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DIY Power Drawbar (hydraulic)

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spumco

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#1
Ok, so the question just came up in the TTS toolholder thread about a PDB I made for my CNC mill. Figured I'd avoid muddying the TTS thread and post details here.

Hang on, this is going to be staggeringly long. "TLDR" doesn't even touch it...

First - basic concept is similar to other push-type drawbars. A stack of belleville spring washers bears against the underside of a drawbar washer and the spindle top. This puts tension on the drawbar, drawing the collet up in to the spindle taper. Pre-load is set by tightening the drawbar to a certain torque. Same advantages & disadvantages as similar systems with no pull stud as on 'grown-up' commercial machines with CAT40 or similar spindles.

Second - I went with this design because I simply didn't have room for a traditional 3-stage air cylinder right on top of the spindle. I replaced the spindle motor with a massive lump and ran out of real estate right over the spindle. A traditional arrangement would have been about 2 feet tall to clear the motor. So I needed an off-set lever arrangement, but I didn't want to hang a giant 6" air cylinder off the head, nor did I want to pay the $300-$400 for a 3-stage cylinder.

Using an air over hydraulic arrangement means you can get serious pressure using a small, cheap air cylinder running at low air pressure and low volume.

Third - there are other options besides push-type PDB's. I didn't want an impact gun thing because I would like an ATC eventually. I needed something that could be controlled by the computer. There's another arrangement for the Novakon mills that uses a stepper motor and gearbox to torque the drawbar - it's way, way slick. But it's expensive and by the time I integrated it on to my non-Novakon mill I'd probably have more work in it than with my current arrangement.

Components:
1. Spindle is standard R8 taper, cartridge type. Double angular contact bearings at bottom, nothing special. Top is threaded for the pulley retaining nut with some oddball metric fine thread. Single timing belt pulley, no gears, although I'm sure someone with a stock gear-head mill could accomplish the same basic thing.

2. Air cylinder is an 80mm pancake type. About 1.5" stroke. Bought new from Automation Direct.

3. Frame is a plate sitting on top of the mill head with two uprights for the lever. Frame serves as a cylinder mount & lever axle mount, and also bears against the bottom of the hydraulic cylinder. The frame is floating so when the thing activates it lifts up about .025" and 'pinches' the hydraulic cylinder so the only load on the spindle bearings is the weight of the assembly. There is a "U" shaped cut-out where it fits around the spindle and bears against the cylinder mount lower flange. I also incorporated a spindle index sensor for rigid tapping, but that's not important to the basic design.

4. Lever is simply a couple steel plates with roller bushings at each end. Think of a V8 engine valve lifter.

5. Drawbar is a chunk of 7/16-20 Gr-9 all-thread with a heavy nut pinned to the top. There is an extra-heavy washer under the nut, and below that is a stack of belleville spring washers. The springs bear against another heavy washer/bushing on the top of the spindle The springs push the drawbar up closing the collet. More on the springs later.

6. Cylinder mount is a steel tube with a threaded and flanged top end. This is what is is threaded on the spindle top and serves to retain the pulley, keep the bellevilles from moving around, and serves as a mount for the hydraulic cylinder.

7. Hydraulic cylinder. This is the tricky bit that does the magic. This is a solid cylinder about 2.5" long that threads on to the top of the cylinder mount. Inside the cylinder is a lower .875" piston and an upper .3125" piston. The lower piston is seated against the top of the drawbar nut. The upper piston sticks up above the cylinder about .5" and the air-operated lever pushes down on the upper piston. The void between the two pistons is filled with heavy hydraulic oil.

First thing to understand about operation is that the hydraulic cylinder rotates with the spindle. It is part of the spindle - yes, it adds rotational mass, but it's pretty compact and close to the rotational axis. No detectable increase in vibration using the 'balance a nickle on the head' test.

Both pistons have double o-rings. The lower piston has o-ring grooves, and the upper piston bore has the o-rings because I can't cut grooves in hardened dowel pins. Lower piston is cut from ground bar stock but un-hardened. Upper piston is a 5/16" hardened dowel pin - nice surface and probably more round than I can accomplish. Neither piston is captured - the lower just bears against the drawbar and the upper doesn't fly out because the lever roller is there to stop it from wandering out. Never had a problem with that, however.

