FIN JIG FAIL

I am working on a 54mm MD build, and wanted to be able to properly bevel the leading and trailing G10 fin edges. I reviewed many plans, particularly liking John Coker's version shown here https://jcrocket.com/finbevels.shtml. I recently purchased a harbor freight bench top belt sander https://www.harborfreight.com/1-in-x-30-in-belt-sander-60543.html, and wanted to make a beveling jig to fit. I attempted to simplify the design and came up with this-

Simply a piece of 1/2" plywood secured with screws from the underside to a 2x2, cut down the long edge on a table saw @ 10 degrees. The plywood base offers anchor points for c-clamp(s) as shown in the picture. The idea being you attach the 10 degree jig edge at a distance from the sander such that ~1/2 the fin thickness is made into a bevel as the fin is moved from side to side along the jig.


The astute builders here will immediately notice the lack of multiple anchors on the jig in the picture, and are probably already chuckling to themselves. I was so proud of this newly completed tool, the fact I could only find one c-clamp to fit seemed trivial. I attempted to use vice-grips for approximately 30 seconds, but such use lifted the bottom of the contraption so the angle was no longer set properly. Eventually I decided clamping it in the middle would be satisfactory, after all thats where the force is applied . I'll be gentle I thought . So I grabbed a fin from another project where bevel precision is not critical or even necessary, and put the G10 on the belt (PPE ALWAYS USED INCLUDE PARTICULATE RESPIRATOR AND GLASSES).
I could tell right away the bevel was to deep, but was cutting very smoothly UNTIL about two thirds of the way down the leading edge about 5 seconds from finishing, feeling giddy with success, I applied to much pressure forward of the anchor point and AHHHH **** :bang:


I think the jig will work fine with a second c-clamp, the bevel produced is suprisingly uniform especially considering this was done with a 60 grit sanding belt. I'm planning to re-cut the 10 degree jig angle in a few minutes (sander took a little of the jig with the fin), purchase a second small c-clamp, and try again with the two remaining fins from the set to try and find the right grit belt and develop a repeatable technique (ordered another set for the same as the cost of a c-clamp).

Moral of the story...Don't get the quick @$$ :2:

Up until the mistake, the finish looks fantastic! A few more clamps and it'll work fine.

Yeah, this is harder than it looks. Once you have your jig and technique perfected, send me some pictures and text and I'll add it to the jig page.

I had a similar problem using the JC Jig. Turned out that the deck I had it clamped to had flex in it. When I leaned over to watch the process, I was putting pressure on the deck and it forced the fin closer in to the sander. Ruined a couple of fins before I realised what was happening. The effort was worth it, though. The bevels added nearly 2500' to my altitude.

JohnCoker said:

Yeah, this is harder than it looks. Once you have your jig and technique perfected, send me some pictures and text and I'll add it to the jig page.

Click to expand...

Will do.

I recut the jig and purchased another c-clamp. I began with the jig parallel to the belt, but occasionally the belt would begin eating into the fin and jig coming slightly off track. It is worth noting the belt sander being used is not high quality: got what I payed for and expected. The way it works out if you try to get the belt tight the angling of the pulley (tensioner pulley) causes the belt to run off track. So you must compromise belt tension for a straight running belt, which is necessary for a nice finish. The belt it shipped with (80 Grit not 60 as indicated before) is the only belt so far that fits tight while also being straight. Ace brand belts are too loose for comfort, but the spare belts I ordered from harbor freight are better but not ideal.

I ended up pretty much shredding the first fin.

So I switched technique to something more like JC's, setting the jig at an angle relative to the belt. No doubt this is the way to go, and immediately started getting better results. I am still working on the technique, but so far an angle allowing about 1/3 of the belt to make contact seems the most smooth and effective.


