Modified Bolt Together Frame Inquiry (12' long, 1' taller, metal rail, and more)

A variation on a frame for my Metal Maslow CNN w/ Makita Router:
For about $100 in material (with a $20 off coupon that I have), I can build a modified version of the Bolt Together Frame. Let me describe my variations / why, and see if the collective experience here can look for any issues I should consider.

RAIL: 12’ long, using the metal rail from the Metal Maslow CNC.
EXTRA A-FRAME: Adding a third A frame to the center (or just off center to one side or another if this location is a problem.
A-FRAME SPACING: Moving the A Frame locations to 8’ apart, reducing the overhang at the ends to 2’ at each end.
RAISED RAIL: The 12’ rail is raised 1’ higher than the default / basic a frame design
WASTE BOARD: Replacing the 3/4" plywood with 1/4" thick Hardie Backer Fiber Cement Backer Board, counter sunk screws, and a layer of 1/2" MDF on top of that to act as an easily replaced fairly cheap waste board. The overall thickness should be about the same, so the Z geometry doesn’t change. This backer board is not the same as “standard” fiber cement backer board. It is stronger.
MATERIAL: I’ll be using 12’ 2x4’s for this design, so the 2 10’ 2x4’s can become one 12’ one since only ~64 inches is used from each one.
Add cross members between the outer A-frames and the center A-frame to provide more support in the Z direction and rigidity between the framing.
I may add a cross member from the overhang of the rail down to the A-frame as well, but will gauge that after it is built as that will be easy to add.
The top surface of the cross member where the wood that sits in the workarea rests, I plan on routing the top of that cross member lightly and setting aluminum angle material there to create a long lasting work surface. It will be inset just so far as to be flush with the original uncut side of the 2x4.

The backer board will be heavier than plywood, but an extra ~50lbs give or take on the frame doesn’t bother me. I plan on adding offset lift wheels to be able to move the frame around and then set the frame down for the added stability. The lift wheels are not factored into the cost at this point and would be added later (probably the week after I build the thing).

I like that the cement fiber backer board doesn’t burn. I will watch the CNC machine when it is running, but making the backer board out of something that doesn’t catch fire is preferable to me. I am not sure how flexible that 1/4" backer board is, so the extra A frame and extra cross member should help keep it nice and flat. If when I go to buy it, if it is not stiff enough, I’ll pickup a 1/4" piece of very smooth plywood instead of the fiber cement backer board.

Another option would be to use their 0.4" thick backer board. If I went that route, I would cut into the 2x4’s just enough so that only 0.25" of the board stuck out in the z direction, so that when you add the 1/2" MDF waste board to the top, it would still only be 3/4" like in the original plans, not changing the geometry.

I’m very interesting in your thoughts on this and concerns.

Your plans for hardy backer should be fine. Your geometry (being the spacing of the beam out from the plane of the work surface) is determined by your ring height that is determined by how your sled center of gravity is positioned. Since you start with the sled and work backwards, whether you use an inch hardy backer or 1/4" should not matter because you want to put the beam on last to get the chains parallel with the surface. The instructions have you make the sled last, but after you make the sled, you may need to adjust he beam position to compensate for the center of gravity. If you start with a premade sled, it is easier because you can line things up first.

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I’m using the Metal Maslow, so it comes with a premade sled. The 1’ height adjustment comes from their recommendation.

Definitely go with their recommendation then. Keep in mind that +/- 1/2" won’t kill it. If you deviate too far you risk the chain jumping or wrapping up around the gear, but parallel is best and it sounds like you have a reasonable plan to achieve that.

Thanks. I was assuming that their design was parallel so my goal was to mimick their 3/4" thickness one way or the other to maintain that same geometry. I do understand that if I want to cut something thicker I should just be able to use some spacers to keep it all parallel, but there is only so much stick-out a hanging router can handle, anyway. I really appreciate you taking the time to read and comment. Thank you very much.

Yes. you should be ok to space it in or out. The workpiece is at a 15 degree angle from vertical. The chains will “hang” with the sled on that work piece. You want just to mount your top beam out from the work surface so the chains are parallel to the flat work surface the sled rides on. The sled will determine what the distance is from the work surface to the chain and the beam placement will determine if the chain are parallel to the work surface but it doesn’t have to be exact. It needs to be reasonably close.

sounds like a reasonable plan. I have seen people use 1/2" styrofoam as a waste board and it keeps things light.

we recommend the lighter makita or dewalt routers, the ridgid is a bit overkill and raises the center of balance.

the only problem with cement board is if the bit plunges too much it will make you bit really dull when then increases the chance of fire.

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you may want to experiment with tilting closer to vertical.

15 degrees is what Bar first tried and it works, so he didn’t try other angles

during the design of the new frame, Bar misunderstood instructions and built a
frame at 20 degrees and it didn’t work, so we no we don’t want to go further
from vertical

we’ve had mixed results going the other way, with an indiction that 5 degees may
be too far the other way, but going from 15 to 10 (and testing towards 5) would
be worthwhile.

David Lang

I got the Metal Maslow with the Makita because of the weight / balance.

I hadn’t considered foam, but not sure I really want to cut up foam. At least MDF is still somewhat wood, not plastic. Definitely worth the option though.

I believe I read that there were problems below 10 degrees, so 10 to 15 was the range to experiment in. I was thinking of a couple options to modify the engagement of the A-Frame. One idea is to use that angle iron that has holes in it. Sort of like plumbers tape in angle form. Another option is to use unistrut (I might have some unistrut on hand) and bolt that on instead of wood arms. By changing where the unistrut or angle iron attaches, I can change the angle. Didn’t really feel like touching on all that in the original post as it was long enough as it was.

I did 10 degrees and 15 degrees. I built the frame with 10 degrees and to get it to 15, I put the front feet on a 2x4 it cuts better at 15 on my setup.