It has 5 interchangeable contact wheels that can be indexed into position–these 3 plus the other 2 corners of the triangle have larger wheels…

They also make a hollow grinding attachment that’s pretty neat…

Below I have included some drawings and models that should hopefully help you with the dimensions and hole layouts. I included the drawings as pdf's and the .step file for the entire assembly so you can see how it all fits together. If you do not have a 3D modeling software I have also included a 3D pdf that will anyone with adobe acrobat (its free) to be able to view the assembly as well.
Note: While making this I made some changes and these drawings and models are not 100% the same as what I show being made. I will address the changes in each step as they differ from the drawings.

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Material List:
2in x 2in square tubing in the following lengths:
One 5in long tube
Two 10in long tubes
One 11in long tube
1.5in x 1.5in square tubing in the following lengths:
One 7in long tube
One 17in long tube
One 12in long tube
Two pieces 2in x 6.5in x .25in (this can be thinner but it is nice if it is thicker because it will be threaded)

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Two pieces 4in x 4in x .125 (this can be thicker if you want but not any thinner)
Four 2in long pieces of 2in angle iron
One piece at least 2.5in x 8in x .25in for the tool rest
One piece 2in x 2in x .25in (I made mine more around 2in x 4in x .25in which you will see later)
One piece 1.5in x 1.1in x .375in for one half of the hinge
One piece 2in x 2in x .375in for the other half of the hinge

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Parts list:

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Lots of bolts (each step will show the bolts you need, most are 3/8in bolts but they don't have to be)
Lots of lock nuts (these are important as this vibrates a lot and you don't want it coming apart)
Two adjustable position handles (I used these 3/8 x 1 1/4 ones)
Flat platen tool (I bought this one but the second link I provided has the plans for one if you can make it)
Guide, tracking, and drive wheels (I bought mine form here but the third link shows how you can make your own)
A gas spring (I used a 7in 100N/22lb spring from amazon but it is no longer available)
A handle (I turned a 2 x 2 piece of wood into a handle but you could buy whatever you want)
A motor (This completely depends on what you have or the power you want, I have a 1hp motor with a vfd to provide variable speed control)