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How to make an automatic blast gate.

Listing #95   Listed on: 08/03/2011

This thread describes my method of fabricating a homemade automatic blast gate for use in an extraction system. Please see the photos below.

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How to make an automatic blast gate.

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Posted By: Mario Aldape     [08/03/2011]

thinking, It sure looks easier than I thought it might be. I googled "perpex" and appears to be some sort of acrylic. Totally doable. Thank you for posting it. Mario

Posted By: thinking     [08/04/2011]

Hi Mario. Yes, perspex is a type of clear plastic sheet which some people call perspex or acrylic. In the UK we call it perspex, mainly. I chose perspex to cover the moving parts because it is very important to see that the gate is opening and closing. Also, it is a good idea to mark around the shutter with a permanent marker. This is so it can be seen that the shutter is completely shut and not stopping short for whatever reason. See the photos below.

Posted By: thinking     [08/04/2011]

These homemade auto gates are the best I have come across. They are so reliable. I've seen many autogates that can be purchased off the shelf which are not as good and they are made by professionals. The good points about my autogate are:-

1) The gate is very strong as it is made from cast. Because of this it cannot be deformed by the connecting ductwork. I've seen a few gates made from thin plate which can bend and jam the shutter.
2) The shutter can be seen to open and close so you are not left wondering whether the gate is working or not. There is a particular auto gate out there which is completely covered.
3) There are no dust traps in my gate. I've seen other gates where the dust has gathered around the mechanism and jammed the shutter. I've also seen gates with square shutters where dust could accumulate.
4) It is relatively air tight. There are gates out there that are completely air tight but cost an arm and a leg. The only place where air can leak on my auto gate is where the shutter enters the gate. This small leak is negligent for our purpose.
5) They are so reliable and easy to maintain. You can basically install these gates and forget about them once you have ironed out any teething problems.
6) They open and close quickly unlike some gates.

Posted By: thinking     [08/04/2011]

Below is a photo of the sides of two auto gates that I have fabricated. It shows how I have attached the flat steel, timber and perspex.

Posted By: thinking     [08/04/2011]

The type of pneumatic cylinder you will need is a 'double acting cylinder'. See link This type of cylinder has two ports (one at either end) which lets the driving air in and out. The size of ports doesn’t really matter but they are usually and eighth or a quarter of an inch.

You will need a solenoid valve to drive the pneumatic piston. See photo below. There are many types of solenoid valves. This one is a 5/2 way solenoid activated valve. The ‘5’ means that it has ‘5’ ports and the ‘2’ means that the valve has two states. See the diagram below which shows the valves function. I'm not sure what the symbols mean on the far left of the diagram but I'm sure the rest of the diagram is correct. See this link for definitions .

Coil voltage. The coil that activates the valve is located at the end of the valve which can be removed and swapped for another one. The coil can be seen in the photo below; it is the black unit with the wire coming out of it. Coils can be purchased separately and valves can be bought without coils but they often come together. Coils have differing operating voltages depending on your control circuitry voltages. I chose 24V because my ecogate GB12 greenbox runs on 24V and you can handle it with minimal risk of electric shock.

I don't want to go into too much detail as there is so much information about cyclinders and valves all over the net. I just want to give you the basics to work from.

Posted By: Mario Aldape     [08/04/2011]

thinking, the information you provide is very valuable. I am going to set out to find where I can get the parts you describe to get a couple made. I truly appreciate you taking the time to post, as many other conrtibutors do also. Thanks again.

P.S. I stopped by the Woodweb both at the recent AWFS to chat with the fellow at the booth who turned out to be the president of the organization, and he asked me how I liked it. I told him that it was both good and bad. I get tons of great information on all sorts of topics but I am addicted to the thing and I spend far more time than I should going through the different forums. It probably sounds familiar to some, if not a lot of you. Oh well. Mario

Posted By: thinkingcnc     [08/04/2011]

Thank you Mario. In my experience the best place to find a pneumatic cylinder is from ebay. I have purchased many cylinders through ebay and they have all been either 'as new' or slightly used for a fraction of the cost of a new one.

Prices for solenoid valves can vary widely. I paid around £100 for my first valve but now I'm buying Chinese ones for less than £20 through ebay.

