Which end of linear rail is ground?

[JetForMe]

I'm assembling my CNC Router Parts PRO4896 (it's actually a foot longer). The instructions say to be sure to butt the two ground ends of the linear rails together (for the long axis), and the drawing shows one end to have a bevel, and the other to be flat. But mine have no bevel, so I can't really tell which end is precision-ground. Are both ends ground? Is there a way to tell? Thanks!

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[GigaWatt]

You need a smooth transition between the rails for the small ball bearing in the bearing blocks. I would check the rail ends with a good square and mate the best ends together.

[JetForMe]

One one rail, they both seem smooth-ish. On the adjoining segment, both ends are quite rough.

[JetForMe]

I just got a reply from CNC Router Parts:

This is an older note in the instructions - we no longer grind either end of the rails, as you can use either end for splicing without any practical consequences. The only features that really matter are the grooves for the ball bearings. You can start with the rail jig to align the rails for splicing, and this is often sufficient. However, if the transition seems overly rough, you can loosen 3-4 bolts on either side of the splice and use a bearing block running back and forth across the splice to set it more precisely. You will likely still notice the transition, especially at lower speeds, but this will not impact your cuts with the machine.

[mactec54]

I just got a reply from CNC Router Parts:

That was not a very good reply, the rails are either a matched pair usually have a markings etched on top or indicating grooves on the bottom of the rail as to what way they can be joined they always have one face as a datum face and this must match

So look for some indication on the rail most will be on the bottom of the rail, it may not be a groove but they will have some different machining on the bottom that will indicate which edge is the datum, just some snips to give you an idea of what to look for, they definitely can't be just butted together without knowing what side of the rail is the datum this goes for fitting the bearing blocks as well they to have a datum side to them

[Richmaple]

I don't have one of these machines yet but to be honest that's a disappointing decision. Whether or not it affects the cut quality, if there's enough of a discontinuous surface to cause any sort of noticeable feeling when you roll the bearings over that part of the rail, it's pretty terrible for those bearings. Seems like a decision made to save CNCRP some time but overall reduce the lifetime of those bearings.

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[mactec54]

I don't have one of these machines yet but to be honest that's a disappointing decision. Whether or not it affects the cut quality, if there's enough of a discontinuous surface to cause any sort of noticeable feeling when you roll the bearings over that part of the rail, it's pretty terrible for those bearings. Seems like a decision made to save CNCRP some time but overall reduce the lifetime of those bearings.

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Yes if there is no Datum edge to align the rails and a machined surface to mount them on then good alignment is basically imposable to do

[dgage]

I paid extra to get full length rails and I don’t regret that decision one bit. So there is definitely an option for no splices.

[mactec54]

I paid extra to get full length rails and I don’t regret that decision one bit. So there is definitely an option for no splices.

The only thing that should be extra is shipping it cost more for the rails to be cut or to have matched ground ends than to get the full length you need, it should not be a option in this case

[Squatch]

I just assembled mine. No marks as indicated above. I hit it with a finger sander to make the edge super smooth. Works good so far. I would prefer to have a single piece.

[ebrakespin]

So, I am going through the exact same thing right now building my 48x96. I got linear rails that were not cut square, the ends were visibly angled. When I put a square on them I was shocked to see that they were all different angles. They sent me out replacements (well, they sent me 2 correct and 2 incorrect rails). The replacement rails they sent were also not cut square.....They told me they went through their stock and it is all the same but it is what they have been using forever and I should just try and use it. (obviously I did not like this answer but I figured I would give it a shot). I tried to match the random cut angles with each other but I have to force the bearings to go over the splice. I just sent them an email asking for them to send me full size rails. IMO, a new build should come with full size rails to avoid this, unless they come up with a better method of actually machining the edges to splice together. The only time you should splice rails is if you are adding on to the machine, and even then I would buy full size rails after going through this. This has set me back weeks so I'm hoping they come through next week and I can put this behind me. See attached image for the drastic angled edges.

[ger21]

IMO, a new build should come with full size rails to avoid this, unless they come up with a better method of actually machining the edges to splice together. The only time you should splice rails is if you are adding on to the machine, and even then I would buy full size rails after going through this

There's nothing wrong with butting rails, provided they are properly ground. We had a $150,000 machine with butted rails, because they were 15 ft long. When properly ground, you can barely see the joint, and you can't feel it. If there's a gap, it has to affect the life of the bearings.

[Squatch]

From your picture, those are really not very square. Just received mine two weeks ago. This is how they fitted.

[andrewwong2000]

I’m about to get my kit after clearing customs

Will see how bad mine are!

For me exchanging rails is not an option.

How hard & expensive would it be to get someone to grind the rails flat and square ?


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[ahren]

All, there's enough concern here that I'd like to step in.

First, we always stand behind our products, and will happily assist with issues related to rails, or any other products, purchased from CNC Router Parts.

Most of our systems have a rail splice. There is normally no issue with this, and it is a common practice. We ship over a dozen new machines every week, and provide extensions for expanding the work area of existing machines regularly. The machines in our own shop all have a splice in the rails as well.

To clarify the change to our instructions - at the beginning of 2018 we began providing rails that were finished for splicing on both ends to simplify the assembly process (prior to that, they were only finished on one end).

However, we do acknowledge that there have been a few recent instances where the end-condition of the rails has led to some difficulty. We are currently working with our rail supplier, and have also implemented some additional in-house QA steps to make sure the rails our customers receive can be easily spliced as intended.

Ahren
CNCRouterParts