What size profile rails for Y axis?

[BigBrand]

Apologies I've been scrolling through the forum that long my eyes are going square.

What size profile rails should i use on a 3.5m Y axis for a 2.2m long X axis gantry weighing approx 150kg max total with ATC spindle.

I was originally planning 30mm rails but the more i look at them the more i think they are probably OTT.

Should i drop down to 25 or can i get away with 20s?

The router will only be cutting signage materials such as ACM, foam, 2-3mm ali, bit of MDF etc.

Kent

[BigBrand]

also whats the deal with the preload? am i right in saying that its how 'snug' the fit is on the rail? Whats ideal for the X Y and X?

[pippin88]

For a wood router 15mm profile rails are more than enough load wise.

Bigger rails are sometimes easier for a couple of reasons:
Bigger mounting screws - easier to tap the holes

Slightly more room around the mounting holes (to deal with misalignment). For example my Hiwin 15mm rails use M4 screws and have 4.4mm holes, not much room for error. Larger rails have bigger clearance).

Height compared with ball screws / ball nut. Ball nuts are quite bulky, and higher than rails+cars. Going a step up in rail size can be useful.



Preload is how tight on the rail. Zero or low preload moves with very little resistance but are not as stiff. For most CNC machines you want a medium preload.

[BigBrand]

Awesome thanks.

Yeah i might go half way house @ 20mm then for the reasons you mentioned. The smaller blocks than 25 will give me a smidge more clearance room where they are sitting.

[m_c]

Pippin has given a good answer.

Profile rails can take a surprisingly high load for their size, but it's often easier to use larger rails. Smaller rails generally involve smaller fasteners which can be a bit of a pain to deal with. IIRC 15mm will mostly use 5mm fasteners, which can be a bit 'delicate' to drill/tap/tighten.

[BigBrand]

i think your down to 4mm on 15s if ive read it correctly which is getting small.

The 20 rails use 5mm i think which is that bit beefier and still gives me a bit of adjustment in a 8mm rail which is what they will be fixing into.

[Chaz]

Yep, I used massive rails on Thor (32mm I think) and sorting out M12 bolts x lots of holes and tapping was no fun.

[Neale]

I'm fairly sure the Hiwin manual, available online, gives recommended loading for these rails. From a personal experience perspective, though, you can probably use smaller rails than you would first think, as you are finding from comments above. While I was assembling my welded steel CNC router, at one point I accidentally ended up with the gantry supported at just one end. The gantry is also welded steel and at that point probably weighed around 30kg and all this was hanging off two carriages at one end, on 20mm rails. It dropped maybe 2mm at the far end. A really unfair test of clearances and point loading but it didn't seem to do any long-term damage and persuaded me that worrying about whether I should have gone up a rail size was entirely pointless! Like a lot of builders, though, my Z rails are oversize for their loading - 15mm would be more than adequate but 20mm gives more spacing and only needed small machined recesses to clear the ballnut.

I tapped most of the mounting holes in 6mm steel (3mm wall thickness plus 3mm backing strip) using a spiral-point tap in a handheld cordless drill with a bit of tapping compound. Saved my wrists a lot of strain! I did make a little tapping guide to help keep things square - just a short steel cylinder faced square at both ends with a clearance hole right through. I could hold that with one hand and cordless drill in the other - worked well.

[JAZZCNC]

I wouldn't fit anything smaller than 20mm on it even though the ratings are more than you'll need. 15mm carriages are weedy fiddly things, if it was me I would fit 25mm as it better suits a machine of this proportions, and when/if you come to my place I'll show you all 3 sizes with both 20 and 25mm on machines and you'll see exactly what I mean.