How to make a Venturi?

[Paul Taylor]

OK,
I tried my hand at making a venturi. I would show a photo but it is not real pretty.

Any way I thought I could just drill a hole and flair the end out a little. Not sure if this is going to work.

Any one have some in put on how to turn a venturi?

[Bill Gruby]

OK,
I tried my hand at making a venturi. I would show a photo but it is not real pretty.

Any way I thought I could just drill a hole and flair the end out a little. Not sure if this is going to work.

Any one have some in put on how to turn a venturi?

 Not enough info Paul. Did you use the lathe? Wish I was closer looks like you need someone who has lathe experience to guide you in proper surface sppeds and feed rates, These two combined give you the end finish result. You also need to drill the hole at the proper feed rate to ge a smooth surface. If you used the lathe to get the bell shape at the intake you needed to set the top slide at the proper angle and use a boring tool to cut the angle. Are you lost yet. 

 "Billy G"   

[Paul Taylor]

Not enough info Paul. Did you use the lathe? Wish I was closer looks like you need someone who has lathe experience to guide you in proper surface sppeds and feed rates, These two combined give you the end finish result. You also need to drill the hole at the proper feed rate to ge a smooth surface. If you used the lathe to get the bell shape at the intake you needed to set the top slide at the proper angle and use a boring tool to cut the angle. Are you lost yet. 

 "Billy G"   

Hey G-Man.... yes I was on a old small lathe.

I think it is one speed. Fast!
Maybe you can do a you tube video.

PS- I saw a back plate that Dan Banjock made at Brodaks with a O-ring. I would like to try that too. Looks easier then doing a venturi.

[Bill Gruby]

Hey G-Man.... yes I was on a old small lathe.

I think it is one speed. Fast!
Maybe you can do a you tube video.

PS- I saw a back plate that Dan Banjock made at Brodaks with a O-ring. I would like to try that too. Looks easier then doing a venturi.

  The best tape I know of for learning to use a lathe is by Rudy Kouhoupt. It is geared for the beginner and it takes you through cuttilg threads. I have a copy and will send it to you. Return it when you are finished. I hope you realize what you are getting into.  A lathe is a very addictive machine once mastered. Then you move up the ladder to the milling machine. Mama is gonna love you. 

  "Billy G" 

[Rich Federici]

So... do you use a shaped tool or a regular boring bar to get the flared opening? Does the opening need a gentle curve or can it be straight sided. The reason that I ask is that my son and I have been  contemplating a model airplane engine as a project, and the venturi has been one of the more tricky parts to figure out. You are right... The lathe is very addictive. I think for months after getting it all we did was make more tooling or adapters for tooling. We have a Harbor Freight 9 x 20, and luckily there is a very informative yahoo group on this machine.
Rich

[Bill Gruby]

So... do you use a shaped tool or a regular boring bar to get the flared opening? Does the opening need a gentle curve or can it be straight sided. The reason that I ask is that my son and I have been  contemplating a model airplane engine as a project, and the venturi has been one of the more tricky parts to figure out. You are right... The lathe is very addictive. I think for months after getting it all we did was make more tooling or adapters for tooling. We have a Harbor Freight 9 x 20, and luckily there is a very informative yahoo group on this machine.
Rich

 The Steve Bedair Harbor freight 9X20 site is over the top on this type lathe. I have a Jet 9X20 that is basically the same machine. It is also Chinese or was, I Americanized the whole machine right down to the Bed.  I also have a Hardinge 14 inch Turret lathe and a Bridgeport J-Head Mill. Right now I am barganing on a Moore Jig Boring Machine.

 A straight boring bar will give you a straight angled venturi. If you make a low angle cut then gradually increase the angle and make a cut each time you will generate the bellmouth you are looking for.

 "Billy G" 

[Tim Wescott]

So... do you use a shaped tool or a regular boring bar to get the flared opening? Does the opening need a gentle curve or can it be straight sided. The reason that I ask is that my son and I have been  contemplating a model airplane engine as a project, and the venturi has been one of the more tricky parts to figure out. You are right... The lathe is very addictive. I think for months after getting it all we did was make more tooling or adapters for tooling. We have a Harbor Freight 9 x 20, and luckily there is a very informative yahoo group on this machine.
Rich

Check out this site: http://www.modelenginenews.org.  The guy is an amateur engine builder, restorer, and collector.  There are a ton of step-by-step engine build projects documented on the site, and if you dig for them there are even some sets of engine plans.

