What to buy: SO Pro or SO4 for cast acrylic?

Struggling to decide between a Pro XL or a 4 XL during the current sale. I will almost exclusively be machining acrylic. Speed is a lesser concern than accuracy. I will be routing complex 3D designs with ball mills. Is the Pro more accurate than the 4 in this regard? I have a Nomad already for smaller jobs where accuracy is even more important.

Right now the Pro XL is $2600, and the 4 XL is $2050, with both having a bitsetter and hybrid table, so I am basically trying to determine what actual benefit, if any, I will get from having linear rails vs. v-wheels, and is that benefit worth $550 and the higher maintenance needs of the linear rails. Any advice would be greatly appreciated! Thanks!

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You may contact sales@carbide3d.com to get their opinion, but here’s mine for what it’s worth.

I used a SO3 for a few years and recently upgraded to a Pro, the SO4 sits somewhere in-between. The linear rails help minimize the deflection of the router under cutting load (especially the front/back deflection, around the X axis), so they matter most when using aggressive feeds and speeds to maximize productivity, or cutting hard material (aluminium, hard wood).

@LiamN has done a fantastic job of explaining/measuring that in this thread, it’s a long read but a very interesting one if you want to have an idea about the actual deflection values, there’s a part of the post specifically about v-wheels.

Linear rails are most useful for folks who cut lots of metal OR need to maximize material removal rate in other material to boost their productivity / minimize cutting times.

I see you intend to machine exclusively acrylic, with a focus on accuracy versus speed, so I would tend to think that a SO4 would work great for that purpose. I don’t have any accuracy figures for SO4 vs Pro in acrylic at reasonable feeds and speeds, but my intuition is that since you will be cutting complex 3D designs, you will use a roughing pass to get most of the material out of the way (and accuracy is not a concern there), then follow-up with finishing passes to mill the fine/final curves, which by definition put very little load on the tool/machine, so there will be virtually no deflection, or at least none that the SO4’s vwheels can’t handle.

As usual, the level of future-proofing you want to do comes in the picture too, if you think you will have other usecases down the line. But for the scenario you mention, I would go for a SO4.


Really appreciate you taking the time to write this. Thanks for the linked thread, that was super informative and amazingly well composed.

I ended up going with the SO4 as you advised. I did reach out to sales and they said the same things you did. I do indeed do all my jobs via a roughing and finishing pass (using MeshCam) so you’re right that I have little tool engagement on the finishing pass so should have minimal deflection. I’ll try to run some of the same jobs on the Nomad and SO4 when it comes and see if I notice any difference in the two pieces, but I’m already feeling confident now the SO4 will be able to handle them just as well in far less time.

I do intend to occasionally use it for other things, but those projects are just for fun and I really can’t envision a use-case where I’d need to be faster and stiffer than the SO4 can handle. Virtually all such side projects would be one-offs. The acrylic scenario is my actual day-to-day business so that’s what matters most by a long shot.

Anyway, I really just wanted to follow-up and say your advice was indeed seen, taken, and greatly appreciated! Cheers Julien!

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Great! Welcome to the Shapeoko club then (and to the more exclusive club of folks who own BOTH a Nomad and a Shapeoko, that’s a great combo)

I’m looking forward to reading posts about your creations once you have the SO4 up and running.


Hi @lurk, got an example of the stuff you’re making?

I have both, and have cut acrylic, polycarb, ABS, and HDPE quite a bit on both. You’ve already purchased, so I’m late. But the good news is either is awesome. As you’ve stated, you have the first part figured out with roughing and finishing passes. I would add that you want to do an offset pass to aid in chip clearing. Also, single flute endmills are your friend.

I have noticed that the SOPro seems to get ever so slightly better edge finish than my SO4. But that is me being REALLY picky with a magnifying glass. :slight_smile:

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Sent pictures via PM! :slight_smile:

Thanks for the input @SLCJedi! I’m glad I seemed to have made a good decision, or at least not a mistake haha! I’ll admit I don’t really know what I’m doing beyond my single use-case. I’m certainly no machinist. I use the roughing toolpath and unified finishing toolpath in MeshCam and let it autocalculate the speeds and feeds. I don’t even know what an offset path is :frowning: but I do seem to get good results with my Nomad. I planned to do the same with the SO4 (still waiting for it to ship), just changing the machine in MeshCam and letting the wizard adjust the speeds and feeds accordingly. We will have to see if the SO4 is as forgiving of my ignorance with its much faster spindle and aggressive removal rates or if I start killing bits and melting acrylic. What do you think?

I can’t use single flute endmills for the most part, my jobs are all curves so I use ball mills, my workhorse being cheapy Amazon 1/8th 22mm FL for most of the roughing work. They’re 10 for $20 and have lasted forever in the Nomad. I also use longer 0.1 degree Amana tapered ball mills for deeper jobs, 1/4" balls for big jobs, and tiny tapered balls for precision finishing on tiny jobs. Only time I get to use my single spirals is for drilling holes in acrylic (partway holes and I need a flat bottom, though I worry it’s killing my bits because it makes a terrible racket and seems horribly inefficient, but I don’t know how else to do the odd-sized holes I need to get tight fits with irregular diameter acrylic rod into the acrylic stock.)

Thankfully edge finish from the CNC isn’t a big concern since I do lots of handwork with a rotary tool after the CNC has done its thing, and I vapor polish the cut areas once I’m all done. Otherwise that would be a big deal, because I definitely do care about the quality of the edge finish to a magnifying glass level of obsession in the end product!

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