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Forum topic by Bieser posted 02-04-2021 09:11 PM 626 views 0 times favorited 31 replies Add to Favorites Watch
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Bieser

215 posts in 3088 days


02-04-2021 09:11 PM

Ok so I bought a used 4×8 avid cnc.

It took me a while to get all the setting correct as far as motor tuning etc. I think I am pretty close.

I sketched up some basic cabinet sides in vcarve and it started cutting all my dados etc then on the final pass where it would be cutting out the actual parts away from plywood it took a strange cut path on its first time around then came back on a different path. I looked at my tool paths. No reason it should have done this. Any thoughts?


31 replies so far

View rtbrmb's profile

rtbrmb

753 posts in 3442 days


#1 posted 02-04-2021 10:12 PM

I am no expert- I do own a small CNC Shark (sold by Rockler) that I have owned for almost 8 years.

The two times I have had this issue;

One time I had some small scraps on the back side of the CNC that the gurney was running into & stopped it from going on its designed path…..once that happens it can’t recover to the original path

The other time (last year) the issue was a failing/failed controller. I scratched my head with that & even bought a used controller off e-bay…same results. Once I upgraded to a new controller ($500) all the issues were solved. Even though my CNC was about 7 years old at that point- the controller technology was about 13 and very outdated in computer years (no chance to repair it at that point).

Is it possible that any of the cables are being pulled taught and not allowing the router to complete it’s path?

Hopefully this helps.

Bill in MI

View Mike_D_S's profile

Mike_D_S

700 posts in 3268 days


#2 posted 02-06-2021 03:07 AM

Well it may not be the case, but my guess would be insufficient clamping and feed speed too high/rpm too low.

This assumes the parts are shifting which may not be the case. But running too fast without corresponding spindle rpm can generate significant side force.

Have you done a basic chip load calc for your feed speed and bit?

Also does your design have tabs included to keep the parts stable during the final pass?

-- No honey, that's not new, I've had that forever......

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Mike_D_S

700 posts in 3268 days


#3 posted 02-06-2021 03:12 AM

Other option is it may be losing steps on long cuts or transits.

You can poor boy test that by setting up a tool path to drill four holes in a big rectangle maybe three feet apart by five feet long. Drill the holes on the first pass then repeat on the same path four or five times. If your losing steps or have some mechanical issue the after a few passes the holes will be getting reamed out some as you lose position accuracy. If you can make four or five laps in the same run with the holes being perfect circles then I’d look other places than the mechanism/steppers.

-- No honey, that's not new, I've had that forever......

View Madmark2's profile

Madmark2

2498 posts in 1642 days


#4 posted 02-06-2021 03:16 AM

As it was being cut free it shifted in the holder. Leave a couple of ties to hand cut for final release. This will keep the cutout from moving.

Alternately you overdrove your servos and lost steps.

Try not to cut too close to an edge. Narrow edges are prone to blowout.

-- The hump with the stump and the pump!

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Bieser

215 posts in 3088 days


#5 posted 02-06-2021 03:43 AM

This is all new to me. Today I figured a few things out. I bought this machine used. So far I ran a 4 hole test for square and it was off, I calibrated the steps and the square was spot on, however after cutting a spoil board I ran a grid for vacuum clamping and found the grid was out of square again. I am wondering aside from adjusting the gears on the tooth is there a better way to adjust for square on the avid setup? I don’t have homing switches, after looking at the back of the internet maybe that would help it stay square. I have ran the x axis to its rubber bumper limits and then hear the gear skip a tooth and wonder if that is whats causing me to go back out of square? Sorry for the basic questions again I am a computer illiterate cnc beginner.

View Mike_D_S's profile

Mike_D_S

700 posts in 3268 days


#6 posted 02-06-2021 04:06 AM

Anytime you jump a tooth on a geared belt setup you’re going to be off. The machine position is essentially inferred from the stepper motor. The motor moves a step and the software assumes the machine moved by the equivalent distance based on the mechanical setup. If the stepper moves and the belt jumps a tooth, then the real physical position of the machine doesn’t match what the computer thinks it is.

Homing switches will definitely help with keeping you from hitting the mechanical stops which is a definitely on the list of things not to do. They can also help with setting a repeatable zero. There are also x/y touch off setups that you can use the get a repeatable zero.

But if you aren’t running into the mechanical stops and still getting off track then you may have a bad stepper, bad wiring, or a questionable controller. When you realize its out of square, does it does it keep drifting further out of square or does it change in sudden jumps?

-- No honey, that's not new, I've had that forever......

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Bieser

215 posts in 3088 days


#7 posted 02-06-2021 04:12 AM

Mike, Thanks for all the info. I noticed tonight that maybe I did not have the tension on the springs set tight enough on the one side. I could fairly easily push it to the next cog. I am going to go down and reset everything tomorrow and give it another try. Is the way you square this machine up best is by making sure your on the same tooth on the gear? Any fine adjustments must be made by adjusting the track? I think I am going to have to get some homing switches.