You can make a venturi with a simple angle cut instead of a nice smooth bell mouth.  You may lose a few percentage points of power (or you may not), but by and large the engine will run.  Get that working, _then_ start worrying about making everything pretty and smooth.

Three ways that I can think of to make a nice bell-mouth venturi:

1: Angle cut with the compound, making a whole bunch of cuts at shallower and shallower angles, then clean it up with some abrasives or a file (i.e. wrap some sanding paper on a stick and finish it by hand).

2: Make a nifty spherical cutting fixture and hand-make a boring bar that'll fit.  Then making the bell mouth will take less than a minute -- but you will have spent a week at least on the fixture.

3: Make a "D" bit reamer from drill rod, with the profile of the bell mouth built in.  Harden, sharpen, and make your venturi in one step.  This will actually take much less time than the fixture in step 2, but you'll only have that one profile when you're done.  On the other hand, if you wanted to make 100 identical venturis with it the job could be done by a monkey.

[Tim Wescott]

If it works then it's pretty.  Looks are for people who don't understand the mechanics of the thing (and your wife/girlfriend thinks engines are ugly anyway).

[John Stiles]

All this talk ain't worth a hoot without pictures!

[Dick Pacini]

You can use a bell-shaped center drill (type B) and get a nice inside radius.  The largest pilot is .250, so you could follow it up with the appropriate size drill.  I always finished the bore with a reamer.  Most of the ones I made were .281.

http://www.centerdrills.com/centerdrillBtype.htm

[W.D. Roland]

I am not a Machinist but play one at home.

Several weeks ago when test flying a new airplane with Enya .45  found it has way more power than needed so made 4 different size ventures in about 3-4hrs.

Will try to keep it short.

First dug around to find some stock to work from. Found a piece of .75od round aluminum stock.

Then found 4 drill bits that were the biggest, smallest and 2 in between in the size range needed.

Centered the stock in the chuck(4 jaw) and with drill bit in chuck drilled with the smallest of the 4chosen  bits.

As long as you do not remove stock from chuck it need not be accurately centered as id and od will be cut and it will become true as work progresses.

Feed the smallest bit into stock, can go to the depth that will make all 4 ventures as each next one will be bigger.

Then machine the outside to the necessary dimensions.
NOTE:
Set cutting tool just barely below the center line of part for good cutting.
The part will gain heat and expand so let it cool or dump some water on it for the fine measure final cut.
If left slightly big it can later be worked/polished down to fit.

After outside is finished I have a long tapered burr ( that I use porting auto cylinder heads) that gets chucked in tail stock and fed into part to form the inside 'flair'.

After that I use a bigger than od of part drill to make the top of the 'flair'

Now with 80 grit paper(or something) rolled into a tube to blend the 'Flair"
going to finer grit until i like it.

Now part it of at the right location.

Now go to the next size up drill bit and do it all again.

They came out nice.



For spray bar that passes through venturi find drill bit that comes closest to fitting spray bar hole in crankcase then slip venturi in and hold tight while inserting and turning bit by HAND to 'center punch' the location.
The deeper the better.
Remove venturi from case.
I then drill with smaller bit and the to correct size.

Like I said ' not a machinist'
I do have several types of lath and milling machine and other odd stuff.(can do all auto type work except turn crank.)
My South bend screw was shipped to the Alameda naval air station at the outbreak of WWII!

Have also made them from brass tube one inside the other and also made sprinkler type using brass tube.
When one(or several) is inside the other flaring with brake line flaring tool works and makes a stop as well as distorting things in the barrel area  making it basically a solid piece.

Hope this helps some!

David


WOW!
I must be really slow typist! there was only 2 post when I started!

[Bill Little]

Hmm.... I ain't no machinist, and I would NEVER try to tell anyone anything on a subject that Billy G has talked about, but I will give you *my* experiences at making a venturi, or three.

I got some nylon tube pieces that work good and already have a center hole drilled through.  The sides have enough stock to drill the hole out a good bit with no problem.  I sue a *Dutch Mill* () and a lot of hand tools.  I also use a "numer drill bit" like a #13 or such.  It all depends on what the venturi opening has to be, other size bits, fractional and "alphabet" are used.   Just work it down like a piece of wood on our model planes.

A good friend of mine *carved* several venturis out of aluminum stock while watching TV for several nights.  The gentleman is an artist, anyway.  The venturis, when finished, looked like they were CNC cut!

Now this isn't meant to be any kind of scenario remotely compared to a real machinist using the correct lathes, mills, etc., but it works.

