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Re: A Power Drawbar Like No Other....
« Reply #10 on: May 16, 2012, 04:24:15 PM »
Interesting arrangement. Proving the old addage 'more than one way to skin a cat'.

Being one of the guys with a 'toe in the water' on this topic, I have a box of parts waiting for me to find the time to put together my own PDB. I am taking an entirely differnet approach (BT30) and what has me backed up is the cost of drawbar collets; $400 to $500 US.  Ouchies. 

Many moons ago I had a furnace and did my own heat treating. I may have to go back to that in order to be able to make some of the parts I want . . . or . . . I could just stop  :'( and spend the money and get on with it.



You tried making your own gripper using Dave DeC's drawings, didn't you?  Or am I confusing you with someone else.

I would like to eventually change to ISO30, but since I do 99% of my work with TTS, this is a nice intermediate step.  And a Belleville drawbar is a really tough thing to do on a converted knee mill....

Regards,
Ray L.
Regards,
Ray L.

Offline simpson36

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Re: A Power Drawbar Like No Other....
« Reply #11 on: May 16, 2012, 05:58:04 PM »
You could use a servo motor and thereby control the torque.

Just a suggestion.

Offline simpson36

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Re: A Power Drawbar Like No Other....
« Reply #12 on: May 16, 2012, 06:25:03 PM »
You tried making your own gripper using Dave DeC's drawings, didn't you?  Or am I confusing you with someone else.
Nope, wasn't me. I can make one easy enough, but then it needs to be hardened and there are not too many shops that will harden one item for anything approaching a reasonable cost . .  and I can't  blame them.  I can get stuff nitrided by just 'riding the tailgate' when they are doing other stuff, but not precision hardening and tempering.
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And a Belleville drawbar is a really tough thing to do on a converted knee mill....
I've watched a few of these PDB projects and they all seem to suffer from the same problem . . .  I hasten to say 'in my opinion'. The bellevilles are not the problem. Early on I noticed that people had no idea how much force to use. Then if someone did discover a target force, they were unable to figure our how to generate enough force to release it.

A pneumatic PDB is very doable, even with R8, that's not why I switched over. A PDB is a stepping stone to an ATC, otherwise, not enough benefit for the effort, again in my opinion. I can find no way to make an unattended, reliable, ATC with R8, and I've looked at it a bunch of ways and studied a lot of different approaches currently out there . .  including Tormac's stuff. Tormac's is workable, but has disadvantages, starting with the old R8 problem of needing gobs of force to retain and turn the toolholder. BT30 is short, only needs a few hundred pounds to retain the holder because the torque transfer is taken by dogs and not friction.

 
Re: A Power Drawbar Like No Other....
« Reply #13 on: May 16, 2012, 06:52:44 PM »
You tried making your own gripper using Dave DeC's drawings, didn't you?  Or am I confusing you with someone else.
Nope, wasn't me. I can make one easy enough, but then it needs to be hardened and there are not too many shops that will harden one item for anything approaching a reasonable cost . .  and I can't  blame them.  I can get stuff nitrided by just 'riding the tailgate' when they are doing other stuff, but not precision hardening and tempering.
Quote
And a Belleville drawbar is a really tough thing to do on a converted knee mill....
I've watched a few of these PDB projects and they all seem to suffer from the same problem . . .  I hasten to say 'in my opinion'. The bellevilles are not the problem. Early on I noticed that people had no idea how much force to use. Then if someone did discover a target force, they were unable to figure our how to generate enough force to release it.

A pneumatic PDB is very doable, even with R8, that's not why I switched over. A PDB is a stepping stone to an ATC, otherwise, not enough benefit for the effort, again in my opinion. I can find no way to make an unattended, reliable, ATC with R8, and I've looked at it a bunch of ways and studied a lot of different approaches currently out there . .  including Tormac's stuff. Tormac's is workable, but has disadvantages, starting with the old R8 problem of needing gobs of force to retain and turn the toolholder. BT30 is short, only needs a few hundred pounds to retain the holder because the torque transfer is taken by dogs and not friction.

