Finally finished my underslung plow frame project. Plow is an 8 Fisher Speedcast I picked up on Craigslist for $300, a steal considering it also included all the truck side frame and hydraulics.
The quick attach plate is a 5/16 thick version. The horizontal frame tubes are 3 inch 1/4 inch wall square tubing. I welded a piece of 4 inch 5/16 thick angle flat to the bottom bent part of the QA plate and across the face of the plate to give strength to the QA plate. I then welded the 3 square tube to the bottom of the angle. The support gussets are 3/8 thick plate that was water-jet cut based on a template I had made. I filled the space at the top of the gussets with 1/4 inch flat bar. The rear cross member where the plow attaches is the original truck side lower member that is an I-beam with the mounting pins attached. The verticals between the square tube and the I-beam are pieces of 4 channel notched to fit the I-beam and welded to form a rectangular tube. At the top rear of the QA plate I welded in a piece of 2 inch x 1/4 angle which centers the QA plate between my loader mounts and also reinforces the QA for the chain lift point.
I did a lot of research and looked at A LOT of other designs by users on this site when I was planning my project. I determined that shifting the plow attachment points rearward by using this type of mount removes some of the lateral leverage the plow has on the loader and also shortens the length the plow adds to the tractor. This plow is mounted 14 inches rearward of where it would be if I just attached pin mounts to the face of the QA plate. I could have gone further back but was limited by the two pieces of 3 square tubing (since I found them in a scrap bin and they were free).
Another key factor was the plow had to have a chain to lift it which allows the plow to float on the ground without having to constantly adjust the loader height while plowing. If the plow is rigidly mounted to the QA plate then you either have to keep adjusting the height or use the loader float function. Using the float function is not desirable because it shifts the weight of the loader lift arms to the plow from the front wheels reducing front end traction.
Total money invested is just under $600 which includes the plow, paint, hoses, and crossover relief valve. I probably have about 40 hours into it as well.
To anyone looking to build their own underslung plow I am posting this here so you can steal any of the ideas I used to build mine.
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