Taller LR3 build

LowRider CNC Your Builds
148

I’ve installed these couplers today together with 2 start lead screws. The new lead screws don’t crash on power loss or reset and move faster than 1 start screws. IMHO 2 start is the best option for LR3. Besides convenience, another big advantage is the ability to cut the power in case of emergency without the risk of completely damaging the stock.

I still had some skipped Z steps every once in a while even with 1 start screws. Obviously, in this configuration LR3 has more than enough power to lift the gantry so I think the problem must be somewhere else. My current guess is that the dust build up on the lead screws starts causing some friction and since the motors had to be spinning like crazy with 1 start screws, motor torque may have been insufficient at high speed to handle that friction. I could be wrong. It’s very difficult to say for sure. All the noise from the vacuum and the router doesn’t make it any easier to spot exactly when the steps are skipped. After installing 2 start screws, I set top Z speed to 35, which is <50% of the max tested speed. I also cleaned the MGN rails with brake cleaner spray and lubed them slightly with lock lubricant (probably silicone based). Maybe I should use some thicker bearing lube for the MGN rails, not sure. I’m afraid that thick lube could mix with all the foam dust and get all that dirt inside the bearings. After cleaning everything, I dry-ran my usual program with 2 start screws for about an hour and didn’t get any Z issues. Fingers crossed.

Another idea for the skipped steps is that I have acceleration ramped up too much for Z axis to get smoother movements. My acceleration for XY axes is set to 300, and Z is set to 250 (so almost the same), while the top speed of Z axis was 16 times lower than X or Y. The skipped steps could be happening due to inertia of spinning very fast in one direction, then reversing and spinning very fast in another direction. I usually get skipped steps on the last stages of my cuts, where I use high speeds and lots of Z movement is happening. After about 1.5 hours of cutting, my Z is sometimes 2-5mm off, so not a whole lot. But it usually means stock going to the trash.

I will try to lower the Z acceleration in Marlin, lower the speed for my finishing passes and try one more cut tomorrow.

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Probably from trying to spin the steppers to fast and running out of torque. the trade offs are speed vs power. The slower they are the stronger they are.

35 to you is like 70 to a 4 start(extremely fast). Try going slower. RPM vs Torque.

Good idea, take my stock settings and divide by 2.

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150

2209 drivers rated @ 28V means that 28V is the max voltage I can send to the motor? Asking for a friend :))

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You need to worry about your control boards max voltage before the drivers.

But yes they will work from 4.75v to 29V. Might just want to stick to 24V to be safe.

Instead of trying to speed it up just try to use less or shorter Z moves in your CAM.

Even with 4 start screws Z speed is the absolute last concern I have in any of my Jobs. 100x more XY moves to optimize for me.

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Well, to get a nice 3d finish the endmill kinda needs to move along the face of a model. In my case Y moves ~157cm tail to nose and Z moves ~8cm top to bottom during the same time. That’s almost 20:1.

In general, I can lower the Z speed no problem. That will probably add 2-5 minutes to my 90 minute job, so not a big deal. I started playing with increasing Z acceleration when I had those jerky movements from Fusion’s G code. It appears to be mostly related to the scallop method in Fusion, other methods produce a more fluent gcode. On the other hand scallop gives me the best board finish with the least post processing afterwards… so in the end after trying all the other 3d methods in Fusion, I went back to scallop and agreed to live with the jerky motion. Increasing Z acceleration helped a bit with that, so I left it there.

What surprises me is that X and Y are flying around at high speeds with quite a bit of resistance and don’t skip. And Z has always been a problem since day 1. Hopefully reducing speed/acceleration will fix it.

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Here is what I tested as a max, Real world testing of 12v vs 24v - #14 by vicious1, 12v Z 4 start 63mm/s, 24v 4 start 80mm/s with stock screws and accel. Z accel is very low, but needs to be.

So with 4 start screws you will get 4x’s slower than XY, with 1 start 16x slower, 2 start 8x slower.

Steppers are just not high speed motors.

With less start screws you can get higher accel, so in the end they might be quicker for tiny moves, but 4 start will always have higher top speed.

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154

You could move to ball screws and get higher speeds.

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155

I tested for max Z speed again today. With Marlin setting at 70, it worked fine. At 80 one of the motors skipped on the way up. So I set my max to 35, which is half of last working test. Maybe thats too optimistic. I will go lower and see if it helps.

I think the biggest challenge is keeping the screws clean and free of dust. If its covered in snow, ball screws are probably not going to last long :slight_smile: Today when one of the motors skipped during the cut, it was on the side next to the wall - a difficult to reach place with a vacuum while everything is moving. I clean the other one periodically while milling.

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I must admit cnc milling is a bit more complicated than I thought.