(BTW: anyone got suggestions on a good, inexpensive, lathe I can get to make parts from?? )

Big Bear

[Norm Faith Jr.]

The venturi that I'm running on my Novi's .46LA...was made using my drill press, 1/2" 6061 T6 round stock, flat file, some numbered drills and 180 silicone carbide paper. "Worked finer than frog hair." 
Norm

[Dallas Hanna]

Been a while since posting anything but can't help myself when a bit of machining comes up!  In the past, some of you may have seen my posts on SSW re spinner turning etc.   Being a Toolmaker and Trade Teacher for 40 odd years does help but none of this is beyond the hobby machinist.   Mike Nelson (UK) put all those instructions in PDF format if anyone wants that to have a go at their own spinners as well.

If you have access to a lathe you don't have to have any whiz bang tooling to make venturis.  Aluminium, or delrin if you choose the plastic way, of suitable size is held in the chuck then:
.
1. Turn to the size on the OD to suit the engine, the spigot which plugs into the case is away from the chuck.

2. Cenrte drill and then drill to the required size of your venturi.  Best to use a roughing drill maybe about 0.010" under finish size to make sure the finish drill cuts on size.   Don't dazzle yourselves whith science here when drilling the hole.  As long as you keep a nice constant fine feed all will be ok.  When hand feeding it's only guess work as to the amount so no use quoting any "book figures".

3. Part off to length.

4.  Grind a triangular scraper from a triangular file or buy a commercially made one.

5.  Using this scraper, support your arm on the toolpost and bell the opening to a smooth curve.

6.  Polish the entry with some oiled wet/dry emery paper and you are ready to put it into the engine and drill the spraybar hole.    

Pics below in production order.   Venturi on right is for the old Merco 49.  Left is ST 46

HH

[Rich Federici]

Thank you to all that replied. Now I think we have something to go on. The pictures helped a great deal.
Rich

[Rob Killick]

Nice to see Dallas has it all under control ...

Proper techniques and use of tools .

Great seminar , Dallas 

Rob K.

[John Stiles]

Outstanding!

[Mike Greb]

The tang end of a file works for me to bellmouth a venturi.

[don Burke]

I use a corner radius router cutter clamped in the tool post to make the inside bellmouth.  1/4 R makes a very nice bell.  That's if the venturi bore is large enough to accommodate the center spud of the radius cutter. 

[proparc]

Great, now show the part where you make the hole for the spraybar so that it compresses the O-ring just right!!!

[Phil Coopy]

I follow the same procedure as Dallas except that instead of using a tapered scraper I use a tapered burr chucked in a Dremel tool and running the lathe and Dremel at the same time, form the bell taper shape I'm looking for.  To get the spray bar right, I put an O ring on, and lightly hold the venturi in the engine with a small C clamp, mark the spray bar holes on the venturi with a proper size centering punch, remove it, and drill it while holdiing it in a V block.

Phil

[Steve Helmick]

What Dallas sed. But I almost always use black Delrin. The generic name for Delrin is Acetal...but it doesn't machine nearly as nicely, for some reason, tends to heat up more, grow more, and then shrink to an undersize condition. I usually get 5/8" rod, and a 10' length is maybe $10-$15.

One nice thing about Delrin is that you can actually sand it with wet/dry or other appropriate sandpaper, if you need to spiff it up some, or take .0005" off the spud to fit the case. Most R/C engines we convert have the bore for the throttle/venturi cast in, so they're not wonderfully round, nor are the setscrew holes perfectly centered. Don't worry a lot about it, just push the venturi/o-ring in as hard as you can, and blast through the setscrew hole with a .160" drill. The .156" spraybars will fit nicely. Turn it around and drill the other side.  

I measure the length of the throttle, but also the depth of the case bore. I make the spud as long as I can (and don't forget to allow for the o-ring!). I usually use a larger section neoprene o-ring.

My procedure is logical. I turn the spud (or spigot, in machinist lingo) to fit in the crankcase. If it comes out too loose a fit, I cut it off and very little material is wasted. I make it a nice push fit. I have a tendency to make the fit too tight, and hard to remove when it's finished...not good. Once the spud is done, I use a center drill or 82 or 100 degree counter sink to get the drills started straight. Either works, but the countersink is good for roughing the bellmouth, which is then finished with a #11 Exacto knife. Getting the rpm and angle right so it doesn't chatter is a bit of a knack, but it looks good and works fine. Brian Gardner says that having a chamfer, angle or bellmouth at the bottom of the venturi insert is a bad thing, reducing vacuum. I don't know about that, but I'm happy to not bother, and they work fine with just a light deburr by hand.   Steve

PS: I never part off nothin'. Instead, I pull the part, turn around, and saw it off in the vertical bandsaw in a sec. or two. Delrin is nice stuff!

