Fin cutting

[cjp]

How do all you scratch builders cut your own fins,and can keep from getting a slanted cut.I tried for the first time making fins and used a straight edge,got a straight cut along the root edge and sides,but when the razer knife went thru the balsa,I had slanted edges on some sides.Nothing like laser cut fins.This can't be to hard to do.(I hope.)

 

[jflis]

You have hit upon one of the main reasons that so many FlisKits kits come with "cut your own fins" rather than laser cut. So that modelers can gain some experience in fin cutting before they begin designing their own rockets

What is happening with your fins is that you are holding the knife at an angle as you cut. I have attached a figure to explain what I mean. It is very common for folks to pay more attention to keeping the knife against the straight edge but then not pay close attention to the angle of the knife as they cut.

In the attached figure, the pink area is the balsawood, the black rectangle is the end of the straight edge and you can see the correct and incorrect way to hold the knife while cutting (I hope this figure makes sense... )

The solution?

Practice, practice, practice

 

[Micromeister]

You have hit upon one of the main reasons that so many FlisKits kits come with "cut your own fins" rather than laser cut. So that modelers can gain some experience in fin cutting before they begin designing their own rockets

What is happening with your fins is that you are holding the knife at an angle as you cut. I have attached a figure to explain what I mean. It is very common for folks to pay more attention to keeping the knife against the straight edge but then not pay close attention to the angle of the knife as they cut.

In the attached figure, the pink area is the balsawood, the black rectangle is the end of the straight edge and you can see the correct and incorrect way to hold the knife while cutting (I hope this figure makes sense... )

The solution?

Practice, practice, practice

Jim Hit the nail squarely on the head.... Practice, Practice, Practice.

keeping in mind that this also means paying attention as your learning a technique, Practice does not make perfect, it makes permanent, or perhaps "consistant" is a better choice of words
You'll need to find a "proper for YOU" material angle or placement on your cutting board that helps maintain proper Hand/knife/straightedge/materal alignment while pulling the blade only toward yourself. Adjust the material after completeing a cut so as to maintain that comfort angle. This is a little more difficult on oval or rounded planform fins but with practice you'll know when to stop and turn the material. Keeping the blade in front of you and only pulling toward yourself is most important.
Hope that helps alittle.

 

[Swampworks]

I have one suggestion, at least when it involves straight cuts. If you haven't already, drop in a decent hardware store, or even Home Depot for that matter and invest in a 3-4 ft length of 1/4" aluminum angle. Probably the most popular use for it is to draw fin alignment lines on the BT's, but it can also help make those fin cuts closer to perpindicular. Depending on the fin thickness, I will use either a single edge blade or a "razor saw" but the method is similar. Use the angle as the straight edge with the leg sticking upward. Press the blade flat against the face of the leg while drawing it along the balsa, this will keep the blade very close to perpindicular. Might take a little practice, but it shuoldn't be real tough. I've seen much better cuts this way.

 

[Pem Tech]

How do all you scratch builders cut your own fins,and can keep from getting a slanted cut.I tried for the first time making fins and used a straight edge,got a straight cut along the root edge and sides,but when the razer knife went thru the balsa,I had slanted edges on some sides.Nothing like laser cut fins.This can't be to hard to do.(I hope.)

It isn't, but takes some practice. Personally I use an 18" metal straight edge, keep the blade as straight as possible and pull the knife towards towards me. Make several light cuts, gently cutting through the material. This is your first step to becoming a craftsman.

 

[cjp]

I do have the 3/4 3ft. long aluminum angle and it's use for my fin placement as well as launch lug placement.I'll try that with the next fin cutting.Jim, I understand why you perfer people cutting their own fins.It's great practice.So many things I want to do,but lack the know how.Thank you for making me a better rocket builder.That thank you goes for everyone on this forum also.cjp.

 

[MarkII]

I have a few different steel rulers that, in addition to their primary purpose, I also use as straightedges for cuts. I do have a few sizes of aluminum angle to use for marking fin lines, but I feel that it is too soft for use as a cutting straightedge, but that's probably just me. One other thing that unless the fin material is quite thin and soft (like 1/16" balsa) to do the cutting by making a few light passes with the knife. Until you get the knack of holding your knife handle vertical and perpendicular to the material, the multiple passes can help to cancel out any misalignment of the blade. It also give you more chances to get the blade in the right position. I have cut fins countless times, but I still check the alignment of my knife blade each time that I start a stroke. And I usually still have to even up the edges and get them perfectly square by stack sanding the set on a piece of sandpaper after I cut them. Once in awhile I have to cut a fresh one to replace a fin that is just a bit too far out of spec. (That's why I like to keep a lot of fin stock on hand. )

MarkII

 

[Handeman]

I have a few different steel rulers that, in addition to their primary purpose, I also use as straightedges for cuts. I do have a few sizes of aluminum angle to use for marking fin lines, but I feel that it is too soft for use as a cutting straightedge, but that's probably just me. ....
MarkII

No, it's not just you. I also think that aluminum is too soft to use as a straight edge when cutting. It is easy to shave little pieces of aluminum off when using a sharp steel blade.

