ER collets vs. R8

If you already have a decent R8 set, there's no reason not to use them. I have an ER40 chuck installed in the Bridgeport as I have a larger selection of good quality ER40 and the chuck was a similar cost compared to filling in the gaps in my R8s with quality collets. I have some end mills that I lack R8 in the correct size for, so I usually leave the chuck in there and use the ER40s. I also already have the ER40s and a chuck for the lathe, which gets a lot of use. My collet blocks also use ER40 for the through hole capability as another post already mentioned.

Rather than using the drawbar, you can change collets using a spanner on the ER chuck. Some people like this better, it's about even in my mind. I'm about 6'4", so the drawbar nut on the top of the machine isn't a big deal to reach up for. That could be a bigger issue for the more vertically challenged. A down side is you need more torque on an ER collet than you do with the R8.

While the chuck does add some runout, it's a couple tenths in my case. That is acceptable for the work I do. If it ever isn't, I won't hesitate to get more R8s as needed.

I don't notice any difference in rigidity, though I suspect it's there. I may just have not pushed things hard enough to notice. More stickout almost has to drop some rigidity.

You do lose some space in Z as well, a couple inches. This isn't an issue for me, but I have a fairly large Z travel. It might or might not matter for you.

If you were to use the ER collets, you would need to purchase ER collects AND an ER to R8 adapter chuck. In your case, I would say not to spend the money unless you were already buying the ER collets for another reason, such as the lathe or collet blocks. At that point, it might be worthwhile to have another option for tool holding in the spindle.

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Post by toastydeath » Fri Jun 12, 2009 12:09 am

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I know the issue is pretty much closed, but I use ER-40 at work.

ER-40 is enormously rigid in what, in my opinion, it was sized for - a CAT-50 collet chuck and nothing smaller. In a CAT-50 chuck, there is very little overhang, and that's necessitated by the locknut that holds the ER collet in place. So you have a 3" diameter locknut, adding about 1" to the spindle distance. You can't do much better than that in CAT-50, even with endmill holders.

Even CAT-40 is too small - the outside of the chuck is bigger than the taper holding it in the machine spindle. Consider that one 50-taper chuck weighs 15-20 lbs without any tool, and a 40-taper weighs less than 5 lbs. Huge difference. And a pissant R8 weighs ounces. ER-40 holds big tools, it's forgiving on diameters, it has friggin' gobs of grip. I use them all the time on a 20 hp horizontal mill at work. No reason to use them on a 5 hp bridgeport or similar.

ER-40 is way too damn big to put on a R8 shank, but obviously someone would do it. The overhang is just unacceptable. R8 shanked machines just don't have any good reason to be using tooling that large. Really, there's not a good reason to be using anything but R8 tooling. I could see something like ER-16 or ER-25 for repeatability after toolchanges as a matter of personal taste. Still, I reach for R8 collets on any mill with an R8 taper.

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