I have not figured out how to set limits which I suppose would stop the machine from running out to the bumps? why do you need homing switches if you have limits in place?

View Mike_D_S's profile

Mike_D_S

700 posts in 3268 days


#8 posted 02-06-2021 06:07 AM

You don’t specifically say, but I assume you followed the leveling, gantry and tramming documentation from the Avid CNC site?

Limit and homing is really the same function on the zero side of the machine. It’s the same sensor, just performing different functions at different times. When the machine is actually running and cutting they are in “limit mode” to keep you from running off to the mechanical stops.

When you’re actually homing the machine, they serve as known reference points on the physical frame.

If you think about what I said about that the computer only knows how much it moves the steppers and determines the position of the axis based on adding that motion to the last known position. So when you first turn everything on, the computer doesn’t have a last know position and doesn’t know where the machine physically is. So the home process moves the machine until it gets detected by the zero side limit switches. Since those limit switches represent a known physical position, the software now knows where the machine is.

-- No honey, that's not new, I've had that forever......

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Bieser

215 posts in 3088 days


#9 posted 02-06-2021 06:17 AM

Ya I have been through the website, The problem I am having is they are setting up a pro machine with sensors where mine doesn’t have any of those so its pretty vague on squaring without the sensors

View Madmark2's profile

Madmark2

2498 posts in 1642 days


#10 posted 02-06-2021 06:27 AM

The zero and limit switches are just small mechanical switches at the four sides of the machine. Most any type of switch will do. Small lever switches or even push buttons can be used. Mounting is more of an issue than electrical characteristics.


Classic lever switch is a little over an inch long

-- The hump with the stump and the pump!

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Bieser

215 posts in 3088 days


#11 posted 02-06-2021 06:30 AM

Ya looks like I will need to get some, it seems like this would make things easier

View DS's profile

DS

3727 posts in 3474 days


#12 posted 02-06-2021 06:30 AM

OP, you’ve received lots of good advice here so far.

I would recommend following the setup instructions for a new machine from the avid website. This will ensure there isn’t something amiss in your physical setup.
Whenever I get any new tool, I like to spend a lot of time to carefully set it up properly to ensure I will get the best performance that tool is capable of. I am known to completely disassemble bandsaws, planers, table saws etc, and put it back together with better tolerances than the factory. (Some might consider this excessive, but I like reliable tools)

Also, check your controller settings as well.
if you attempt to overdrive your motors by having a too aggressive acc/dec setting, you can stall a stepper or skip a tooth. Both of these things are bad. Compare your current controller settings with the factory defaults to see if something is extreme out of range.

Since the X axis is driven with two motors, if one side skips or stalls, this creates the out of square issue. Homing should reset square as it relies on the calibration of the left and right switches to set square on the machine. (Calibration of square is covered in the docs on the website)

If you continue to get out of step, there is something else wrong. Like you said, the tensioner might be loose, belts could need replacing, feed rates could be too aggressive, etc.

Also, the folks at Avid have been fairly responsive to any issues with their machines. You might try calling them and they could walk you through the issues you are having.

I wish you all the best with your new-to-you CNC.

-- "Hard work is not defined by the difficulty of the task as much as a person's desire to perform it.", DS251

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DS

3727 posts in 3474 days


#13 posted 02-06-2021 06:39 AM


Ya I have been through the website, The problem I am having is they are setting up a pro machine with sensors where mine doesn t have any of those so its pretty vague on squaring without the sensors

- Bieser

Also has 293 pages of standard machine setup

Lots of setup instructions for standard machines here

-- "Hard work is not defined by the difficulty of the task as much as a person's desire to perform it.", DS251

View Mike_D_S's profile

Mike_D_S

700 posts in 3268 days


#14 posted 02-06-2021 02:21 PM

DS has the right idea. Treat it like it’s a pile of parts and walk through the new machine setup. This will get you back to “factory” condition and you may catch a few things along the way.

Also, just to clarify OP has the 4×8 so I think this the is the right link for the standard machine setup instructions.
https://www.avidcnc.com/support/instructions/standard/4896/instructions/

Also, don’t be afraid to email the Avid people and ask for any documentation for your machine.

-- No honey, that's not new, I've had that forever......

View Mike_D_S's profile

Mike_D_S

700 posts in 3268 days


#15 posted 02-06-2021 02:36 PM

Going through the standard machine instructions it still calls for limit switches. If you don’t have any (and assuming the previous owner didn’t remove them) then the machine may be one of the older kits.

I think contacting the Avid CNC guys may be really helpful.

I see they offer the current proximity sensor kit as a separate add-on. Depending on which controller you have it may be a pretty straight forward installation.

-- No honey, that's not new, I've had that forever......

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