Get a good steel ruler. I use the one from my combination square or a carpenters square if it's a bigger job. Having a square face on the edge of the ruler instead of a slightly rounded face also helps.

 

[mkadams001]

I have used aluminum straight edges and have never had the cutting edge of a blade shave off bits of the aluminum. That would require angling the blade edge towards the straight edge rather than cutting parallel with the straight edge.

I normally us a stainless steel straight edge that is about an 1/8" thick and the edge is beveled to about 1/32" thick. Super nice to use.

 

[jflis]

Oh heck (though I wouldn't recommend it), I've even used another piece of balsawood as a straight edge... Once you get familiar and comfortable with your tools and the techniques it all just works. But it takes time to get *that* comfortable.

oh, and John, I love that "practice makes permanent" I will have to remember that one with my kids at the B&GC

 

[Micromeister]

Thanks Jim! The "Practice makes Permanent" analogy has always been a great teaching aid for as long as I can remember. Not sure if it originated with Mom or Dad, I really can't recall.
I do recall it was brought to my attention while learning some early skill; "If we pay attention while practicing a skill method following proper and correct guidence we will develop and have repeatable good results, if on the other hand we practice with abandon, or incorrect method; we will still retain those ill learned skills continuing to end in unsatisfactory results. Always keeping in mind no matter how well whatever is done...nothing on this earth is or can be perfect" I kinda hear Dad when speaking those words but could just as easily have been Mom...funny how things fuzz after they are gone


Straight edges and guides of all kind are a great help. I have and use a number of different items for various straight cutting tasks. Stainless rules in 6", 12", & 24" but I also use clear 30/60/90° acrylic triangles and 3" x 30", 48" and 60" x 3/16" clear acrylic straight edges for cutting very long stiff. As well as some short leg 3/8" x 3/4" x 3/8" x 1/8" alum channels and 1/2" x 1/2" x 1/8" alum angle. I've often heard of some using wooden corner angle moldings on edge as well.

Something to keep in mind about all extruded, folded or routed shapes. They all have a bit of a twist "built-in" by working the material. While it isn't much it can be enough on larger objects to cause a little concern. Thats one of the major reasons for using various sized Rulers and Formed Triangles as these objects are fabricated with tighter "square" tolerances.

Even the most precicely cut fins, from a single fin pattern will always have some slight dimensional differences caused by all kinds of things as small as which side of the pencil line did you cut on and how wide that line was to start with.
Stack sanding your fin sets quickly and easily returns the group to uniformity. Even a single badly under sized fin can be used to reshape the rest of the stack to a single tho slightly smaller planform. I've never had or seen even the worst undersizing of a fin or fin set cause a flight perfomance problem, nor will "the man of a flying horse" ever see the difference cause by such a minor alteration.

 

[dedleytedley]

I find that the type of blade used makes a difference. Most hobby knives come with a long pointed blade. Your hobby store likely carries other shapes of blades. I like the shorter straight edge with an angle of about 45 degrees to the 1/4 long cutting edge. With this shaped blade you can also cut with the heel and it doesn't bend as easily as the longer ones. Also a very sharp blade is essential, as soon as the balsa bends instead of slicing it's time to replace it. Ted

 

[shrox]

Maybe a mirror in front of you to see if you have the knife straight up and down?

 

[Buckaroo]

A good work bench light helps me a lot, I've cut many a fin with slanted edges due to the shadow coming off the blade from whatever overhead light source I was using.

Micromeister has the truth of the matter though: Cut them the best you can and then stack'em and sand'em to get the edges square....

Of course now we will have to start a thread on the best way to make sure you are holding the fins square to the sanding block...

 

[MarkII]

I'll second what Buckaroo said. Set yourself up with good light so that you can see what you are doing. It makes a world of difference!

MarkII

 

[luke strawwalker]

I don't usually have much problem with this sort of thing, but then again, it doesn't really matter when I do...

I always stack the fins, align one edge, pinch them tightly, and sand the stack on each fin edge until they are all identical anyway...

Later! OL JR

 

[dave carver]

When your cutting across grain the blade can have a tendency to want to wander if you try to cut with the grain going away from the straight edge. You need to orient the board your cutting so when your pulling the knife towards you the grain is running under the straight edge at an angle less than 90 degrees.

Working against the grain pushes the blade into the straight edge, cutting in the grain direction makes the blade want to wander as it follows the grain.