First, I noticed by boards don’t look 100% as I designed them. Looks similar, but something is just not right :smiley: So I started measuring. And in some cases, the board is thinner by… quite a bit, like 4mm… than it should be.

So I started driving LR3 around the table and measuring the distances and yes, the table was not 100% even. I found a few spots that were higher by 1.8-2mm than my zero position. Apparently humidity played a certain role in that, as I moved the cnc to the basement which is a lot more humid, so the old wasteboard absorbed some water and started sagging etc.

Thing is… it’s off by 2 mm on one side, then after cutting the first side I flip the surfboard over and cut the other side, and it’s 2mm off on that side as well. So 2 becomes 4. And the surfboard nose is less than 1cm thick, so basically I loose half of the nose thickness. Not cool!

I bought some thicker plywood and replaced my table top with that, and then surfaced the whole thing with a surfacing endmill. Doesn’t look pretty, but at least it’s even(er).

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Next surprise: I tried to make a stand for the surfboard lamination. Found a few ideas online, made a fusion sketch and here it is:

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This one was cut with 6mm endmill, 2m/min feedrate and 3mm 2D contour stepdown. Looks pretty nice. Except the hole in the center is supposed to be 18.3mm, and it is somewhere around 17.3mm or so. Roughly 1mm narrower than it should be. And the other 2 holes seem to have similar issues, although they are more difficult to measure. Then I started measuring some other plywood parts that I cut, and it appears that all the holes are up to 1mm narrower than they should be, and all the solid parts are ~1mm wider than they should be. I was like WTF maybe my endmill is 5mm instead of 6 and I didn’t notice it! Measured the endmill - it’s exactly 6! :smiley: Damn.

Then I made some tests and apparently cutting speed plays quite a big role in accuracy. If I cut reallllly slow, the tolerances are more or less what they should be. Or at least so it seems, I haven’t had the time to test it very thoroughly. So I guess I should use a finishing pass and make it go sllllow. You live and learn. This is a time consuming hobby.

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You do not need to go slow you do need to use a finishing pass 100% of the time for dimension cuts.

In wood something like 0.5mm at full depth is a good starting place for the finishing pass.

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With that same note though. Make sure you have reasonable tolerances for wood. It moves a lot. Most projects, 1/32"-1/16" is way more than enough.

Plastic and metal can be held to higher standards but as you see humidity can change dims very easily in wood.

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If I’m using 2m/min with 3mm DOC for the milling of the contour, what would be the recommended speed for the finishing pass? Leave it the same? I actually used 2x speed for the finishing pass as I thought there would be hardly any resistance, maybe not a great idea.

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=33.3333mm/s That is fast. You can go deeper and a bit slower. I use teh same for teh finishing pass just full depth 0.5mm cut.

The firmware is limited to 50mm/s. Speed does not equal accuracy. You will only save a few seconds so just stick to the same speed.

Beyond that do some actual test, everything is completely up to YOUR build, no two are the same. so real tests give you your numbers.

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I found a mistake in my model, where one of the hole edges was offset 0.15mm in the wrong direction. That’s definitely part of the problem. I will try a new cut tomorrow with the fixed model and adjusted speeds, will see how it goes.

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162

You should be able to do a full depth finishing pass. That should give you a good finish and accuracy. I’m not sure what speed, but it is only just barely touching the material (if things went right). So I would start with the same speed and just full depth.

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It seems like the problem was with the milling bit in the end. I ran the finishing pass 3 times at different speeds and couldn’t get the accuracy anywhere close enough. After changing the milling bit it worked from the first attempt.

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The one at the top was causing problems, the one at the bottom seems to be way more accurate for cutting plywood. The downside is that it seems to be much louder as well.

When the cuts started coming out accurate enough, I was able to assemble the stand.

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I will still need to figure out the proper dog bone hole diameter, as the endmill is making strange sounds when it enters the hole. I used 6.5mm for the 6mm endmill.

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Your CAM program should offer to do that automatically for you. Usually you get a few options as well, I use the smallest one.

The straight mill also tends to need much slower federates so play around with it a bit. Odd that your upcut isn’t the better one. The next one you should get is a single flute upcut for the wood. Our routers are generally too fast for two flute or more.

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I’m using Fusion 360 and I don’t think it has any support for dog bones, at least not officially. I heard there are some 3rd party plugins, but they seem to be buggy and only offer one position for the hole.

What CAM program are you using that does this automatically? Creating those holes manually every time is indeed a bit time consuming…

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It isn’t quite automatic with estlcam, but you can choose the overcut tool and click on the corners you want to overcut. It doesn’t do the same dogbone you have in that picture. If the ones in your picture are at 90degrees to the side, the estlcam ones are at a 45. That ends up making them just a little smaller.

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That is it, overcut in ESTLCAM. Works great.

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