OOZnest CNC router going off course

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OOZnest CNC router going off course

Postby mplsartist » Sun Jun 04, 2017 4:44 am

While test cutting some Baltic birch plywood for my sculptures recently, twice the router has gone off course resulting in some loud screeching and burning of the wood and ruining the job. I’m wondering if my feed rate is too high and or getting too much bit deflection

Here;s the technical info:

Newly assembled OOZnest 1500x1500 OX CNC
Dewalt 611 1.25 hp router set at minimum 16,000 RPM
Baltic Birch with a plastic laminate on the face side for a total thickness of 0.712 inches.
Routing from the back side (plastic laminate face down).
¼” carbide compression bit with approx ¾” to 1” stick-out
Feed rate 290 ipm (derived from trial version of G-Wizard)

Software: Vectric Cut2D Pro with following parameters:
Plunge rate 30 ipm
6 passes at 0.1187” at a time
Lead in @ 45 degrees, 0.1” length with lead out of same length, overcut distance 0.1”
Checkmarked: Ramps smooth
Checkmarked: Ramp on lead in
Order selection: L to R, Bottom to Top, Grid, shortest
Checkmarked: sharp external corners

Nesting setup:
Tool diameter 0.25”
Tool clearance 0.50
Border Gap: 1.0
Rotation step angle
Allowance offset: 0.02
Final pass 0.00

The pieces being cut are very curvy (Bezier) . Both times that the diversion occurred, it was one of the deeper cuts of the six cuts. No other adjacent nested pieces were cut before the diversion and no objects were present to hinder the routers path. On one piece, I was able to see that there was a slight diversion at several higher steps but the router continued on course until the lower cuts and then it went off track. The other piece went off track on the deeper cut without any previous diversion at the higher level cuts.

Any suggestions would be greatly appreciated.
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Re: OOZnest CNC router going off course

Postby Adrian » Sun Jun 04, 2017 8:45 am

Does everything look fine in the Cut2D preview? If so it's a mechanical or control software issue almost certainly.

The feed rate does seem very high for that sort of machine but you are taking very shallow cuts. You are restricted a lot with such a high minimum rpm though.

Has the control software been configured properly for cutting curves? The acceleration/deceleration rates might be out.

Are the areas where it's going wrong consistently in the same area of the board and/or orientation to one of the axis? If so that could indicate a physical binding problem.
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Re: OOZnest CNC router going off course

Postby ger21 » Sun Jun 04, 2017 1:03 pm

What size motors, and what drives are you using? I wouldn't expect one of those machines to be able to cut at 290ipm reliably.
What are your acceleration and rapid speeds set to? That may also be the issue.
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Re: OOZnest CNC router going off course

Postby mplsartist » Sun Jun 04, 2017 5:02 pm

Adrian:
As far as I can tell, everything looked fine in Cut2D Pro preview.

I specified bezier curves in Cut2D Pro but I don't know if the controller has the capability to specify things like that. I'm using everything standard equipment with the OOZNest including the controller which is a X-pro version 3.

The standard code in the controller shows:
$100-112=2500(x,y,z max rate, mm/min)

$120-122=50.00(x,y,z accel mm/sec ^2)

The deviations occurred on opposite sides of the board but both on the right side of the part. There were no adjacent parts cut on that side of the parts. Also, it looks like the deviation occurred in the opposite direction of the climb cut I had specified. I did have the first 5 passes with an offset of 0.02 and the final pass with an offset of 0.00 in the same direction as the previous passes. I was wondering if the offset could also be a cause of the problem and maybe I should not use a final clean up pass.

Other pieces in the same area were unaffected.

Ger21:
The motors are standard Nema 23
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Re: OOZnest CNC router going off course

Postby IslaWW » Sun Jun 04, 2017 5:18 pm

Gerry is most likely correct, you have simply specified a feedrate greater than the machine can deliver. This most likely happened during the last pass then the bit was fully engaged with the material. If you test, you may find that most fibrous material (wood or plywood) cut better using conventional rather than climb direction.

Will the problem repeat when air cutting the same file?

Did it happen more than once in the same place?

Does the bit have a long enough CEL to accommodate a full depth last pass?

Have you tried recutting the file with the XY feedrates at 50%?

IF you are a new user of a small footprint machine, you might as well get out of the habit of setting proper chiploads for the bit and start setting them appropriate for your machine. Most desktop and a good number of table tops are not rigid enough to achieve optimum chipload. Those using a handheld router as a cutting head will seldom have the power at recommended RPM to achieve optimum chipload, even if the machine frame could withstand the force.

A screenshot from one of my "CNC Basics" class:
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Re: OOZnest CNC router going off course

Postby mplsartist » Sun Jun 04, 2017 6:05 pm

IslaWW:

I'm going to try another file this afternoon reducing my feedrate from 290 ipm to 200 ipm and use conventional cutting instead as you suggested. What is your opinion on cutting with an offset of 0.02" and then final pass at 0.00"?

The first time the diversion occurred, I was not close enough to yank the USB controller cord fast enough and things continued on until the next pass when it occurred again. The second time, I caught it right away when it occurred and yanked the USB cord and shut everything down. Both occurred on the 5th or 6th pass and tried to take out a full depth pass that strayed off the designated toolpath which caused the screeching and burning.

I didn't think to try a retry with an air cut.

The CEL is approx 1 inch, so I should have enough.

