15 mm or 20 mm linear guides for a BF30/PM30/TM30?

[ikbendusan]

Hello friends

I plan to convert a BF30-type benchtop mill to cnc (840x210 mm table). My original goal was to achieve at least 10 m/min rapids, and after some thought I realised I probably wasn't going to reach those speeds with these ways. Rather than spend half a quadrillion years scraping the ways to perfection only to come to the conclusion that 10m/min still isn't possible, I've decided to go the linear guide route. I want to fit Hiwin RG-series rails; and I've got a design modeled for 15 mm rails (https://i.imgur.com/lq9psPH.png).

The z-axis column would probably to be shaved down some more to fit the bigger blocks, so --ridigity from that, and ++ridigity from the bigger rails; cancels out, not worth?

The y-axis would require the saddle to become thinner so I can even get the bolt holes anywhere near a flat surface on the saddle (https://i.imgur.com/ZLC9ZPy.png), and I kind of want to avoid having to make a completely new saddle (because I don't know where to get the material from for a reasonable price)

The x-axis is the most likely candidate where 20 mm rails can be installed since there's plenty of room on the saddle and table for that.

My question is, before I go and redo anything, is it worth the effort? Will I even notice the difference in practice? Do the x-axis, leave y and z as is?

Idk, just want to know what you guys think

Cheers

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[peteeng]

Hi Ik- I expect there is no difference between 15 and 20mm in your case. To get 10m/min with a stepper and direct drive you need a ballscrew pitch of 10000mm/min/500rpm=20mm. This is not good for force. Your likely to use a 10mm or 5mm pitch screw. So your rapid will be well down. Steppers loss torque at 500-600 rpm. Hybrids can go a little faster. If you use servos you can go to 3000rpm so 10000/3000= 3mm pitch so a 5mm pitch would be good. You will also need to use medium or heavy preload cars. If you use "normal" preload you will get hysteresis when you change direction. Peter

The rails do not contribute a lot of stiffness to the system so I would not count on them for stiffness.

[ikbendusan]

I have three 400 W AC servo motors ready to rock, and I did plan to combine those with 5 mm pitch ball screws. I don't yet know if I'll be direct driving them with couplers or with a 1:1 ratio belt drive (should I need more torque I could change the gear ratio). The RG-series blocks I planned to use only have a medium or heavy preload as an option, so that works out then eh?

Thank you for the reply Peter; I can proceed with the rest of my design now.