Power draw bar idea - Feasible?

[PsyKotyk]

I've been mulling over how to pull off a power draw bar. Looking at Hoss's very cool air-cylinder + lever design, and Tormach's triple pancake cylinder stack, I couldn't help but feel both options put extra weight on the head, and/or required expensive pneumatics.

Then I thought, what if I took a small disc brake calliper from a car or motorcycle, put a thrust bearing in the pot and used that to push the drawbar down.

It would require having a master cyl. that was driven by either pneumatics or possibly a motor with lead screw, as well as some plumbing. The advantage is that the master + power source can be located anywhere, so size and weight aren't a real concern.

I was assuming series stacked Belleville washers (7000N force to flat)

Feasible? What are the kind of forces can a brake calliper exert?




Cut off the one side, insert thrust bearing, bolt to mill (might need extra support bolts)

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[SCzEngrgGroup]

Power drawbars on commercial machines often use an "air over hydraulic" system similar to what you describe. You can generate as much force as you want, with proper design. The idea is to use a small air cylinder to move a small hydraulic piston which pressurizes the hydraulic fluid, to move a larger hydraulic cylinder. The small cylinders, of course, must have much longer travels than the large cylinders, as both must displace the same volume of fluid.

A brake caliper can easily tolerate in excess of 1000 PSI.

Regards,
Ray L.

[Fastest1]

Sounds very feasible to me. A clutches slave cylinder would work too. I investigated using one and blew it off. Not because it didnt or wouldnt work just lazy. You really wouldnt need the thrust bearing. The spindle wont be turning during a change. Try it, I will watch. You might try a simple 17-19mm motorcycle master cylinder with lever as a test.

[hoss2006]

sscnc did something similar to what you are proposing.

http://www.youtube.com/watch?v=0HmdWtJOkPU]X2 Electric-Hydraulic drawbar - YouTube

He ultimately decided

Even though this works, I've decided not to use it and instead go with a single small air cylinder mounted on top similar to Hoss's setup.

Hoss

[SCzEngrgGroup]

There is also this approach:

http://www.cnczone.com/forums/tormac...d_drawbar.html

Replace the lever and cam arrangement with either an air cylinder or electric linear actuator , and you have a nice, compact, light-weight solution. The cylinder on the spindle shaft is a dual-hydraulic cylinder with the belleville springs inside it, so the only force the spindle sees during release is the small force applied to the small piston at the top of the cylinder.

Regards,
Ray L.

[hoss2006]

PsyKotyk,
I prefer this approach that I did on my G0704 than what I believe you were referring to from the X2.
The scissor like action puts no strain on the spindle and acts fast.
g0704.com/Projects.html#pdb
You could make your own multi-stage air cylinder like I did if you want a custom, smaller, lighter setup.
A more compact setup here.
g0704.com/Projects2.html#pdb2
Hoss

[hndswthtshdws]

I've been mulling over how to pull off a power draw bar. Looking at Hoss's very cool air-cylinder + lever design, and Tormach's triple pancake cylinder stack, I couldn't help but feel both options put extra weight on the head, and/or required expensive pneumatics.

Whats wrong with extra weight on the head? The Z motor can handle it. I don't think the acceleration in wear is significant. One could argue the extra weight helps with dampening. Rapids would be slightly decreased but its not going to make a measurable difference to your cycle times.

You can buy a cheap new air compressor for $100. The "expensive" pneumatics can also be used for a misting system and cleaning.

I do think your idea is very innovative. I think its true value would be shown in lighter weight mills where added headstock weight can be detrimental to performance.

[DonKes]

If you are using it with a manual tool change, you can use a foot powered master cylinder.

I used a foot master cylinder on a motorcycle dyno to vary the load.

Don

[AiR_GuNNeR]

I just made one for my Novakon mill. It is a 2" air cylinder acting against a hydraulic booster. Go to the latest posting where I have pictures and a youtube link.
http://www.cnczone.com/forums/novako...ld_thread.html

[TiagoSantos]

After exchanging a couple of PM's with Air_GuNNeR (love his power drawbar setup!) I had some ideas as well..

