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Messages - simpson36

501
all the denford atc triacs after 1990 use either bt35 or bt30 toolholders without the drive dogs, so they rely solely on the taper for drive.

That could be because someone determined that the dogs were not needed (say if the mills are all low HP spindles),  . . . . -or- . . .  it could be that they were unable or unwilling to provide the required indexing for the spindles in order to use the drive dogs.



502
Yes, I have an Emco F1 bench top mill and it uses BT30 holders without the flats and the spring force is sufficient to hold the holders not allowing them to slip.

It would be helpful to know the application.  There is a lot of distance between driving 1/4 roughers thru aluminum and pushing a big face mill across an interrupted cut on stainless.

503
That's a good point, and I like when I see the air blast to clean the tooling during toolchanges, however, sometimes it ends up blowing chips/dust right into the critical surfaces.

It has been said that you can easily train a cat to do whatever the cat wants to do. This applies to swarf as well, I would imagine. Having reviewed a lot of different ATC designs, I elected to set a goal of completely encasing the adapters in 'pods' to keep the tapers clean. I even have a soft o-ring at the bottom of the pod . . . OK, maybe a bit of overkill there.  :-\

504
Thanks, fellas, and yes, Ray, most guys I know usually yank that set screw/key out as soon as they get a mill. More of a nuisance if anything.

I can vary the force on the spring pack by turning the threaded cap on the stack, so anywhere from zero to about 2,000lbs with the current springs. A different spring is available in the same size (ID/OD) the would take it into the 3k neighborhood, but I don't anticipate needing that.

A little over .200" is needed for release, to that limits the amount the stack can be compressed and still release the tools. I would need to look that up again before I quoted, but as it stands now I have it set to 1,000lbs and release is approx 1,400 lbs. All numbers are calculated as I have no way of directly measuring these forces.

505
Actually I thought of something I always wanted know about these type of interchangeable setups. How does the individual toolholder keep from spinning in the socket when under extreme loads? Is it simply the spring/air pressure from the contacts of the balls?

Transferring torque thru balls is not unheard of. Motorcycle transmissions use this method. In your toolholders it seems like something of a 'band-aid' for a stubborn problem, but without seeing the internals on the parts, it would just be speculation.

506
Steve,

Is that really correct?  I would think tapers are purely a matter of friction.  If you know the material and surface finish of the inner and outer tapers, you know (or can measure) the coefficient of static friction (assuming a completely clean, dry fit, as it SHOULD always be).  Knowing the drawbar tension and taper angle, you can easily calculate the normal force.  Multiply the two, and you know the total frictional force.  Integrate that over the entire taper, and you can calculate the total torque that can be generated before the taper slips.  I can see how stretch might be factor on low-angle tapers like Jacobs, but have a hard time believing it's a factor on CAT/ISO tapers, especially given the relatively low drawbar tensions, relative to the cross-sectional areas of the toolholder tapers.
Yes, I mentioned that the formulae are similar, but there is no threshold that I am aware of where the rules change after a certain angle. As you pointed out, the force pressing the surfaces together is what creates the friction. That force can come from many sources. In the taper, the force is not applied perpendicular to the surface but rather, as you correctly point out, thru an angle. If you consider each surface as non moveable, then the calculation is not complicated. But all materials are elastic. It is only a question of degree.


Bolts are a lot easier for people to comprehend. While  the calculations are again straightforward, they are not directly concerned with the amount the bolt stretches, but rather the tension generated. The stretch is definitely there in any case. Take the practical example of putting 100ft lbs of torque on an ungraded 'hardware store' 3/8" bolt. It is unlikely the bolt will ever reach the 100lb spec. It would just stretch to failure. A grade 8 bolt would generate an extreme amount of tension. The difference is in the material. If manuf #1 us using bolts from Home Depot, he may well have a different spec than the manuf who use using graded fasteners.

My intention was to point out some of the non-so-obvious, but still very much present factors that might provide some insight into why one manuf would have different specs than another manuf for the same CAT40 taper and why both might be correct in their application. I think it would be safe to say that my X3 spindle would never survive a 4k or 5k pull on a taper. Considering the material that spindle is made from, we would call that a swaging operation . . . LOL!

I think we all do the same thing, Ray. 1) We run the numbers and then 2) build a prototype and then 3) try the prototype and see if the calculator lied or not. Some folks skip step 1. 

507
Surface FINISH also comes into play. The drawbar force IS important to maintain force on the taper it is NOT like a morse #2 where the taper is so long it has a lot of holding power from the force exerted on the taper by the pin(tool holder).  Remove the tension from the BT30 holder and it WILL fall out under cutting loads. Let it get light and it will chatter the toolholder in the spindle under heavy loads
I don't recall saying that the drawbar was unimportant. Certainly I did not suggest running a BT30 without the drawbar.