[Bill Gruby]

 There you are Paul, many ways to accomplish the same task. Dallas pretty much nailed it down for you with the pictures. Great tutorial. I understand that you are about as new as they get to turning in the lathe. I will still send you the tape as it covers tool grinding and a lot of other basic things you will need to know as you progress. When you run out of that 10 feet of delrin I sent you let me know I have plenty more.  Most of all be safe and have fun, don't be afraid of the machine but do respect what it is capable of. Let me know if I can help you in any way.

 'Billy G" 

[Dallas Hanna]

"Most of all be safe and have fun, don't be afraid of the machine but do respect what it is capable of."[/b]

 'Billy G"  

Very true Bill.  I've never seen a real serious accident in all these years but did pull a guy out of a lathe, luckily no broken bones BUT how do you tell your ex boss he was an idiot for polishing in a lathe WITH GLOVES ON.  

Here's one to remind what can happen with lack of training on machines.  Happened here in Sydney a few months back.

http://www.smh.com.au/nsw/questions-over-teens-work-death-20100507-ujp2.html

Regards Dallas

PS.  Steve H, have you retired yet!!!!   

[Bill Gruby]

 That is awful Dallas. I have seen a few that shuold not have happened. Most due to just plain being in a hurry and neglect. I remember in 1986 whin John Striver was finisshig up a job in the drill press. In a herry he neglected to take the gloves off after using the cleaning tank. He now only has three fingers on his right hand.
 Paul, no matter how much you are in a hurry, we all get to that point at one time or another, it is better to slow down and think thru what you are doing.

 "Billy G" 

[Greg McCoy]

I saw a someone stop a 12" Rockwell disk sander with his fingers, they had to disassemble the machine to get his hand out. The material went into the gap along with his hand.

[john e. holliday]

When are people going to start taking resposibility for their actions.  I have worked at a corregated paper box factory, before that an injection molding plastic plant, on the farm and retired from the phone company.  In all the accidents I seen it was the person not paying attention or in a hurry.  Now when it comes to model planes and competition that is another story in which I have scars.  No loss of fingers or hands yet. Where's the knock on wood icon?

[Tim Wescott]

In college I had a pony tail that almost reached my waist, when braided.

Then I started working with machines again, and I decided that I didn't want my face remodeled...

[Bill Little]

When are people going to start taking resposibility for their actions.  I have worked at a corregated paper box factory, before that an injection molding plastic plant, on the farm and retired from the phone company.  In all the accidents I seen it was the person not paying attention or in a hurry.  Now when it comes to model planes and competition that is another story in which I have scars.  No loss of fingers or hands yet. Where's the knock on wood icon?

Hi Doc,

When I was in college, I had a summer job at a bottling company.  Back when the bottle went through a washing machine, got inspected and filled, then capped.  After that they passed on down the conveyor until they hit the packing machine.  The old wooden crates would roll down under the line and the bottles would slide into the packer.  This was all *open to the world*.  No cover, or protection.  The arms slammed in and slid the bottles over the holes and the bottles fell into the crates.  The sliding arms were pneumatic.  All of this was an automatic schedule from the time you loaded the dirty bottles until the bottles were crated.  Now the crating machine had two settings, one for 10 oz. bottles, and one for 16 oz. bottles, and the built in counter would activate the gate and sliders.  Really a pretty good set up!  One day the machine jammed and I was on that end of the line.  This was not that of an irregular occurrence for minor jams, but this time there were about 6 out of th e12 broken and jammed in the machine..  When this happened, you just hit the alarm and the line stopped (all the way back to the dirty bottle loader!).  All you had to do was move the broken bottles, reach down and pull that crate out of the way (a person would hand fill the remainder), and when all was clear, turn the switch back on.  Well, they didn't like the line to be down long so you hurried to get everything cleared.  A guy beside me decided to hit the switch while my hand was still amongst the broken bottles.  A pneumatic sliding piece of steel under a lot of pressure can really hurt.  Especially if it pushes a broken bottle into your hand.  Almost lost most of my right pinkie.  If another guy had not grabbed me, the fellow on the switch would have lost a LOT more...... this was back in my *unbroken days*.  Still have the scars as a reminder.  Funny thing was, the Doctor around the block had a standing order to let any of us in if we needed stitches!  He did a LOT of business!    

Mongo