Cuts consistently under sized - Help Please

I’m about to do a larger project that will see me cut about 5 x half sheets of plywood to build something. Before I started I wanted to double check everything so I checked the tightness on my v-wheels and also the lead screws. Double checked the Y parallel situation. But something is wrong that I can’t find.

I decided to do some simple test cuts with 100mm x 100mm boxes, and the last two were the 100mm box with a 100mm diameter circle in the middle. I’m using the Sienci up cut bit and this is the first use of it (although I had the same issue with my Yonico compression but, but I have eliminated that from the test now). The boxes are NOT square and they are out by a consistent amount.

In the X axis my 100mm distance comes in at 98.9mm. In the Y direction it is 97.85.

I’m cutting 19 MDF at 2520 mm/s with a 5mm DOC. |Material is screwed down.

I can’t make boxes for the project if even a small 100mm box is out of square. Any ideas what it might be?

The file is a simple Fusion 360 file and there is no Stock to Leave set.


I’m realizing that it might be helpful if there was a spot designed in to the machine to easily take a reference measurement. For now what I did was bring the XZ gantry to the front and back it off the feet a known distance and then try and use the edge of the front foot to the edge of the gantry side plate as a distance to check both sides. I found there were out just a tiny bit. I adjusted one side by hand very slowly and got them to virtually identical. The problem is it is impossible to know if your caliper is measure in the same spot height wise each time. Maybe a flat ledge could be added to the foot to reference off for checking squareness of the XZ gantry against the front feet.

I’ve tried the cut again and my Y distance is now 99.1 so less than 1mm off, an improvement. My X axis cut is almost identical at 99.05mm. Not sure what is going on there. I think I may try starting the larger project and see if it cascades in to a bigger issue or not.


Two suggestions: 1) you might want to measure your endmill to make sure its diameter is what you expected and entered into Fusion 360. 2) You could try to cut a 200x200 square to see if the error multiplies by 2 or stays the same. I suspect the latter which would mean this would be even less of an issue for larger parts.

And I almost forgot, I think that Scienci recommends feed rates somewhere between 800 - 1000mm/s, at least as a starting point. You are running over twice that.

Good idea, I’ll try the 200x200 square and see what happens. I’ve switched back to the Yonico compression bit and it is 6.35 on the nose. I’ve had no issues with this one in the past.

I did start the job and instead of under sized, the 12.3 mm Dados are coming out at almost 13mm which is going to be an issue for fit. I had a suspicion something was wrong when there was a thin strip of material left up the center of the dado, suggesting it stepped too wide. Argh.

I have been successfully cutting baltic birch at these feed rates and depth of cut, accurately, already. As has someone else on the forum (he was the source for the feeds and speeds).


Holy smokes. Mid job I was inspecting things and I noticed something dark in the sawdust. Thankfully the vac shoe hadn’t grabbed it. The bloody nut/screw from the anti-backlash nut on the X has come completely free and fallen out. The cylindrical piece that you adjust with. Well that would certainly explain odd behaviour. Holy hell. Not sure how to get that back in there or how it could walk it’s way out of there?!

Yikes! the anti-backlash devices came with a set screw (some folks call them grub screws) and a nut. In the Longmill assembly instructions (at least when I used them in January) replaced the set screw with a longer regular allen head screw and no nut. I think this made assembly easier but with no nut the screw could work it’s way out and undo the adjustment. Later I decided to add the nut and went back to using the set screw so I could use a socket to tighten the nut.

This could account for wonky dimensions in the X axis

Well the plot thickens. I did the full bed cut of 9 parts last night from a single sheet and everything was undersized by about 1mm-1.5mm. I sent the machine back to zero, Z up, as I do and shut down. I came back today to run more of the parts in a new sheet and now a part that is 21mm and came out at 19.9mm last nght is coming in at 23mm. It’s a contour cut. Nothing changed over night, same laptop, same everything, same Fusion data. Although I did adjust some of the contour cut settings, nothing that should have widened it, as that wasn’t my intent. I was going to make it work as long as everything was under sized by the same amount.

I am using some truly horrible G1S 1/2" ply from Home Depot. Never again. It’s too much work to design and set things up to cut garbage material. I didn’t know quite how bad this stuff is. I’m actually wondering if the quality of it and potentially some flex in the wood (due to the narrow width of some of my cuts and the poor material quality) may be contributing to it. But if that were the case I wouldn’t expect such consistency across the table either.

I’m really at a loss as to what has happened. I’m going to work my way through the other 5 sets of cuts and see if I can salvage the project at the end. As it was designed to be precise I have my doubts but I need the project done before I can mess with the machine again. Very frustrating.

I did replace the grub screw with an M5. Not sure how I missed that during assembly. I did check my Y axis ones and they are fine, so unclear how the X got so out of whack but it seems better with the M5 in there. Very tight and stable now.

I’m starting to feel like I can’t start any material project without going through a pre-flight checklist of all the major fasteners and adjustments. I’m tempted to partially disassemble the machine and build it back up from the (now updated and very nice) manual. I used Chris’ video the first time, which I found very good, but I can’t help thinking maybe I missed something? Is anyone else finding they have to do a lot of adjusting to maintain consistency even over a few days of use?


@jwoody18 I’m not doing anything that big, Jeff, so my results may not be relevant. Most of what I have been doing lately are double sided picture frame jobs. The largest is about 21 x 28". I’ve not had any issues with size or registration. I don’t mess with the machine much. I push and pull on the gantries to make sure they are not sloppy and turn the wheels from time to time to ensure that they are OK. I’ve not adjusted them in quite a while.
I wish I could offer some advice, but I’m stumped.

I’m pressing on, perhaps not a good idea, and we’ll see how the job fits together. After fixing the backlash on the X I think I feel a bit better about it. I do think this totally crap wood is throwing things off a bit and making it hard to really get a proper measurement with the caliper. If I had extra baltic birch I would do a small job in that to compare.

For now I’ll just continue double checking everything until I get more confidence that something else doesn’t slip. Now that I have the proximity switches installed to detect crashes I shouldn’t be at risk of shaking things by accident. Loving those.


What sort of work holding are you using? Parts shifting around while cutting can also affect your tolerances.

Hold down was via screws.