Making a really really fast fincan.

[Aksrockets]

I'm making a fincan for a friend's project. Without giving too many details away (not mine to give), I can say it'll be going REALLY fast. Just documenting my process here.

Here are my CAD designs based off his fin specs


Oh, and here's a drawing of the fins.
View attachment Fin-Drawings.pdf

Time to pretend I know what I'm doing.

Alex

 

[fyrwrxz]

ohhhh, a shiny thing it is...my precious.....

 

[patelldp]

Time to pretend I know what I'm doing.

You may be out of the realm of pretend! Best of luck.

 

[Aksrockets]

The fins are 7075-T6 Aluminum. I had a plate of .226 thick lying around and that looked about perfect for this.

I started by roughing them out on the bandsaw.


I then ground them to final shape using a belt grinder and a flat platen. (edit: I would not have done this if I hadn't already done it almost exclusively in the past)




I then milled the 1/16in lip using a face mill. I did them all at once. I also bolted the Bridgeport to the wall. Or maybe I forgot to rotate the photo.


This is my beveling setup. An adjustable angle plate on a Bridgeport. Lots of tramming and tapping to get everything all lined up.


I can hear you machinists screaming at me for being so lazy with my clamping setup. It worked just fine.


Finished bevels look alright!


Now the Aerospace engineers are screaming at me for using such thick fins. Don't worry. They'll get thinner. They'll get thinner tomorrow.

Alex

 

[Incongruent]

They'll get thinner. They'll get thinner tomorrow.

Alex

Don't say that. Now the dietitian is screaming at you.

 

[fyrfytr310]

I think you need some contract work...

 

[dford]

"A poor college kid" UROC
Are you utilizing UofU WSU USU or BYU to do all of this? Your shop doesn't look too poor to me. I'm with fyfytr, you may be on to something career wise. Utah is definitely known for entrepreneurship and fabrication!

 

[fyrwrxz]

Please send me.a list of fasteners you used on the Bridgeport. I need to save some space, too, (after I get a Bridgeport, that is...) Lol! Nice work,mate!

 

[NateLowrie]

I'm making a fincan for a friend's project. Without giving too many details away (not mine to give), I can say it'll be going REALLY fast. Just documenting my process here.

Here are my CAD designs based off his fin specs
View attachment 306511View attachment 306512View attachment 306513

Oh, and here's a drawing of the fins.
View attachment 306516

Time to pretend I know what I'm doing.

Alex

I have some questions on the design.

• Why doesn't the can go all the way to the end of the fin? Wouldn't the lack of attachment in the front 1/3 contribute to increased flutter potential?
• It looks like you milled the bevel the same thickness all the way down the fin, but the CAD appears to have a variable width bevel. I surmise the fin will be tapered from the root edge to the outboard edge, which would make the variable width bevel on the leading edge?
• What fasteners are used for attachment?

Looks awesome. Keep up the good work.

 

[BuiltFromTrash]

Nice shop!

 

[ksaves2]

Ummmmm, that project looks like it's up to snuff to handle some serious aerodynamic forces. Is that pray tell for an L3 project? If not, it should be 'cause you're doing all the work yourself. A lazy M motor for a shakedown certification flight and then get
to a venue for some serious altitude. I envy the machinists in this group. Best of luck to you. Kurt Savegnago

 

[ChrisAttebery]

Everything you did looks good to me. I'm not a machinist by trade but I've been playing machinist for about 20 years.

 

[Aksrockets]

"A poor college kid" UROC
Are you utilizing UofU WSU USU or BYU to do all of this? Your shop doesn't look too poor to me. I'm with fyfytr, you may be on to something career wise. Utah is definitely known for entrepreneurship and fabrication!

Yeah, That's my school's shop and not mine. Im poor in money and not in equipment. (which, ironically is what I would buy if I HAD money).

I have some questions on the design.

