[DDM]

Ok most of us know how a water jet/abrasive jet machine works, one problem wood and water don't always work well together and obtaining 60,000 PSI water pressure is somewhat hard to obtain. Lasers can be expensive and they burn through the wood although it isn't that evident. EDM's and plasma won't work on wood. Rotary tools are cheap but the smaller the tooling, the easier it is to break and when you want to cut through 3/4" wood with a 1/16" cutter things will take some time or plan on breaking bits.

So what if a person was to make a very high velocity sand/bead blaster to cut through wood like an abrasive water jet machine. You could then get cuts through thicker wood with a very small kerf and tooling would be fairly cheap.

A person is going to need fairly high pressure compressed air and an abrasive delivery system to the stream of high velocity air. Now comes the big quesiton, how fast to we want these beads ripping the wood apart. Air pressure, orfice size and volume are going to come into the picture but how do I get from Pressure to orfice size to volume to "Ouch, there goes another finger"

I'll look through my industrial hydraulics book later tonight but I'm not sure about how compressed air changes the equations. Let me know what you guys and gals think.

:cheers:
Carl

[DAB_Design]

Can't really answer your questions, but wouldn't the dust just be EXTREME? Any metal mating surfaces would have to be covered. Not to mention breathing the stuff.

Sounds like a cool idea though. Hope you get it to work.

[CJL5585]

What were your thoughts on the material used to make the orfice? Ceramics would be eaten away in a very short period of time. Maybe some type of exotic metal?

Jerry

[Ken_Shea]

DDM I am thinking out loud here but I would suspect it would take substantial air volume and would not produce a particularly clean cut since the orifice would of necessity be large and the cut would widen as the distance increased.

[DDM]

DAB_Design-
I was thinking of a totally enclosed machine with a vacume to take care of the very small dust particles, don't need to sound like I've been smoking a pack a day for 50 years. There should even be a way to recycle the sand or beads from the dust using a calibrated vacume system like they use in foundry settings.

Jerry-
Ceramics are pretty tough, one of the companies in my area had to used ceramics for an elbow for exhausting ash and carbon from a wood burner system (not sure on specifics) but it ate right through the metal piping that was there before and darn near cut a guys hand off that decided to use the feel test for leaks.

I was thinking along the lines of T1, 4340 or 4140 for a metal and heat treated to say 60 rockwell. I'm a machinist by trade and I've got the capability to machine the piece but I'm afriad they would be consumable parts for the orfice due to the abrasive nature of sand or beads. Ceramics or carbide would probably be a better material to use but I don't have an EDM to make the orfice for the cutting tip out of carbide. If there is anyone out there with experance with exotic materials I'd love to have your input.

Carl

[DDM]

Ken you beat me to the post, I've been thinking about this for a few days now and the key I think is to look at it like guns. I know I'm showing my true north Idaho side but I'm not against the government or anything like that. A sand blaster is like a shot gun or a machine gun. By putting a choke on a shot gun you can thighten your pattern, a choke basically forces the shot into a smaller area depending on the choke type. You could also look at it like a hand gun vs. a long range rifle. You wouldn't want to try hitting a pop can at 300 yards with a 9mm pistol with any accuracy. On the other hand a rifle with a long barrel and a scope you could probably hit the can every time. Also with the greater velocity you have less of a chance of the bullet (sand particle) to stray from where it should be going.

To take this into our idea of using a sand blaster as a cutting tool; We would probably want to make the barrel fairly long for the air to accelerate the abrasive to as fast as possible. I would want the orfice to be fairly small for accuracy sake (kinda like the choke on a shot gun). And air pressure is going to have to be decently high to get everything moving at a high velocity. I don't know where all the trade offs exist but I'm sure someday we can sure try and find out.

If there is anyone out there with experance with the physics of paintball guns, air rifles, sandblasters, waterjet, abrasive jet, ect. I'd love to have you chime in. Some of the best input comes from the most varied experance.

