Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: HimyKabibble on May 12, 2012, 09:27:13 PM
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I had a crazy idea for a completely different kind of power drawbar a couple of years back, and yesterday I finally had the time to build one. I specifically wanted to be able to use both TTS tooling, as well as R8. This meant Belleville spring designs were out, as it is very difficult to provide enough drawbar tension to properly retain TTS tooling under worst-case conditions (about 2500+ pounds drawbar tension, or 25+ ft-lbs drawbar torque is required). To change out a TTS tool, the drawbar only needs to be backed off about one turn, while for R8 tooling it needs to be backed out 8-10 turns.
So, here's my crazy idea - use a stepper or servo motor, driving through a high-ratio planetary gearbox. This provides very high torque capability, and makes it easy to control how much the drawbar is turned, by driving the stepper as another axis from the CNC controller. A little E-Baying got me a 55:1 NEMA34 gearbox capable of 75 ft-lbs. I picked up a 400 oz-in NEMA34 stepper, which lets me use the whole 75 ft-lbs if necessary. Mechanically, the thing is dead simple - The stepper and gearbox are mounted to a floating plate which is moved up and down by air cylinders. A 3/4" socket is attached to the gearbox output shaft by by a simple home-made coupling, and either a mechanical or pneumatic mechanism is used to actuate the spindle brake. I got a dual-shaft stepper so I can put a rotary encoder on the shaft, to monitor actual movement, if needed. Everything can be controlled by a simple macro (or, in my case, a simple KFlop C program).
Here's how it will operate:
1) Spindle stops, spindle brake is applied
2) The PDB is dropped down, with low air pressure, and the stepper is rotated slowly, until the socket and drawbar hexes align and engage. A micro-switch will signal that the socket is fully engaged.
3) The stepper is turned at high current/torque to loosen the drawbar by the required amount - one turn for TTS tools, 8-10 turns for R8 tools.
4) Once the new tool is mounted, the stepper is turned at reduced current/torque (corresponding to 25 ft-lbs drawbar torque), until motion stops.
5) The PDB is raise up, and the spindle brake is released.
The photos below show the complete mechanical assembly, missing only the engagement microswitch and spindle brake actuator. Here's a short video showing the PDB moving up and down while mounted to the mill:
http://www.youtube.com/watch?v=saGv_deZmL0&feature=youtu.be
I just spun up the stepper for the first time. On Monday, I hope to get the software going, and be able to "use it in anger" for the first time.
Regards,
Ray L.
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HIYA RAY interesting approach but it is not new. The problem was getting enough torque to consitantly tighten the collet and having enough torque to LOOSEN it back up. SOMETIMES it takes more torque to loosen than tighten and THAT is the reason everyone WENT to an impact wrench (air or power) to ensure it got tighten enough AND that it would loosen it back up consistantly.
Even saw one that used the spindle motor to rotate a NUT that tensioned the drawbar. The machine shifted into low range for more torque and A fixed plate dropped down on the nut and held it and then the spindle rotated to loosen it then retighten it. Then shifted back into high range.
Just a thought, (;-) TP
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Terry,
That will not be a problem at all with my scheme. The motor/gearbox combination is capable of 75 ft-lbs - about 3X the max required torque. Tightening torque will be limited to ~25 ft-lbs by current limiting in the stepper driver. Current will be max for loosening, so the full 75 ft-lbs will be available. So, I will have ~3X as much torque when loosening as when tightening. There's no way the loosening torque requirement will be 3X the applied tightening torque.
Regards,
Ray L.
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SOunds like you have it figured out. Please keep us up to date on how it is working.
(;-) TP
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Well, I'm happy to report the power drawbar seems to work *perfectly*! I got it all plumbed and wired, and did the 20 or so lines of macro code to make it work, and it grabbed, and released, the tools 25-30 times in a row, without so much as a single hiccup! Limiting current when tightening seems to work perfectly, tightening the drawbar very consistently, and no problems whatsoever loosening it again with max current. This is going to be REALLY nice, and gets me one giant step closer to an ATC!
Regards,
Ray L.
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Nice craftsmanship, Ray.
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Nice craftsmanship, Ray.
Thanks, Sam! I'm really pleased with how it came out, and how easy it was!
Regards,
Ray L.
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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.
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Pretty neat Ray, nice concept.
One question ... When tightening, do you run the motor for a specific time to assure proper tightening and let the motor stall ? ? or are you sensing the load as an input to Mach ?
Thanks,
Russ
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Pretty neat Ray, nice concept.
