I'm interested in trying to make one of these:
Mostly because of the price, but also for the challenge. The basic idea of this attachment is to allow coolant (water) to be pumped into a tool bit which is attached to the bottom of the shaft. A motor drives the top of the shaft. Water is introduced at the side of the body and presumably flows through perforations in the shaft and out an axial hole at the bottom.
The construction and machining seem pretty simple. But what kind of bearings or seals can be used between the body and the shaft that will handle, say, 20-50psi without without leaking? The shaft needs to rotate somewhere in the range of 3,000 to 15,000 RPM.
Does anybody have any clues about this? Am I better off to spend the 140 bucks and just buy the thing?
Ian Stirling wrote...
But then how would water be prevented from coming out the top, between the body and the shaft's motor attachment? A bearing is needed there, too.
Oh right. For some reason I thought you were talking about a submerged application. Oops.
I believe there are ways to use the shaft speed to pump the water that leaks past the seal out to where they won't get to the motor. Think a disk past the seal which slings water outwards, for example.
On Fri, 09 Jan 2004 11:32:33 GMT, Ian Stirling
Unfortunately, SS bearings will corrode if this is done. Furthermore, flooding a roller bearing will cause it to skid and will destroy the bearing. I built a centrifuge with ss bearings because that was the spec. They lasted 1 day. Only pure water was used. Failure was due to corrosion.
Corrosion or cavitation? Sometimes they are indistinguishable.
"Gun Control, the theory that a 110lb grandmother should fist fight a 250lb 19yr old criminal"
Corrosion. Even areas where there was no motion encountered on the outer race corroded. You can imagine what this pitting does to the rolling elements. Eric
I've never seen one of these, but I would expect that the water passes thru a hole in the spindle (between the bearings, and out the cutter end, like you said). I don't think it needs to withstand much water pressure, the barbed hose connector is small, which will restrict the water flow and the valve could be adjusted to just supply a flood of water for the cutter.
I would expect the bearings to be standard sealed ball bearings, with a lube port in the outer race, since there appear to be zerk fittings on the housing.
Automotive water pump bearings withstand a lot of harsh conditions, and about 16 psi of hot water, and relatively high speeds. The model on that particular page is rated for 8500 rpm max.
Water pumps use mechanical seals. They usually consist of one face being carbon and the other a hard material like carbide or ceramic. They rub against one another.
O-rings can be used in this application. A little water must leak out to lube the seal. This is a tiny amount that can be captured and drained easily. I have built rotary unions this way for gundrills operating at over 2000.00 psi. They were using oil. In your situation even a lip seal may work because the pressure is so low. Parker has the specs you will need to use o-rings. Because of the rpm and pressure the groove for the o-ring will be different than for a static seal. Another way to accomplish the rotary seal is to use carbon discs which are lapped flat and pushed together by a spring. The rotary unions I use in my shop work this way. However, the ones I use are rated at about 3000.00 rpm at 150 psi.
The technical term for these devices is 'rhotostat'. They are commercially available most everywhere. The tolerances and intricate machining required to produce one is usually best left to people set up to make them. That said, however, it is possible to produce one. The bearings inside the unit never come in contact with the fluid. The shaft is sealed with (usually) quad seals (sometimes o-rings), and on some, very intricate and proprietary sealing systems, depending on the application. But, at a cost of $134, you are probably better off just purchasing this item. You will have AT LEAST that much in producing one yourself.
Helical labyrinth. Half seal, half pump. They often leak slightly when not rotating, so there may be an O-ring as backup.
NB - their rated speed is 8,500 rpm, which is slower than small angle grinders run at.
Why would anyone want one of these anyway ? What's the application ? I've seen plenty of wet and dry core drills, but never one that ran at this sort of speed, or was driven by an angle grinder.
-- Smert' spamionam
Andy Dingley wrote...
Cutting faucet holes in granite counter tops.
Nominal cutting speed for a diamond cutter in hard granite is 5050-6250 SFPM. So a typical 1-3/8" core drill would like to see 14,000-17,000 RPM. At that speed in a material as hard as granite, the diamonds are be quickly knocked out of an electroplated (nickel bond) bit. Newer technology brazing has the strength to hold onto the diamond if the feed pressure is moderate and they are run wet, but they are single-layer tools and have a short life.
The long-life solution is a sintered, or "metal bond", core drill, but they can't be run dry. Submersion is possible for shallow holes (< 1")but is not as effective in deeper ones. Another drawback of submersion is that the dust and chips aren't cleared as well as when the coolant is pumped into the center of the bit.
Thanks for the ideas, everyone.
I'm a tad surprised there isn't an obvious simple solution, other than perhaps the O-ring option. So much of the tooling in the granite cutting industry is so wildly overpriced (owing to low volume, I reckon) that I'm betting there's an easy work-around. If not, the $140 won't kill me and I'll learn something from it.