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?
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
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.
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.
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.