[PCar]

I have a Plasma CNC build I'm working on and have some HWIN linear rails along the x axis - around 2.4m long.

So each side has one linear rail and 2 guides, with a rack and pinion for the motor.

The rail is being mounted to box section. I'm reading through the manual for the rails and they suggest high accuracy gauges and machined surfaces. Does anyone have any experience with this kind of a setup?

Thanks a lot.

[peteeng]

Hi Pcar - from another post they are 20mm? 20mm rails will conform to the foundation when pulled down. So the "box" section has to be very straight and thick. Structural sections are rarely flat and level. So either be prepared to machine the surfaces or try to pick one from the stack that is straight. For a low tolerance plasma, std sections maybe OK. There are some threads that have used std sections and the cars have jammed when the rail is pulled down. Tread carefully along this path. Peter

[joeavaerage]

Hi,
some years ago I made a 'trammel mill'. That is a machine not dissimilar to a gantry style CNC but is manually controlled. I used a 50 x 50 x 6 RHS section with bearing rollers onto which to
run the spindle back and forth. I too was concerned about how straight and flat regular RHS would be. I found that it was pretty good. I found about 0.1mm variation in section thickness
and about 0.2mm 'out-of-straight' over the 1.5m length. This required that some of the bearings be compliantly mounted, but the result was entirely good enough for my purposes.

peteeng is correct that RHS is seldom flat and straight, but neither is it too bad. It would certainly not be good enough for my mill where I hope to get better than 0.01mm accuracy,
but for a wood working machine or a plasma table 0.1mm accuracy might be entirely reasonable.

May I suggest try it out. Get a piece of RHS and screw your rail down to it. If the RHS is too 'out-of-straight' it is likely that the subtle curves pulled into the rail will cause a small amount of binding
of the cars as they traverse the length. I rather suspect that a decent piece of RHS will NOT cause any binding, and will probably therefore be good enough without machining.
If there is significant binding in the car, you either selected a poor piece of RHS or it will need machining, but certainly no harm in trying without pre-machining.

If 'cheap and cheerful' is good enough then use 'cheap and cheerful'.

Craig

[peteeng]

Hi Pcar - Also consider structural channels vs hollows. Hollows are rolled cold and through a multistage process then welded. This traps lots of stress in the section and the sides are not flat. If you use a hot rolled channel this has been formed both sides and hot so the trapped stress is much less and it can be flatter then a hollow. Hand pick your section don't rely on the sales team or storeman to pick your metal. At around 2m long you can also consider structural aluminium channels. I have found them to be straighter and flatter then steel sections plus no need to paint. Peter

[joeavaerage]

Hi,
all the larger, thicker wall RHS sections are hot rolled. The only cold rolled stuff is smaller and much thinner, commonly electro-galvanised finish.
The piece of 50 x50 x 6 section that I was talking about earlier was part of a regular 8m length that we would stock for making the beam of a trailer axles. Nothing special,
just a hot rolled RHS section.

Craig