Joe90's BIG RAT Build.

[JAZZCNC]

In regards to carriages, I understand that Hiwin is the brand of choice but I just wondered when you would chose to use the W kind, with the side flanges as opposed to the regular square type? So when for example you'd chose to have HGW15CA or HGWCA20 instead of the what seems to be the more common choice of HGR15CA or HGR20CA.

Ok well first HGR is the rail. HGH15CA or 20CA are the Slim carriges.
When to use the W or wide carriage depends on the design. Personally I always use the W on the X & Y axis when ever possible and Slim type on the Z axis to save room. If I have plenty of room on the Z axis I'll use the W type again because being wider gives that little more support.

Also, is it common to have the 20s on the X & Y Axis' and have the smaller 15s on the Z? This seems to be quite a common choice

Regards 15 or 20mm on the Z-axis then I strongly urge you to go with 20mm. The 15mm is fiddly and mean you have to have big standoff spacers or machine deep into the rear plate. 20mm rails and bearings give you much more support which you need in a Z-axis.

To be honest, only partly watched the video and didn't see his other machine but he made comment to the rails on the front plate being stronger.? This isn't strictly true and it does depend on the type of cutting your doing and the length of the tool being used and the machine design.

I've built machines small and large using both methods and there's very little difference between them when using short tools, as the tools get longer the rails on the front start to have a slight advantage because the Z-extension reduces. However, this only applies up to a point because it also limits the length of tools and the material height you can use. Whereas the other method with rails on backplate allows much longer tools to be used, it allows much taller material to be used and still use long tools or drills.

So here's where machine design comes into the mix. With the rails on the front plate then because it limits material and tool size due to the fact it can only lift the spindle up until the rails reach the bearings, meaning the tool and spindle hang below the gantry. This means the gantry sides need to be taller if you need to cut taller material with longer tools. This weakens the machine and takes away any advantage it offers over the other method, which as I say isn't a great deal.

Whereas the rails on rear plate mean that the spindle can be lifted high up the rear plate so the tool clears the bottom of the gantry giving the full height from bed to underside of gantry even with a long tool. This means you can make your gantry lower and still have the same material clearance as the other method but it makes a much stiffer machine and better cutting so eliminates any advantage the rails on front plate offers.

So as you see there's more to it than just bearing type.!

Finally, are people happy to go with the self-made ball screws that Fred @ BST supplies, or do people still stick to Hiwin branded wherever possible?

Fred is a great guy to deal and I fully recommend him, tell him Dean sent you and it might help on the price. Also, I suggest you invest in some C5 grade ballscrews and ask for the Better BK/BF bearings which I believe are C3. These are more in line with Hi-win grade ballscrews.