One question ... When tightening, do you run the motor for a specific time to assure proper tightening and let the motor stall ? ? or are you sensing the load as an input to Mach ?
Thanks,
Russ
Russ,
Right now, I have only partially implemented software to drive it, so I'm taking a brute-force approach - To loosen, I command a move equivalent to 1.25 turns of the drawbar. To tighten, I command a move equivalent to 1.5 turns of the drawbar, ensuring the motor will stall before it goes that far. Seems to work just fine like that - I've been using it all day today without a single hiccup (well, other than Mach3 weirding out one me twice....). When I move it over to the KFlop, and ditch Mach3 (Mach3 is currently running ONLY the drawbar, everything else is running on the KFlop), I will put an encoder on the stepper, so I can detect when the drawbar stops moving, and stop the stepper right there.
Regards,
Ray L.
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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.
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You could use a servo motor and thereby control the torque.
Just a suggestion.
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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.
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.
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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.
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.
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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.
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
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!!
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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
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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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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 :)
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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?
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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
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Steve here is the original You TUbe that started it all(;-). Dave Dec is a sharp cookie.
http://www.youtube.com/watch?v=l2PWuqgviS8&feature=relmfu
He went on to produce his OWN new version of a scratch built Mini CNC machine center the UMC10
(;-) TP
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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.
After two full days, mine is still working perfectly, and I am VERY pleased with it. Soooooooooo much nicer than the execrable impact wrench drawbar....
Regards,
Ray L.
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"Torque is not an accurate way to determene the amount of stretch. Yes it acan get you into the ballpark(;-)"
We don't agree completely on how to get here, but in this statement, we agree completely. In my youth, I raced. Part of racing is building motors. Rod bolts are arguably THE most critical fastener on the motor and while most people (as well as all OEM) are satisfied with using a torque wrench, they do not work well with UBER strength bolts form ARP or equiv (if I remeber that brand correctly). We used a stretch gage to measure the actual stretch on the bolt. Still using a torque wrench (the old beam type), to turn the nuts, you could still see the torque being applied when you reached the correct stretch. THat will make a beliver out of the worst skeptic (usually I fill that role :-)) However, a lot of rods used cap screws, so it was back to the trusty torque wrench for that.
OK, enough remenising, back to the debate; I get the part about holding on to a cutter with a collet vs holder, but you ahve not addresssed my comment on transferring the torque from the spindle to the taper, which is done by friction alone on R8 . . as to the pin? Don't even go there. Many people remove it.
One more comment and I'll leave you alone. It seems curious that one would re-grind every CAT holder instead of regrinding the spindle? Perhaps that will come clear when I get the link. I reground my R8 spindle to BT30 no sweat . . in fact it is due for a touch up.
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Steve here is the original You TUbe that started it all(;-). Dave Dec is a sharp cookie.
http://www.youtube.com/watch?v=l2PWuqgviS8&feature=relmfu
He went on to produce his OWN new version of a scratch built Mini CNC machine center the UMC10
Thanks for that link! As soon as he said the word 'prototype' I understood why modify the holders instead of the spindle. I will check out his other vids when I get a chance.
Very nice machine, I must say. Of the pieces of the puzzle, I have the BT30 spindle, the multi piston Pneumatic cylinder, The wrong Bellevilles (they are sized for R8 . . way too big), and most importantly, as repeated numerous times by Dave, indexing on the spindle.
My spindle is a big DC servo motor and I have on my desk the newest member of Copley's AccelNet drive series. Jumping up from a 10A PWM to this 30A servo drive will be quite a nice change. Hard tapping, here I come!
Now for the problem child; the drawbar collet . . . last night I purchased a furnace to do my own heat treating again. Lots of new possibilities open up . . . including making my own 4th axis spindles . .
The last piece of the puzzle will be the hardest one to find . . . .time.
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ON the important stuf you preset the bolt with a torque wrench to a low point just to take up the slack. Then rotate the bolts a set DEG of rotation until you have turn the bolt the Prescribed amount. Rods Mains Heads are the important stuf(;-)
YES friction is what holds the R8 holder in place.
The idea on Daves Catr8 was it is EASIER to buy a premade toolholder over removing the spindle shipping it out and then grinding the spindle and reassy the spindle. He used a standard Grizzly unit in his prototype machine. ALSO using the SHORT CatR8 holder gives you a little more room in the spindle for the Gripper and pull stud assy.