Carl

[foamcutter]

Hi,
I have a friend who works with a abrasive water jet system. He and I have been dreaming about a hombrew abrasive jet system. Here's some of what we think. I am not an expert on this subject and have not tested or tried this, so use at your own risk. Most abrasive water jets use crushed garnet stone as an abrasive as I understand it. this is available in different grit sizes. Why not try air pushing garnet not sand? Also the water abrasive jets use "focus tubes" and orafises (?spelling) which they are considered consumables, which means they are availble to purchase from a supplier at a reasonable cost hopefully. What this means to me is one can now purchase quality focus tubes, orafises and garnet abrasive that will work together. then homebuild the rest of the system to match those parts. Kind of reverse engineering from the cutting tip back to the rest of the system. What ya think? This only my opinion and before using any of this information use your own judgement as to to it's suitability or safety for you as I am not responsible for your actions or how you use any information I have written. Just my pennys worth. Ron

[4104]

I use a 180 cfm compressor driven by a Diesel engine with coarse media to clean metal. If I happen to aim it at a piece of wood, like a 2 x 4 support structure, it blasts the wood away real quick. The nozzle is about an inch in diameter. It goes through a 100 pound bag of media in a few minutes. It is dirty messy work but very effective.

Mixing abrasive material with water and pumping it through a small nozzle is what waterjets do very well. Flow and Omak are two local manufacturers. The latter builds a very nice small machine. It costs $80K. A rebuilt pump and motor is $30K. Nozzles are expensive and last 100 hours or so. When I win the state lottery I'm going to buy one.

Regards,

Leonard

[DDM]

Foamcutter-
Do you by chance know how much the orfices are running for price, just a though to incorperate something that is already being made into the design?

4104-
Waterjets are expensive for sure or else everyother person might consider buying one. Cutting metal can get expensive but wood is fairly cheap and as you said the media that you were using distorys any wood in its path.

I've been thinking about the system for some time now and I think that high air pressure is going to be the key but where to get that kind of pressure required to eat through wood is the tricky part. A lot of air compressors max out at about 125 psi and if you're working with the big boys you can get 175 psi but I'm not sure if that is going to be enough to get the velocity needed to send a glass bead through wood. This is assuming that pressure not volume is going to be the dictating force on the velocity of the air and the beads/abrasive. I've looked around and I can't find many high pressure air compressors for a decent price, so building the pump unit might be the key. I don't know if there is anyone out there that has this kind of experance but if so please chime in or tell me that my redneck thinking is wrong.

One last thing, I would like to wish everyone here a safe and happy Chirstmas.

:cheers:
Carl

[4104]

First - go to the Yahoo group titled "waterjets" and read some of the interesting posts and some of the links. They give a good idea of the principles of design. Note also that those things operate at 30,000 to 50,000 psi. Some of the people who post on that site have academic engineering credentials and might be aware of any past attempts to develop a system like you'r working on. You might save a lot of experimenting, which can be costly, if you find out what has gone on before.

My blasting work is done at about 125 psi. Thats the easy part. The hard part is getting 185 cfm at 125 psi That flow rate is achieved through the large nozzle opening I previously described. If I had a small nozzle, in the order of .005, I would no longer be able to maintain that flowrate but a very small nozzle is neccessary to limit the kerf size and to achieve a high velocity dischage. If I moved my present blasting system with a large nozzle slowly over a piece of wood to cut all the way through I suspect the kerf would be a few inches wide.

I cut metal on a cnc plasma cutter and a cnc mill. I'm slowly accumulating parts to build a small router to cut wood. These are all relatively inexpensive but are accurate enough for my hobby work.

Incidentally if you find you need to be able to have a lot of air for your project you might consider buying a well used diesel powered compressor like you see driving jackhammers for busting up pavement. That is what I did.

Good luck in working on your air/abrasive cutting system.

Leonard

[CJL5585]

The Air Force had self-contained air compressors which they used to start jet aircraft in Vietnam.

You might want to look at some Vietnam or later era Military Surplus. The compressors were three stage (if I remember correctly) vane type. The unit had a 4 cylinder jeep type engine which used 87 or 89 octane gas.

The units were about 10 feet long and 6 feet tall and were towed behind a truck. There were either 4 or 6 tanks similar to Oxygen tanks, which were about 8 feet long.

