Bit Sizes - Bit Shank Sizes and Additional Bits

I have a couple questions regarding bits.

  1. Are all sizes of CNC bits manufactured in all shank sizes?
    Say a 1/16th" bit - better on a 1/4" shank?
    2 I have the starter array of bits. end mills, ball nose, v-bits, surfacing bit, several sizes and extra collets.
    I’m thinking about adding a 3/8" bit and something that engraves better than a v-bit but runs on a basic, C3D spindle. I’m only 6 months into this and I make mostly signs and containers.

Any suggestions appreciated.

Thank you. JC

  1. No, that wouldn’t be possible — the specific dimension you note probably does exist, but it would be expensive and doesn’t have a reason to exist when there are 1/16" tools on 1/8" shanks which require removing less material.

Do you have a 1/8" precision collet?

  1. 3/8" collets are tiny things on a Standard Compact Router and are usually used only for metrology — might be able to make them work, but you’d need to work up suitable feeds and speeds — as an alternative, why not consider 8mm (5/16") tooling?

The only way to get a collet in odd sizes is with ER type collets. The ER type collets come in SAE and Metric sizes and depending on the ER collet there is a range of sizes. The Makita, C3d Router (Standard one) and the Dewalt routers are not compatible with ER collets due to the thread pitches of the router shafts. Carbide3D does sell a router that has an ER11 Collet and is in their shop for $150.00.

If you want you can buy precision collets for Dewalt 611, Makita/C3d Routers at Elair.corp. If you bought your router from C3D then you have a 1/4 and 1/8" precision collet as well as the original 1/4" collet.

Almost all router bits regardless of the cutting bit size come in either metric or SAE shank sizes. The most common SAE are 1/2", 3/8", 1/4", 1/8" and for Metric the most common sizes are 8MM, 6MM, and 4MM. Never mix SAE and Metric collets. Use SAE collets with only SAE bits and Metric Collets with Metric Bits. A 6mm is close to 1/4" but they are not the same and are dangerous to interchange.

Here is an example from the of ER collets sizes.

ER Collet Length Diameter Clamping Range
ER-11 18 mm (0.71") 11.5 mm (0.45") 0.5—7 mm (0.020—0.276")
ER-16 27.5 mm (1.08") 17 mm (0.67") 0.5—9 mm (0.020—0.354")
ER-20 31.5 mm (1.24") 21 mm (0.83") 1—13 mm (0.039—0.512")
ER-25 34 mm (1.34") 26 mm (1.02") 1—16 mm (0.039—0.630")
ER-32 40 mm (1.57" 33 mm (1.30") 1—21 mm (0.039—0.827")
ER-40 46 mm (1.81") 41 mm (1.61") 3—26 mm (0.118—1.024")

If using an ER type collet you have to have multiple collets to fit inside the ER collet. There is a limited range of bits that will fit in individual collets inside of an ER collet. Generally if you want to use a 4mm shank bit you have to get a 4mm collet to fit inside an ER collet.




Thank you for this awesome information. I wish I could determine what bits to buy next. I have a Shapeoko Pro xxl with a basic C3D spindle and the collets you mentioned. Still a tad confused with usage of downcut versus upcut bits.

Thanks again for taking the time to post this!


Downcut endmills are for when the tool pulling up at the surface of the material is a problem — they can also be useful (w/ good dust/chip extraction) when cutting sheet goods (the downcut geometry pushes the material down, rather than pulling it up).

There are also compression tools which are upcut at the bottom, downcut at the top, but we don’t sell any.

Everybody works on different stuff. One thing in common are a few bits.

#251 1/4" downcut bit An alternate is Whiteside RD2075
#112 .0625" 1/16"
#102 .125" 1/8"
#302 60 Degree Vee Bit Frued 20-152
#301 90 Degree Vee Bit
#201 1/4" upcut 3 flute bit. This is the basic bit but I seldom use it because it leaves fuzz all over the cut. I mostly use the #251 down cut but there are uses for the #201.
Fly Cutter Whiteside 6210 Used to flatten your spoilboard. C3d sells the mcfly but if you ever wear out a carbide fly cutter you would be doing a lot of cutting. So for me the replaceable cutters on the mcfly is a gimmick but individual requirements need to be thought over.

After these basic bits there are ball nose which is useful for 3d carving. There are also specialty bits that c3d does not support like bowl bits, 45/22 degree champer bits. Some use dovetail bits keyhole bits but these bits are only needed for special cases.

Some people like to buy the bundles but I think they are mostly a waste of money. For instance there are 5 bits in the bundle and two of the bits you use and 3 you may never use. So buy the individual bits you will need and get the rest as needed. A good deal is only a good deal if you USE the bits.


That is one of the most helpful guides I’ve come across to date.

Thanks so much!


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