Homebrew Advice

[dank]

I use the HB2 overhead CNC router often, but not as much as I would like. It has at least 50 hours of
run time and probably more like 75-80. My only problem with the machine has been the coupling
failures. It seems like it happens a lot (3 times). In my thoughts even once is too often given the
operating time so far.

80 hours is a lot of 40 IPM (average) cutting distance. That is like "city driving" with a LOT of starts, stops
and directional reverses. That figures out to over 3 miles of directional travel when I do the math.

That doesn't justify the coupler failures. I'd hate to change a flat tire on my car every mile of travel.

Most of the time the HB2 runs just fine, so I start feeling all the coupler problems are resolved.
Though, I have become somewhat paranoid about leaving the room when the machine is operating.
Some projects take hours to cut and nature occasionally calls in that time spell.

I WILL solve all the problems as I intend to use the machine to perform in a light manufacturing role. I
would just rather replace worn parts every 500 miles than broken parts every one mile.

My thought (not yet a plan) is to develop enough business that I could someday justify a larger
professional machine. I now know from building my own, exactly what is important as far as features.
A rigid double drive gantry is one of the requirements. The other is superior axis screw end bearings.
The proper spindle is always a consideration but is not a problem to solve.

From the start I intended the HB2 to be a learning experience. I certainly do not recommend building
from scratch as the way to go for a professional purpose machine. Scratch building is perfect practice
for an experienced hobbyist who likes to build and troubleshoot machinery. If the plan is to reduce
cost but have a professional level machine, choose a proven product. There are many flavors available.

If I don't ever go professional at CNC routing, I will have a great little home shop machine that I know
intimately. It just takes a little while to develop that relationship to perfection.

[ace]

whats failing ? the coupling from lead screw to motor? are they aligned as good as you can get them ? maybe a different type coupling is in order? can ya post a pic of whats breaking?


LOL can i ask any more questions??? ...

im on my way to re- read your build log and see if i can find any thing i think might cause the failure..

================EDIT=================

ok...
you used the flex coupling instead of the solid ones that were available for your machine at the time (looks like they now offer the same type couplings that you have on your current machine) only thing i was wondering is. your couplings look like they have smaller (thinner sectional area) than the ones they offer (to me) maybe you can get some of the ones they have now and use them on yours ? yours look more flexible but maybe not as strong? the new couplers say they are rated at 495 oz in.

if your motors are aligned with your lead screws well, then you should be able to actually use the solid couplers ,with out worry of misalignment causing wear on your motor shaft if that doesn't fix the problem of failing couplers then maybe you can get some of those oldham type couplings?

lemme know what ya think...


========EDIT====== (again) ========

i saw your machine on the main page when i saw the change gears in the background on the shelf, i thought "hey thats dan machine" , also saw the gallery section when there were lots of pics

[dank]

The couplings are failing right at the end of the spiral cuts. All three cuts break when they totally fail but I have seen the cracks before total failure. Part of the problem is the square cut of the spiral. It creates a high stress point at the end where it fails. However, I don't believe it is the coupler fault. I just exposed its weakest and therefore its failure point.

I have backed way off on my acceleration ramps. That should help in the sudden stops and reverse stress loads. There is no definitive setting of what is proper. "If it doesn't break something it must be alright" is the rule.

The original thin couplings are rated for more that the new ones. Since I have broken the new ones too, I don't think it is an issue with the ratings. The old ones are actually a better design from what I have read in the specs. The X coupling (old style) never had a problem and it transfers the entire X direction torque at the same speeds as the double Y.

I don't like the drag on the bearing shields and will fix that. It just has to be alignment problems. I have worked with motor couplings all my life and coupler breakage is always alignment issues. But I know the shafts are now perfect so I think I have fixed the breakage problem. Flex couplings are the proper method and I have no interest in solid couplings, I want to keep drive train problems out of the motor bearings.

The new X coupling broke because of gantry skew when one Y stepper stopped. That definitely created a severe shaft alignment problem as the other Y kept driving. The break was in the same place as the others so I am double certain the Y failures were also alignment problems. Case closed (I hope)

Now I am trying to fix the Stepper stall problem.

There is a known resonance problem in steppers, right smack in the middle of the power curve. Generally it is insignificant and micro stepping avoids the issue. Newer controllers like the Gecko's and others have a built in electronic damping for this resonance hole. I don't see that mentioned for my controller.

The controller I am using (a unipole chopper) has been wonderful on single stepper axis. The Taig has never failed a motor and the HB2 has actually run equally well most of the time on the same type controller. I am thinking the twin drives may be adding to the resonance. I will do more testing as my jury is still out on this issue.

[dank]

I think I have resolved the CNC controller from any part of the problem. I have all the bearings replaced and a (thrust) washer between the screws and the ball bearings. The thrust washer is not doing anything for thrust except transfering it to the ball bearings.

I am getting 600 RPM reliably and that is good. With my five turn screws that's 120 FPM rapids. The original PDJ design was for 2.5 turn screws and it would be easy to do over 200 FPM rapids. I opted for more push than speed. With a machine the size of the HB2 I have plenty of speed and tons of torque.

The steppers I picked are not the fastest specification either. 4.2 volt at 3 amps. More of a "brute" than a speeder. All in all I think I have a sweet set-up.

What I have learned is set up and attention to even the smallest detail is critical for long life without issues. My HB2 machine gets used and there is nothing like hours of constant opperation to bring out the flaws.

The machine is still solid as a rock with no slop anywhere. I am sure it will run for many years to come. Yea!