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er20 collet question

dmcnally

Aluminum
Joined
Jun 12, 2012
Location
Lompoc, CA
I have an older CNC mill with NMTB40 tool holders. The mill has a manual drawbard. I've been using er16 collets in collet chucks with 1/2" shanks for drilling. Swapping the collet chucks is a lot easier, faster, and cheaper $$$, than swapping the NMTB40 tool holders.

I bought 5 er20 collet chucks with 3/4" shanks and I'd like to use them to hold my end mills (1/4, 5/16, 3/8, and 1/2), edge finder, and 90 degree chamfer bit. I'd like to swap the collet chucks in sort of a poor mans power drawbar.

I experimented with the 1/2" end mill. I was trimming (facing) some stock to get it to the proper size. I zero'd the 1/2" end mill and then attempted to trim 0.100" off the top of a piece of stock. This setup trimmed 0.110". I tried with a 0.020" cut and the resulting cut was 0.024".

I'm assuming the difference is due to collet flex. Does that sound correct, or am I way off base (i.e. clueless)?

I'd like to use the collet chucks for the poor mans power drawbar, but I need to have predictable depth of cuts.

Any advice would be greatly appreciated.

thanks,
Dave
 
.....I tried with a 0.020" cut and the resulting cut was 0.024".......

That does not sound like "flex". What are you cutting for these tests?

Are you holding the straight shank collet holders in a side lock holder mounted in the spindle? There's a reason for a steep taper. It locates radially and axially pretty positively. The straight shank has the potential to not positively locate axially.

Are you sure the machine positions accurately and repeatably? After the cut, is the tool length still correct. .02" cut should not cause a tool to pull out of a collet, but sometimes strange things happen.
 
Thank you for taking the time to reply.

I've attached a picture of the toolholder and the collet chuck.
er-20.jpg

I'm cutting 6061 T6511 aluminum. 1/2" 2 flute HSS endmill. 0.100" depth of cut and 0.25" stepover. When the 1/2" end mill has been mounted in a 1/2" NMTB40 toolholder, the cuts are very predictable and repeatable( ~0.001").

thanks,
Dave
 
From what OP wrote, I would be looking at:
a. Are you picking up Z height accurately? (How are you doing it?) And is the top of the stock flat to start with?
b. Is your stock held properly.

That said, even in fancy machines with renishaw probes and laser tool setters, endmils sometimes cut a little higher or lower than expected in Z. But we're talking order 0.001" - and it changes with speeds/feeds.
Your 0.100 cutting 0.110 or worse 0.020 cutting 0.024 seem very large to me.

You might be better served by switching to a power drawbar add-on - and maybe convert the machine from NMTB to CAT or BT while you are at it.
 
a. Are you picking up Z height accurately? (How are you doing it?) And is the top of the stock flat to start with?
I zero the endmill the way I was taught at the junior college that taught my into to machining class. I run the endmill down to a precision ground 1-2-3 block on parallels and use a 0.003" shim (from McMaster-Carr) instead of a piece of paper.

b. Is your stock held properly.
The stock is held in an old Bridgeport vise.

You might be better served by switching to a power drawbar add-on
Yeah, that would be nice... but it's not in the cards right now. :(

thanks,
Dave
 
I agree that you seem to be going the long way around the block, but - three lefts doo make a right, so ... ???


Bryan's power drawbar was not exactly my first thought, but might be the first place to look into.

My thought was to convert to a "Quick Switch 40" spindle.
Your current spindle may be able to be modified, but I doubt it.
But maybe you could get, or produce a spindle for it yourself?

FWIW - "Quick Switch" spindles use NMTB tools, but "QS" designation tighten up the tol of the flange location to the taper.
Many times a standard NMTB holder will work, but other times you may need to modify them - or maybe not be able to use them.


----------------------------

Think Snow Eh!
Ox
 
ER20 is very much on the low end of rigidity at the 1/2" tool size, even on my Tormach I won't rough with a 3/8" or bigger in an ER20 holder. Set screw is the preferred. The difference in rigidity shows up in the cut in the form of chatter normally.
 
Thanks for all the comments. I'll go back to using the endmill in the NMTB40 tool holder and I'll use the er20 collet chucks for drilling operations.

Thanks again,
Dave
 
I zero the endmill the way I was taught at the junior college that taught my into to machining class. I run the endmill down to a precision ground 1-2-3 block on parallels and use a 0.003" shim (from McMaster-Carr) instead of a piece of paper.

The stock is held in an old Bridgeport vise.

Yeah, that would be nice... but it's not in the cards right now. :(

thanks,
Dave

There is NEVER "enough money, NOW". Consider;

- Used power drawbar you have to make some bits for and adapt.

- "Native" 40-taper holders, if you please. Even TG or ER on 40-taper integral tail is not an improvement over ignorant side-locks. You were aware that either of ER or TG are meant to see SERIOUS-GOOD cleaning and accurate use of a torque wrench, each setup? Side-locks are simpler.

- Stacking straight-shanks of either is not even close to Best Current Practice.

- Seek used Quick-Switch or such, as Ox suggested. By luck - later expanded - I have the PDQ-Marlin "VS" and "S", each with Welden-style side-locks. Grab 'em if you find 'em, used, add more. Because you can. Marlin Tool still makes those "PDQ" new, and right here in the USA. Just not cheaply.

Hunt for this stuff, be patient. You can find it.

"Meanwhile" a genuine BirdPort vise is not a plus. At all. Notorious for allowing s**t to move too easily.

Hunt you a used Kurt or decent clone that at least has a form of angle-lock pull-down.
 








 
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