 

It hjas been pretty well established at this point that to use TTS to its full capability (which I do every day), requires upwards of 2500# drawbar tension, or 25+ ft-lbs drawbar torque.  That's hard to achieve without a really large pneumatic cylinder, a multiple-piston cylinder, or an air over hydraulic system.  Then there's the headache of packaging on a knee mill, where the Bellevilles either have to fit inside the spindle, or you need to build some kind of holder for them above the head, which is a major PITA.  Hence my decision not to go that way.

Are you sure a few hundred pounds is adequate for BT30?  Sounds low to me.  I know ISO30 requires on the order of 1300#, and I would've thought BT30 would be the same.  I think the issue is not so much torque capability, but loss of rigidity caused by un-seating of the taper, due to the holder being pulled down due to cutting forces, and leverage through the tool holder and tool holder to spindle interface.  I have seen the spec for ISO30, but not BT30.  I would not think the drive dogs would be of any real value until you get up into the 5-10 HP range.

Regards,
Ray L.
Regards,
Ray L.

Offline simpson36

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Re: A Power Drawbar Like No Other....
« Reply #14 on: May 17, 2012, 07:41:14 AM »
It hjas been pretty well established at this point that to use TTS to its full capability (which I do every day), requires upwards of 2500# drawbar tension, or 25+ ft-lbs drawbar torque.  

Funny you mention that. I offered to help a guy who was trying to figure out the force needed. I told him to use a torque wrench to tighten his drawbar and give me the number and a couple of other params and I would calculate the force for him. His response was that "you can't calculate force from torque". Sounds like you have this figured out though.  
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That's hard to achieve without a really large pneumatic cylinder, a multiple-piston cylinder, or an air over hydraulic system.  Then there's the headache of packaging on a knee mill, where the Bellevilles either have to fit inside the spindle, or you need to build some kind of holder for them above the head, which is a major PITA.  Hence my decision not to go that way.
Obviously, you have been doing some homework on this. An articulated multi piston pneumatic cylinder can generate the 2500 lbs on normal shop air. The Belleville stack is a large topic that is too frustrating to discuss on a forum :-X

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Are you sure a few hundred pounds is adequate for BT30?  Sounds low to me.  I know ISO30 requires on the order of 1300#, and I would've thought BT30 would be the same.  I think the issue is not so much torque capability, but loss of rigidity caused by un-seating of the taper, due to the holder being pulled down due to cutting forces, and leverage through the tool holder and tool holder to spindle interface.  I have seen the spec for ISO30, but not BT30.  I would not think the drive dogs would be of any real value until you get up into the 5-10 HP range.

BT30 has two specs. above and below a certain RPM. I would have to look it up again (old now,  memory not so good) but it is something like 625lbs for high RPM and somewhat less than that below a certain RPM threshold. BT30 is small and very high speed. If one compares the surface area of a BT30 taper with that of R8, things become clear. Currently I am running a 140V 37A DC brush servo motor for the spindle which I run at 180V.  Right now I only have 10A to feed it, but sitting on my desk is a new Copley model 180V 30A. That's more than 7HP.

I don't know anything about the ISO spec you mentioned, but most likely there are advantages and disadvantages to every spec and to every PDB and ATC arrangement. As a development engineer, I've had a life of setting out to build my 'perfect on paper' designs only to find as I go thru the build process all of the things I *should* have thought of  . . . . LOL!!
« Last Edit: May 17, 2012, 07:47:50 AM by simpson36 »

Offline BR549

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Re: A Power Drawbar Like No Other....
« Reply #15 on: May 17, 2012, 03:37:05 PM »
A 30 series holder should run at about #800 - 1000 drawbar force.

A r8 collet holder made need a great deal more to hold a tool in the collet for heavy cutting. Solid tool holders require less (;-)

THE drawbar force based on torque is a guess. It depends on thread pitch , friction AND the strength of the drawbar as to generate force it has to stretch to hold force on the toolholder. THere are drawbar force testers made just for that reason. AND it would scare you the amount of force some people put on a R8 collet to attempt to hold the tool from slipping in the collet.

The motor driven drawbar idea has been around for a while as well as the impact method. The motor idea never survived for various reasons. But the impact version is still active.