You're screenshot was comforting. I was going bald scratching my head trying to figure out chiploads etc.
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Re: OOZnest CNC router going off course

Postby IslaWW » Sun Jun 04, 2017 6:16 pm

Have you answered the questions above? 200 may also be too fast.

Approach achieving proper cutting speeds like you would taking your car around "Dead Man's Curve". Start slow and gradually increase. No good comes from going too fast the first time.



Insanity is doing the same thing over and over, while expecting different results. The inverse it true of troubleshooting. Keep repeating the file, changing one item at a time until you isolate the root cause of the problem.
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Re: OOZnest CNC router going off course

Postby Leo » Sun Jun 04, 2017 7:08 pm

As to speeds and feeds I find it better to start slow and slowly and gradually bring the speed up.
I have a far more rigid and powerful machine and I don't really cut faster than about 150 IPM, and at about 10,000 RPM.
I DO cut at 250 IPM and higher, but then clamping can become an issue.

I listen to the sound of the cutting and watch the chips.
You do not want to be making powder, you do want to be making chips.
Dust means you are feeding too slow.

I would suggest you start around 50 IPM and try working up from there.
I wish you could turn the RPM down, but there is a RPM to Torque curve you need to be aware of.
Routers are not generally real good at low RPM's

Unless you are a production shop and charge machine time and NEED to be fast - why go that fast?
Some of the people here NEED fast, but a hobbyist generally does not need, fast.
Yes, there is a need to control chipload, but that does not mean fast cutting.

I am very conservative on cutting speeds because I am not a production shop a really not in need of time, and I like to me conservative with the wear on the machine?
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Re: OOZnest CNC router going off course

Postby mplsartist » Sun Jun 04, 2017 8:17 pm

Thank you everyone for your input. I greatly appreciate the help. I'm going to start by cutting my speed in half and see what happens and adjust from there. Normally, I wouldn't mind slowing down even more, but I'm trying to get a lot of pieces done before my first show in a couple of weeks. I don't know if I'm going to accomplish that. Previously, I had been getting my CNC work done by some outside vendors but was disappointed with the inconsistent quality (one was a 5'x10' 100K pro machine!) and inability to return emails/phone calls in a timely fashion.

I am now learning the hard way about the nuances of the CNC world!

Another question regarding using slower feed rates: I am nesting 3-4 dozen pieces on one 4'x4' piece of Baltic Birch. Given the slower feed rates, this could run all day. Would it be better to section the 4'x4' piece off into 4-6 separate tool paths to let things "rest" and also give me a chance to go to the bathroom, etc? Given what's happened recently, I don't want to let the machine run unattended.

What's the best strategy when the router goes off course? Currently, I am yanking the USB cord from the controller and that stops the motors in their track. I would like to guide the errant router back on track and resume the job, but am wondering about accuracy, etc.
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Re: OOZnest CNC router going off course

Postby Leo » Mon Jun 05, 2017 1:23 am

The best strategy is to get the process under control.

Rather than start out taking the large steps, perhaps starting out with smaller steps until you learn what your machine is capably of is best.

You "could" cut your job size down to single pieces ans then step up to 2 pieces then 4 and so on.

At least if things go "off" you don't ruin so much - just one piece.

I don't think it's a good think to pull the plug on the usb if the process is not working. That is a pretty dramatic action.

Does your machine have a feed hold, or an "E" stop?
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Re: OOZnest CNC router going off course

Postby Leo » Mon Jun 05, 2017 1:45 am

I looked up the ooznest machine.

If you bought the full kit and went with the standard parts, the drives motors are 177 oz/in Nema 23 with Acme lead screws. The rails are some "V" bearings, the frame is extruded aluminum. The router clamp holding the router to the "Z" axis is not all that strong. With that said, you cannot push that machine as hard as you are suggesting.

I'm not saying the machine is no good, but I am saying that you need to have your process to fit the constraints of the machine.

The controller is an open source, and I assume a bit limited.

If you cut you feed in half you will end up about 140-150 IPM.
In my estimation that is a bit too fast for that particular machine.
I know people with similar machines and they are cutting in the 40-60 IPM speed range.

I hear you that you want to get an inventory ready for your 1-st show, but you will be constrained by the ability of the machine.

You cannot compare to a larger commercial machine.
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Re: OOZnest CNC router going off course

Postby mplsartist » Mon Jun 05, 2017 2:21 am

Leo:
The more I hear from everyone, it looks like I should be running well below 100 ipm. The control software does have a pause command. I have already started planning smaller tool path runs even if it means not nesting as efficiently as I would prefer.
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Re: OOZnest CNC router going off course

Postby IslaWW » Mon Jun 05, 2017 2:56 am

Once you find the feedrates that yield good parts and are within the abilities of your machine, then increase the nest size.

You might want to test at a more appropriate pass depth for a compression bit. Few are designed for less than 1/4" penetration. Deeper pass depth allows a larger chip and cools the tool better. You should be able to cut 3/4" material in 3 passes. In reality the bit is designed to do it in one.
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Re: OOZnest CNC router going off course

Postby mplsartist » Mon Jun 05, 2017 3:04 am

Gary:
That's something I didn't know! I thought that the general rule of thumb was to have pass depths at half of the bit diameter. My concern is that the deeper pass would increase bit deflection and might bog things down a bit. Is that a concern if my feed rate is 50-90 ipm?
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Re: OOZnest CNC router going off course

Postby mplsartist » Mon Jun 05, 2017 3:05 am

Also, the bits are barely warm to the touch right now.
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