Math:
1. Desired collet retaining force - I got this from Tormach's white paper on the TTS system, and added about 50%. I didn't want pull-out, period.
2. Drawbar torque - also taken from the white paper. This is what gets you the retaining force.
3. Drawbar tension - needed to determine the spring rate. If X-torque gives you Y-tension, then the Y-tension is the minimum you need for the belleville stack once you torque the drawbar against the springs.
4. Drawbar/collet travel - I figured I needed about .125" of movement in the drawbar to release the TTS tools effectively. Turns out you only need about .025"-.050" to release as long as the collet and tool shanks aren't scored or buggered.
5. Hydraulic ratio - you can fiddle with the piston sizes based on your air cylinder force and lever ratio (if you choose to use a lever). Larger difference between the pistons give more force, but at the cost of reduced lower piston travel for a given input (upper piston) travel. Once you determine the desired drawbar travel, that sets your lower piston travel requirements. Upper piston diameter and length will be set by your desired ratio. Smaller, longer upper piston give you better advantage and permits operating the air cylinder at a lower pressure - but at the cost of needing to make a longer lever and all the geometry problems that go along with it.
6. Belleville spring rate. This is fussy as belleville springs are not discussed much in elementary school. I stumbled on a superb calculator from Key Bellevilles that allows you to input different spring thickness, diameter, material, height, and stack arrangements. The objective is to come up with a stack arrangement that has the minimum tension when preload compression (torque) is applied, but doesn't have too steep a spring rate while still having enough travel before flattening to give required drawbar travel.

NOTE - for those not familiar with belleville springs, when I mention 'stack' I mean the arrangement of springs. You can stack springs up facing the same direction "(((((" - this multiplies the spring rate by the number of springs (minus friction). You can stack them opposite from each other "()()()" and this increases the travel but with the same spring rate as a single spring. You can also do "((()))((()))" to get the spring rate AND travel you need, which is what most drawbar stacks are like.

You can get a belleville that has all the tension you want in one spring, but it won't have the travel you need. You can also get some that look just about right, but the effective spring rate rises so quickly that your air & hydraulic cylinder run out of steam before being able to push the drawbar down enough for tool release.

As I understand it, most springs are happier and live longer lives the less you deflect them. You may be able to get away with 4 single springs (Tormach), but if you can adjust the assembly so that you can use more springs, then each spring will deflect less for the same travel. Downside is a taller stack, upside is - theoretically - longer spring life.

Picking the springs:
1. Drawbar tension
2. Drawbar travel
3. Maximum OD (to fit inside of cylinder mount)
4. Minimum ID (determined by drawbar that passes through them)
5. Maximum height of stack (adjustable, but there's a limit. The reason those commercial CAT40 VMC's have a stack of 100 springs is that they need a huge amount of travel compared to the TTS system. The drawbar has to move quite a bit to release the pull-stud).
6. Spring rate at your desired travel distance for a given stack arrangement. (this is how much force you need to design in to your air cylinder/lever/hydraulic cylinder assembly. You have to be able to hit this, plus some, in order to move the drawbar enough.

I wound up with a 5x3 stack. I won't bother with dimensions because everyone's mill is different, but it was a compromise based on what was available from McMaster. This gives me plenty of travel, even if the TTS shank is cruddy, and way more holding power than the basic Tormach springs.

Safety - 5-way air valve is triggered by a relay. Relay is triggered by both either a panel button or a foot switch. Relay is cut off if the spindle is turning, and there's a 3 second delay after spindle off before the air relay is enabled. My VFD has output terminals that can be programmed to send a signal when the spindle is energized - that's where the delay is programmed.

For those wondering about whether the DIY hydraulic cylinder can withstand the pressures generated - I did the wall thickness calcs according to Section VIII, Div 1 for my materials and with a 6:1 safety factor at the highest my air supply can possibly reach. Nothing is hardened except the upper piston, so I should get leaks or progressive, ductile failure at the cylinder to mount threads rather than an explosive fracture. Besides, the air cylinder is between me and the hydraulic grenade. If you're going to design and build a hydraulic cylinder - please don't guess at it.

Problems
- only serious problem I've had was with the hydraulic cylinder upper piston leaking. This was solved by going to double o-rings and using Bardahl super-thick oil. Activation is low speed, low heat, so traditional hydraulic oil was unnecessary.

Basic cause of leak(s):
1) my lathe tailstock wasn't perfectly centered and my reamer cut a little oversized. Had to drill out and tap the upper piston bore and thread in a blank plug so I could bore a new, better hole.
2) cutting o-ring grooves inside a 5/16" hole is a pain. De-burring them is also a pain, but cut o-rings don't seal very well.
3) I wasn't fully cognizant of the difference between a typically 'good' fit and the kind of fit & surface finish you need to withstand a few thousand PSI of fluid. I'm new at this, so lessons learned.

If I were doing it again, I'd try to ream the upper piston a couple tenths undersize and then push a precision bearing ball through the bore with an arbor press. That would - hopefully - improve the surface finish and make the bore as round and to size as possible.

