The Home Machinist!

OK Harold, just what does "tight enough" mean? Torque it reasonably or give it all you've got?

Jerry

How I wish I could answer that one. To be perfectly honest, I don't know that I can put it in words. It's something one acquires over years of practice, and may not mean much when described. It's much like tightening a grade 2 ¼" hex head cap screw. You just seem to know when it's time to stop turning the wrench.

There's a world of difference in how tight things should be. As an example, if you've ever operated any precision grinding equipment, and it happens to have the wrenches provided by the maker, you'll notice that they are short handled. When you make adjustments on a grinder, and that's every time you use it, you must learn to snug things, so stresses aren't introduced to the setup, or the action of tightening a fastener doesn't move the table. In fact, when a grinder is in operation, it's a good idea to not even touch the machine, especially any part of the carriage or wheel head. The slightest touch offers the opportunity to scrap the work, due to deflection.

As far as the drawbar of a drop spindle mill (Bridgeport type of machine) is concerned, you certainly want it tightened enough to prevent movement of the end mill, keeping in mind that the right hand helix of the common end mill is trying to pull the cutter out all the time it's cutting, with the deeper cuts and feed exerting far more effort than a slight finish pass. By the way, that's the reason left hand helix cutters are made. They don't pull, but push up instead. Problem is, they have negative rake on the ends, so they're not good for end cuts.

So then, how tight to tighten?

Well, how big is a rock?

If you're using a 1" end mill taking a full width cut, plunged .600" deep in aluminum, it has to be quite tight, I discovered. That's the only time I ever pulled one that I can recall. I could be wrong about that, but it certainly hasn't been a regular occurrence for me.

The draw bar on my BP is the factory original (purchased new in '77). I have never pulled the threads, nor have I pulled any threads in the collets. I have to assume that I am using reasonable judgment in tightening collets. The only way I could answer the tightness question beyond that is to use a torque wrench, but that isn't the same as using the provided wrench, as there's a distinct difference in leverage, so the feel wouldn't be the same. Further, the tightness is load dependent, so a hard rule just doesn't work.

I can appreciate this question because tight is a relative term. To a guy who's ham handed and lacks a basic understanding, breaking small bolts when tightening is routine. Such an individual simply doesn't know when to let up. By sharp contrast, I have no doubt that there's an individual somewhere, likely even here on Chaski, who has had a small end mill move while in a cut, possibly because there's a thought that over tightening is a bad thing (and it is). However, the flaw is in his understanding of what's tight enough, and what isn't.

I wish I could say.

H

Right or wrong, I'll take a stab at it, because I had the same question in the beginning.

Lots of folks really push hard on needing EM holders and Weldon tooling as opposed to collets because of the risk of EMs pulling out of collets. But I run up to 1/2" carbide EMs in steel using collets, fed harder than most, and taking pretty heavy bites when useful, and I've never had the problem. Early on a friend with lots of experience explained to me that you needed to keep the tooling, collets and spindle taper very clean and oil free. Wipe it down each tooling change. Seat the EM deep in the collet, but not so the flutes go in. And if coated carbide EM, then the shank is generally not coated, and that bare area is where you need to grip. Obvious adjustments for long EMs, or when needed for spindle clearance.

Anyway, when all is right, tighten the draw bar until everything holds itself, and then continue tightening. As you tighten, you feel it taking up the slack, and everything tightening up, but required torque to continue goes up fairly gradually in a somewhat linear fashion. Then at a certain point (usually a bit after I was comfortable initially), you feel that all of a sudden the resistance to further tightening (increased torque requirement) goes up very rapidly. That's the point where all the flex/spring/space/whatever is taken up, and everything is basically solid with no opportunity for flex remaining. Much more than that, and you are going to start entering the yield range for the draw bar (like torque to yield head bolts). It is really quite noticeable when you get there, and just solidly into that range that's where you stop. If everything, including drawbar threads are in good shape, there seems to be no risk, at least not that I"ve encountered so far.

Note that on less than ideal tooling interfaces, the limit point can be harder to gauge. I had a big fly cutter with an as manufactured 3/4" bolt shank, and it made the "tight" point sort of mushy without the distinct, "wow, that's it" rapid increase in required torque. I assume that's because it's not a precision parallel surface like the EM shanks. Or it could be the 3/4" collet not having as much meat. Fortunately there isn't much (any really) pulling force on a fly cutter as opposed to a helical end mill which can have quite a bit.