The manual gate itself is cheap enough to buy new. I get mine from this company

I have a word of warning. When you first hook up your auto gate to the air and have it on your bench to test, please make sure your hands are well away from the gate opening. It can act exactly like a guillotine and would chop your fingers off. It certainly made me jump.

Posted By: thinking     [08/05/2011]

I would recommend doing the work yourself. You will be making this gate through trial and error so you don't really want to be paying someone to do it for you as it will increase the cost and time of the project.

The cutting and drilling of the steel is straight forward and I should imagine most people could do this. The problem in doing it yourself comes when welding needs to be done as not all joiners can weld. If you have never welded before I would thoroughly recommend learning this skill and buying a welder. I have a MIG welder and use it on a regular basis for all sorts of things. If you don't have time to learn to weld you can still probably fabricate the gate without it just by drilling and bolting the steel. I prefer welding though as it is stronger and more rigid than drilling and bolting.

Attaching the shutter to the piston end: I have two ways of doing this which I have described below.

1) In figure 7 below you will see that the piston end has been bolted to the lip of shutter. This isn't ideal as it flexes the shutter but if you don't want to weld then it is the only way of doing it. On the other hand the gate in figure 7 has worked fine for nearly 1 year so this method is probably good enough.

2) My second method is welding. See figure 8. The idea is to get the shutter to align in the middle of the piston so the force of the piston doesn't flex the shutter. I have circled the relevant area in red.

Figure 6 is a photo of a 150mm gate that I have fabricated and installed. Notice the solenoid valve and the pipes leading from it to the cylinder.

Posted By: thinking     [08/05/2011]

In figure 7 above I solved the problem of the shutter flexing by making sure the piston hit up against some angle iron so the force was diverted away from the shutter.

Posted By: Mario Aldape     [08/06/2011]

Thanks thinking, for the thorough description. I do also have a mig welder an so it will come in handy. I am checking on e- bay this weekend to see what I can find in terms of the parts. Thanks also for the link to the gates supplier. They look pretty sturdy. I think they have the10" (250mm) but not the equivalent to 9". I will give them a call. Again, thank you for your valuable contribution to this community.

Posted By: thinking     [08/07/2011]

Mario, I have a fair bit more to post on this thread yet. There are the electrics and pneumatic components to cover yet, it's just finding the time to post. I'm glad you have a welder, this will make things easier for you. I know I shouldn't say this but I have actually used a smaller size gate than the ducting it is attached to. My reasoning for this is that because there is so much more vacuum generated from having automatic blast gates that having a smaller gate doesn't really matter. The extraction on the associated machine will still be larger than what is was, hopefully anyway.

I would like to point out some safety tips on using an angle grinder and welder. Sorry if this is slightly off topic but I wouldn't like to have disgruntled users because they've had to have time off work through injury. When cutting metal with an abrasive disc always use goggles. Glasses, alone, are not good enough. I know some one who was wearing glasses only while drilling. A metal splinter got behind his glasses and burnt into his eye and has permanently degraded his eye sight. I have also had a metal splinter melted into my eye through grinding but I was lucky and the medics managed to remove it with no damage. The reason why it damaged my colleagues eye so much was because he didn't realise he had something in his eye for about a week when his eye started watering. The medics didn't find and anything in his eye the first time he went so he had to go back 3 days later with the same problem. This time they found the splinter and removed it. The problem was that it had left rust rings in his eyes which cannot be removed. One thing I've learnt is that it doesn't matter how much you squint, this is no substitute for goggles and debris will still get into your eye.

When welding, there is something that I was not told about which I had to learn from experience. We all know about 'arc eye' and that we have to use a welding mask to dim the arc. What I didn't realise was that a second or so after welding has stopped, the molten weld can actually explode and spit out hot metal. If the person welding stops welding and removes his mask straight away to examine his work, he could end up with molten metal in his eye. What I do now is to wear goggles and a welding mask.