Minor gripe is that I'd like the ability to fill & bleed the cylinder without disassembly. Possibly a couple of ports could be drilled in to the cylinder, but those are additional leak points and there's a LOT of pressure inside the cylinder. Also, without bleed ports setting the upper piston height & oil fill level is fussy, requiring a couple of attempts to get the upper height right.

I've uploaded an excel file that has my calcs for picking the air cyilnder, hydraulic piston dimensions, and lever ratio. Feel free to plug in some numbers and fiddle with possibilities.

Fire away with questions.
-S

Overview and frame. Air cylinder is double-acting with speed valves so it doesn't slam.
IMG_3392.JPG


Drawbar (DIY), 5x3 belleville stack, and cylinder mount. Bottom of mount is what the frame pinches against. Aluminum collar is there just to hold the set screw that triggers my spindle index sensor. Note the liberal moly grease on the stack and everything else - friction is your enemy here. Flats on cylinder mount so I can tighten the hydraulic cylinder more than hand tight. Hasn't come loose after 6 months of operation.
IMG_3382.JPG

Magic cylinder, plus upper and lower pistons. Threads on top of cylinder mount and the lower piston is seated against the drawbar nut. Lower piston has a pocket in it so the upper piston can telescope in to it - gains full travel while keeping to overall height of the cylinder a bit shorter (slightly less rotating mass).
IMG_3387.JPG

Installed on spindle ready to rock. You can see how I ran out of real estate with that ridiculous motor. The cylinder turns with the spindle, and I've got about .010" clearance between the lever roller and the upper piston top, depending on how much oil is in the cylinder.
IMG_3393.JPG

Spindle index sensor triggered by set screws. The thing with the spring is a 5/16" shoulder bolt screwed in to the mill head. This allows the whole frame assembly to float up and pinch the hydraulic cylinder to keep the load on the spindle bearings down. Only moves about .025" up.
IMG_3398.JPG
 

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Eddyde

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#2
Wow, Very Impressive!
I have been wondering how I might accomplish an auto collet arrangement on my mill, in the future. I will give this approach serious consideration.
Thanks!
Eddy
 

spumco

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#3
I will give this approach serious consideration.
It was really based on the lack of real estate on the head. A simple 3-stage cylinder shoving straight down would have been way simpler and possibly more reliable, but I just didn't have room. In addition, my air compressor is a little single-stage thing and I was reluctant to have to operate the system at 100psig just to get it to work. It drops the tools reliably at 55-60psig, giving me tons of reserve in case something gets a little stuck.

There are plenty of other PDB plans (Hoss, for one) that use single stage cylinders and lots of mechanical leverage. However, due to my cautious nature and the desire to increase the drawbar preload torque well above 'standard', an all-lever mechanism would have been seriously wonky. Like needing a 10:1 or more ratio - I didn't want some Rube Goldberg dingus hanging off the mill head - so the hydraulic actuator got me the force I needed.
 

Eddyde

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#4
I too have space issues with my mill being located in a low ceiling basement, I even had to remove a section of floor joist to clear the motor... I am also afflicted with similar obsessions of wanting to max the torque on the drawbar etc...
Do you have any drawings you can share? I would be greatly appreciated but totally cool if you don't want to.
 

spumco

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#5
I would be greatly appreciated but totally cool if you don't want to.
It would be totally uncool if I didn't share drawings. Keep in mind that I did the CAD in Fusion360, but the end dimensions of the hydraulic actuator are not what got cut on the lathe. There was quite a bit of fiddling that didn't make it back to the drawing.

Here's a few PDF's of the components. If you want CAD models let me know. If you are a Fusion member, I can send you an invite to the project folder and you can root around to your heart's content.
 

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Eddyde

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#6
Many thanks, those are most helpful. I am a Fusion 360 member but haven't really gotten into it yet.
 

mitcheip

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#7
It would be totally uncool if I didn't share drawings. Keep in mind that I did the CAD in Fusion360, but the end dimensions of the hydraulic actuator are not what got cut on the lathe. There was quite a bit of fiddling that didn't make it back to the drawing.

Here's a few PDF's of the components. If you want CAD models let me know. If you are a Fusion member, I can send you an invite to the project folder and you can root around to your heart's content.
Hi Spumco,

I would like to try and adapt your design for use on my Syil X4 Mill. Is there any chance you could share the Fusion Model with me? Most of the things I can figure out but I'm struggling with the actual cylinder and a section view that I could produce in Fusion would help.....
 

spumco

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#8
Hi Spumco,

I would like to try and adapt your design for use on my Syil X4 Mill. Is there any chance you could share the Fusion Model with me? Most of the things I can figure out but I'm struggling with the actual cylinder and a section view that I could produce in Fusion would help.....
Certainly. Im away from my computer for a few days but i can help when i get back. Best way is for you to create a new project and invite me. Send me your email in a pm and we will work out the details in a few days.
 
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