But for the 1/2" and smaller EMs, there is never any guess work about "is it tight enough". And even with 1/2" carbide EMs fed hard, I've had no slipping since my early education on collet tightening.

Side Note: Larger ER collets have somewhat the same issue. Most folks never get close to the required torque to fully seat them properly, particularly in the common ER-40 size often used for mill tooling. Here's a hint. I'm pretty confident no man who ever lived could achieve near the proper torque using the handle that comes with those sets...

Lots of posters will not agree, but with this setup of a #40V flange EM holder that is keyed to the spindle and holding the EM with a set screw on a flat, I have never had an EM move or slip. Break, yes. I have a friend who used to work for SGS Tool, testing cutting tools. His job was to push the tooling as far as it would go before it broke. He ALWAYS used an EM holder in a Vertical Mill with the set screw(s) tightened up as tight as possible. I assume that the EM holder was a keyed R8 type, but I can ask if anyone is interested.
--earlgo

I think one thing that has been overlooked is the quality of the collet. Some Chinese collets can be off by a thou or two which would affect tightening.

earlgo wrote: ↑Wed Apr 29, 2020 9:41 am Lots of posters will not agree, but with this setup of a #40V flange EM holder that is keyed to the spindle and holding the EM with a set screw on a flat, I have never had an EM move or slip. Break, yes.no slip tool holder.jpg
I have a friend who used to work for SGS Tool, testing cutting tools. His job was to push the tooling as far as it would go before it broke. He ALWAYS used an EM holder in a Vertical Mill with the set screw(s) tightened up as tight as possible. I assume that the EM holder was a keyed R8 type, but I can ask if anyone is interested.
--earlgo

I can't get any Cat tooling to fit in the spindle of my Bridgeport clone, so I can't test that.

Dave

One limitation that many have is that there's often not a good way at hand to hold both the spindle and the drawbar for tightening. You really need a splined wrench on some machines to hold the spindle. My old gear head didn't have a break, so you put it in low gear and rap on the end of the wrench like an impact wrench would do. 4 taps and it was "tight enough" - generally. Not good on the gears, but never broke any either. A splined wrench would have let me hold the spindle but I never got one made.

While I wait on brake shoes to arrive, I am locking the back gear on the Bridgeport clone to tighten the drawbar. Also not a good practice since forgetting that it's bound up could break something if the motor gets turned on. Funny thing, I watched a video a couple weeks ago of some "kid" explaining how to use a Bridgeport mill. He kept referring to the backgear knob as the brake!

Dave

My Bridgeport clone mill (South Bend) actually has a spindle brake knob.
You can see it hanging down on the left side of the belt housing case.
And I've operated Bridgeport mills that did have a similar spindle brake. I think... Or maybe not.
Hmmm...
RussN

Most bridgeport clones have an actual spindle brake, I think. The point was that not all milling machines do. Many mill/drills for example don't have one.

Dave

Gee how tight do you make the collet....it's much the same as tightening bolts, spark plugs or anything else...tighten it till the threads start to pull , then back off a quarter turn....
All kidding aside, I was told many years ago that 3/4" is the maximum diameter end mill recommended on a Bridgeport....this coming from someone who ran one for 50 or so years. As everyone knows you can get larger collets than that, and Weldon shank holders up to 1-1/2", and lets not talk about shell mill holders that can handle up to a 6" face mill.
I've never had an end mill start to pull in an R8 collet, but on my Cat40 CNC mill I have had 1" dia carbide end mills pull from the collet holders a few times.

Nyle

The Pratt & Whitney mill has a spindle lock, not a brake. You press the little button on the side near the back of the spindle and spin the spindle until it goes into the hole, like a bull gear pin.



The wrench is about 10" long and you can really get a collet to tighten down.

On the P&W, collets are the ONLY method to hold tooling, as they never came with end mill holders. I was thinking of making my own end mill holders anyway, just for my own piece of mind.

Steve

Thank you all for your "tight enough" answers. Kinda what I figured, experience and judgment.

Jerry