Joining linear guides?

[MechanoMan]

One thing that seems to be a cost factor in getting (or distributing) a long router axis is the shipping cost of the linear guide rail length.

Now the rack, the rack CAN be joined from shorter lengths. Moore Gear, which makes racks, even describes the recommended process.

If a kit from one mfg were being shipped though, it doesn't mean much to have two 4ft racks if an 8-ft linear rail must already be shipped from the same place. I can get 8020 in town. The rail, the shipping is quite an issue in these lengths.

Are there any practical, effective methods by which say an 8-ft axis could be made by joining shorter rails?



I was picturing these rails on top of 8020. Actually, I can see how it would be simple enough to round off the edges and butt them together, but alignment would be less than perfect and there's a lack of stiffness at this joint right in the middle of the router which certainly sounds like a potential problem.

Any thoughts? Or has this already been solved somewhere, somehow?

[jalessi]

MechanoMan,

I wonder if the rail could be disassembled, chucked in a lathe and center drilled so that a dowel could be inserted to maintain position between the round rail sections.

Jeff...

[ger21]

I've seen THK type profile rails butted together, but never round ones. Jeff's idea sounds like it would work fine.

[CarveOne]

Originally Posted by jalessi MechanoMan, I wonder if the rail could be disassembled, chucked in a lathe and center drilled so that a dowel could be inserted to maintain position between the round rail sections. Jeff...

In theory, yes. In practice it will not be so easy to get a perfect transition at the joint. (My opinion) The round rails are usually hardened at the surface and are chromed for hardness. The core is more easily machinable. The ends must be turned for a perfectly flat butt join, and the dowel hole is bored for a snug fit to the dowel. Anything less than perfect will make a bump, bind, or lockup depending on the severity of the misalignment. There are probably people here that can routinely do it though.

Racks are much easier to join.

CarveOne

[MechanoMan]

That did occur to me. It would require some decent equipment and skill to machine a rail of say 4-5ft.

There's three factors here. Keeping the 2 pieces on center, keeping the pieces in one very straight line, and keeping the outer surface friendly to the bearings.

Can the support be unbolted from the round rail? This would make more sense to have the support break in a different place than the rail break.

What ARE those linear bearings like inside? I can't picture how they'd deal with a discontinuity. I mean we could lightly round off the edges but there would probably be a dropout in it.

[ger21]

Yes, they can be unbolted.

Inside, there are usually 6 to 8 rows of balls that run along the length of the rail.

Rounding the ends would be a bad thing, as each ball would "grab" in the edge. If the bearings are tight enough to not have any play, there's a good chance they'd get stuck in the groove formed by the rounded edges.

[MechanoMan]

Well, I'd assume the radius of the discontinuity could be made much smaller than the radius of the ball.... what's the ball size like inside these?

If the round part is unbolted from the support and the support is offset so it's continuous underneath the break, then the discontinuity could be quite small. You'd grind down the end where it chamfers down until the whole side is flat and the rim's almost sharp, and then mate them flat with just a really small chamfer.

[CarveOne]

Originally Posted by MechanoMan Well, I'd assume the radius of the discontinuity could be made much smaller than the radius of the ball.... what's the ball size like inside these? If the round part is unbolted from the support and the support is offset so it's continuous underneath the break, then the discontinuity could be quite small. You'd grind down the end where it chamfers down until the whole side is flat and the rim's almost sharp, and then mate them flat with just a really small chamfer.

I have some Thompson Super 12 bearings with 3/4" x 23-3/8" shafts. There are 6 rows of recirculating balls of something around 0.1", almost half the diameter of a BB (0.177"). I'm not volunteering to dump out the balls and measure one.

CarveOne

[mmcp42]

rather than a straight cut, would an angled cut be better - like they do when joining rail tracks?

[CarveOne]

Originally Posted by mmcp42 rather than a straight cut, would an angled cut be better - like they do when joining rail tracks?

I don't think so. The angle on each shaft will be much more difficult get right to avoid an angular gap than a straight across trim cut on a lathe. It would also increase the length of the joint due to the resulting elliptical shape.

The ball bearings probably wouldn't know the joint was angled and there will be a series of little bumps as they all pass over the joint, same as a 90 degree joint, just spread out over a longer distance.

The idea of using a chamfered support across the joint for alignment sounds best to me, but that is only an option for open bearing glides for fully supported rails. It does little for the edges of the joint itself. They must be as continuous as possible and a smooth transition. The more snug the balls are against the rail the more apparent the joint will be. These Thompson bearing assemblies have set screws for zero clearance adjustment.

CarveOne

[LeeWay]

While it might be possible to join these round rails with a joint good enough not to effect the travel, it could prove to be more costly than paying the extra shipping if you fail on even one.
You might want to attempt the machining on standard bar stock first to see just how close it can be done on given equipment. I think the round rail might be fair less forgiving than the typical square type rail.
Another thing I have noticed is that the longer lengths on the square type rail get cheaper by the inch. Though shipping might cost more, the end cost might be about the same. Less the work too.

[cliffy]

Everyone has been on a train and I bet everyone can remember that bloody noise, each time we run over a joint in the track.


Anyone that cuts a guide rail in two will regret it forever.