Here is a long winded post and looking for some input:


I have been looking at going to a R&P setup and re-building my Joe's Hybrid 4x4 with a steel base on the machine. Being cold in Wisconsin and having limited time in the shop, I haven't wanted to turn on the heater out there and try to warm it up to sit down and come up with some ideas for what I want to do. I want to change from lead-screws to R&P because when it is below 55 degrees in my shop, the AB-nuts get tight and I loose steps. Over the last few week I finally put on paper my brainstorming on what I think I may do, and I am looking for some input on placement of the rails on the X gantry and sizes of rails to use. Here is my thought process in no particular order:

My upgrade will be most likely the following:

Linear slides on all axis, 15,20, or 25 series with 2 bearing blocks per axis - but may go with only 1 per slide on the X gantry if I go with the large 25 series rails.

Use one of the linear actuators that I have for a new Z -most likely one of the 2 KR33 with stepper mount and ball screw. I also have a linear stage that is really nice, however I need to build a mount and get a screw and AB-nut for it so I will be most likely selling it.

I will be using 1/2" aluminum side plates for the Y carriages, and R&P on X and Y from CNCrouterparts.

A new 8020 X gantry beam - a 15-3060 3"x6" fractional to replace the metric 25-5010 that the Joe's uses right now.

It looks like the 15mm series linear slides/rails may be a good choice mathematically. Each bearing block can support 80 lb-ft of moment force on them so 2 bearing blocks per rail gives 160 lb-ft of carrying capacity per side for the Y. If I step up to the 20mm series it could support a moment of 165 lb-ft per block - so 330 lb-ft per side on the Y. I am unsure if I really need the capacity of the 20 series rails The 15's on the X gantry can support a 98 lb-ft moment per block - so with 4 to support the Z almost 400 lb-ft- I see no problem with these on the X with 4 blocks holding the linear actuator of the Z.

The 15mm works out rally nice for the Y on the 8020 because they are 24mm thick from the face of the 8020 to the mounting point. A 15mm rail and block mounted to a 1/2" aluminum side (13mm) = 38mm - This is good because that means if I mount the rack on the 8020 and use a CNC router parts R&P drive that I will get nearly full engagement of the pinion on the rack.


Now looking at the 20mm rails - it would push the R&P out an additional 6mm to 42mm total. A 3/16" aluminum spacer would be needed be be added to the back of the rack to bring it back out to match the location of the Pinion.

Then throw in the 25 series - may be overkill - but I would then need bring the rack farther out from the Y axis 25-5010 extrusions - most likely mount on top of the 25-5010 to avoid that.


I have a few pictures of what I am thinking of for the X. Either mount one slide vertical and the second slide on top of the 8020 to support the R&P drive, or both on the horizontal face - these are labeled 3&4 in the picture. I am leaning towards #4 to support the horizontal member that attaches the R&P drive. This is similar to Greolts build.


I have some more sketches of the 15 series and 20 series for the Y axis gantry side plates. the 20 series will require some additional machining to make sure that I can clear the blocks with the R&P drives. I am not sure if I can get this to work out. The 25 series end out working Ok with the 2-hole blocks mounted on the side of the 25-5010 and the 4-hole blocks would need to be mounted on top or bottom of the 8020 to allow engagement with the rack on the side of the 8020.

My issues:

- the 15 series appear to be just too darn small. They connect with small fasteners - 4mm for rail to 8020 and 5mm to the blocks. I haven't done the math on the shear forces yet but I am sure that it is close to exceeding the connections. Look at the pictured of the comparison of 15 to the 25 series!

The 20 series are used on some other commercial machines and use 5mm fasteners for rail to 8020 and the rails, and they just appear to be beefier. But these cost!

The 25 series may be overkill but they use 6mm fasteners to connect the rail to the 8020. This has a 7mm hole and fits a 1/4" fastener fine.

4mm and 5mm t-nut hardware from 8020 is EXPENSIVE. I need 128 to attach the rail and many of them are $2.00 each

so an additional $250 for the hardware to attach the smaller rails. I could get 1/4" nuts for $50 for all 4 rails

if I use the 25 series and use the $200 towards the bigger rails.... trade offs everywhere...

Load capacity of blocks and rails I am looking at:

15 series 80-lbft per block - 4 hole
20 series 165-lbft - per block - 4 hole
25 series 268-lbft - per block- 4 hole
25 series 56.3-lbft - per block 2 - aka - it's a smaller block

So the big question is, what combination? I know that the 15 work on the X, and Y but are too small IMHO, the 20 series work - but fasteners get expensive, the 25 series are expensive but the fastener costs are reasonable. The 25 2-hole blocks have a substantially less load rating than even the 15 series. But on the Y sides - this gives me 112lb-ft per side. My gantry weights 50 lbs (about 1-ft high)- so the questions is - will I generate an additional 50-ftlbs of cutting forces? The documentation that I have been reading say that the 2-holes blocks on a wood machine should be adequate - we will see what everyone else thinks

My break down: (don't choke on the costs)

Y&A listed first, then the X, cost is with shipping and fasteners

#1 25-2hole and 15 cost = $865 - cons: the 15 are awfully small!

#2 20 and 15 cost = $890 - cons: fasteners and again the 15 are still small

#3 20 and 20 cost = $1,025 - cons: one of the most expensive options

#4 20 and 25-2-hole= $930 - cons: may look weird since their is a larger 25 on the gantry

#5 25-2hole and 25-2hole cost = $905 - cons: smaller load capacity of the 2-hole block

#6 25-4hole and 25-4hole cost=$995

#7 25-2hole 25-4hole long sides (1 per rail on X) cost = $905 - this option has a longer single block on X axis rail to support the Z slide

#8 25-4hole 25- 4hole long sides (1 per rail on X) cost = $948

I am leaning to #5,#7 and #8. I just need some reassurance on the 15 series - again they are so small in comparison to the 25's! (picture!)

Thanks for any input that anyone could provide! I am having a heck of a time with this forum allowing me to upload photos - see these are just linked: