Drawbar Tension....

[HimyKabibble]

I want to design an automatic toolchanger, using Tormach toolholders, and a spring-loaded drawbar. Does anyone know what the actual drawbar tension should be? I just measured the drawbar torque, as I normally tighten it, and came up with about 25 foot-pounds. By my calculation, that means a drawbar tension of about 2500 pounds. Can anyone validate that as a reasonable number?

Regards,
Ray L.

[ViperTX]

I went to zerofast.com/torque.htm and for a Class 2, lubed, 1/2 inch bolt it looks like the tension is 43000 x .14?

Paul

[chevdrgtrk]

look up Mach 1 quick change tool, they have a spring loaded system,and they have a spring rate of 600lbs....if I remember correctly

[HimyKabibble]

The drawbar is 7/16"-20. Bolt tension is normally estimated as:

Tension = Torque / (diameter * K), where K is 0.15-0.20, depending on the materials used, whether the threads are lubed, etc. This gives about 4500 pounds for a 7/16" bolt at 25 foot-pounds. But, I keep seeing numbers in the 500-600 pound range talked about for power drawbars. That's a pretty big discrepancy....

Regards,
Ray L.

[BobWarfield]

A CAT40 uses 1800-2500lbs:

http://www.clamprite.com/productinfo.asp

600 sounds low, 4500 sounds high. 2500 might be just right. LOL

But Ray, why not just keep things as they are and do an impact wrench style? Getting to the optimal force so the tool doesn't slip appears to be a non-trivial and recurring problem with the collet-style systems. Solid R8 toolholders are repeatable, more reliable, and off the shelf.

I suppose you could just conclude you won't worry about it with enough spring tension.

Cheers,

BW

[HimyKabibble]

Originally Posted by BobWarfield A CAT40 uses 1800-2500lbs: http://www.clamprite.com/productinfo.asp 600 sounds low, 4500 sounds high. 2500 might be just right. LOL But Ray, why not just keep things as they are and do an impact wrench style? Getting to the optimal force so the tool doesn't slip appears to be a non-trivial and recurring problem with the collet-style systems. Solid R8 toolholders are repeatable, more reliable, and off the shelf. I suppose you could just conclude you won't worry about it with enough spring tension. Cheers, BW

Bob,

My ultimate goal is a full auto toolchanger, which is hard to do with the impact wrench style power drawbar. I can see how to do it, but it just seems kinda kludgey to me. If I were building *just* a power drawbar, the impact wrench would be the obvious choice.

Regards,
Ray L.

[Geof]

Originally Posted by BobWarfield A CAT40 uses 1800-2500lbs: http://www.clamprite.com/productinfo.asp 600 sounds low, 4500 sounds high. 2500 might be just right. LOL BW

And a 5C is around 2000lbs; 2500 could be an upper limit.

[BobWarfield]

Originally Posted by HimyKabibble Bob, My ultimate goal is a full auto toolchanger, which is hard to do with the impact wrench style power drawbar. I can see how to do it, but it just seems kinda kludgey to me. If I were building *just* a power drawbar, the impact wrench would be the obvious choice. Regards, Ray L.

There's gonna be a kludge of one kind or another either way you go unless you swap the spindle for a taper designed for an ATC.

I think people are going to find the Hoss-style collet and springs works fabulously well for X2's, and is problematic for larger mills with more horsepower.

Just choose your poison in terms of what it is that is being kludged.

Cheers,

BW

[HimyKabibble]

Originally Posted by BobWarfield There's gonna be a kludge of one kind or another either way you go unless you swap the spindle for a taper designed for an ATC. I think people are going to find the Hoss-style collet and springs works fabulously well for X2's, and is problematic for larger mills with more horsepower. Just choose your poison in terms of what it is that is being kludged. Cheers, BW

Bob,

I gotta disagree with ya there. I've been using Tormach toolholders since day one, and I push my 3HP knee mill to the limits on a regular basis - doing full slotting cuts over 5/8" deep with a 1/2" endmill. Never had a problem. This setup will be more than capable of applying more tension to the drawbar than I normally do when I hand-tighten it. The only downside I see is there will occassionally be tools I need to use, like a boring head, that will require a manual toolchange, since the collet will have to be removed to swap in another R8 tool. Other than that, I think it will work fine. Having a CAT30 or CAT40 spindle would be nice, but ain't gonna happen (at least not until I swap my knee mill for a real VMC, which is a very real possibility....).

Regards,
Ray L.

[BobWarfield]

Originally Posted by HimyKabibble Bob, I gotta disagree with ya there. I've been using Tormach toolholders since day one, and I push my 3HP knee mill to the limits on a regular basis - doing full slotting cuts over 5/8" deep with a 1/2" endmill. Never had a problem. This setup will be more than capable of applying more tension to the drawbar than I normally do when I hand-tighten it. The only downside I see is there will occassionally be tools I need to use, like a boring head, that will require a manual toolchange, since the collet will have to be removed to swap in another R8 tool. Other than that, I think it will work fine. Having a CAT30 or CAT40 spindle would be nice, but ain't gonna happen (at least not until I swap my knee mill for a real VMC, which is a very real possibility....). Regards, Ray L.

Not surprising that cut would work. My feed and speed calculator says it requires a little over 1/2 HP. It'll be the more aggressive cuts where there is trouble, but perhaps they'll never come up.

The cut PoppaBear had trouble with from the Tormach video was a 3/4" endmill, 2" axial engagement, 1/2" DOC. That would be more like pushing your mill to the limits. Tormach themselves say not to use TTS for it.

I get a lot of use out of my face mill and my 5/8" indexable end mill, not to mention Silver and Deming bits. Those are the sorts of things that would bother me with a collet-based system.

As I say, you kludge one thing or another. Just a question of what and when, LOL!

Best,

BW

[HimyKabibble]

This is amusing.... I contacted Tormach tech support, to see what drawbar tension they recommended. The response I got was:

"Funny you should ask that question. We have been trying to determine the optimum draw bar tension for a R-8 collet and have not been able to put an accurate description together for it yet. If I had something I would be happy to give it to you but I don’t. If you find something we would appreciate you letting us know. It seems to be a mystery of the industry that no one can put a real number on, but develop a feel for it working with it every day. "

Oh, well! I've got my power drawbar all designed, and should be able to build it up in about a day, once I have all the materials on-hand. So, it should be working by sometime next week. I've designed it so I can adjust the tension from 500-1500#. It should be under a second to release and re-engage the drawbar, since it's all done with two sequenced air cylinders. The long pole in the auto toolchange will be returning the Z axis to the home position (quill full up), then returning to the working position afterward.

Regards,
Ray L.

[vmax549]

HIYA RAY, Take this for what it is worth. IF there was a way to create a successful atc from the R8 collet format you would see them everywhere. THere are serious flaws to the normal R8 system that have yet to be overcome when trying to develope a true springpack pnematic release drawbar. I have seen many designs tried(;-)

The other problem to overcome is the pressure placed on the quill/stem pushing the springpack to release the collet. It can be done but requires a pushme/pullme design where the load is NOT imposed on the quill or bearings.

The only sucessful long term successes I have seen have ALL been the impact type screw drawbar.



Just a thought, (;-)TP