The Mach1 system reveals the secret here. There are really two issues to consider.
First is locking the tool holder to the mill. That's going to be a function of the surface area of the R8 taper and the pull force. I am not too surprised that 600 lbs suffices for that force, even with quite a lot of "work" being done by the spindle.
But there is a second force, and the way it works is hugely counterproductive to the first. That second force is the squeeze on a collet to hold the tool. It doesn't exist with solid tool holders, and it is the reason the Mach1 system has that special R8 collet holder. It's purpose is not only to squeeze the collect down on the tool and not only to provide a reference datum so Z is repeatable. It's alternate purpose is to provide a clean r8 taper surface on the tool. You can see this clearly on the patent illustration:
http://www.google.com/patents?id=8mgWAAAAEBAJ&printsec=abstract&zoom=4
Why is this so important? Because the deformation of the collet as it locks down on the tool really interferes with its ability to make good contact with the R8 taper. If you think of bluing tapers, there is no way in heck that there is much precision in that interface. So now the drawbar force must not only provide sufficient clamping, but it must also combat the reduced surface area and hence friction of the collet in the taper. The Mach1 system avoids all of that.
Folks get started on these air-cylinder only systems because they seem simpler than an impact wrench system. But they're really not unless you're prepared to live with a huge amount of drawbar tension, and even then I wonder how well they are going to work with a facemill or a large silver and deming bit. People keep saying that this has been tried over and over, and it has. The drawbar manufacturers would love a simpler cheaper mechanism, if only one would work. Yet they keep shipping impact wrench based systems for R8, or special patented tricks like Mach1.
What I will tell you is that a rookie machinist can build an impact wrench system in an afternoon and it won't suffer from any of these problems. It can be completely automated for use in an ATC if desired. It's simpler and cheaper. Your biggest challenge for the ATC is that you'll be using solid R8 holders which don't have a standard interface for the ATC carousel. That's no big deal. You'll need to fab some collars for the tooling that serves that purpose. Meanwhile, you will be saving a fortune on TTS holders and you'll have a more rigid system to boot.
Cheers,
BW
Bob,
You seem to "know" things that nobody else seems to know. For example, the required drawbar tension. I looked high and low, and could find *nobody* that could give me anything more concrete than numbers they pulled out of the air. I even asked Tormach - had several e-mail exchanges with several of their engineers, in fact - and THEY have no idea! They asked me to send them any information I come up with! So, I went with what I had, and best guesses, which turned out to be too low. I looked at what Hoss and others did (only 500#, BTW), and assumed I'd need at least twice that, due to the far more aggressive cuts I can take. Even that estimate was too low, by a factor of 2-4. So, had I had a good number at the outset, I never would've gone down this road.
Saving a fortune on Tormach holders? I always planned to make my own, so cost was almost $0. No savings at all there. The commercial drawbars out there are just that - drawbars. I don't think any of them, even the Mach1, are intended for use in an ATC, so it's apples and oranges. Were I after nothing more than a power drawbar, I would've built the impact wrench kind a LONG time ago. It's cheap, simple, and works. But it's *not* well suited to an ATC, which was always my goal.
As for "this has been tried over and over" - I've spent most of my career doing things most others told me could not be done. And, more often than not, I've succeeded. So, for me, the fact that nobody else succeeded is hardly reason to scrap the whole idea, unless I know *why* the others failed. In this case, I did not, so I decided to give it a try and learn for myself. If nothing else, I've learned a few things that I'm sure will come in handy down the road somewhere. And I still believe this approach *can* be made to work, and the cost would not be THAT much more than the impact wrench kind. I can now see good solutions to *all* of the problems., even the high drawbar force requirement. It's now down to whether the additional time, money, and effort to do it is worthwhile, given that I have no shortage of other projects I need to get to. I may well go ahead with something else for now, and complete this in the background. It absolutely CAN be done - I have no doubt of that at this point.
Regards,
Ray L.
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