Please put your "I am new and need help" question here.

[Dallara]

@ matiz

Thank you so much for that excellent advice. Obviously I want to not just throw money at this hobby unless I'm at least reasonably confident I'm buying the right stuff. What diameter wire would you use, 0.6 or 0.8mm?

The nearest scrapyard to me is over an hour away, and they tend to crush everything almost immediately. I'm good with an engineering shop nearby but all their 304 is structural and at least 8mm thick (I know I used to work there) although they will be laser cutting my sheet for me. Ive seen their laser cutter cut 20mm S355 like it was butter and the finish left was not far off a machined edge, so my 0.9 should be nothing to them

https://core.ac.uk/download/pdf/71674951.pdf page 31/32 covers the use of effective length and mean diameter. I'm in no way endorsing the content within, I was simply trying this and any other method of measure to govern the said scaling of designs. I'd love to know the true gospel method of calculating thrust and frequency from the fine members of this forum

[metiz]

I use 0.8mm wire. The paper you linked discusses a valved engine. Valved engines do not have a protruding intake or a coned exhaust so effective length=engine length (maybe not including the small length at the front of the engine taken up by the valve) and mean diameter is what it sounds like. It's important to note that, although many principles between valved and valveless engines are interchangeable, you can't just use this paper for your valveless design. Just stick to Pyrojoe's recipe for beginners or one of the known designs for now. If your scrapyard is an hour away, go there and take a bunch of sheet with you (and some stainless or copper/ brass brake lines while you are at it) so you have enough material for a couple of engines. Stainless steel sheet (304) will cost you about €2/kg, which is WAY less expensive than buying it new. A cheap angle grinder can be found for €20.

[AlexanderAndrosov]

One observation from hours on this site already, a lot of the links no longer work, I would say half or more, and that's a terrible thing to lose that knowledge. Maybe something can be done about this going forward.

Regards,
Dallara

Hi Dallara.

You can check out my collection of PJ materials at https://drive.google.com/file/d/1JxaNa3 ... drive_link.
I update it occasionally in my topic at https://www.pulse-jets.com/phpbb3/viewt ... f=3&t=6785.
I have filtered the entire valveless section as well as a part of the valved section and also checked researchgate.net and other similar sites.

[Dallara]

@ AlexanderAndrosov thanks for that, that looks like the stuff I was after!

@ Matiz, and others, I need to know if I'm picking the right PJ's for starting to intermediate difficulty. My previous employer has given me a quote to supply me with a 3m x 1.5m x 0.9mm sheet of 304, INCLUDING laser cutting the 6 jets below, for around €120!!! I'll even get the useful remainder of the sheet for future cuts, and all the in between bits cut into strips for learning how to weld stainless... I think this is an amazing deal, and I want to strike while the iron is hot, as I have been quoted €125 for 1 jet worth of parts or €500 for the same full sheet cut, and 70km from where I live so I'd have to pay extra for delivery... my previous employer is only across the road from my home.

Have a look at the following, in no order but my plan is to start with the caliber designed jet, then the rest of the linear jets, then the lockwood and finally the giant chinese:

I hope this works













or this might work:

https://photos.app.goo.gl/wJQPBhuQAMKSG6WS6

Thanks in advance!

[metiz]

That's a great deal, go for it. Don't forget you have to compensate for the sheet thickness in your flat patterns: all dimensions in these drawings are internal so if the sheets are cut with the dimensions as stated, your engines' diameters will be off by 0.9x2=1.8mm. Also have the intakes cut slightly longer so you have room to hammer the flares without consuming intake length.

It's "metiz" btw, not matiz

[Dallara]

Apologies Metiz, I was so excited about the deal I forgot to check the spelling of your name

All the dims I have been dealing with are internal dims, are you saying I should add additional width on to the dimension of the flat pattern, based off a part modelled on the ID? I've never had to work sheet metal to this accuracy before.

I have already added 1mm in width to all flat patterns as Solidworks only allows revolve 359deg for sheet metal parts, and measuring the difference it appears to be very close to 1mm across all geometries.

I included extra meat on some inlet flat patterns, but I will check once I get home that all inlets are 12mm longer to allow for a flare.

Its a bloody good deal, that's €20 a jet!

Thanks again for your input, I really appreciate it even if I didn't get your name right

[metiz]

"All the dims I have been dealing with are internal dims, are you saying I should add additional width on to the dimension of the flat pattern, based off a part modelled on the ID? I've never had to work sheet metal to this accuracy before."

No, increase the diameter of your pipes and cones by 1.8mm in CAD, then generate flat patterns from those. Say you have a cone, big diameter is 100mm, small diameter is 70mm, length is 150mm. in CAD, make your cone dimensions 101.8 - 71.8 - 150 (length stays the same) and then generate a cone from that model. Same for straight pipes.

[Dallara]

I have made the critical dimension as the ID, so the thickness of my material is additional to that. Solidworks then generates the flat pattern from that 3d model. For example, I could change all the material thicknesses across the entire model, and the internal diameter would always stay the same in the model. Are you saying I need to add extra material onto the width of my flat pattern due to the material thickness? I would be under the impression my flat pattern would remain the same, as the ID hasn't changed?