Posted By: Mario Aldape     [08/07/2011]

Thinking, keep it comming, by all means. I'll keep mind your advise, especially when an employee is welding. They are usually much less concerned with safety issues than owners.
When you mentioned to look out when testing the gate to avoid having your fingers injured, as well as the stop brace to deflect the force of the piston, it occurred to me that a pressure control gauge would slow down the piston. Have you tried it, or is it even an issue? Thanks.

Posted By: thinking     [08/07/2011]

Pressure regulator. I hook my solenoid valves direct to my compressor which gives out about 150 to 170 PSI. To drive the shutter with a 32mm bore cylinder you really need that pressure especially with a new gate. As we all know a manual gate can be quite stiff. A pneumatic piston will need enough force to overcome this. To make things more complex, the gate will have stiff spots which will stop the shutter until enough pressure is built up in the cylinder to get it to move again. Then the shutter gets to a loose spot the it will suddenly lunge forward so you end up with a jerky movement. A pressure regulator wouldn't alleviate this.

If you dropped the pressure with a regulator a bigger bore would be needed. If the gate is opened and closed a lot like with some of my machines, the gate becomes worn in and and the shutter becomes very easy to move in and out but this is irrelevant because you will have already fitted a cylinder to drive a stiff new gate.

Also, you have to bear in mind than when the DC is on, the shutter can become forced into the side of the gate which makes the shutter harder to move. What happens here is that when you test an installed gate without the DC on, the shutter will move in and out fine but when you turn the DC on the shutter can get stuck if the piston doesn't have enough force. I experienced this with a 25mm bore cylinder and is why I say 32mm is a minimum bore size. So all in all I would say that a pressure regulator wouldn't really contribute anything worth having, IMO. It's all very much trial and error and please don't always take my word for it, you might find something I've overlooked.

Posted By: thinking     [08/09/2011]

Pneumatic components. In the pictures below are some of the pneumatic components you will need. Figure A connects straight onto your cylinder ports and controls the flow of air into the cylinder which will control the speed of the shutter. You will need two of these. Figure B attaches to the outlet ports of the solenoid valve and stops dirt getting into the valve. You will need 2 of these. Fig E attaches to the valve ports that drive the piston. You will need 2 of these. Figure F attaches to the inlet pressure port of the valve(1 of these). Fig C connects Fig A and Fig E together. Fig D is the hose which fits onto Fig F and connects to your compressor. Fig G shows a valve that is connected up with these components.

You will need to get the correct sizes of theads to match the ports i.e., 1/4 or 1/8 of and inch. The nylon pipe I use is 8mm. The hose I use in 1/2 inch. I purchase all my components through ebay.

Posted By: thinking     [08/09/2011]

I don't know whether I answered your question correctly, Mario. The speed controller elbos(pictured above) will control the speed of the shutter but you wont be able to set them until the gate is functioning so you still have to watch out for the speeding shutter. These elbos do not vary the pressure, just the flow of air into the cylinder. You could use a regulator to slow down the shutter instead of the elbos but the shutter may get stuck due to a lack of force on a tight spot.

Posted By: Mario Aldape     [08/09/2011]

Well thinking, this is awesome information. I am sure questions will pop up as I start to build the first gate. I certainly have enough information to get going. I actually have a 10" gate and two pistons I might be able to use. It would be a matter of getting all the parts together and a little time to get it done.
I am very much obliged. I will post my progress, as my work load permits. Mario

Posted By: thinking     [08/12/2011]

Keeping the inside of the gate clear of debris is another important consideration. If a piece of timber gets stuck inside the gate, this will prevent my gate from fig 1 shutting properly. There is another type of gate that I have seen that is referred to as a self cleaning gate. One of these gates can be seen in figure 10 & 11 in the picture below. Figure 10 and 11 are the same gate, opened and closed. This type of gate takes up more room that my gate because the shutter extends out of the bottom of the gate. The advantage of using this gate is that it pushes any debris out of the way. My gate in figure 1 doesn't do this; it’s shutter would just jam up against the debris leaving a gap for air to get through. My solution to this can be seen in figure 12. I basically cut the ducting the fit over my gates outlet instead of going inside. If the ducting had gone inside the gates outlet, this would create a trap for debris which we don't want. With the ducting fitted in this way the debris will flow straight through the gate and not get stuck.

