Arc/Bend in X rail

Hello All,

I have found that my machine cannot make straight lines across the X axis, there seems to be a 1mm arc in the rails, is there a way to adjust this does anyone know?

Hey Brenden,

in the rail? You mean a bow in the chrome-plated, hardened steel hollow shafts with 6 mm wall thickness? If so, do you have a long straight edge?

Yes, those rails, which are straight when I put a straight edge up to them, so I’m stumped. I wanted to put a new base on it anyways so I’m taking this opportunity to dissamble it all and reassemble, crossing my fingers the voodoo goes away.

Hey Brenden,

if you have some play in the linear bearings, the router/Z assembly could be deflected during load. The radial forces at the end of the bit can be quite high when milling.

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What router or spindle are you using? What size bit? The x rails being bent would be… highly improbable.

Is your table dead flat?

Bit deflection or Z axis deflection seems far more likely! Is your Z carriage snug and is the router reasonably trammed?

Regardless of the spindle being on or not, it arcs in the middle as I jog the x axis across.

Yes table is flat, haven’t trammed, when I installed the spindle I just leveled the top of the z carriage. How are you thinking that effects the linear x movement? Hyuanyang 220v spindle, great unit, had zero run out. No I don’t think the rails are bent, since I checked, I just can’t account for this…but I will check the z carriage for snug, good idea.

Thank you both for taking the time to respond

Hey Brenden, I had the same problem, I added the stiffy rail and it helped, still not perfect but it helped

Hey Brenden,

are you sure there is an 1 mm arc in what the machine milled? Or is your wasteboard edge (or whatever reference you use here) simply not straight?

Yes–that is a ~1mm groove made from jogging the machine manually from side to side, the straight edge is a piece of 3/16 brass flat bar I had, but I’ve also referenced against my Festool rails, Stabila levels, all show it. I have it at .6mm with feeler gauges.

Thanks for the heads up, mine is a foreman with the stiffy ( just the third rail on the x correct?) what were your tolerances across? I am getting .60mm of an arc

Here is a video of what I am doing. After I cut this groove in the wasteboard I put a straight edge across and measure the arc in the middle…

Carriage is fine, Im wondering if tramming might somehow be influencing it but I don’t see how in theory

Hi, cant remember the exact numbers but as I remember it was close to half a mm in the center, the stiffy reduced it to a few thou, close enough for me now. I have a 21 lbs spindle hanging on a modified 80mm holder, so it needed some help

Yeah, Im wondering what is spec. Most people seem to make signs not anything that needs tight tolerances, Id be happy with a few thou but I’m stuck at .60 despite all my troubleshooting.

Do see the same arc when trying it a different Y locations - front , middle, back?

Most of these hollow hardened tubular rails that I have looked at have straightness tolerances in the tens of microns, even for the longer lengths, so what you are seeing is quite a bit. If your tubes were installed twisted with the ‘tramming’ bolts, I don’t think it would result in a bow.


To rule out any deflection on the Z axis whilst moving across, I did a few lines at differing depths, still the same, and to be conclusive, I used a 1/8" bit to mark three holes, one at each end and the middle and stuck 1/8 bits in there as pegs to see if the gap could be recreated via drilling and no lateral loads, I have .5mm

I would think the stiffy would mitigate a bow in any one tube. Is it difficult to remove the stiffy and run the tests again - just curious, not based on science or engineering.