Source: 2023Q4 Beartracks, Todd Stock
In early September 2023, I was contacted by Carlo Cillier, a serial plans-builder that had seen a Patrol at Oshkosh, purchased a plans set, and found me just 20 miles away on the Bearhawk Builder Map (brilliant site…simple and it works!). With wing rib production under control by mid-October, and our spars freshly bent at a local sheet metal fabricator, Carlo started looking for spar cap stock, finding that it had not only tripled in price since I had sourced mine in 2022, but both the 1/8” x 1” bar stock for the main spar and 1/16” x 1” angle stock for the aft were out of stock at multiple vendors. A quick check on 4’ x 12’ 1/8” and 1/16” 2024-T3 sheet showed in stock and could be had for just over $1700 (!!!) delivered… which was still less than the cost of a single set of cap strip bar stock and angle aluminum delivered. We elected to mill up three full main spar sets and six aft spar sets and offer the excess sets to other builders in similar straits at cost plus shipping.
If contemplating a similar course of action (group buy or reverse group buy), here’s what we learned about milling spar caps from sheet stock.
Setup
A 10” cabinet saw or well-tuned contractor saw will do the job with a dedicated aluminum blade such as the Freud Diablo D1084L (available at big box stores for about $77). The Diablo has a 0.100” kerf in aluminum, making the math simple on the width lost per pass to the cut. A clear space of 14’ in front of and behind the saw is needed for infeed and outfeed, and enough space to the left side of the blade to make the initial cuts to reduce the 4’ wide sheet to three ~16” wide sections. Trying to move a 4’ wide sheet through the saw is difficult, and the waste generated by ripping the stock into three narrower, easier-to-manage pieces did not reduce overall yield.
An ideal setup would be long infeed and outfeed tables for stock handling, but we made do with four portable flip-top saw stands and three pairs of hands. We did not run the cyclone dust collector because past experience had shown that the chips generated stayed in the bottom of the saw, while the aluminum dust was difficult to clean out of the cyclone filter.
Also worth mentioning that any milling process is going to scuff the stock, so paying for the plastic protective sheet on both faces is a work-saver. Our quality standard was no scuff or scratch in the primary section of a cap (the portion to be used after cutting to length) which could not be cleared with maroon Scotchbrite. The extra cap strip generated per section (13 needed for the main spar; 14 caps out of each piece) on the 1/8” and 2 per section on the 1/16” gave us some opportunity to cull for excessive bow, deeper scratches, or narrower-than spec width.
Milling the Stock
As mentioned, we reduced the 4’ wide sheets to 15.590” nominal, leaving a straight-edged center section of 15.9” width. This was the most difficult part of the job, followed by milling out the 1/16″ strips, then finally the 1/8″ strips. We milled the strips 0.010” oversized (1/8” caps to 1.010” and 1/16” caps to 0.760”, assuming we would lose about half that to edge cleanup. In retrospect, we could have gone
0.020” oversized and still gotten out the 14 caps per 1/8” piece and the 18 caps per 1/16” piece. The critical dimension for the cap strips is distance between upper and lower and then nominal width, so slightly oversized caps are easily managed by either bringing them down to nominal measurement with an aluminum file or just living with the extra weight.
We measured every other strip when milling the first ~16” wide piece of both 1/8” and 1/16” stock, then every 5th strip as we milled the remaining pieces. This helped us in recognizing any problems such as a slipping fence setting or blade damage before it resulted in waste of the entire piece. Division of duties was an infeed person that managed the position of the stock prior to the cut, my job, which was to maintain pressure against the fence when the stock was being cut, and our outfeed person that carefully pulled the nearly-cut strip through the last bit of the cut, as well as stacked the strips. We wore A5 rated high-grip cut-proof gloves, eye, and ear protection, with a flip down face shield for the person managing the saw cut.
Our goal was no scuff or scratch deeper than could be easily dressed with maroon Scotchbrite and no more than 1/2″ of end-to-end bow on the 1/16″ material and 1/4″ of bow on the 1/8″. Both the 1/8” and 1/16” inch sheets were covered on one side with plastic to avoid the need to do individual strip wrapping to prevent fretting in shipment.
With that 200% Russian aluminum duty in place and other global sources tapped out (aluminum is like oil… last few percent of supply determines market pricing), our final cost for material plus shipping from vendor to Carlo was $308 per set of 1/8” cap strips and $102 per
set of the aft spar 1/16” sets.
We were fortunate to be able to use a corporate account for less-than-truckload shipping, with the first order – one set of main spar caps and two sets of aft spar caps costing just $71 to ship from Frederick, MD to Indiana. For those contemplating a group buy and then shipment
onward to participants, a set of caps fits nicely between two spruce 2×4’s milled to 2-5/8” width, with 3/8” ply sides to close the crate. The first shot was taken as I started clean-up- lots of sweeping and time spent cleaning out the bottom of the saw (filled to within a few inches of the motor). The second shot is as we packaged the deburred stock in plastic and prepped the crate. Finally, our caps did sustain some light scratches and scuffs, but those were easily cleaned up. Compared to my set of cap strips sourced from a major supplier, our milled strips are much, much cleaner and will require far less prep to address surface condition.
Bob adds: He has done it this way but has found he prefers to keep the aluminum stationary, and cut the strips with a handheld circular saw with a fence. He also uses a non-ferrous metal cutting blade.