Hello, I recently upgraded from a SO3 XXL to a Pro 4 XXL and am having an issue with cut dimensions that I did not have with my 3. My SO3 had a dewalt router, and my Pro has a Carbide router.
On my SO3 if I created a 1.375" square it would cut it out nearly perfectly. 1.3750 to 1.3756 on the various pieces I measured (outside cut), and right on or slightly undercut by .004" for an inside cut.
I am using the same router bit that I used on the SO3.
On my Pro 4, I will be oversized by .01" in each dimension (X and Y) for an inside cut, and .01 undersized for an outside cut. I am cutting HDPE btw (Starboard).
I have played with the belts, measuring the cuts after each belt adjustment. I went from 46 hz average to 70 hz average without any measurable difference in accuracy.
The machine is square, set screws are tight, rails are clean, everything is tight.
At this point I am starting to think the issue is the trim router / runout.
To address runout I have run various speeds and feeds to see if there was a measurable difference, but it was negligible (when RPMs were much slower than recommended).
I know I can make GRBL adjustments, but I feel that is more of a bandaid then a fix, and I know the machine is capable of greater accuracy. Additionally, I did not have any GRBL customizations on the S03.
Unfortunately I sold my SO3 in a day after getting the 4Pro. Wish I wouldn’t have done that but it didnt expect it to sell right away.
It is possible that run-out is off. I had issues with one Makita I had years ago that required them to replace it due to bearings being loose. And then I had to rebuild it later in it’s service life. Our S4Pro at work started out with a C3D router and now has a C3D VFD spindle. I did have to do significant adjustments to the tram, which indicates the router was less concentric.
Honestly if you’re looking for dimensional accuracy, get a spindle. End of story. They are designed specifically for the use, whereas a trim router is not. Trim routers work great for the majority of projects people do, decor, or +/- 0.010" accuracy in wood. But if you’re trying to get better levels of accuracy, then you’re going to have to have to get a spindle that matches the accuracy of the rest of the machine. They have MUCH, MUCH better bearing arrangements. We run a good amount of HDPE, polycarbonate, acrylic, delrin, and composites on our S4Pro for scientific experiments. More power, less noise, and software control, and honestly I wouldn’t have a router without a spindle now. I have one on my S5Pro at home and wouldn’t trade it…unless C3D releases a 2.2kw one. But their current VFD spindle is a perfect match for the S4Pro motion system. We even run it on the standard Z-Plus Z axis that came with the machine.
Another thing to note is that adjusting the GRBL settings is a 100% valid way to go, and is in no way a band-aid. Especially with belts that themselves have tolerances. I have adjusted the GRBL settings on our S4Pro at work and we regularly hold less than 0.005" tolerances. We also employ roughing/finishing strategies when we need to be more precise. I have been involved with the installation and verification of multi-million dollar, 4 axis, multi-pallet machining centers in aerospace. They required many such software adjustments to meet tolerance requirements and be verified to enter service. ALL machines have tolerances, and adjusting software is a 100% valid way to account for variances from one machine to the next, or in some cases I witnessed, after a part was replaced.
I have not done any additional calibration as noted in the link that Will posted. Partly because I did not need to do this to get acceptable accuracy with my SO3, and partly because I don’t believe it is a calibration issue since the error is exactly the same in the X and the Y direction, and it is unlikely that I have induced mechanical error in exactly the same amount in both axis via the belts, pulleys, stepper motors, etc.
Because the error is exactly the same, I believe it points to the trim router as the issue. I do believe now that my bearings are bad or are going bad as SLC suggested could be the issue. The other tell tale is that I am getting slight lines in the cross section of the material being cut on each pass. While faint, the depth of the lines is consistent around the entire piece being cut out. If it was a tramming issue, I would expect the line depth to be deeper on one side than the other. Because the line depth feels consistent, it makes me think the bit itself if spinning out of round.
Long story short, I believe a new trim router is needed. I’ve got a few jobs to finish in the meantime and the .01" tolerance and faint lines on the cross section will be acceptable for now.
I may switch over to a makita trim router so that I can get a new one in a moments notice should I have an issue like this again.
Hello again. I wanted to update everyone. After thinking about it more I was pretty certain it was the router. Looking around my shop, I noted a harbor freight bauer trim router sitting on the shelf…
After a quick swap of the Carbide router for the Harbor Freight router… I am cutting back at the expected tolerance and the horizontal lines are gone. Man I was chasing my tail for awhile on this one.
The HDZ uses a ball-screw which has a greater weight-bearing capacity, finer pitch, longer travel for some iterations, and since it is ball-bearings against steel, and lubricated, doesn’t wear with long-term heavy use as the lead-screw in the Z-Plus does.
I have one on my S5Pro at home and wouldn’t trade it…unless C3D releases a 2.2kw one. 
But their current VFD spindle is a perfect match for the S4Pro motion system. We even run it on the standard Z-Plus Z axis that came with the machine.