How to Cut a Long Extrusion Lengthwise?

[Hobby Racer]

I'm going to mount a plastic fairing extrusion around the top roll bar tube to smooth out the air flowing back to the rear wing.

IMG_20171026_133727.jpg IMG_20171026_133534.jpg

I am looking for some ideas on how to cut the extrusion down the entire length. I want to remove the minimum amount of material and make the cut as straight as possible. I want to cut it along the trailing edge like you see in the following pictures.

profile.jpg split.jpg

I cut the small sample on my band saw, but that won't work for a 4 ft. piece! The only idea I have gotten is to use a 4 ft. hot wire to melt through it.

What ideas you got?

[frankc5r]

Pretty hard to setup up any kind of alignment that can cut
both sides at same time, the slighest of angle change is way off 4 in away. I would be tempted to use 0.035 thick abrasive wheel very carefully following a line you mark
on fairing. I have had good luck with that and any tiny flaw
can be sanded out without really changing shape. Plus
the 4 in dia wheel helps keep the cut straight.

[flynntuna]

You could try ripping it on a table saw with a tall fence.
Here's a YouTube video on how to make one...https://www.youtube.com/watch?v=s1s1IvRVHOA

[UnhipPopano]

Try using a table saw and cutting it on the bottom where the material is thin.

[Jetfuel]

The airfoil needs to stay fixed to the working surface by means of a airfoil bed cut out off wood, you already have the shape and the bandsaw,no need to be full length, just every foot or so, and as mentioned by frank a steady set of hands and a cutting wheel will do the trick, at the end any imperfections will be glued/filled with methacrylic and sand to shape...

Just a thought...

[Hobby Racer]

Pretty hard to setup up any kind of alignment that can cut
both sides at same time, the slighest of angle change is way off 4 in away. I would be tempted to use 0.035 thick abrasive wheel very carefully following a line you mark
on fairing. I have had good luck with that and any tiny flaw
can be sanded out without really changing shape. Plus
the 4 in dia wheel helps keep the cut straight.

Not trying to cut both sides, only the trailing edge. This is possible with a steady hand. I was hoping for a solution that required less dexterity and would result in a very straight cut in the end (like a table saw with a super thin blade).

You could try ripping it on a table saw with a tall fence.
Here's a YouTube video on how to make one...https://www.youtube.com/watch?v=s1s1IvRVHOA

Thought about using my table saw but I can't find a thin enough blade. The thinnest blade will still completely remove the trailing edge of the air foil

Try using a table saw and cutting it on the bottom where the material is thin.

I want to cut along the trailing edge, making for a lap joint, so the shape comes back together naturally when slipped over the roll bar. Plus it makes it easy to join the foil back together. Cutting on the thin part would make a butt joint and be nearly impossible to put the foil back together again.

The airfoil needs to stay fixed to the working surface by means of a airfoil bed cut out off wood, you already have the shape and the bandsaw,no need to be full length, just every foot or so, and as mentioned by frank a steady set of hands and a cutting wheel will do the trick, at the end any imperfections will be glued/filled with methacrylic and sand to shape...

Just a thought...

Nice idea about making a jig/fixture to hold the foil while cutting.

[Hobby Racer]

Keep the ideas coming, someone has to have a super slick way to cut this!

[Jetfuel]

OK here we go...mount a scroll saw onto a board with the blade sticking out of it that can be secured to the working surface with the blade in a horizontal position and just high enough as the height where the airfoil is to be cut while in the bed/jig.
Wish I was home to show you pictures of my set up that I use to do all kind of crazy cuts..

[Hobby Racer]

OK here we go...mount a scroll saw onto a board with the blade sticking out of it that can be secured to the working surface with the blade in a horizontal position and just high enough as the height where the airfoil is to be cut while in the bed/jig.
Wish I was home to show you pictures of my set up that I use to do all kind of crazy cuts..

Not sure how that would work. Scroll saw blades are connected on the top and bottom. How would you feed the foil in as to only cut the trailing edge like I did in the picture with my band saw?

Maybe a jigsaw mounted to a table might work.

[Sgt.Gator]

A router table / fence with a very small bit?