As well as taking up more room the gate in Figure 10 & 11 is flimsy and leaks a lot more air than my gate in figure 1.

Posted By: thinking     [08/12/2011]

When I was looking to buy my gates ready made I found this gate that I have pictured below. The first thing that struck me about it was that the force from the piston pulls on one side of the shutter. I thought, surley this could jam the gate so I didn't buy it. The piston on my gate is applied to the middle of the shutter so this effect wouldn't happen. I can see that the gate saves space but I would question it's reliability.

I asked another member of the forum what he thought about this design and he was actually using one. He told me that he has to oil it regularly otherwise it jams up.

Posted By: thinking     [08/13/2011]

The picture below is another off the shelf auto gate that I found. It looks very neat and shiny but there are two things I don't like about it. The piston end bolts into the shutter lip which I have talked about this in my previous posts. Also, it looks as if the bottom of the shutter is square. This means that the shutter slide way groove must be square at the bottom. To me this would act as a dust trap and would inevitably fill with debris and stop the gate from closing properly. I don't know this for a fact and is just my opinion.

This wouldn't happen with my gate because the bottom of the shutter is round and there is no groove for this part of the shutter to fit into thus no dust traps.

Posted By: guitarguy     [08/15/2011]

Great idea. You have my wheels turning with ideas - I'll be able to use my "cool parts bin" to build some automatic gates!

Posted By: Gene Maiorano     [08/16/2011]

'Thinking' has provided very valuable and insightful help here and it made me think about my own use of remote door lock controller solenoids to control automatic starting of a hybrid-diesel engine alternator.
These controllers would be perfect for other uses like the blast gates as done by 'thinking', particularly because they can operate with 12 volts and don't need compressed air.
I haven't done this with the blast gates (yet) but I put the idea out there as an alternative.

Posted By: Alan Schaffter     [08/18/2011]

Since I don't have a commercial shop and don't have gov regs to deal with, I took a little easier approach to my autogates- I made them from MDF. I lined the inside faces with plastic laminate (Formica) and made the slides from the same material so they are very, very slick and slide easily. Because of that it only takes 15-20 PSI plumbed with cheap, lightweight vinyl tubing, to operate my gates. I even had to use flow restrictors (simple bleeder valve the came on many of my surplus solenoids) to slow down the actuation slightly to keep from tearing up the gate. The slides go all the way through, so the gates are self cleaning. To save space I mounted the cylinders alongside the gate and used a straight linkage to the slide. Again, since my slides move so easily, I have had no problems with binding, racking, and never need to lubricated them. I used small bore, bi-directional cylinders (mainly Bimba) and 24V 5/2 SMC valves from Ebay as well. I did not have or use an ECOgate "Green Box" but instead use small current sensor switches (torroid w/PNP transistor) to control the solenoid. They sense when current flows from each outlet where a machine is plugged in. Turning a machine on opens its gate (and starts the DC.) When the machine is turned off the gate closes (by design, the DC stays running.) Pics below are of my gate design (SketchUp drawing is from a magazine article I wrote), first prototype gate, installed gate (all 12 are mounted behind my walls), and a current sensor switch. There is a link to a video tour of my setup that shows a gate in operation.

Posted By: thinking     [08/19/2011]

Thanks to everyone for all the encouragement and appreciation. This is a big topic to cover and it's nice to see that some people are finding it useful.

Alan, IMO your system is technically excellent. I have watched you video. I love the way you have hidden all the unsightly ducting. You must have an extremely tidy workshop. I like the idea of a cyclone separating out the dust from the shavings. At the moment my shavings are hitting my extractor fan and overloading it's inverter from time to time. Maybe a cyclone would be a solution to this. Although, I'm hoping that I can change the parameters in the inverter to compensate for this.

Do you intentionally want the DC to stay on after a machine is off? It would be relatively easy to wire your machines so your DC turns off when a machine is turned off. If you are concerned about your fan being turned on and off a lot and wasting electric, an inverter would solve this problem. For the size of your DC an inverter for it would be quite inexpensive. The greenbox is ideal for turning DCs on and off if you don't want to use an inverter. It's Ok to run an inverter but it’s not ideal and very fiddly to do this. I have written a thread about the greenbox.

