3/8” endmill on shapeoko 4?

Hi everyone!

I currently have an huanyang 800w water cooled spindle with an er11 collet and I need a way to chew through material faster and cut larger pieces. I’m doing large horns for loudspeakers so I’m doing lots of roughing and fine contours to shape the flare.

I was looking on Amazon, and they have collet adapters that could convert the spindle to a er16 collet and I would be able to use 3/8” bits. Is this feasible? My spindle draws no more than two amps during hard cuts so I know I have more power available. Can the machine handle a 3/8” bit? The one I’m looking at has a 150ipm federate.

Also, what is the consensus on these adapters. Are they safe to use? I’ve heard conflicting information but it seems most people don’t think they are safe.

This thread from folks who know their stuff seem to indicate that this would be a Bad Idea™

If you want to maximize MRR, I would maybe start with playing around with @gmack’s spreadsheet

and figure out if you can optimize your toolpaths based on 1/4" tooling?

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The collet adapters have been used by some folks, but they limit the RPMs, so can be counter-productive.

I will note that there is a (tiny) 3/8" collet available for the Makita RT0701, but most folks just use it for measuring tools.

I share your frustration that the commonly available spindles on Amazon seem to universally downsize the taper along with the current rating. AliExpress doesn’t have that problem, BTW, and I think 1.5kw/ER20 is the sweet spot for home machines; the current draw is fine with regular domestic outlets and you can put just about whatever tool you want in it.

While I have no experience with the taper adapters you listed, I am naturally suspicious of how they reference on threads.

When I was searching for a solution to a similar problem, I bought an ER20 extender from Amana Tool and cut down the shank. You’d probably need the 1/4”-shank version, since you’re starting off with ER11: https://www.amanatool.com/te-100-cnc-high-precision-tool-holder-extension-1-4-shank-5-7-8-inch-length-1-7-64-inch-dia-1-4-inch-inner-dia.html I would try this over any other modification, but I would recommend an ER20 spindle as a permanent fix.

I have the non-flange version of this: NEW Arrival 1.5KW ER20 Air Cooled Spindle Motor Square Spindle With mounting flange 0.01mm Accuracy 4 PCS Ball Bearings|Machine Tool Spindle| - AliExpress on a different machine and I have zero regrets (you probably need the flange to mount it to Shapeoko).

I’m already using fusion 360 adaptive toolpaths and I’m cutting 100 ipm with my Amana 1/4” 46376-k upcut 2-flute endmill.

I do 100ipm at 9mm doc and 2.54mm engagement.
It’s much faster and efficient than what carbide create can do and it is fast, but I’m making large horn flares and it takes lots of time to carve away the bulk of the material. It takes usually an hour to cut each “slice” that makes up the flare (cut from 1” pine). Each top and bottom flare is 4 slices total. Here is a photo of three slices glued together.

A lot of the time is in the finishing paths to create the fine contour which I can’t change (it needs to be a fine contour)


What is your RPM? A general consensus on these machines is to go for high stepover and low DOC in order to maximize material removal. I almost always run everything at 70-80% stepover and the depth changes based on material. You could also switch to a 3 flute. I don’t cut wood much on my machine so I don’t have any specific recommendations.

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If you are at 24.000RPM, that’s a 0.002" chipload. In pine there is probably scope to be more aggressive on the chip (but reduce DOC, sometimes MRR improves)

If you are not at 24k, you should (and bump up the feedrate proportionnally)

Like Will said the adapter approach will likely severely limit your max RPM (for example that Amana adapter @veteranbicycle linked, is rated for 10k RPM max), so in the end it’s unlikely you would get a higher MRR with 3/8" tooling.

Silly question: what is the geometry of the piece of stock you start from ? is there a chance you could do most of the rough material removal with power tools before you load the stock on the Shapeoko to give it is final shape ?


Here’s another question, why do you need to go up to 3/8" exactly? ER11 collets go up to 8mm, you should be able to order 8mm tools as they are very common. Otherwise, you could use 5/16" collets and order these from Lakeshore Carbide for a substantial improvement in MRR. That would make the biggest change with the smallest effort.


Specifically for finishing the 3D surfaces and curves, I can sympathize with the understanding that bigger diameter = faster. Have you considered using a bowl bit or a cove bit? Those are pretty easy to find in larger sizes but 1/4" shank. It might even be good for roughing.