[JohnK]

Aerodynamics have been a hot topic in the bicycling community for some time now, particularly for time trial and triathlon bikes (I'm also a triathlete). The tendency for the better part of the last decade was to push the boundaries of longer and longer airfoil-shaped tubes on race bikes. This made them very aerodynamic but also very expensive to manufacture and less than optimal in other regards (susceptibility to crosswinds, lack of places to mount water bottles, etc.) More recently, there has been a trend toward 'virtual airfoils' like the Kammtail design. Here's some info on it from Trek's website:

https://www.trekbikes.com/us/en_US/i..._virtual_foil/

KVF_1-up_16x9.jpeg

The net effect of this type of design is shorter tube cross-secctions with similar aerodynamic properties to a full airfoil shape. In your case, this may be useful in that you could get the same performance as a full airfoil with a much easier fabrication process by simply cutting the tail off the tube and devising a way to close up the end of the tube with a flat section. Just an idea...

[AZPete]

I picture an abrasive clothes line threaded through your fairing. Maybe cut a bandsaw blade, thread it through, mount it tight and move your fairing back & forth. Can it be any more redneck/low tech?

[Hobby Racer]

I picture an abrasive clothes line threaded through your fairing. Maybe cut a bandsaw blade, thread it through, mount it tight and move your fairing back & forth. Can it be any more redneck/low tech?

Oh yeah!!! I think we have a winner. I have tons of old wood carving band saw blades with very thin kerfs. I think I'm going give it a try.

[NAZ]

If your method doesn't work, take it to a machine shop and have them cut it on a mill. There are thin kerf blades made just for that purpose that are designed to chuck up in a vertical mill. Hey, you said you wanted a straight cut with minimum material removed. Thats what machinists do.

[Jetfuel]

Not sure how that would work. Scroll saw blades are connected on the top and bottom. How would you feed the foil in as to only cut the trailing edge like I did in the picture with my band saw?

Maybe a jigsaw mounted to a table might work.

You got it...brain thought jig...hand typed scroll...
The jig saw mounted on the board and the board secured to the table...

[aquillen]

I'd tackle this with an oscillating saw, you can get very thin blades that don't do too bad getting clogged cutting plastic. Setup a wood or metal guide to run the blade against, and it won't be in the teeth of the blade. You can run it by hand but still be very straight. May want to take smaller cuts, moving along fast enough to prevent plastic melt-clogging, then just do multiple passes until done. If you don't have the saw, cheapest is from HF and works just fine.

[jmccoh]

How about a Dremel table with a cutoff wheel. Those things are pretty thin.

[Zach34]

A little late to the party, but 2 more cents: The bandsaw blade idea is great as long as you can rig it up TIGHT. Your hot-wire idea is good, too. The only other way I can think of using common tools would only work if you can get a second airfoil. You could mark your line and rough-cut on both ends with a grinder, then sand to your line and bond the two halves together around the bar.

[Just puttering]

Cut the leeding edge? The cut will not need to be as precise and some deformity in the front is probably better than the rear

If you did this you may be able to cut it with a sharp blade

[Hobby Racer]

How about a Dremel table with a cutoff wheel. Those things are pretty thin.

I am also looking at this option, but it would require a very steady hand to make a straight 4ft. cut!

A little late to the party, but 2 more cents: The bandsaw blade idea is great as long as you can rig it up TIGHT. Your hot-wire idea is good, too. The only other way I can think of using common tools would only work if you can get a second airfoil. You could mark your line and rough-cut on both ends with a grinder, then sand to your line and bond the two halves together around the bar.

Yes, I'm still thinking about how to rig the blade up really tight. I want to only cut one half so the there is no alignment issues. By cutting only the trailing edge, the foil snaps around the bar like a clamp and keeps its shape well.

Cut the leeding edge? The cut will not need to be as precise and some deformity in the front is probably better than the rear

If you did this you may be able to cut it with a sharp blade

The foil is a hard ABS plastic and would not cut with a sharp blade. I thought about cutting the leading edge, and I may still do that if I can't cut the trailing edge well enough. Luckily I have another piece if I screw up!

[DSR-3]

How about a hot-wire tool that you can mount to table and fence(s), and push the foil through? Something like this in G10 maybe.Part1.JPG

[Hobby Racer]

How about a hot-wire tool that you can mount to table and fence(s), and push the foil through? Something like this in G10 maybe.Part1.JPG

In my original post I talked about maybe using a hot wire. I would fish the wire through the foil, string it tight, hang the foil by the trailing edge and pull the hot wire through it. My concern is that the hot plastic might remelt back together too quickly. I thought about having a helper keep a wooden dowel wedged in the foils interior to keep the sections slightly separated until they cool enough so they don't stick back together.

[Big Blocker]

Along the same thought that Hobby Racer suggested, a heated wire run thru the length would slowly cut thru in a perfectly straight line and only be as wide as the wire you used.

We used to do this with Plexiglas - a heated wire to soften the plastic for bending to make 6 or 8 sided barrels.