I'm thinking of a pneumatic cylinder, about 2" bore and maybe 1" travel.. This would be mounted on the side of the head, and would push on an head mounted hydraulic cylinder, something with a small diameter but fairly long travel (well, about 1" to match the pneumatic cylinder), let's say a 0.25" piston. I'd connect a stainless braided brake line (I have a few lines I removed from the race car as preventive maintenance that would be perfect for this) to this master cylinder and connect it to a large diameter, small stroke cylinder on top of the drawbar, with a scissor type setup similar to what most people do with the pneumatic drawbar actuators.

2" bore pneumatic cylinder at 50psi = 157lbs. For 3000lbs at the belleville stack, that's a 19:1 ratio. The 0.25" piston has 0.049sqin of surface area, so I'd need a big piston with 0.9359 sqin. Let's make it 0.940, which gives me a diameter of 1.094"! And for 0.050" of movement to collapse the bellevilles and push the drawbar down, I'd need.. 0.955" of travel at the small piston! How about that.. I swear I was making it up as I went.

Hmm..

[TiagoSantos]

Of course it just occurred to me that with my dimensions above, there would be over 3000psi in the system, which may not be a great idea. Still, I think scaling up the system to reduce the pressure to 1000psi or so would be feasible.. And one other thing - the only reason I'd go through the trouble of doing something like what I just described, is if I didn't have enough headroom to mount a more conventional setup.. Which I don't :\ My basement has a really low ceiling and I can't really mount a multi stack cylinder on top of the head without losing a fair bit of Z travel. Same reason I want to mount my motor upside down on the side of the head..

[SCzEngrgGroup]

After exchanging a couple of PM's with Air_GuNNeR (love his power drawbar setup!) I had some ideas as well..

I'm thinking of a pneumatic cylinder, about 2" bore and maybe 1" travel.. This would be mounted on the side of the head, and would push on an head mounted hydraulic cylinder, something with a small diameter but fairly long travel (well, about 1" to match the pneumatic cylinder), let's say a 0.25" piston. I'd connect a stainless braided brake line (I have a few lines I removed from the race car as preventive maintenance that would be perfect for this) to this master cylinder and connect it to a large diameter, small stroke cylinder on top of the drawbar, with a scissor type setup similar to what most people do with the pneumatic drawbar actuators.

2" bore pneumatic cylinder at 50psi = 157lbs. For 3000lbs at the belleville stack, that's a 19:1 ratio. The 0.25" piston has 0.049sqin of surface area, so I'd need a big piston with 0.9359 sqin. Let's make it 0.940, which gives me a diameter of 1.094"! And for 0.050" of movement to collapse the bellevilles and push the drawbar down, I'd need.. 0.955" of travel at the small piston! How about that.. I swear I was making it up as I went.

Hmm..

Tiago,

That is precisely the "air over hydraulic" system I described earlier that is used on large VMCs. There are a few people on here who've done that, but it gets complicated and expensive for the hydraulic parts, and you have to make sure the hydraulic system stays topped up. 3000 PSI in a hydraulic system is not a problem at all - many/most cylinders can run more than that.

Regards,
Ray L.

[TiagoSantos]

Yeah, I've seen parts rated for 3000psi, but the system I described and imagined in my head would be made up of parts made.. by me At this point in my "career", there's only so much trust I'm willing to put in my own parts!

I guess there is very little to be invented anymore*, but I'm glad what I "came up with" isn't as far fetched as I had feared!



*ok, that's not true. But boy does it ever feel like it!

[hndswthtshdws]

After exchanging a couple of PM's with Air_GuNNeR (love his power drawbar setup!) I had some ideas as well..

I'm thinking of a pneumatic cylinder, about 2" bore and maybe 1" travel.. This would be mounted on the side of the head, and would push on an head mounted hydraulic cylinder, something with a small diameter but fairly long travel (well, about 1" to match the pneumatic cylinder), let's say a 0.25" piston. I'd connect a stainless braided brake line (I have a few lines I removed from the race car as preventive maintenance that would be perfect for this) to this master cylinder and connect it to a large diameter, small stroke cylinder on top of the drawbar, with a scissor type setup similar to what most people do with the pneumatic drawbar actuators.

2" bore pneumatic cylinder at 50psi = 157lbs. For 3000lbs at the belleville stack, that's a 19:1 ratio. The 0.25" piston has 0.049sqin of surface area, so I'd need a big piston with 0.9359 sqin. Let's make it 0.940, which gives me a diameter of 1.094"! And for 0.050" of movement to collapse the bellevilles and push the drawbar down, I'd need.. 0.955" of travel at the small piston! How about that.. I swear I was making it up as I went.