There is just one major problem I can see with your system: It is made from PVC which is flammable. This would invalidate my insurance. I've heard one or two stories about shavings catching fire inside ducting. I would definitely consider changing to metal ducting or at least informing your insurance company.

Posted By: Alan Schaffter     [08/19/2011]


Since I often jump quickly from machine to machine- jointer to tablesaw, etc. I didn't want the DC motor cycling frequently- it is not good for a motor (some manufacturers say starts should be limited to no more than 6 per hour). Even if I'm not so quick, I don't want to wait for the DC impeller to spool up to full RPM if I don't have to. Contrary to what many think, running a DC with all blast gates closed is less stressful on the motor than with them open.

Like I said, no government regs (and no insurance folks) telling me what I can and can't do. Even in the face of overwhelming evidence to the contrary, the urban legend about fire hazards in a DC and/or ducting won't die.

The beauty of my homemade blast gates is that they can be made with metal inlet/outlets sized to perfectly fit any metal ducting or fitting.

For easy operating metal gates, you can make the slide from any slippery material (Nylon, Delrin, UHMW, etc.) and/or line the faces with laminate like I did with my MDF gates.

I made all my gates in a batch, production stye. Later when I converted them to auto gates, I did the same thing.

Posted By: thinking     [08/20/2011]

Alan, I must admit that I have never sought proof of wood chippings catching fire in duct work. Could you please show us this overwhelming evidence? I googled and found this:-
Fire damages high school wood shop
By MARK GUNDERMAN | | Posted: Wednesday, April 13, 2011 7:00 am
The wood shop at Chippewa Fall Senior High School will be out of service for a time after fire did significant damage to a dust collection system and ventilation ducts.
The Tuesday afternoon fire caused a brief evacuation of the high school, but no one was reported injured and the fire was confined to the shop area on the north side of the building.
Chippewa Falls Fire Chief Tom Larson said someone saw smoke coming from the dust collection unit, which sits outside the building, connected to woodworking equipment inside through ductwork. The fire department was called at 1:23 p.m., Larson said.
Firefighters saw flames coming from the unit and attacked the fire with water and a compressed air foam mix. But the unit wasn’t the only problem. Sawdust was burning inside ductwork leading to the collection unit.
“We put out about five different fires in duct work,” Larson said. “Every time we took the duct work apart there was actually fire in sawdust inside the duct.”
A thermal imaging camera was used to find hot spots in the duct work.
Larson said it all came back to a dull router bit. The school has a computer-controlled router that cuts wood according to instructions programmed into the computer. It was operating and cutting wood, but with a dull blade. That increases friction and heat generated from the rapidly-turning bit.
The wood began to smolder then caught fire. That was noticed and the small fire quickly extinguished and the burnt wood placed outside a side door from the shop.
“They thought that was the end of it,” Larson said.
However, the dust collection system is designed to suck up sawdust through the cutting process, and it was doing its job. It pulled smoldering sawdust from the wood and spread it through the duct system all the way back to the collector unit, leading to the larger fire.
Firefighters were on the scene three and a half hours.
“The fire was very stubborn to go out and kept re-igniting in the sawdust, wood chips, and dust collection bag system,” Larson said.
“Firefighters had to dismantle several sections of the metal ductwork and cut some access holes into the duct work to gain access to extinguish the various spot fires. The building was ventilated of smoke.”
Chad Trowbridge, the school district’s business manager, said the wood shop area was evacuated during the fire, with students moved to another part of the large Chi-Hi campus. However, there was enough smoke created by the fire that it set off fire alarms, leading to an evacuation of the entire school.
Larson gave clearance for students to return after only a brief time outside.
He said the school district estimated the damage at $50,000.
The immediate impact for the school district will be on the educational programs that use the wood shop.
“The shop will be inoperable for a period of time. How long we don’t know,” Trowbridge said.
The link for this story is

Even if this story is untrue, it is still believable to me. All the bag houses I have ever seen have fire dampers on them including mine. There are many types of in-line ducting fire dampers that are available. Why is there all this hardware available if fire in ducting doesn't happen? OK, it could be an unnecessary industry based on fear but I wouldn't take the risk to find out if it were true or not.