One of these units could get you up to 5000 psi, but I don't think that it could furnish this pressure on a constant basis since it was designed to get the turbines rotating on a jet engine, when everything else didn't work.

Maybe this might be an option for you.
Jerry

[lerman]

Doing a quick calculation, I find that with a .1 inch orifice, the flow at:
100psi -> 14 cfm
200psi -> 26 cfm
500psi -> 63 cfm
1000psi -> 124 cfm
2000psi -> 245 cfm
5000psi -> 611 cfm

This calculation is for air, I assume that abrasive mixed with the air will change the flow. Also, the velocity of the air as it leaves the nozzle is independent of the pressure -- it is the speed of sound (1080 ft/second).

A source of orifices is Bird Precision -- www.birdprecision.com. They make precision ruby orifices -- I don't know if they make any this large, but they are surprisingly inexpensive.

Ken

[DDM]

I think you guys are looking on the extreme side of air compressors but I'm looking a little smaller along the lines of something that could be ran off of single phase power. I was thinking along the lines of maybe a 3-5 hp setup pushing around 1000 psi. Although air acts like fluid in many respects, it is compressable where as fluid can be compressed but the volume only changes by 3%. I'm not too sure on the abrasives available on the market but I was looking at an orfice size of .020 to .050, something small enough so that small work could be done. It's late but here is my thinking when the compressed air reaches the barrel it accelerates (loosing pressure) to the mixing chamber and the air/abrasive mixture is then shot out the tip into the work. There's a little more than that but it's the basic idea.

When calculating the velocity of the air or the air/abrasive mixture can I use the same equations that I would use in fluid power or does the compressability of air cause this to change? After I know that what is a preferred velocity of the abrasive to distroy the wood in it's path? I mean what's the difference between 100 f/s and 1000 f/s? What is the equation to determine orface size to pressure and volume? And from that you should be able to determine the velocity. My thinking is that after you have these numbers it would be a good starting point for designing and eventually building a machine.

Well it's late and I'm going to bed. Thank you everyone for your input, I'll see if I can find some equations somewhere for all of these nagging questions.

Carl

[lerman]

I think you guys are looking on the extreme side of air compressors but I'm looking a little smaller along the lines of something that could be ran off of single phase power. I was thinking along the lines of maybe a 3-5 hp setup pushing around 1000 psi. Although air acts like fluid in many respects, it is compressable where as fluid can be compressed but the volume only changes by 3%. I'm not too sure on the abrasives available on the market but I was looking at an orfice size of .020 to .050, something small enough so that small work could be done. It's late but here is my thinking when the compressed air reaches the barrel it accelerates (loosing pressure) to the mixing chamber and the air/abrasive mixture is then shot out the tip into the work. There's a little more than that but it's the basic idea.

When calculating the velocity of the air or the air/abrasive mixture can I use the same equations that I would use in fluid power or does the compressability of air cause this to change? After I know that what is a preferred velocity of the abrasive to distroy the wood in it's path? I mean what's the difference between 100 f/s and 1000 f/s? What is the equation to determine orface size to pressure and volume? And from that you should be able to determine the velocity. My thinking is that after you have these numbers it would be a good starting point for designing and eventually building a machine.

Well it's late and I'm going to bed. Thank you everyone for your input, I'll see if I can find some equations somewhere for all of these nagging questions.

Carl

A while back, I was trying to determine the flow thru an orifice and found a spreadsheet using google. The name on the copy I have is: [email protected] I believe it cost around $25, but I'm not sure. The equations are reasonably complex.

It gives:
REFERENCES: MARKS' MECHANICAL ENGINEERS HANDBOOK; PERRY'S CHEMICAL ENGINEERS HANDBOOK; FLUID FLOW by Sabersky & Acosta.

One important thing to realize is that the limiting velocity thru an orifice is the speed of sound (1080 feet per second in air). So, more pressure will not give a higher velocity; it will just give a higher flow rate.

By the way, I'm not qualified to say anything I've said about flow thru orifices. Just because I can type this doesn't mean I'm an authority.