ShopBot, and the frustrating case of Z axis drift

[MichaelChamplin]

Hey all. I need some help on an issue I’m experiencing with the ShopBot. The short of it is: on long operations, the Z-axis seems to cut deeper and deeper.

Could be the same issue that was discussed a few years ago on this thread: https://talk.makeict.org/t/shopbot-tune-up/6103

The long of it:

I’m new to v-carving, but have had plenty of success with the ShopBot with other projects like cutting out templates from plywood. I used Easel to generate the g-code for the project. I homed all 3 axis on the bottom left corner of my workpiece:

First letters cut as expected:

However as the machine continued to cut, it progressively cuts deeper and deeper on the z axis. Here’s the same letter, but 90 minutes later near X=50:

After this I jogged the machine back to the corner I zero’d on and here we see the z-axis is off:

I’m happy to run more tests and collect more information if anyone has ideas. I think I can work around this by splitting my project up into smaller jobs, but that’s not ideal. It’ll mean a lot more set-up time and slightly less efficient use of materials and I’d like to avoid both.

Thanks in advance for the great support this maker community always provides.

[DanS]

I haven’t used the ShopBot, but I assume you’ve checked that the collet is tight enough and the bit isn’t working down so it’s hanging farther out of the collet? I’ve seen that before on other CNC equipment.

[doug.wilson]

It looks like a problem that I tried fixing years ago; the z-axis spring is carrying too much. The online recommendation i found to double the support spring did not work like they suggested. I’m starting to think we just need a lighter spindle.

···

On Mon, Sep 23, 2024, 11:38 AM Michael Champlin via MakeICT Forum <noreply@talk.makeict.org> wrote:

| MichaelChamplin MakeICT Member
September 23 |

• | - |

Hey all. I need some help on an issue I’m experiencing with the ShopBot. The short of it is: on long operations, the Z-axis seems to cut deeper and deeper.

Could be the same issue that was discussed a few years ago on this thread: https://talk.makeict.org/t/shopbot-tune-up/6103

The long of it:

I’m new to v-carving, but have had plenty of success with the ShopBot with other projects like cutting out templates from plywood. I used Easel to generate the g-code for the project. I homed all 3 axis on the bottom left corner of my workpiece:

First letters cut as expected:

However as the machine continued to cut, it progressively cuts deeper and deeper on the z axis. Here’s the same letter, but 90 minutes later near X=50:

After this I jogged the machine back to the corner I zero’d on and here we see the z-axis is off:

I’m happy to run more tests and collect more information if anyone has ideas. I think I can work around this by splitting my project up into smaller jobs, but that’s not ideal. It’ll mean a lot more set-up time and slightly less efficient use of materials and I’d like to avoid both.

Thanks in advance for the great support this maker community always provides.

---

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[whateg01]

Or retrofit a screw in place of the rack and pinion

[MichaelChamplin]

This was actually my 2nd failed carve. After the 1st failure I suspected bit pull-out. I used a combination square to record the bit protrusion before running the machine. Bit was in exactly the same position afterwards.

[whateg01]

Generally speaking, if the router collet is sufficiently tight, it should not pull out. Also generally speaking, only spiral bits tend to pull out.

[DanS]

I figured you probably checked that, but I guess it doesn’t hurt to ask. Have you had any other helpful advice on the problem? It’s always frustrating when something like that happens and you don’t see the fix, or what’s causing the problem. Good luck with figuring it out. I know there are lots of members with a lot of experience with the ShopBot, so someone should be able to help.

[Kim_Kirwan]

There are things we can do both with the Shopbot and with LinuxCNC. If it’s OK with the Woodshop Lead (isn’t that @doug.wilson now?), I’ll check on several possibilities the next time I am there and come up with some suggestions.

[doug.wilson]

I don’t know how to do that and match the z-axis steppers. Does anyone here know how to do that?

[whateg01]

Aside from the mechanical parts to adapt it, the changes for steps per inch are made in the configuration file (can’t remember if it’s ini or hal)

[whateg01]

While watching glue dry, I took a moment to see how effective the spring on the z axis is at holding the z position. In short, it isn’t. I really like the idea of something other than rack and pinion for z, but a quickly implemented solution might just be a counterweight. There’s plenty of room above the z axis for something. There are holes that can be used to attach something already.

[Kim_Kirwan]

I am supposedly working on this, but I have been distracted by other things. I will try to get back on it. Thanks for the reminder.

[whateg01]

Having used the Shopbot this past weekend, I have a few observations. First, through many Z movements, the position was never lost. That is not to say that OP didn’t experience lost Z steps, but that the machine is apparently capable of repeating.

I have some thoughts on potential causes of lost Z. We have, over the years, had several users observe that the machine lost Z when they used it, yet many others never experience this issue. @MichaelChamplin can you provide the program for the items you were routing? I believe there may have been issues in the program that contributed to the problems you had.

I would like to run the part again, even if just cutting air to see if we can replicate the problem. Understanding why it happened might lead to steps to prevent it. It is possible that even if some other means is used to control the Z axis, if there are other contributing factors, the problem might still occur.

In any CNC machine, one of the limitations that exists is acceleration. The machine can only move axes so fast, but it can also only accelerate to those rates so fast as well. If the acceleration is too high, it can lose steps and the trend will be to lose them in the Z- direction. This isn’t something that is part of your program as it should be in the HAL or ini file, IIRC, but perhaps something in your program was causing the machine to struggle with it.

You posted pictures of the bit touched off, but it isn’t clear if this is a picture of the initial touch off or after the fact. Logic says that you wouldn’t have taken a photo before as there was no reason to, but I would like to get confirmation of that.

The bit used doesn’t really contribute to Z forces aside from some slight Z+. Spiral bits can pull the router into the work or push it away.

Do you know if the router bit pulled out of the collet during use? This, like pulling the tool into the work, isn’t something that typically happens with a straight flute bit, but if the collet isn’t sufficiently tight, the bit can spin in the collet and if this happens, it will almost always move one direction or the other.

I believe that although a counterweight system, whereby the stepper only has to move the axis and not maintain it (it really does both) has merit, there are also drawbacks. It might be best to try to keep the existing spring to avoid unwanted side effects.

[whateg01]

Ran another job last night with no issues concerning z height.

It would be helpful to get a sample from somebody who has had problems in the past or now to evaluate. Otherwise, I think I’ll stand by my opinion that we leave the machine configured as it is for now.