I could very well be wrong, but wont the ID determine the flat pattern moreso then the material thickness?? I have taken all dims as internal across the board. I must calculate some circumferences and adjust all my flat patterns if this is the case

[Dallara]

Ok ok, after some quick googling and circumference calcs, it appears Solidworks may compensate for the 359 degree revolve and it looks like the thickness of material. An example is on my drawing linked above titled Thermojet 5.
The inlet pipe has an internal diameter of 44mm, so an internal circumference of 138.23mm. The outside diameter = the ID + 2 X material thickness, so 45.8mm giving an external circumference of 143.885mm. The solidworks flat pattern measures 140.7mm across the edge to be rolled...

The neutral axis should lie on the ID + 1 X material thickness, so 44.9mm giving a circumference of 141.0575mm..

OK, I'm lost now. I had already made every flat pattern 1mm wider to compensate for the 359 degree revolve... Was this a mistake?? I'll have to check everything tonight, I want to get sheet cut next week and I dont want to end up with a load of scrap!!

Can anybody clear this problem up for me?? I have never worked with sheet metal to this tight a tolerance before...

Solidworks surely has to be right, the smaller (0.35mm) length either has to be a product of the 359 degree revolve that solidworks forces on all curved sheet metal parts, or a product of the compression of that side of the neutral axis due to bending, right?

I don't even know anymore. Every other sheet metal part I have designed it never mattered if it was out by a mm or thereabouts, but a 5.5+mm discrepancy is quite worrying, to say the least

Thanks in advance

[metiz]

For layouts, I use Sketchup. I "unfold" the models myself so I have to compensate for material thickness. I don't have experience with sheet metal in solidworks, but if the program compensates for sheet metal thickness, you don't need to change anything. Can you print a pattern for a straight pipe on some paper? you can measure the width and see if solidworks has corrected for the thickness.

[Dallara]

So I was reminded I had investigated this same matter previously, with the package Solid Edge. SW uses either gauge tables for various thickness materials with the same bend radius, or uses the K factor of a given material to compensate for stretching/compression either side of the neutral axis.

In short, the compensation is calculated on a "one rule" for all bends and thicknesses, unless specified by a table of constantly repeated bends and bending radii. This means I wont know the real world dims until I bend the parts, which is less than ideal.

On the somewhat positive side, the flattened parts are always a margin larger than ideal dims, maybe that margin is too much, maybe its just right. I think with this and the additional 1mm I added to the width of each flat pattern, I think its safeish to say the parts are oversized. I'm thinking I can get around this by having a small overlap of the curved edges, which I will then weld.

Has anyone else done lap joints on the curved parts? Or did everyone use butt joints? At the moment I'm leaning toward trusting the software, its bigger if anything so I should be ok there, but if its too small then its scrap.

What do you think, Metiz? How do you calculate your flat patterns, specifically cylinders and cones?

Thanks again

[metiz]

I don't recommend overlapping edges, it impedes air flow. I use Sketchup with a lot of "CAD-CAM" stuff because it's very flexible. I use a small plugin "unfold" that allows you to unfold faces. Sketchup has a fairly low resolution so I just click on every face of the cone, or whatever else kind of model to unfold in a flat pattern.

A sure fire way to figure out if your flat patterns are correct is to print a small, straight flat pattern out on paper, scale 1:1. Say it's a 100mm diameter pipe for example.

If your CAD model did not compensate for the material thickness, the width of your flat pattern would be 100*π≈314mm
If your CAD model did compensate for the material thickness, the width of your flat pattern would be 101.8*π≈320mm

If your 100mm pipe is correct, and you generated all the flat patterns identically, then you're good to go.

If you can't get it to work, https://file.io/TAtYYI2OZjEe this url links to a 300-ish kb program that I've used when I started with pulse-jets and it can generate cones for you (you have to add the 1.8mm yourself) and it can export to an autocad file so it can be used in a plasma cutter.

[Dallara]

Well to take an easy example from my folder above, 0009 Therm 5 has a combustion chamber ID of 100mm, OD of 101.8mm. If we take the circumference at the ID, its 314.159mm. At the outer diameter its 319.814mm and through the neutral axis is 316.987mm... Solidworks lists it as 316.1mm??? So where does that leave me? I know the neutral axis also changes location as you bend a part, so should I trust the software?

[metiz]

The ID sounds exactly right. The outside diameter is a bit sus, but it doesn't really matter, only inside diameter matters. If you want to be sure, print the pipe and measure. you could also post an autocad file of the flat pattern here, then I'll check it. If you do, note which part it is.

[Dallara]

@Metiz the rest of the geometries have been calculated the same way, so I'll trust the software, verifying the ID's as I go... One more look over the scaling so and I'm ready to cut metal

I just tested my sheet roller too, it works like a dream up to 75mm ID, so that should get me the majority of the way through my parts. My stainless welding gear should be here midweek next week, I tested my welder without wire and it still works, it had a recent enough rebuild so it should be grand. My bbq spark ignitors will be here from china in about 1.5 to 2 months, I'll source a propane cylinder and I have the necessary pipework to get the job done...

Lets cut some metal and build some jets