Need Advise - Can Linear Rail be extended without problems


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Thread: Need Advise - Can Linear Rail be extended without problems

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    Default Need Advise - Can Linear Rail be extended without problems

    Over the past couple years I have accumulated parts and pieces to build a more substantial CNC for woodworking. My goal is 300 ipm for 3D models, 400 ipm for solid wood. The working area ideal is 40" X, 30" Y and 6" Z. (size is limited by my shop, but also what I plan to do)

    I have;
    (4) THK HRW21 carriages and (2) 19" lengths rail. My intent is to use this for the Y axis, but really want (2) 34" length of rail.
    (2) Schneeberger MR24 carriages and (2) 5" lengths of rail. My intent is to use this for the Z axis, but really want (2) 10" length of rail.
    All of the above are NOS.

    My question is if I buy the additional length of the same brand rail, can I just add to the end? I assume alignment is critical, but is durability affected?

    Also, do I need another set of carriages for the Z axis, two on each rail? It will start with a 2.5 hp router, but eventually go to a spindle.

    The X axis will be 1" Thompson round with double ball bearing NOS Pillow Blocks. (I have (2) 8' sections, plus (3) 6' sections of the Thompson round.)

    Steve.

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    Default Re: Need Advise - Can Linear Rail be extended without problems

    Steve,

    Kent Williams here from Automation Overstock. Over the years I've sold thousands of rails where folks butt them up to get longer lengths. There are a few general principles I'll point out to you that might be helpful.

    First is that there are "butt joints" and what I refer to as "true butt joints." A butt joint is where you put consecutive rails together to get a longer overall length with out regard to maintaining the "P" pitch hole spacing on the rail. What I mean by that is if your rail is produced at the factory with mounting holes 60mm apart, done correctly segmented rail should be cut so as to maintain that 60mm spacing across the joints. But it is not essential to do this. If you have 60" (1524mm) rails now, it's likely that you have equal measures from the end of your rail to the center of the first mounting hole on each end. If you get some additional 5' pieces of rail and put that together with the rail you have, you will probably end up with some odd measure across the mounting holes there. THERE IS NOTHING INHERENTLY WRONG WITH THIS. I would not recommend it for someone who is mass-producing a machine, but for a guy just building his own, I think it's fine. So in summary, you may not get a "true" butt joint, but you'll be using a butt joint.

    Now here's a warning. When we cut someone a rail that measures, for example, 1524mm, it is our assumption that it will be used at that length. After cutting we use a "Scotch Brite" buffer pad on a die grinder to "ease" the corners where the rail has been cut to remove burrs and lessen the chance of a customer cutting his hands. It also can make it a bit easier to install the carriages. However, it does create a little chamfer. Imagine that you put to rails together that have this chamfer, and now you have a slight "V" groove that can be a bump point for ball bearings traversing the joint. If I know ahead of time that a customer is going to butt rails, I give our cutting department special instructions to NOT buff the ends of the rails. This means that the rails will have better alignment. In other words, no V groove. It could mean that some fine-tuning would need to be done once rails are installed (emery cloth or some minor work with an abrasive stone or similar on a Dremel to just ensure things are as smooth as possible where the rails come together). Not rocket science. Just some good clamps to hole the joined rails together while mounting bolts are installed. Also be sure NOT to use mounting bolts for the rail that fit too closely to the diameter of the holes in the rail. Rule of thumb is bolt diameter should be 1mm smaller than diameter of hole in rail. This give some "adjustment" possibility after rail is installed.

    A couple more things. Many linear guideway manufacturers have one side of their rail etched or otherwise marked as a "reference" side. General rule is to face all reference sides of rail on a butted leg the same way. This accounts for the rail facing the same was as it came out of grinding machines at the factory to lessen the chance of microscopic misalignments when put onto a machine. In other words, if your rail has markings along one of its edges, if you but another piece to it, be sure that the edge with markings faces the same way on all pieces.

    It is also a good idea, whenever possible, to stagger joints on axes of a machine. If I understand your potential butting, you would end up with something like this:

    ____________________ ____________________


    ____________________ ____________________

    In other words, you would have equal lengths of rail, and the joints would be directly across from one another. When a carriage on one side is traversing a joint, it would also be traversing a joint on the other side. Better to avoid this if possible with unequal lengths of rail that are flipped. It would be something like this:

    _________________ _______________________

    ______________________ __________________

    I don't know if that makes sense to you or not. But if you had the choice, instead of having two rails made up of four 60" pieces where your butt joints would be directly across from one another, it would be better if you had perhaps 70" and 50" pieces (which still add up to your desired 120"), but if flipped in the layout would not align the joints directly across from one another. When bearings are going over a joint on one side, ones on the opposite rail probably won't be going over the joint at the same time.

    Truly it's best to completely avoid butting if you can and construct using continuous rail pieces. Most manufacturers supply raw sticks of rail at 4000mm, so it's easy to get 10' (3048mm) out of a stick. Of course, rail is not cheap, and shipping is not cheap when rail gets to that length. But when you factor in the other hassles that come with butting rail, it might be in your best interest to bite the bullet and get rail the length you need.

    The technical section of our website has a lot of information about rail butting and other topics, and you might want to check those out. If you have other questions, let me know and I'll try and answer.



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    Default Re: Need Advise - Can Linear Rail be extended without problems

    Kent,

    Excellent information and you addressed my most significant concern, the corners have been "eased" on the rails I have. I do have access to a Jet Mill that could machine one end of each to flat. Offsetting the joints on each side if adding length makes perfect sense.

    I want to end up with (2) rails of the THK HRW21 that are 34" (850 mm) length each for my Y axis. I want to end up with (2) rails of the Schneeberger MR24 that are 10" (260 mm) length each for my Z.

    Today my HRW21 rails are 19" each and the MR24 rails are 5" each.

    Already having adequate carriages, my intent is to increase the rail lengths or just build a smaller foot print machine at this time.

    I had looked at your site offering and impressed with the BLH series offerings you have. Have considered putting my existing parts on eBay and then buy the sizes I truly want.

    Steve.



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    Default Re: Need Advise - Can Linear Rail be extended without problems

    Glad I could help. I'm real excited about the BLH material, and before I ever went down that road I sent out samples to several of my customers to get their opinions. If you read my overly detailed page on the site about the BLH, you'll know that it is intended to be an alternative to the old "Legacy" Hiwin AG and LG in the smaller sizes (15mm, 20mm and 25mm) that I have basically exhausted. Or at least I've exhausted 1/2 of the components. It's almost comical that I have over 1000 pieces of LG15 rail (2000mm long each) and absolutely no carriages to go on them. On the other hand, I have a couple of thousand AG20 carriages but for all intents and purposes, no rail. There's not really any possibility of getting either rail or carriages made so that the components I have can be used. Trust me, I've looked high and low. Just too costly, and companies that could do it really don't want to because they're concentrating on making their own guideway products. If anyone out there has a creative way to utilize any of the big quantities of orphan bearings or orphan rail such as I've described above, I'm all ears.



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Need Advise - Can Linear Rail be extended without problems
Need Advise - Can Linear Rail be extended without problems