As it happens my blast gate, being made from metal only, gives a certain level of fire dampening. Come to think of it, I could probably put several smoke detectors in the ducting which would trigger all the blast gates to close thus cutting off air to the fire. I could put an auto blast gate in the main duct to separate the bag house and DC from the internal duct work in the event of a fire. This is something for the future.

Yes, the load on the DC is lower when all the gates are closed. I have measured this myself with my ammeter. With all the gates open the DC drew about 31 Amps. When the gates were closed it drew about 26 Amps, if I remember correctly. With an inverter the DC can be turned on and off as many times as you like; it does no damage and doesn't cause the motor to over heat. I have two 15KW extractor fans and they are instantly turned on and off at the same time of a machine being turned and and off. This is even true for my cross cut when someone might only need to do one cut.

Posted By: Alan Schaffter     [08/20/2011]

I probably should have qualified my statement and said in "home shop" systems and fire initiated in PVC ducting due to static discharge. This keeps popping up time and time again, despite some rather scientific explanations why it is highly unlikely (generally impossible). Google "static and Rod Cole"

As to the fire in the duct- my reading of the article is that it was a fire started at some CNC equipment and was transported to the dust bin or filters. Then, since appears there was not a blast gate in the main it migrated back through piping when the blower was off. It sounds to me like it was a poorly designed or maintained system that allowed dust to collect on the inside of piping. That should not happen. In a properly designed and maintained setup the flow should be high enough to prevent buildup on the ducting walls (it should actually scour the duct walls) and during operation the concentration of dust (combustibles) in the air stream should not be sufficient to sustain fire in all but the most demanding industrial applications (with undersized blowers). A bag house with blower off is another story and much like a grain elevator!

It also sounds like they fought the fire the wrong way- I would have parted the duct from the blower outlet and turned it back on to blow and duct fire out, and handled any dust bin/bag house fire(s) separately.

I'm sure your inverters (VFDs) are programmed to control motor start-up though wonder why you need to turn more than just a few times a day. One thing to be aware of- if the ramp up is too slow and you start generating too much dust too quickly, a cyclone separator will not work efficiently right away. The velocity will be too low. If you are running a bag house then it is not so much a problem. It just means it will require shaking more often.

Posted By: thinking     [08/20/2011]

Ah, this is a very valid point IMO. So, for people with cyclones, which need a minimum amount of air volume to work efficiently, a certain number of gates will need to be open at anyone time to let this minimum air flow through. This doesn't apply to me and my bag house. Although, I do get a build up of chippings in my ductwork work which is cleared out every hour on a cleaning cycle. I suppose that having any build up of shavings in a ducting system is a fire risk and a cleaning cycle just minimises the risk. I could run a smaller cleaning cycle particular to every machine everytime it is switched off i.e, when a machine is turned off open only the gates that are needed to clean the 'fall out' caused by that machine. My Greenbox is not sophisticated to do this and I will be looking for a replacement controller in the future that will be able to carry out dedicated cleaning cycles for individual machines. When I have achieved this I wont need these all out cleaning cycles every hour that I have at the moment. Thank you for making me think about this.

We are a small joinery shop and only have 3 joiners working for us so the DC doesn't need to be on all the time. There might be a couple of hours in between the use of machines.

Posted By: thinking     [08/20/2011]

I found a company that specialises in detecting and extinguishing fires in duct work. Basically their system detects the spark or burning embers in the ducting and then extinguishes it further down the line. See picture below. Their link is which is worth reading.
This link is also interesting

Posted By: Alan Schaffter     [08/20/2011]

With DC air moving at 4000 +/- fpm you need a system that responds quickly or have enough downstream duct to allow time for the extinguisher to react. I wonder what effect the mist has on the duct?

An a better alternative to misting that would be as far as possible from the detection point and would totally remove the source of fuel might be a diverter located in the duct just before the air reaches the filter or bag house. Once activated, it would divert the spark laden air to the atmosphere instead of the filters.

Posted By: terry green     [03/20/2015]
Our retirement community has a large woodshop. We need automatic dc gates
but need tech help.

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