Quote
I do not KNOW of a STANDARD value or calculation to estimate the force needed.  Each Manf I have dealt with has their OWN NUMBER working range they want you to set the drawbar tension to. They know the basic values needed then add in a fudge factor from in field experinces over the years.

See Ray's post

Quote
Most are similar for each taper.  THICKER spindle shafts may need more, harder spindle shafts may need more.

Agreed. Now, why do you suppose that is?

Quote
The standard taper used for all the sizes is not used by accident it it a compromise of torque transfer  AND being able to release it from the spindle

Agreed. Remember I started out by sating that tapers is a large subject. The same forces are at work in all tapers. An MT is no different that a CAT40 as far as the physics are concerned. They behave very differently as you correctly point out, but that is because of the shape. For example a 1 degree taper and an 89 degree taper are going to have very different properties and every angle in between will follow suit, but still follow the same rules.

You have long pointed out that there is a range of tensions quoted and that each manuf has their own numbers and there is 'trial and error' and in this post you mention 'fudge factor'. All true. What I offered is an explanation of some of that factors that make it true.

508
Drawbar tension has been argued ad-nauseum, so instead, how about some physics and 'why everyone can be correct even with vastly different numbers'. First; there is no linear relationship between drawbar tention and the friction generated. The way tapers work is a very large topic, but in a nutshell, the force on the taper (pull on the drawbar in this case)  is generating a press fit. If you want to calculate the torque transfer, you would use similar formulae. The inner part of the interface (the adapter) will be considered incompressible and thrown out for the purpose of this discussion. So the amount of interference fit depends primarily on the material of the outer part of the interface (in this case, the spindle itself) and the amount of drawbar force. That may seem contradictory, but the same drawbar force will create different interference with different materials because the material stretch is vastly differnet from one material to the next.
 
This is where the 'trial and error' would come into play as was mentioned in the last couple of posts. By way of example; my current mill has an X3 spindle which is just soft mild steel. May have been Toyota frames in a past life, who knows. As a guess, I would (generously) give it 50k - 70k yield. My new Mega-Duty 4th axis spindle and the BT30 spindle are 110k yield. A deeply hardened 4000 series steel spindle could easily hit 190k. It is going to take a lot more tension to spread the taper on that last spindle than on my crappy X3 spindle. It would not surprise me if the drawbar tension for a VMC spindle would literally exceed the yield strength of an X3 spindle and permanently deform the taper. A big hole in the nose and the adapter would be up inside the spindle somewhere . . . .LOL!


To address another question: Surface area of the taper is a consideration, of course, but the gorilla is the taper angle.

So before anyone argues that the drawbar alone is retaining the adapter, consider that if the taper angle is very low (i.e. MT, JT, etc) the leverage is great (lots of stretch for not so much force) and the grip is tremendous for that reason. Note that these tapers do NOT need continuous drawbar tension once the initial force has caused the interference fit. It would be a decidedly bad idea to remove the drawbar from R8 for example after the adapter/collet was seated, but the effect of the pseudo press fit is evident when you have to hammer on the holder (thru the drawbar) to free it from the spindle.

In a later post, I  will comment on some other topics and show the completed iteration #2 ATC claw . . working perfectly and will be the final design.

 


509
Look forward to hear what the Copley's can do :)
Hood

Turns out the Mitsu has it also. Not a big surprise there. I purchased a second identical gearbox and the Mitsu 400 fits it so perfectly! I would like to get another Mitsu for the carousel, but they are a bit pricey for that application, methinks. Maybe I could justify it by getting a 750 watt Mitsu for my Z axis and recycle that 400 watt guy to carousel duty.

A good friend of mine had an interesting strategy that so far has not worked for me. If he wanted it, he bought it and his universal justification was 'I needed it to make me happy'. Must be nice . .  ::)

510
Nice work!.
It is also amazing to me that people will send you an email or PM but they won't post in the forum so everybody can participate.

Forums are the place where the small can be giants and the weak can be bullies. People have a bad experience with some a-hole who ridicules their ideas or comments and they don't post any more. With 3,400 reads and the same handful of people participating, I have to believe there are some great ideas out there that we will never hear about. It is foolish and short sighted to trade a moment of bravado for losing a resource, but a lot of people do it.  Great ideas often come from unlikely places.

If somebody posted a comment or idea in this thread and got slammed for their trouble, I'm afraid I would have to go postal on the slammer.