Hmm..

If you are going to mount it on the head anyway; why not just get the right size pneumatic cylinder right off the bat and skip all the hydraulics? I just don't see the advantage of mounting all those things on the head.

[TiagoSantos]

I'm limited in height. I can mount anything to the side of the head, just not on top.. There are plenty of solutions, I just thought this was an interesting option.

[PsyKotyk]

Thanks for all the great info!

After reading that a smaller machine like my X2 only needs about 600-800lbs of force, it makes the pneumatic solution a little easier for me.

I'm thinking of using this guy Aluminum Single Acting Spring Return Cylinder | Princess Auto which should be good for about 120lbs of force at 100psi. Then a 8:1 lever, which puts it at 960lbs, but sill allows for 1/8" travel.

[neilw20]

Consider not just enough movement to squeeze the stack, but you need say 0.030 either end of the stack for clearance plus some extra to pop the collet.
This extra movement means considerably more force than the holding force on the stack is required to pop and eject a typical TTS tool.
Doing it manually, you undo a turn or two, and have to give a firm rap on the drawbar to release.
You also end up with some lost travel from the drawbar elastic elongation under tension. Quite measurable!!
A single 2" cylinder at 80 PSI needs close to 2" travel convert to about 0.25" travel at the stack to release AND eject.
1" travel at the cylinder just won't give the extra pressure required to release and overcome the extra bits of 'lost travel', practical clearances, etc.
If the pressure on the stack was constant as the travel changed all would be fine, but the pressure must increase considerably past the required holding tension in practice.
At this point consideration must be given to ensure the fatigue life of the belleville washers are not exceeded.
If you squash past about 75% they will fail during cycling, as many have found, and how many of the 'real professional' packs need their broken washers replaced?
Fatigue life should be taken into account if you want to keep the maintenance low.
Looking at washer data, much over 75% compressed from the free state is asking for problems, and you probably need them at 65% to get the required holding force.
Any comments?

[SCzEngrgGroup]

"how many of the 'real professional' packs need their broken washers replaced?"

Actually, the Bellevilles on a VMC are considered a "wear item". The spring stacks, often 100-150 Bellevilles, DO need to be replaced regularly.

But, everything you've said is absolutely true. The Belleville force increases almost exponentially once you go beyond the rated compression, which can easily make the required release force 2X the working tension. And, when working with such large forces, mechanical multiplier, like levers, can get VERY inefficient, due to high frictional losses.

The many headaches associated with Belleville drawbars are the reason I went a completely different way on mine - I built a motor-driven drawbar. This has the added benefit that I can swap out an R8 tool just as easily as a TTS one. You can't do that with Bellevilles! Since I have several tools that I use all the time that are just not suitable for TTS (like a 4" face mill), this is an important feature. Drawbar tension can be set almost arbitrarily high - My PDB is capable of up to 75 ft-lbs, but I run the equivalent of 25 ft-lbs of drawbolt torque, and it is absolutely consistent. Never had a single issue with TTS pull-out, even on the most aggressive cuts, and the tool has never once failed to release.

Regards,
Ray L.

[TiagoSantos]

edit - nevermind, move along

[hoss2006]

Thanks for all the great info!

After reading that a smaller machine like my X2 only needs about 600-800lbs of force, it makes the pneumatic solution a little easier for me.

I'm thinking of using this guy Aluminum Single Acting Spring Return Cylinder | Princess Auto which should be good for about 120lbs of force at 100psi. Then a 8:1 lever, which puts it at 960lbs, but sill allows for 1/8" travel.

Yes the lever type does work very well on the X2.
I've done thousands of cycles over several years with not one bit of trouble.
It takes very little drawbar travel to actually open the collet and release the tool, around .025 iirc.
The bellevilles I used had a working force of 654lbs and a flat force of 858.
If your setup can exert 900-1000 at min shop pressure you have plenty of headroom to account for any losses.
Proved this with many videos showing tool holder retention tests, plenty strong for the X2.
Mcmaster.com carries a 1.5 inch bore cylinder for $35, pity surplus center doesn't have any more of the $4 ones I used.
They do have a 2" bore for only $17.
http://www.surpluscenter.com/item.as...29&catname=air
Hoss