# ER20 collet advice

**URL:** <https://forum.onefinitycnc.com/t/er20-collet-advice/39412>\
**Category:** Onefinity Spindle Kit by Redline CNC\
**Created:** [July 27, 2026, 8:43pm UTC](https://forum.onefinitycnc.com/t/er20-collet-advice/39412 "2026-07-27T20:43:38Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![HardtailSr](https://sea2.discourse-cdn.com/flex022/user_avatar/forum.onefinitycnc.com/hardtailsr/32/35326_2.png) [@HardtailSr](https://forum.onefinitycnc.com/u/HardtailSr)\
**Post date:** [July 27, 2026, 8:43pm UTC](https://forum.onefinitycnc.com/t/er20-collet-advice/39412/1 "2026-07-27T20:43:38Z")

</div>

I have been looking now for hours on line for an ER 20 collet that will hold a #7 drill bit. 13/64ths Amazon’s algorithm is so stupid all it shows in 13/32nds and pretty much the same with Zoro and tools today etc. Is a #7 that rare? I mean come on #7 is the standard hole size for 1/4-20 tapped holes. Talk about a standard size? what am I missing?

---

<div class="post-metadata">

**Author:** ![TMToronto](https://avatars.discourse-cdn.com/v4/letter/t/7ea924/32.png) [@TMToronto](https://forum.onefinitycnc.com/u/TMToronto)\
**Post date:** [July 27, 2026, 9:05pm UTC](https://forum.onefinitycnc.com/t/er20-collet-advice/39412/2 "2026-07-27T21:05:59Z")

</div>

They are that rare…

Option 1: use 6mm ER collapsing collet (not exact 6mm type or mislabeled 6.35mm/1/4") which can clamp down to the 5.11 mm of the #7 drill

Option 2: Use a 5 mm collet and drill, but be careful if in steel as the smaller hole size means greater thread engagement

Option 3+: let’s see what others have to suggest

---

<div class="post-metadata">

**Author:** ![joejk](https://avatars.discourse-cdn.com/v4/letter/j/aeb1de/32.png) [@joejk](https://forum.onefinitycnc.com/u/joejk)\
**Post date:** [July 27, 2026, 11:31pm UTC](https://forum.onefinitycnc.com/t/er20-collet-advice/39412/3 "2026-07-27T23:31:56Z")

</div>

You might find a few places with 64th increments but most place that sell collets, go by 32nds. And they can hold a smaller size by about a 0.030 from the actual size of the collet size. Here is a page at the McMaster-carr website to show you the range they can hold. These collets have a lot of slots in them to compensate for the variation of size they can hold. The collets with only 4 slots will not do this. See here for the range of sizes at the McMaster-carr website. Just another note the 52100 alloy steel is a very good tool steel and will hold up to this kind of variation in sizes to be held. I use this material to make bearings for industrial machines at my work. Also it is common practice in most machine shops to do this.

> **[McMaster-Carr](https://www.mcmaster.com/products/collets/collet-type~er-20/er-collets~~/)**
>
> McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.

Cheers

Joe

PS. You can use a 13/64 drill(0.203) for a 1/4-20 tap, I do this all the time at home since it is 13/64 is more available in box stores than a number drill.

---

<div class="post-metadata">

**Author:** ![Discounttoolbox](https://sea2.discourse-cdn.com/flex022/user_avatar/forum.onefinitycnc.com/discounttoolbox/32/31115_2.png) [@Discounttoolbox](https://forum.onefinitycnc.com/u/Discounttoolbox)\
**Post date:** [July 28, 2026, 1:05am UTC](https://forum.onefinitycnc.com/t/er20-collet-advice/39412/4 "2026-07-28T01:05:29Z")

</div>

As @joejk mentioned the collets from McMaster Carr have a decent range. I use a 7/32 collet to hold #7 drill bits on my mill all the time and have no problem with slip when drilling steel.