Any increase in feedrate?
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Hugely yes, for aluminum 5052 .04" thick I was going with the following:
Old:
Spiral O Single Flute (Amana 51377-Z)
50ipm
.02" DoC
45ipm ramping
New
Same end mill
80ipm
.03" DoC
75ipm ramping
The new rate has some of the cleanest cuts I have seen and no chatter noise at all. Again donāt take my word for it because at the same time I switched my belts was the same time I also got my new Z axis and 2.2kw spindle up and running. However, given the new Z axis and spindle are definitely heavier it is nice to know these belts are stronger at their core to handle jogging around.
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No idea if reputable but, https://reprapworld.com/products/gt2x9_mm_kevlar_reinforced_timing_belt_per_meter/ 
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Interesting. May be worth checking if I ever wear out my steel core belts
Holy cow the shipping!!! $25 shipping on a $25 orderā¦
AliExpress free shipping, or expedited for less the $10. Ordered yesterday, shipping notice today.
Maybe now isnāt the right time, butā¦
There was a thread about a system from another vendor using two belts, one stationary, the other inverted into the first belt.
The idea was better power, less (no?) stretch, etc.
At the time the thought was there wasnāt enough room for two interlocking belts on the Shapeoko. Well, there is enough room on mine. Two meshed belts can fit under my idler wheels.
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Checkout process will get you a more accurate shipping quote, mine was $6.89 to CO, US. Also shipped next day.
Little more inclined to try RepRapWorld than AliExpress but thatās just me. For all I know the belts come from the same China manufacturer 
Soooo, epic fail tonight why working on calibrating these new steel belts. What I had in mind was cutting a set of small square shapes of various predefined dimensions, located at various positions across the wasterboard, then measure everything, and check whether the calibration factor (measured vs expected) is the same everywhere (which I think would be a good indicator of belt deformations). Silly me did not want to waste good stock, so I used a thick sheet of MDFā¦and shot myself in the foot by also using an upcut endmill and cutting too fast to be done quicker, and got this:
Measuring anything precise on such a messy mess is virtually impossible (face palmā¦).
BUT, since it was there I measured it anyway, and it turns out the dimensions are very uniform across all locations, so there is hope (that if I redo this test with a proper material, this will lead somewhere. My end goal is to compensate for dimensional non-uniformities depending on position (as discussed here))
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I am defiantly interested in what you folks find.
Itās the very first post of this thread. You can choose 8mm, 9mm, or 10mm x 10meters long. But I see you asking for Kevlar. I ordered steel core.
Yup, just some miscommunication. I thought you were responding about Kevlar. I have steel core as well.
I ordered the steel belts a couple of days ago. I will be interested to see how they work out.
As far as calibration, I prefer direct measurement instead of cutting something and measuring it.
This is how I calibrate my machine: https://timf.anansi-web.com/wp/shapeoko-3-upgrades-finishing-up-for-now-2/
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Yeah I also used my indicator and did some tests that way. Since I want to characterize the calibration factor at various X/Y locations, I thought it would be simpler to cut a test pattern and then do all the measuring manually, but I may end up using 123 blocks as you do, measuring, then moving them a bit, rinse and repeat.
Hereās my method for belt adjustment, works great, fast to do once youāve done it once or twice.
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It is coming out of the Netherlands so shipping will be a bit to chew on. Did anyone see what the pitch is on these Kevlar belts? Digging around like others I am sure and have not found any on this side of the pond. I have been thinking about having a backup set of belts. If I donāt i will have one break on a rush run. I would be willing to consider the Kevlar. I like the flexibility aspect versus the steel core. With all the upgrades so far cheap belts is like running low octane gas in a performance car.
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Iām really interested in this too. Iām game to try either the steel or Kevlar ones. Kevlar offers better flexibility but does that translate into worthwhile performance?
The performance will be incremental as most here need to admit we are taking the machine to levels not originally intended. I am happy getting the next level of machine with a little DIY and incremental spending. Most of the testing will be qualified as pass/fail and a little subjectively added in for interest. Anyone who uses their machine a bit will notice changes, chatter, tolerances, surface finish etc. I have a Y+ direction chatter I will pay attention to. I donāt have the equipment or intent to spend the time to apply a scientific approach. We will likely start to reveal the next weakest link, at least I hope I see it before I break something. Actually I have had more frustrations with the electronic side than the mechanical issues. At some point I am hoping to kick the make it better syndrome and just be happy with making stuff. I did sent the kevlar company a question on the pitch.
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The shipping was reasonable (a few dollars) on 6 meters of belts. I got 3 meters of steel, 3 meters of kevlar. I have steel on my X now, so far itās much stiffer than the fiberglass belts in that direction.
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