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Author Topic: BT30 spindle from scratch - with power drawbar and ATC of course  (Read 411013 times)

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #240 on: October 06, 2012, 07:36:44 PM »
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.

Regards,
Ray L.
Regards,
Ray L.

Offline Chaoticone

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #241 on: October 06, 2012, 07:36:54 PM »
Very good points and a lot to digest.

Brett
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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #242 on: October 06, 2012, 08:25:12 PM »
just an example of the different levels of drawbar tension, an emco f1 (boxford 190) has a hand operated crank to release the toolholder which is relatively easy to use, but a denford triac uses a lot more drawbar tension to hold the same size bt30 toolholder, operated from an air cylinder.
The reason is the spindle power, the triac is about 2-3 times more powerfull than the emco f1.

Offline derek

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #243 on: October 06, 2012, 08:28:30 PM »
If your driving it with lugs then isn't it just a calculation of how much downward pull a tool will exert on the Belleviles.

Derek
Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #244 on: October 06, 2012, 08:39:51 PM »
something i've just remembered, many years ago i was operating a 40 int mill and i checked to see if the lugs were actually doing anything. After the job the gap between the drive lugs and the toolholder was still there, so this job didn't cause the toolholder to use the drive lugs.

Offline simpson36

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #245 on: October 07, 2012, 05:27:46 AM »
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.

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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

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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?

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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.

Offline simpson36

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #246 on: October 07, 2012, 06:20:50 AM »
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. 

Offline simpson36

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #247 on: October 07, 2012, 06:23:44 AM »
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.

Offline simpson36

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #248 on: October 07, 2012, 06:30:39 AM »
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.

Offline simpson36

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #249 on: October 07, 2012, 06:39:23 AM »
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.  :-\