The standard kneemill with the reduction gearbox on top of the spindle is a bear to get a auto drawbar working dependably. OTHER than the impact method AND that is NOT with R8 collect but with R8 solid toolholders.

A machine like Dave Dec used with a simple one piece spindle is simple to do what he did drawbar wise . Don't think it would work on a Kneemill setup very well.  THE tool holder claw idea worked very well for him and the use of the modified cat30 holders allowed him to GRIP the tool holder with the ATC and get spindle/tool  indexing correct.

AS long as it works, go for it. (;-)

(;-) TP

Offline budman68

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Re: A Power Drawbar Like No Other....
« Reply #16 on: May 17, 2012, 03:48:46 PM »
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AND it would scare you the amount of force some people put on a R8 collet to attempt to hold the tool from slipping in the collet.


Yep, I've stripped my share of bridgeport drawbars.  ;D

But hey, the cutter slipped, and darn it, it has to stay put!

Dave
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Just because I'm a Global Moderator, don't assume that I know anything !

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Offline Sam

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Re: A Power Drawbar Like No Other....
« Reply #17 on: May 17, 2012, 04:36:34 PM »
Stripped a few myself. Broken a few as well, where the top nut attaches to the bar. Whats really annoying, is when the threaded section of the collet separates from the collet body while tightening the drawbar, leaving you no good way to get the bar unthreaded from the broken collet thats up inside the spindle 3 inches or so. Sorry to jack your thread with stories Ray, but they have to be told when the opportunity presents itself :)
"CONFIDENCE: it's the feeling you experience before you fully understand the situation."

Offline simpson36

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Re: A Power Drawbar Like No Other....
« Reply #18 on: May 17, 2012, 04:53:44 PM »
A 30 series holder should run at about #800 - 1000 drawbar force.
30 series?    BT30, CAT30, ISO30, NMTB30   are you suggesting they are all the same? I don't know if they are or not, just asking.

A r8 collet holder made need a great deal more to hold a tool in the collet for heavy cutting. Solid tool holders require less (;-)

Certainly a lot less to hold the tool, but transfer of torque from the spindle to the R8 taper would be the same, one would think.


THE drawbar force based on torque is a guess. It depends on thread pitch , friction AND the strength of the drawbar as to generate force it has to stretch to hold force on the toolholder.

Those would be the 'couple of other params' I mentioned. of all the factors to consider, only friction is a guess. Material strength is irrelevant . .  unless the bar breaks or strips  :o  1000lbs is 1000lbs regarless of how much the bar stretches to transfer the force.

A machine like Dave Dec used with a simple one piece spindle is simple to do what he did drawbar wise . Don't think it would work on a Kneemill setup very well.  THE tool holder claw idea worked very well for him and the use of the modified cat30 holders allowed him to GRIP the tool holder with the ATC and get spindle/tool  indexing correct.

Dave Dec again . . can anyone provide a link?  Why would you need to modify cat30 for an ATC  . .  isn't that sort of like a Ferarri modified for the Autobahn?    ;)  Don't they come out of the box ready to rock?

 
« Last Edit: May 17, 2012, 05:04:19 PM by simpson36 »

Offline BR549

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Re: A Power Drawbar Like No Other....
« Reply #19 on: May 17, 2012, 05:17:57 PM »
As a GENERAL rule YES they share the basic geometry.

ON a R8 the drawbar force and the clamping force of a collet are 2 different things. I solid tool holder is easily held with light drawbar force. A collet type on the other hand would slip the tool easily with the same light force you can use on a solid.

To calculate the drawbar force one needs to the know the angle of rotation of the  drawbar, thread pitch and the size and type of steel used for the drawbar. Torque really does not come into play.

The angle of rotation X the pitch tells you the stretch of the drawbar .

Torque is not an accurate way to determene the amount of stretch. Yes it acan get you into the ballpark(;-)

When Dave Dec did his ATC he wanted to KEEP the R8 spindle as a cost option. He modify the Cat30 holder by Regrinding it to the SAME angle as an R8 and used a gripper to pull the holder tight into the taper. BUT that is also NOT a Bridgeport type type spindle he was working with.

I'll look up the Utube page for you.

(;-) TP