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3D printed beam mount .049 backplate - Page 5 Babe_b10
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Post  Jason_WI on Thu Apr 16, 2020 9:52 pm

Adding a few more pictures.

Needle valve version.
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Cox TD venturi and NVA assembly.

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Various carburetors on the venturi version.

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Post  fit90 on Thu Apr 16, 2020 10:11 pm

the wasp carb is a great idea
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Post  1/2A Nut on Thu Apr 16, 2020 10:29 pm

What size thread did you use to cut the AP Wasp carb?
Did it require any reduction first?

Good to see you are using socket head bolts too,
easy to adjust just the right amount of torque.

I use socket head wood screws for the beam mounts the hole
size you currently use is a bit big / smaller hole would help on
strength / splitting / cracks.

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Post  Jason_WI on Fri Apr 17, 2020 1:57 am

I turned it down on the lathe for 1/4-32 pitch thread size. I ran the die on the normal way to get a few threads started the turned the die around to cut threads to the bottom. Not much meat left. These carbs were cheap when they were readily available. It needs a thin nut to tighten the carb down tight to the carb body.

I have taps for the .020 venturi, .049 venturi and the TD 15 venturi. Also a cox cylinder tap. All the venturi tools from Ridenfligh on eBay. Piston reset tools for all engines and the .15 reset tool I made. All kept in a handy case.

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I did model the hole size of the mounting ears after the TD but they get egged out a little printing vertically. I did buy #2 small diameter washers for the wood screws I have. I still have a boatload of the wood screws left over from the horseshoe backplates.

I just got in over $100 worth of small screws from McMaster Carr. I didn’t like the thread engagement with the 2-56 x 1/2” hex screws. I bought 2-56 x 9/16” screws. Gives a few extra threads.

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Post  1/2A Nut on Tue Jun 09, 2020 2:00 am

65 deg carb mount with a KB .049
Tatone Muffler with Larry's metal mounts.
Will bench test with a APC 4.6 x 3

Hex head inpoint stud bolt with locking nut.
Much lower weight compared to long bolt / spinner.
The thick beveled washer seats into the prop hubs
recess for improved alignment.

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Post  Basswood on Tue Jun 09, 2020 3:51 am

Brad just have one question, have you had any problem with the Cox .049 .05 Tee Dee RC Carburetor Assembly fuel barb. Ive cracked or broke the barb off two of them. It seems to me the plastic is just old.the plastic seems to be very brittle on mine. I'm thinking about just building them out of brass with tubing. Basswood
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Post  Basswood on Tue Jun 09, 2020 4:26 am

I guess my point is what is this 3D printed material made from?
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Post  happydad on Tue Jun 09, 2020 5:57 pm

Jason_WI wrote:Still 3 left for testing yet.



I have one of each carb type for testing. I won’t need anymore for that.  Thanks for the lead though.

Are there any left for testing? I just started reading this thread. I have been looking for this kind of carb for a few years. I even talked to bob Beecroft about building me one, but he had too many other priority jobs a ahead of that one.
 Happydad RC Plane Huh...
Edited to add data

P.s. I have a 1/4-32 tap
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Post  1/2A Nut on Tue Jun 09, 2020 6:44 pm

Basswood, the version I have to test has a carbon blend to it.
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Post  davidll1984 on Tue Jun 09, 2020 9:11 pm

wit wasp carb y sugest use 051 size cylinder is porting wil fit beter ventury to jet size of wasp carb neadle set up is bit diferent y made my wit couple mod to get right amount of fuel to hard to explain wat is ur test result on wasp carb how u tuned carb for 049 ? my is tee dee 051
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Post  1/2A Nut on Wed Jun 10, 2020 12:51 pm



Killer Bee .049 with 3D Printed Backplate for RC Carb 65 deg.
Testing with muffler and exhaust deflector made reasonable
power considering not open exhaust / very easy on the ears.
SPI engine seemed to breath reasonably for bench testing.

APC 4.6x3 / 20,914 rpm peak / 60 mph pitch speed / 8.9oz thrust.
Stop screw needs to be set for low idle looks like it can handle 6.8k
with a larger prop much lower perhaps 5k I would expect.
25% nitro / 20% castor / 3 shims Cox High Compression Plug.

Engine ran well side mounted the exhaust system may have helped
with the steady running. Butter on the ears will make a great RC
engine project as is or open exhaust for full power. Aluminum mounting
brackets for bench testing ease and to acquire side mounting test.
3D mount was clean no air leaks great design for fuss free use!
Improved 65 deg. angle made access a breeze with the RC carb.

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Absolutely zero vibrations with this set up very pleased with the low weight centered stud / nut method.
The engine did great with the bulky robust combat prop.

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Post  balogh on Wed Jun 10, 2020 1:00 pm

Brad my Tarno carb got loose in the 3D backplate despite the contra-nut. I did not want to overtighten for fears I might cut the threads in the backplate. How did you keep the carb vibe-free?
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Post  1/2A Nut on Wed Jun 10, 2020 2:43 pm

It all starts with a perfectly centered prop. I use fuel tube that
is a snug fit when the stud bolt went thru it sets the stage for
a centered prop. The trick is not to mess it up with a spinner
that slides / kicks out of center when tighten. Threaded bullet
spinners work great. The nut and stud method worked very
well today low mass tucked up close to the prop hub is another
factor to help reduce vibrations. I used a small crescent wrench
to tighten the locking nut no fear of stripping the threads. I put
about 6-7 lbs of torque on the nut.

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Post  Jason_WI on Wed Jun 10, 2020 3:54 pm

For the threads you should be able to tighten it down as much as you would with a delrin carb body on a TD 049.

I’m also going to use a tapping machine with custom mounting blocks for the parts to get the tap perpendicular to the hole and have perfectly formed threads from the start. These first few samples were hand tapped.

I’ll see if I can run some torque tests on these to see what the threshold is. Destructive testing is fun.
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Post  balogh on Thu Jun 11, 2020 3:24 am

Thanks for the advice Brad and Jason.. I also make sure the prop is well balanced and use the fuel tube on studs for APC props that come with larger screw holes than the COX prop stud diameter. Because of the US thread size, I do not have too many options to replace the aluminum spinner with something lighter. I do not often use even the small COX rubber spinners because they tend to fly away from the stud hub and get lost.

Maybe I will also wrap some plumber silicon tape on the carb neck to increase tightness in fitting, then span the contra-nut cautiously a bit more than before.
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Post  1/2A Nut on Thu Jun 11, 2020 1:04 pm

Thank you Andras, have a few of these backplates and thus far the ones with RC
carbs on them have held up well with the locking nut. None have gotten loose on
me / no air leaks observed. Fuel draw has been good although if I use a muffler
I tend to go with a pressure tap to improve steady top end rpm.
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