Doc

[Hobby Racer]

Thank you for all the great ideas. I took bits and pieces from a few ideas and came up with a easy way to get a perfect cut. Here it is snapped in place on the upper roll bar right after cutting it. You can see the long cut down the trailing edge.



The final idea was to make a fixture that conformed to the foil shape and mount it on my hand held jig saw. The zero clearance fixture does two things.

1. Centers the blade on the trailing edge.
2. Holds the blade perpendicular to the cutting plane so it can't wiggle while moving down the cut.

IMG_20181127_120754.jpg IMG_20181127_122149.jpg IMG_20181127_122218.jpg


Next I made three more fixtures that hold the foil firmly while I'm cutting.

IMG_20181127_124053.jpg IMG_20181127_124405.jpg IMG_20181127_124721.jpg

It came out great and was easy to make with a few scraps of 2x4 and a hand held jig saw

[flynntuna]

Wow an idea designed by a committee, using the lowest budget materials, and it worked! What are the odds. ��

[AZPete]

Nicely done. I enjoyed the challenge, suggestions, techniques and your successful cut. Thanks.

[Bob_n_Cincy]

Great job by all.

[AC Bill]

Unless you have access to a wind tunnel, how will you ever be able to figure out, what the best position will be for that airfoil?
The shape of it may also change, from air pressure at higher speeds, unless foam filled, or supported internally somehow, possibly where the material is thinner, towards the rear.

Perhaps this info would be helpful..
https://www.sciencedirect.com/scienc...70963815000802

[Scargo]

After that effort it makes me wonder why the wing itself is not part of the roll bar? Is that crazy? It's behind your head. The roll bar could be higher. The roll bar could mostly form the leading edge. It could provide some downforce closer to midway in the car. Obviously I'm missing something because it's not done. It won't be as wide but couldn't that be made up for in shape or angle? Might be some club racing legality issues of you're violating roll bar rules, but it seems it could still be a one piece roll bar.

[Hobby Racer]

Unless you have access to a wind tunnel, how will you ever be able to figure out, what the best position will be for that airfoil?
The shape of it may also change, from air pressure at higher speeds, unless foam filled, or supported internally somehow, possibly where the material is thinner, towards the rear.

Perhaps this info would be helpful..
https://www.sciencedirect.com/scienc...70963815000802

The air foil has the same streamlined 3:1 profile on the top and bottom so as not to generate down force or lift. The intent is to smooth the air stream so the rear wing is more effective. A round tube is terribly draggy and leaves a turbulent wake making the rear wing much less effective.

Simply align the wing's chord parallel to the air stream, aka level with the ground, to achieve optimal effect.

Once the rear trailing edge is rejoined, via the vinyl wrap around the foil, it is quite strong and should not flex in the air stream. The trailing portion has very little force on it compared to the leading edge.

[Hobby Racer]

After that effort it makes me wonder why the wing itself is not part of the roll bar? Is that crazy? It's behind your head. The roll bar could be higher. The roll bar could mostly form the leading edge. It could provide some downforce closer to midway in the car. Obviously I'm missing something because it's not done. It won't be as wide but couldn't that be made up for in shape or angle? Might be some club racing legality issues of you're violating roll bar rules, but it seems it could still be a one piece roll bar.

I believe many sanctioning bodies do not allow wings or other things attached to the top roll bar.

Others have contemplated doing that but steered away from it for various reasons. From a safety standpoint I did not want a fiberglass/carbon wing close to my head in the event of a crash / roll over. All that shrapnel real close to my head made me uncomfortable

[Jetfuel]

Nicely done Racer...just goes to prove that there's more than one way to skin a cat....

[Zach34]

That was a great idea for the cutting method! Gotta file this away somewhere...

[Sgt.Gator]

Glyn to expand on the idea of using the roll bar as either a wing itself or to mount a wing to it:
Many racing rules require the rear wing to be behind the rear axle centerline. Most also limit how far above the roof they can be. However some classes and organizations are wide open. I've looked into this quite a bit and attached some images below.
I agree with Hobby on the open top car roll over issue, I don't want the wing parts to become a spear. Although that doesn't seem to faze the sprint car folks. On a hardtop though......

Lemons racers love to take what Chaparral and Porsche did to the extreme!

[Sgt.Gator]

This is great work! I would do this if I hadn't gone to a full Halo cage. I have too may cross braces between the front and rear hoops to make it worthwhile.

Fairing in the roll bar is one reason why Radicals and Stohrs are so fast.

Nicely Done.