• Why doesn't the can go all the way to the end of the fin? Wouldn't the lack of attachment in the front 1/3 contribute to increased flutter potential?
• It looks like you milled the bevel the same thickness all the way down the fin, but the CAD appears to have a variable width bevel. I surmise the fin will be tapered from the root edge to the outboard edge, which would make the variable width bevel on the leading edge?
• What fasteners are used for attachment?

Looks awesome. Keep up the good work.

1. To reduce mass. The need for reinforcement decreases as you get to areas along the root where the span of the fin isn't too great (I.E. near the tip). The first node of flutter is generally in bending from the root to the tip, and flutter increases with span^2, so it makes since to have the majority of reinforcement where the span is greatest. Yes, this fincan would probably fail at a lower velocity then one with a fully extended tube, but it wouldn't be as light.
2. Yep.
3. Either set screws or dowel pins. The fins will also probably be press fit and/or epoxied in.

Ummmmm, that project looks like it's up to snuff to handle some serious aerodynamic forces. Is that pray tell for an L3 project? If not, it should be 'cause you're doing all the work yourself. A lazy M motor for a shakedown certification flight and then get
to a venue for some serious altitude. I envy the machinists in this group. Best of luck to you. Kurt Savegnago

Definitely not for an L3 project, or any commercial motor.

About taking on work: I can't get paid to do work for people in the machine shop (department policy (read bandman444's thread on CNC machined AL fins)). But I do work for trade. If anyone wants to trade machine work for something, let me know.

Also, I was not able to work on this today. I was too busy machining the carrier for my school's stupid rocket.


Alex

 

[soopirV]

I'm infinitely jealous of people with machining ability...I'm definitely Icarus in intentions, but Homer in execution: [video]https://youtu.be/EvF9Fx6S5vE[/video]

 

[warnerr]

Nice work! My favorite part of this post is that a machine shop still exists in school... they have been largely eliminated here. Your fitted slot design should prove up to the task- glue and dowels! i look forward to following your progress.

 

[Aksrockets]

It took awhile, but the fins are a little different now. The Root-Tip taper wasn't an easy task. The fins taper from .227 to .0625 along a 2.5in span. A little trig gets 1.84 degrees for the half angle.

I knew this would be borderline impossible to achieve by eyeballing my adjustable angle plate in, so I had to resort to using a dial indicator and sweeping along the angle plate.

I just set my indicator to 0 at around the midpoint of the angle plate, moved the Y axis to -2.500 in and ensured my indicator read -80 thou of difference. If it didn't, I used a small hammer to tap it in. Because of the T slots in the plate, the indicator had to reference off of a parallel resting on it.


Another parallel was then clamped to the angle plate and indicated to be true to the X axis movement of the mill.

The fins were then clamped to the plate. I used a 3/4in endmill to cut the angle. I was planning on using a face mill, but I had little trust in my clamping setup. Not a big deal, but the surface finish was less then ideal. I did a bit of a "Clamp Dance" to get every spot on the fins.

After the taper on one side of the fins were cut, I moved onto the other side of the fins. The angle of the plate had to be doubled because the fins would now rest on the previously cut side. I used the same procedure as before, but instead of using 80 thou for my target, I used 160.


After milling, I wasn't too happy with the surface finish. (I think the head of the mill needs to be re-trammed). So I sanded the surface of the fins down on a surface plate.



Nothing special. Just went from 80 grit up to 500. This is a pic after 220.


After the 500 grit, I touched them up on the buffer.
I really should have spent more time getting the deep sanding scratches out and sanding the beveled edges. There's still some deeper scratches and the final pass of 500 is still visible in some spots. (See pics). You could chalk that up to hastiness. The other 3 fins will probably be a little better.


What I was most concerned about is symmetry. Because of the complexity of the setup and the bevels, any errors in the fin's symmetry would be easily visible. Luckily my setup was pretty spot on so as far as I can tell, both sides of the fins are completely symmetrical.


Now onto the harder part.


Alex

 

[dhbarr]

Uhh, leading and trailing optimized for heatload at M4+ moving to hypersonic; and protruding past the fillet ( bracket ) to avoid the extra drag / heating on the finpoint?

In any case, DANG!