Xzero Raptor CNC build - linear rail and ballscrew selection

[Mattia]

Based on your recommendations, I ordered the rails and carriages from Fred at BST - excellent communication, great pricing and quick turnaround. Time from payment to delivery was 8 days (paid Sunday evening, received Tuesday the following week).



I'm going to test bolt things together this weekend to measure up various bits and pieces and see how she slides. I'm going to get someone local to machine a few bits of aluminium for the extra bracing on the gantry. Do you guys lubricate the carriages before putting them into operation? They feel like they're a little 'dry' out of the box, but not sure if that's the ZA preload I'm feeling?

Now it's down to selecting the ballscrew for the X axis. Some folks on a Dutch forum suggested going for dual ball screw to prevent the gantry from racking. Gantry width is 800mm, and as mentioned I plan to mount a 2.2kw Teknomotor spindle. This machine was originally designed with steppers in mind. Will adding servos (with their potential for greater acceleration/rapids) make a duals screw with individual drive a more stable solution? If so, 2020 or 2010 for both, or downsize to 16mm screws?

Also, is there any significant upside to getting a C5 ground ballscrew (e.g. a TBI from Tuli) vs a C7 screw from Fred here?

[JAZZCNC]

Do you guys lubricate the carriages before putting them into operation? They feel like they're a little 'dry' out of the box, but not sure if that's the ZA preload I'm feeling?

Grease them when your ready to use the machine, there is enough grease inside them for setup etc but not enough for running.
The stiff and notchy feel is quite normal, when greased and in use under weight they run smooth.

N

ow it's down to selecting the ballscrew for the X axis. Some folks on a Dutch forum suggested going for dual ball screw to prevent the gantry from racking. Gantry width is 800mm, and as mentioned I plan to mount a 2.2kw Teknomotor spindle. This machine was originally designed with steppers in mind. Will adding servos (with their potential for greater acceleration/rapids) make a duals screw with individual drive a more stable solution? If so, 2020 or 2010 for both, or downsize to 16mm screws?

Also, is there any significant upside to getting a C5 ground ballscrew (e.g. a TBI from Tuli) vs a C7 screw from Fred here?

At 800mm your at the limit of a single screw and personally I would use twin screws, esp if using servos.
Regards the screw size then it depends on the length, if over 1100mm then I would use 2020 with a 2:1 ratio, again even more so if using servos because of the higher RPM's servos run at. The ratio will half the screw speed but bring the resolution and federate back inline with 10mm pitch.
If between 1000-1200mm You could use 1616 and do the same but the difference in cost isn't worth taking the trouble, and if over 1200mm then 20mm is the best choice.

Just getting C5 rolled screws over C7 is worth it and ground goes a step higher but so does the cost. End of the day it's down to how a good a machine you want and your budget. BUT.!! . . .Just remember the machine is only as good as it's weakest link so even if you fit C1 ground screws they won't take up any slack in other weak areas so it could easily become wasted money. The goal and secret to a great machine is a balanced machine, if your going to fit C5 level then the rest of the machine needs to be built to C5 level.

[Mattia]

Grease them when your ready to use the machine, there is enough grease inside them for setup etc but not enough for running.
The stiff and notchy feel is quite normal, when greased and in use under weight they run smooth.

Thanks for the confirmation!

At 800mm your at the limit of a single screw and personally I would use twin screws, esp if using servos.
Regards the screw size then it depends on the length, if over 1100mm then I would use 2020 with a 2:1 ratio, again even more so if using servos because of the higher RPM's servos run at. The ratio will half the screw speed but bring the resolution and federate back inline with 10mm pitch.
If between 1000-1200mm You could use 1616 and do the same but the difference in cost isn't worth taking the trouble, and if over 1200mm then 20mm is the best choice.

Just getting C5 rolled screws over C7 is worth it and ground goes a step higher but so does the cost. End of the day it's down to how a good a machine you want and your budget. BUT.!! . . .Just remember the machine is only as good as it's weakest link so even if you fit C1 ground screws they won't take up any slack in other weak areas so it could easily become wasted money. The goal and secret to a great machine is a balanced machine, if your going to fit C5 level then the rest of the machine needs to be built to C5 level.

Looks like 2020 and dual screws are in my future! Will shoot Fred a message to ask about pricing on a couple of ball screws with bearings, which I'll mount inside the base up against the 8080 profile. And try to find someone local who can help me machine the frame end pieces to fit the two new ballscrews and add mounts for a pulley system with the motors inside the base, which should save a fair amount of room. I'm also having a bit of a sketch to see if I can mount the Y axis motor so its on the T extrusion I'll add to the back of the gantry to stiffen up the top. Although that will probably be a more long term plan since it would probably require replacing the screw - I don't think the end extends past the side enough to mount a belt pulley...

I think one of the biggest potential areas where there's a risk of loss of precision is the X axis (longest rails, 1200mm) which are mounted to unmachined extrusion. The base face plates have two accurately machined notches that the rail ends rest in, however, so I figure I'll use those as reference points for vertical and horizontal alignment at the two ends. The Y and Z plates are flat and have machined surfaces and edges the rails can mount to, so that's much less of a concern/challenge for now.

Also, anyone have a tip on what timing belts and pulleys are appropriate for this kind of build? Or a good source of basic info on the upsides and downsides of various systems (eg T5, T10, AT5, AT10, HTD??). Even just some photos of a 2:1 geared drive system on a DIY machine would be helpful...

[routercnc]

Joined late so just my summary thoughts.
One of my early machines was 600 wide and used a single screw and you could push each end of the gantry back and forth noticeably. I tended to cut in the centre line to hold a tolerance. So anything over 600 would be a double screw for me.

I used T5 belts on an earlier machine but use exclusively HTD 5 now. I prefer the tooth profile on HTD and believe the shape is optimised for high torque.
It has been a while since I looked into it but it think the AT types are somewhere similar in profile but I find the HTD work well.

Although I’ve only ever used rolled screws I have never found them to be anywhere close to being a limiting factor on the overall machine. So in my view rolled threads can be used and still make a very good machine.