expect from it.  Right now I see no issues with it but I'm not working to .001 inches.  I have the belts quite firmly tensioned.  I'll try to see if I can measure the stretch later today.

Chris

Thanks Chris,

sorry i posted this in the wrong place. not so much stretch, these are very large kevlar reinforced belts, it is more of a vibration issue. when the router head changes direction quickly, like working a tight corner with a 1/64th inch (.015") bit, yes they make them that small, it sets up a vibration you can see in the belt, especially with those nice machine generated G-Code files, the "wave" in the belt is what i think is the culprit. if i could figure a way of dampening this wave, i think it would improve accuracy.

with the laser it seems to work more like a printer making long sweeps with a single direction change at the end of each pass, so this condition may not occur. in fact i think a lighter (less mass) belt may be another consideration. Less mass less inertial force to set up vibration. gawd i sound like a freakin engineer, this is supposed to be art

i'll stay tuned,
Jen
 

Hi Jen,

To test my repeat accuracy I ran the x axis 50 times back and forth over a 20" run and it came back after the 1000 inches less than .001 off of the zero position.  That doesn't really test the accuracy at the exact moment of an acceleration/deceleration and I'm not sure how I would do that.  With the motors locked I can stretch the belt about .005 in either direction before I hit a really solid resistance point.  My belts aren't kevlar so that may be an improvement I can make should I someday find the current accuracy insufficient.  My belts also have very little mass to them and I would never consider them to be contributing to a problem with inertial vibrations.  I imagine though that if this is the situation in your case you could put one or several idler pulleys along the backside of the belt to kill the harmonic tendency you're now seeing.  Also even though I'm new to this and can't speak with much authority I have noticed that some G-code files are more ridden with inertial changes than others.  There seems to be a bit of an art to generating files that are efficient at producing the highest quality part.  G-code generating programs are at best only working with a 'typical' geometric input in mind.  Another point I might mention is that I'm using a Smooth Stepper to generate the pulse stream (currently set at 64k) and this pulse stream is most definitely smoother than the default parallel port output.  Whether the quality of the pulse stream is contributing to your difficulties would be another item to consider.  If the laser unit doesn't run G-code then you won't be using Mach and I'm going to assume that it has it's own pulser which may be the source of the smoother result you're seeing in that machine.  You might want to find out what the spec is for the laser's pulsing engine just to be informed about it.

Thanks and I hope some of this is helpful.  By the way I can't imagine using a 1/64" bit. 

Chris

For anyone considering a Belt drive... I recommend looking into this thread at the zone:

http://www.cnczone.com/forums/linear_rotary_motion/59570-best_belt_drive_ever_if.html

There is a lot of information to digest in that thread but it is a very ingenious system which I believe would work very good,
It eliminates most of the belt stretch and the vibration in the belt which Jen mentioned.

Jen, by the sound of your comments, you are more of an engineer than a few engineers I know.

Take a look i think you'll like it

Thank you both for your comments,

i will check out the CNC zone thread and see what they have come up with. a simplified way to visualize this "wave effect" is to simply push in teh center of a belt with your finger. When you do the gantry will move a few thousandths, but spring right back. Now picture a wave running up and down teh belt from rapid repeated changes in direction, like when routing the apex of a triangle.

my machine will repeat with wonderful accuracy, even after a thousand traverses, thats not really an issue, it is this jiggling of the router head when i am attempting very fine sweeping inlays of abalomne or some other shell that drives me nuts. i have to go back and manually sand off this roughness. once i am involved in doingthat i have lost any time i may have gained by cutting the part with the darn machine in the first place.

i have a meeting, but will check the ink out in a bit... Thanks again, Jen

Hi Jen,

I see what you're talking about.  If I pluck the backside of the belt like a guitar string I get the effect you're describing.  I wouldn't have thought that that light a load on the belt would show up as stretch (for me not much more than .001) but it does.  So this ultimately has to be belt stretch.   Have you tried adjusting the acceleration rate in Mach under Motor Tuning?  I found it necessary to play with this a bit as my initial tendency was to have the acceleration too high for the best cut.  

Good luck.

Chris

This inlay work has turned out to be incredibly easy to do but so so difficult to do well (perhaps that applies to everything) but I am now just starting to learn the key point like matching the aspect ratio of the whole image not just matching the height and width.

Thanks, once again, to Switcher for the DXF of the Rocking Horse.

Tweakie.

Hi Tweakie,

I use a software package called V-carve Pro ( http://www.vectric.com/WebSite/Vectric/vcp/vcp_index.htm ) for my inlay tool paths. it can create two different tool paths for each drawing i make. One for the piece and one for the pocket. It even tales into consideration the radius of the tool you use and corrects outside corners on the parts so you do not wind up with the old square peg in the round hole problem. You can also just specify your offset so the pocket will be a little larger than the piece thus allowing for glue space and finishes. it is really neat stuff.

They allow you to download the entire package for free, you can even save your drawings. it just will not generate the G-Code from them. But if you are nice to me i will let you send me a drawing or two and generate if for you.

they also have many hours of video tutorials on-line i think one of them is geared specifically for inlay basics.

inlay is big fun especially when you get into real intricate patterns, but it can also be maddening. It really is not the best candidate for a CNC solution unless you, like me, need to repeat the same design many times. it is a lot faster to cut your part with a saw and route out your pocket by hand, unless you are getting into a production scenario.

Jen

Hi Jen,

Thanks for the offer to generate the VCP files – I may just take you up on that one.

Most of the things I do, in a way, demonstrate that CNC can and in fact is replacing skill in the workplace. I doubt I have the ability to manually do inlay work, to make even a reasonable job would require many skills that I really just don’t have, artistic ability being one of them. This is where CNC comes into its own and the Turners Cube http://www.machsupport.com/forum/index.php/topic,9755.msg59684.html#msg59684 is a good example of something I can easily make with CNC but not with a lathe as it was originally intended.

Tweakie.