When I re-cut the jig, I also cut a shim about the same width as the G10 (~.064") and using the belt sander shaved one side to about half the width. I thought the placement would be about half the distance of the fins width, but this ends up cutting far too deep. It seems placing the jig about 1 fin width from the belt with the wooden shim cuts the bevel to about half total depth, exactly what I'm shooting for nice sharp equal bevels.:grin: I was so pumped about the success turns out I didn't take any pictures of fin 2, but it isn't nearly the frankenfin show above. Hopefully by the time I'm through the third fin I will have a solid technique down and turn this fail into a win!

I used a disk sander rather than a belt, but I also see a lot of runout when sanding so I need to sneak up on the final bevel depth. Also, it's important to cut no more than halfway through on the first pass, so that you don't lose the edge for the other side.

I revised my setup slightly, further reflecting Johns original design. Basically just added a new plywood base which extends past both sides of the fin guide, acting as both the tool mounting base and an elevated horizontal fin guide. Spent a lot of time just staring at the fins after jig revision two. Although okay, they just weren't up to my expectations for this job.
View attachment 177541 View attachment 177540

I decided maybe I had the set up right handed for a lefty, and switched to pass the fin over the other side of the belt. This was honestly just a shot in the dark.
View attachment 177544
Turned out I was better able to make a smoother and more important, a more repeatable pass in this configuration. The end result isn't near as good as John's, but is much better than any previous version for usability and repeatability.


As the pictures show it is still not far from perfect, and leads me to an opinion question. I want to utilize this set-up for fins twice the thickness for a MD build. The plan is doing traditional (no vacuum) T2T fiberglass with Aeropoxy to the bottom of the bevels, then apply a thin layer of Proline 4500 along the bevels extending just over the fiberglass to try to prevent delamination. Will this attachment method fill the imperfections created during the beveling process? Is there a structural cost?
Most of all, if this was for your MD build would you-
A) Use the method described above.
B) Ditch the bevels and do T2T to the factory edges.
C) Save until you can afford a disc sander, and follow John Coker's method.
D) OTHER SUGGESTION.

Feedback, especially from experienced MD fliers, would be much appreciated. I am itching to make a decision and try my first T2T layup! :fly:

Have you considered adding support to the metal bracket behind the belt? My band saw, like most, has a wheel that applies pressure to the back of the blade to reduce blade flex. That wheel also spins in place so that it doesn't get worn out from the blade running against it. I can take a photo of it if you would find it useful. You can purchase those sorts of parts from band saw suppliers and just find a way to affix it to your sander.

Those grooves can be filled with epoxy, so I don't think they're a waste. I think Pat may be on to something with the idea of making the sanding belt more rigid.

John

FWIW, here's a typical band saw setup. The wheel is adjustable for and aft in order to adjust the tension against the blade. These parts are available at pretty reasonable prices.

Thanks for the replies, I tested the sander belt backplate and it does have some movement. I will look into commercial and scratch reinforcement.
I noticed when making passes, it seemed like if the fin stopped or even slowed too much the belt would wonder slightly into the fin. I am worried it is partially due to the sander setup, as mentioned earlier to get good tension on the belt it must be tightened until the belt is out of track. I was using the side it tracked towards, and actually off the reinforcement bar. Now that I am using the other side I may be able to tighten the belt fully, adjust the jig, and get a better end result. Worth a try at least. I am just a little concerned what is lacking in finish can be best dealt with one of two ways-epoxy or disc grinder.

John, if you use a disk sander, with a platform that can be angled (ie between 0 and 45 degrees), couldn't you accomplish the same thing by sliding the fin across the angled platform, without the need for a jig, or am I missing something??
Greg

Greg Young said:

John, if you use a disk sander, with a platform that can be angled (ie between 0 and 45 degrees), couldn't you accomplish the same thing by sliding the fin across the angled platform, without the need for a jig, or am I missing something??
Greg

Click to expand...

Yes, any setup that allowed you to get the fin at a 10° angle to the disk will work. However, built-in platforms won't get that close to the disk, and you have to arrange to have a base to slide the edge against. Whatever setup you use, must allow you to slide the fin through against a firm fence and base so that you are just sliding it, not guiding it.