Milling: collets v. end mill holders

[trlvn]

I might have a line on some end mill holders in various sizes. So far, the very little milling I've done has been with the cutter held in an R8 collet. I don't think I've ever had one slip but I read that this can be a problem.

Are larger end mills more likely to slip because of the cutting forces? Or are smaller cutters more likely to slip because of the limited surface area engaging in the collet? Etc?

Does the answer change at all for stuff like a boring head or fly cutter?

Thanks,

Craig in Oakville

 

[DavidR8]

I've used up to 3/4" roughing end mills in my ER32 collets and never had anything slip. My criterion boring head has a 1/2" shank and it's never slipped in the MT3 collets I got from you.
I may be wrong on this but I believe that end mills destined for end mill holders need to have flats on the shank for the set screw to land.

 

[Tom Kitta]

The main problem is that the torque needed for say ER32 is huge. Most people do not tighten enough when it spins (and it does).

Larger cutters are more likely to slip.

End mill holders are more "idiot proof" as if cutting hard you don't need to worry about torque.

Also with collets, especially metric ER you have a 1mm range - if you are close to the max the actual holding power is much less if you are close to min. Thus holding a 12mm carbide end mill in 11-12mm collet is far stronger then holding 1/2" in 12-13 collet. Holding 12mm in 12-13 is weak.

Weldon double shank is far stronger hold then any collet system but is also very inconvenient - i.e. lots of changes. It is the strongest "home shop" system vs. say shrink fit industrial system.

A 1/2" tap when machine tapping blind hole will spin in a collet if you go too deep - even when you hammer collet close. If there was a Weldon shank it would break the tap.

TG collet system offers stronger grip then ER.

For home shop most pp use ER 90% of the time.

 

[PeterT]

To continue on some of Toms points, there is the hobbyist machine perspective which is kind of a sub item to ponder. A Bridgeport is a differnt animal than an RF 45 which is different again to mini mill. What do you want to 'break' first if the limits get pushed? The $75 end mill or have it spin in replaceable collet & an opportunity to re-assess? Or the EM is locked in holder via its flat & set screw, collet holder is locked in spindle via the R8 slot & taper seat, so maybe a coin toss between tripping the breaker before shearing something mechanical in the drive train?

 

[DavidR8]

@Tom Kitta and @PeterT raise excellent points.
I purposely don't crank my ER32 collets down to the nth degree because I want the tool to slip as my 'safety'. Especially if using 1/4" or 1/8" endmills.

 

[trlvn]

Thanks guys, that really helps me. I did a quick inventory and most of my end mills have a flat (Weldon shank). Shanks are mostly 3/8" and 1/2" with just a few larger. (I didn't specifically buy any of the larger ones; they came in box lots.) I'm using an RF-30 mill so I won't be doing monster stock removal, anyway.

My boring head, fly cutter, and a keyless drill chuck are all on 3/4" shanks without a flat. Could have sworn the boring head had a flat but this is what happens to the memory after a certain age! Regardless, the lack of flats means they won't work in end mill holders I'm looking at. At least, without modification.

I think I will try to pick up 3/8" and 1/2" end mill holders if the auction price doesn't shoot out of sight. I'm thinking I might just leave a 'go-to' end mill locked in each one for ready access.

Thanks again,

Craig

 

[RobinHood]

Another thing to consider is stick-out. You will add quite some distance by using R8 end mill holders. This will make things less rigid. What you gain in “drive torque” you will lose in increased deflection. So overall, the performance may actually be worse...

 

[Tom Kitta]

The few times my endmills got broken was when they were not tight enough in the collet and spinned in the collet. With power feed on an endmill that spins in a collet == broken endmill. Same if endmill stays spinning but the part moves with power feed on - broken EM.

For R8 one has to see practical system limits of R8 - I think it is like 3hp. Remember R8 system was designed for BP that had rather little power compared to their more industrial cousins - BP was very versatile but not a hogging machine. They started with just 1hp motor. It is current idea of putting large motors on BP and applications such as large face mills for which R8 was never envisioned.

Both of my machines are 40 taper.

 

[Dabbler]

I have a set of R8 collets, and an ER40 adapter. I use the R8s all the time, but hardly ever use the ER40s. Just habit, I guess...

 

[Brent H]

My BP has the 30 taper so I have a mixture of adaptors - the 30 taper collets are very expensive so I only have 2 that are end mill holding- I have a set of OZ25, a set of ER32 and a set of ER20 that also fits my router/shaper.

If the mill was an R8 type, I would probably have just that type collet set as @Dabbler suggests. It would add some space under the quill - as would be very useful on a smaller mill and the R8 family of collets and holders are a lot cheaper.

Because it is a 30 taper any larger boring tools, saw arbors, drill chuck holders, face mills etc are all part of the 30 taper so they are fairly robust. Not quite as large as @Tom Kitta 's 40 taper but it helps having that beefier collet for larger tooling. You can plow off a lot of material without chatter or worrying about slippage.

 

[PeterT]

I know we have seen references to gripping power of ER collets, require X amount of torque & maybe co-related to runout numbers, can't remember.
But has anyone seen an equivalent number to R8 collet grip by torqueing the drawbar to X?

Just as an aside, I am kind of a torqueing Wus on my machine. I have a Milwaukee cordless 3/8 drive that I can reach the drawbar nut on my tippy toes. I just wrench it tight by light feel, not a torque clutch setting. Even so, the odd tool wants to stick a bit in spindle but needs just a light tap with hammer. I'm told this is normal. BTW I mostly use the cordless tool just to quicly rotate on & off as a time saver, not for any torque setting. Some of those pneumatic systems I see - I'm not sure do they adjust it somehow?

 

[Tom Kitta]

In R8 the drawbar torque not only provides gripping power of the collet but also makes sure the collet does not spin itself as the little positioning bolt in R8 is not for power transfer but position (it helps when removing an R8 collet, some people remove it). Thus I would think it is more crucial to get the torque right in R8 under heavier loads as spinning collet will quickly mess up your spindle.

In 40 taper system (and I assume in 30) the torque is transferred not through the taper (which itself is much, much bigger surface area then in R8) but through a set of two bars. Thus there is little need for me to make sure drawbar is drawing the 40 taper tooling in hard. It is snug but no more then that. When removing after some harder work requires few taps of brass hammer (unscrew few turns then tap). The limit of 40 taper system is something like 15-25hp so a LOT. Some people for unknown to me reasons (probably running CNC stuff) remove 1 or both of the torque transferring bars and rely just on surface contact - which appears to be working with low power application but of course is risky.

Most pneumatic systems use butterfly wrench which does have max torque and I am sure you can set the amount of air. I only used such system briefly and I attest that it works great. One press and its on, one press and its off. I did not hold it long to tighten it extra strong.

Here is possible R8 stripping the set key - http://www.chaski.org/homemachinist/viewtopic.php?t=98260
Note guy was running 1.5" end mill with a 0.05 DOC i.e. 1.25mm with "slow feed" hitting some hard spot. Not exactly light milling for R8 taper.

 

[Dabbler]

I find R8 to be more than adequite for all my needs: I'm not a heavy cut person... I agree that 3/4 is on the big side for a conventional milling cutter in R8... I have 1" and 2" insertable face mills I use occasionally. I'm a really big fan of fly cutting instead of face milling. Slower, but I like the finish and control it gives.