Steel core GT2 upgrade on SO4

Hi all!

I’ve been having concerns with belt stretch in my shapeoko 4. On a whim I bought GT2 steel core timing belts. I want to replace the x axis carriage belt as when you push the spindle with any small force, the spindle has a more than noticeable amount of deflection/play. This has resulted in bad v carves and other issues with cuts being off due to belt stretch.

It’s mostly a problem with the x axis because there is only one belt controlling it whereas the y rails have two total belts (more tension). My belts are properly tensioned and I’ve been having minor chatter issues since the start. When the machine is cutting in the x direction you can see lots of chatter but due to having really nice Amana tool carbide bits the cut is still nice.

Has anyone done this on a Shapeoko 4 yet? I’ve heard there might be issues from the motor pulleys being too small and bending the stiffer belts. How long do these typically last?

We tried steel belts for a while on the SO3 — they are initially stiffer and easier to tension, but as you note, due to the tight bend around the pulley suffer from metal fatigue and wear out all-too quickly. We went back to the fiberglass belts — they last for a year or so of average use.

If your belts aren’t working well, please contact and we’ll do our best to sort out what the difficulty is with you.

I’m confused, maybe you could post a video?

Generally speaking, “chatter” essentially means the cutter is literally bouncing around a bit…or a lot, which always results in a less than ideal surface finish.

As for steel belts, as @WillAdams said, consensus on this forum and from C3D, not a viable option. A quick search on “steel belts” will confirm.

A Goldilocks tune on the stock belts using the smart phone tuning procedure posted on this forum is your best bet.



For the 9mm belts there are kevlar cored options which work well, I’ve not seen any source for 15mm kevlar belts though and the 15mm fiberglass are similar to the 9mm steel / kevlar core belts.

As for the vibration, there are several things at play.

The double belts only provide double holding force when the Z carriage is in the center of the X beam, at either end you are back to a single belt worth of force.

There is a substantial difference in moving mass for an X vs. Y deflection, more mass tends to resist vibration.

There are also complex flex modes, particularly the Z carriage nodding around the X beam which get excited differently based on the force direction coming from the cutter.

More here

That’s about the SO3 but most of the same things apply to the SO4


Hi all! I canceled my order for the steel core and decided to get fiberglass reinforced belts. I had credit card points so I used them to pay for my order so I’m basically upgrading for free. I’ll post a video in a bit of it cutting. Will the fiberglass belts make a difference?

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Just curious, does the shapeoko use GT2 or 2GT. My SO4 says 2GT so I’m assuming 2GT. Are the stock belts fiberglass reinforced? If not then I’m definitely going to get some as I can easily apply force to the carriage and wiggle it. I need better stiffness and less play in the carriage.

The ‘naming standards’ for the belts are rather confusing.

Gates (the OEM who actually design them) calls the belts;

Old version of the belts - GT2
New version of the same belts with the same dimensions - GT3

Both old and new versions have the 2MGT3, 2MGT6, 2MGT9 which are the 2mm pitch belt in 3, 6 and 9mm widths. There is no Gates 15mm, this is only made by third parties such as Powge.

Most AliExpress, eBay and other sellers call the 2mm pitch 2MGT belt ‘GT2’ (which it used to be) and do not know the difference.

So, you care that

  • It is the gates GT2 / GT3 tooth profile, not a square tooth
  • It has the 2mm tooth pitch

Then you buy whatever width your machine needs and reinforcing material you prefer.


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