Linear Bearing Rail Size

Whilst upgrading the Genmitsu 3018-pro CNC I started measuring the linear bearing rail size. This has made me realise that the bearings are not a match to the rail.

linear brass bearings fitted to Genmitsu z-axis

I swapped the linear ball bearings out for brass bearings. However, when I made these I did a quick measurement and saw something close to 8mm.

Hence I reamed the brass 8mm, fitted the bearings and put the machine to use. This improved the machine as the ball bearings had effectively failed. Failed at the point they came off the manufacturing line I think.

However, there was still quite a bit of play in the Z-axis.

So, back to looking at upgrades to the Genmitsu 3018 CNC.

After spending some time looking at bearings, feeling the play, making some measurements and then swapping parts. I realised the issue is the z-axis rails are under-size. They are not 8mm in diameter, measuring them with a micrometer I get 7.96mm

checking linear bearing rail diameter with a micrometer
Checking linear bearing rail diameter with a micrometer.

Going through my material box I checked the diameter of some nominal 8mm silver steel and I get 7.94mm

Luckily some years ago I had dismantled an inkjet printer and I had kept the 8mm rails. Checking these with the micrometer showed they were 7.995mm in diameter. That was more like it. The bearings on these ails had no play at all.

Checking the X and Y axis rails on my machine they were 9.995mm and interestingly those axes have no play. There is some flex in the X and Y rails as they are rather long, but that’s another problem for another day.

So, I cut and machined two 8mm diameter rails from the old inkjet printer parts. These z-axis rails are 95mm long. I then used the 45mm long linear ball bearings that I originally bought to improve the z-axis. Now they work perfectly with rails of the correct size. These bearings are longer than the original bearings and so they do restrict the z-axis travel. However, stiffness is more important to me than travel at the moment.

Genmitsu z-axis bearings
New bearings and rails installed.

So, note to myself. Check all of the dimensions and don’t take anything for granted. 7.96mm is not 8mm diameter. Especially true when using linear bearings.


  • 3018 Desktop CNC Supply – this company offers a large list of upgrades including 12mm rails for the X and Y axes.

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