Just a thought, (;-)TP
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ON the important stuf you preset the bolt with a torque wrench to a low point just to take up the slack. Then rotate the bolts a set DEG of rotation until you have turn the bolt the Prescribed amount. Rods Mains Heads are the important stuf(;-)
That is a great technique to use if you have the stretch spec and do not have access to both ends of the bolt.
YES friction is what holds the R8 holder in place.
It's biggest drawback, methinks. Same goes for Tormach's setup. They are short and ATC-able, the newer ones have a groove for holding on to them, but still rely on friction . . . and are even worse in that regard than R8. . . . . probably going to hear from Ray on that comment >:(
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It's biggest drawback, methinks. Same goes for Tormach's setup. They are short and ATC-able, the newer ones have a groove for holding on to them, but still rely on friction . . . and are even worse in that regard than R8. . . . . probably going to hear from Ray on that comment >:(
Depends what you're trying to accomplish. TTS is great, for what it is, and I think Tormach is very clear about what it is, and is not. For small machines (probably no more than 2-3HP max), they work great. I''ve been using them on my 4HP knee mill for 3 years, and never had a problem. And, in reality, most hobby machines (my BP clone included) run out of either power or rigidity before they tax the limit of TTS capability. But for your 7HP spindle, it would be totally inadequate. I believe the taper in R8/ISO30/BT30/CAT30 is quite adequate up to perhaps 5HP, which seems to me about the limit for what you can do with any tool that will fit in a 30-taper holder, no? For something in the 7-10HP range, I would think you'd want to go to 40 or 50 taper, like the big boys do.
Regards,
Ray L.
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To me the TTS is usefull I guess but it suffers all the problems that a R8 collet has holding the tool. Its only real advantage over 30 series is it is cheap(tool holder) and doable on a standard R8 machine(cheaper as well).
(;-) TP
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Nice job on your PDB, Ray. Will have my furnace next week. Need to do more homework to decide between ball type catch or the 'finger' collet to grab that little BT30. Not looking forward to cutting tool steel again.
Total agreement here with both Himmy and BR assessments of TTS.
Since full agreement does not make for a particularly intersting discussion, I'll go earn a living now.
Nice chatting with all of you again.
- Steve
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Just an update, now that I've got quite a few miles on the PDB - I absolutely LOVE it! Other than the quill drive, probably about the best thing I've done for this machine. Over the last several weeks the PDB has proven to be bullet-proof. A few days ago, I did the necessary re-wiring to get it running on the KFlop, under my controller app, instead of running under Mach3. This has enabled me to speed it up considerably, since I'm no longer limited by parallel port and "kernel" speed. That allowed me to triple the speed, with no loss of reliability, and I suspect I could go further still, just haven't had the time to try. But, I can now do a complete manual tool change in under 4 seconds, and it has not misbehaved once in hundreds of changes. It does a wonderful job of very consistently torquing the drawbar, and it has never once failed to engage or disengage, loosen the tool, or pop the collet free. It's also FAR quieter than those hideous impact wrench PDBs. This was a little bit on the expensive side, due mostly to the $150 gearbox, but it was dead simple to build (I could easily build one now in about half a day), and it was absolutely worth every penny. And, it should work, unchanged, when I finally get around to swapping out my R8 spindle for an ISO30 spindle.
Now I can finally get to work on a real tool-changer!
Regards,
Ray L.
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Hi Ray,
I'm Thinking of have a go at building one of these for my TM20VL mill. The only bit I don't like is the pneumatics to lower the Stepper. I want to get rid of the air supply.
Did you look at other means of lowering the Stepper, such as another smaller stepper?
Cheers,
Peter
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Peter,
A stepper seems to me like severely over-complicating things. Perhaps a small AC or DC gear-motor driving a screw. However, keep in mind, the drawbar needs to generate about 300# of down-force to pop the collet free. Also, depending on how you couple the drawbar to the PDB, either the drawbar or PDB dhaft needs to turn some to get aligned, before the PDB will come all the way down. A pneumatic cylinder can easily accommodate the coupling mis-alignment, but a leadscrew would require some kind of heavy spring mechanism to deal with it. Pneumatic really is the simplest, cheapest solution.
Regards,
Ray L.
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Hi Ray,
Thanks for that. I looked around for solenoids that could do the trick but could not find anything suitable.
I still need to sort the brake out, but It looks like I'll end up using pneumatics liker you.
Cheers,
Peter
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Peter,
If my machine didn't already have a spindle brake, I would've just adapted a bicycle disc brake.
Regards,
Ray L.