Using metric Collets with Imperial Endmills

[oskar]

I have five SpeTool spiral end mills 1/8" shank, the cutting end is also 1/8" by 5/8" long. Unfortunately all my collets for my ER11 spindle are metric (I have a set from 1mm to 7mm). The 1/8" shank end mills will fit into a 3mm collet tight but it fits.

Should I use the 3mm collet?

As a side note I do hobby work with my home made cnc. All my life I used Imperial measurements, talking metric makes me dizzy and too late to learn metric now!

Thanks

 

[Karl_T]

A purist will say no, but I have certainly done it myself.

 

[GrifterGuru]

1/8" (0.1250") is between 3.10mm (0.1220") & 3.20mm (0.1260") so I would say be careful.

Tighten it enough to hold the end mill, but not enough to distort the collet.

 

[mickri]

ER metric collets have a 1mm range. There should be no problem using the 3mm collet. It will be at the very upper end of its range. Or you could use a 4mm collet. You would be at the bottom end of its range. Try both to see which you like the best.

 

[Ulma Doctor]

I have used 3mm collets, to hold 1/8 endmills, just don’t wrench them too hard as you might spring the collet

 

[oskar]

Many thanks to all for the clear answers. I'm not a "purist" either so I will try the 4mm and if it feels loose I will use the 3mm.

Nicolas

 

[RJSakowski]

I interpret the 1mm range on ER collets to mean nominal - 1mm to nominal. An ER collet nut has a retention tab to facilitate removal of the collet from the chuck and you can't seat the end mill prior to seating the collet in the nut. Seating an oversized tool in a collet once the collet in installed in the nut can be difficult. However, 4mm ER collet will effectively capture an 1/8" end mill. I wouldn't worry about over tightening as ER collets are designed to fully close on any shaft within their range. However, if you plan on using 1/8" shank tooling in the future, I would spring for a proper 1/8" ER collet. I use ER20 collets on my Tormach as part of their TTS tooling system. I have a full set of metric collets to allow me to accommodate drills and the like but I also have multiple inch collets in 1/8", 3/16", 1/4", 5/16" 3/8", and 1/2" sizes to accommodate all my inch tooling.

FWIW, I have never seen an end mill with a shank larger than nominal size or or that matter, smaller by more than a half a tenth. A larger than nominal tool wouldn't be able to load in standard end mill holders. Smaller than nominal by several thousandths would cause runout issues.

 

[oskar]

Thank you for the information, I agree a set of Imperial ER collets is needed and I will get them soon.