Prettymuch says it all. Im considering switching to the lr from the primo but sometimes need an extra 1/4 inch over the recommended for this machine. Is it really to much for it to handle?
You’d need to use taller side plates. Dan has got a remix. It loses stability up top though.
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Short: Depends, there’s options 
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Long: Depends on what usable X/Y/Z is needed, depends what’s being cut (foam or steel), height impacts speed/quality. Stock LR3 has ~80mm (~3.1") of z travel. People wanting more Z have used longer leadscrews with taller YZ plates. Or, if the stock is thick, but the cuts are relatively thin, then people have raised the Y rails on stack(s) of MDF. The more Y rails are firmly raised the less gantry needs to move (more rigid and faster cuts). Some Makers have designed/modded their builds with Drop Tables and/or gantries with 3+ DOM pipes (instead of stock build of 2) too.
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My table now has a drop built in, lowers the material about 3/4" so the cut ends up going lower than table depth. Will the LR as is have enough Z movement for that deep of a cut? Running the primo, it cuts well up top but things get squirrelly as it gets lower.
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I remixed the side plates to give me an extra 50mm of Z travel.
It works for me. With the Primo, extra Z travel means less stability when cutting deeper (like through material.) With the LR, more Z means less stability at the top of travel, through cuts will be very close to the same.
Without the remixed side llates (and longer MGN12 rails) more than the ~75mm or so of Z axis is flatly impossible. The Z travel is constrained by the side plates and rail length. I get 128mm of Z travel possible, which works well for me. Most of the thick material I cut isn’t dense, so the lesser rigidity of the machine isn’t a problem.
TallYZPlate.zip (3.1 KB)
Edit: the V2 remix is recommended for acrylic, the difference is removing sharp corners.
TallLR3 YZ PlateV2.zip (5.2 KB)
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If you end up remixing YZ Plates for more height, consider modeling/cutting top of the YZ Plate’s profile shaped in a way that adding extra weight(s) for stability is easy.
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Is there a tendency for the bit to push the machine up?
So roughly 3" total travel, 1.75" material and a bit of at least 2"… not a happy combo 
I’m cutting wood, so nothing to hard but not foam either.
In general, no. Most people seem to be using upcut bits that pull router into the material if anything.
However, LR3 isn’t locked down to the table, and does rely on weight/gravity, as well as pull/push force caused by the bit. So, the machine pushing up can happen depending on the combination of machine weight, depth of cut, feed-rate, material density and bit rpm and profile, etc… For example, I have to go slower when using downcut instead of upcut.
For LR3 low, slow feed-rate, with deep cuts, seems to be best way to maximize material removal rate.
My bowflex YZ plate mod helped, but wasn’t the correct fix for my situation… I got the combination/balance wrong and was struggling to get symmetrical V-Bit to fully cut into polycarbonate/acrylic. Adding weight to the YZ plate did help, but using offset V carve bit was a better fix for that task. Am still learning…
Vaguely recall someone in similar situation using MDF stacks to raise their Y rail/rollers. Goal being to have the gantry be as low as possible relative to the YZ plates.
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Are all those little holes 1/16th inch, and are they necessary?
Those holes are 2.5mm and are the holes to attach the Z linear rails. In the first cersion they may be 1/16" and I indented them with an engraving bit into ½" plywood by 1mm just to locate them as pilot holes for #4 wood screws. In the acrylic I have them cut to 2.5mm and then thread tapped for M3×0.5mm screws. Probably the top and bottom 2 are all that’s necessary, but you want your Z rails appropriately aligned for certain.
If you cut them to 3mm or ⅛" then you can use longer M3 screws and put nuts on them on the outside.
There are 2 4.5mm (IIRC) holes alongside the center rectangle that I used for mojnting my LCD. You probably don’t need those.
I’ve only experienced that with the router off with wood. I had the machine drop (lost power) and just the residual speed of the router let the bit go through the material and jnto the spoilboard until the collet nut was resting on the surface.
I’ve had a few projects imperfect because the upcut bit lifted the work off of the spoilboard. Never had the machine ride up.
I cut thick foam, but most of my projects are wood. I use a lot of birch plywood, with some solid cherry, walnut, oak and maple cuts. I haven’t tried milling acrylics yet, but I have an HDPE project on the drawing board. Well, an acrylic one, too but that’s later.
I have done acrylic on the Primo, just not the LowRider.
It will be close. I was able to go roughly 25mm below zero, but with a very long endmill that stuck out too far. 
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Looking at this…
3" of travel is the target for the LR3. Should work fine. Not much more than that though.
With the V1 end mills, which have about ½" cutting depth, the LR3 just gets to table height. With a 2" bit a ¾" dropped table ought to be fine. Maybe 1 ¼" but the extra long bits that I have the ¾" drop would actually be about right.
Adding it up: 1¾" material, plus a 2" bit is 3¾"
My LR3 gets 128mm travel, so standard would be about 78mm,which is a shade over 3" add in your ¾" drop and you’re good to go, though you better watch the clearance plane, you might not get 5mm above the work… but then again, you might. Some people get 80mm Z, there is some variability.
The tall plates do lose some stability at height, but only when it’s actually up high. Cutting at the bottom of travel is the same, or close enough to it not to matter.
As long as I can cross the material and then get full depth it should work.
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Did you need different length lead screws or linear rails? I’m about done printing parts, have the side plates cut out so ordering stuff is the next step
I have 200mm linear rails, and an extra 50mm on the lead screws. (Actually, probably closer to 65mm extra on the lead screws, mine stick up just a bit higher.)
The limits on the travel by the YZ plates matches the likits imposed by the linear rails and lead screws, so extending one means you need longer parts for the others as well.
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