Using gantry tube as a cable carrier

[dmoore]

I've seen a few different methods for routing the cables coming off the Y-cable chain onto the gantry and then onto the X-cable chain. Initially I did the snap-close cable enclosure but after purchasing a bunch of the 30mm ID grommets to fill the holes in the y-car, I though it might be easier, cleaner and cheaper to just run the cables through the gantry tube. It turned out to work quite well.

You just drill two 30mm holes into the gantry on each side of the lower y-car bracket, as low as possible toward the bottom. Pop in the grommets and pull the cable through. I have the following cables running through mine: power, ground, two proxy, two stepper (double shielded), USB and a spare double-shielded cable. Another side effect is that it cleans up your cable exit, so instead of having some of the cables exit the gantry tube (near and far side stops and stepper cables) and the rest exit the slot on the far carrier, they all come out cleanly in one spot. It also allows a nice clean location for a MechMate logo (not covered up by the y-cable.)


http://www.dmoore.com/mechmate/WireThroughGantry.jpg

Here you can see there is plenty of room for all the cables coming off the y-car. You might even be able to fit all the cables into a single 30mm hole:
http://www.dmoore.com/mechmate/WireEntryOnGantry.jpg

Notice the empty cable entry slot to the left where normally the cables would enter:
http://www.dmoore.com/mechmate/GantryToXChain.jpg

Here is a view showing the cables hidden under the carrier plate and then routed inside the gantry tube. In the future I will make them a full 30mm so I can use the same grommets.
http://www.dmoore.com/mechmate/FarAndNearCableToGantry.jpg

[Gerald D]

30mm holes into the middle of a gantry tube is a bad idea for the strength of the tube. Big holes near to the ends are okay, a few small holes in the middle are tolerable, but big holes in the middle are not good.

[Greg J]

Maximum bending stress is located at the edges (Y max), in the middle of the X axis of the beam.

1591

Just make sure the holes are located in the center (neutral axis, Y axis) of the beam.

[Gerald D]

The gantry also carries horizontal forces (cutter load, motor drive).

David's holes will probably only make a small difference to the flex, but one must consider the implication of size and location of holes. I don't want to be drawn into issues of how big a hole can be allowed . . .

[dmoore]

30mm holes into the middle of a gantry tube is a bad idea for the strength of the tube. Big holes near to the ends are okay, a few small holes in the middle are tolerable, but big holes in the middle are not good.

I did give that concern some though before I drilled those holes - I don't believe it's an issue in this application because:

The hole is in the side of the box tube. A good portion of that load is being carried by the bottom of the box tube. This isn't any different from an I-beam with holes or a open web truss - the loads are carried mainly on the bottom.
This is box tubing. I think of box tubing as I-beam with two webs. The hole in one side has minimal impact.
I didn't use .083" wall, I used .120" wall tube - nearly double the "spec"
While completely non-scientific, I've stood on top of the gantry with the holes in the tube and noticed no deflection.
The Y-car weight is less than 60 pounds with the Z slide and router.So, given all that, absent of data to the contrary - I'm pretty confident that my decision is reasonably sound.

[dmoore]

Maximum bending stress is located at the edges (Y max), in the middle of the X axis of the beam...Just make sure the holes are located in the center (neutral axis, Y axis) of the beam.

What might actually work better would be a "oval". The oval wire holes in the ends of the y-car happen to be really close to 30mm also.

The oval would not only move the hole closer to the center but I suspect it would be easier to pull cable through. I think I'll go that route on my next MechMate.

Thanks for the information...

[Gerald D]

You are missing the basic point that the gantry has to be horizontally stiff as well. That is the direction in which it wants to flex when a cutter "chatters".

And then it must also be torsionally stiff because the rail/wheel loads from the y-car are not in the center of the tube - they tend to twist the tube.

[isladelobos]

it's easy, you just have to weld, a washer, a metal plate, Around the hole, for strengthening the area.

[Gerald D]

Ros, remember this is the gantry tube, with the y-rail screwed on top......the tube needs to as straight as possible. Welding such a plate on will make make a kink in the tube.

If you guys really want to put the cables through a hole, make an oval hole in the center of the tube.

1595

16mm [5/8"] high X 48 mm [1.875"] wide
(same as in 1020456P)

[dmoore]

If you guys really want to put the cables through a hole, make an oval hole in the center of the tube.


Be very careful how far up the hole is on the gantry tube - if it is too far up, the Y-car idler bearing will hit it. Of course you could move to a smaller bearing to solve that issue. If you notice in my photo you will see that one of the holes (closer to Y0) is a bit further up. This grommet hit the bearing and I had to trim the top of it.

Of course I have no actual data to back this up, but I don't notice any difference in my cuts of Hickory wood from before when I didn't have the hole and now that I do have the hole. Of course with a 2hp router, it's pretty hard to put stress on anything.

[Gerald D]

A smaller bearing's shaft will hit the y-car.

[ekdenton]

Another thought is that the wider the table, or longer the gantry rectangular tubes, the easier it is to make them flex, so a hole in a shorter tubing may not be quite as critical as a hole in a longer tubing. It may affect a wider table MM more than it would a narrower table because of the length of gantry tubing.

[Gerald D]

Ed, you have a valid point - a short gantry would be less sensitive to holes than a long one. But where does one draw the line?

What is being suggested in this thread is that 1060318S, a PVC duct, is not "easy, cheap & clean" enough to carry the wires that short distance. Maybe I should delete the screw holes in the duct and say to use double-sided tape? I really cannot see a convincing argument that the drilled holes should be Easier? Cheaper (material yes - labour and tools no)? and Cleaner (aesthetically, that is in the eye of the beholder - practically, there could be sharp burrs and gunk inside those tubes)?

[dmoore]

I don't see anything "wrong" with the conduit. As my original message title stated - I felt that running the cables through the gantry was "easier, cheaper and cleaner".

Maybe I should delete the screw holes in the duct and say to use double-sided tape?
I did drill and tap holes on the first one and it wasn't hard. I do see a problem with painting/re-painting with the double sided tape.

I really cannot see a convincing argument that the drilled holes should be Easier? Cheaper (material yes - labor and tools no)?
To drill my holes, I used a simple hole saw - the same one I used on the other parts of my MechMate build. Nothing complicated. Also, if you drill holes under the near/far carriers to start with, it's just as easy to drill a hole or two in the gantry for the y-car cable. It took less time to drill the 30mm hole than it did to drill and tap just the one 5/16" hole for mounting the conduit. Since I did it both ways (conduit with bolts and the hole method) I think I can comfortable say the hole method was faster. As to cheaper - the duct is about $15 USD, the grommets are $.50.

Cleaner (aesthetically, that is in the eye of the beholder - practically, there could be sharp burrs and gunk inside those tubes)?
I personally feel that less wires spread out over the machine, looks better (I feel the final photos of my finished MechMate will confirm this...) As to the "sharp burrs and gunk" - I don't completely uderstand this point as your orginal MechMate has cables run through the gantry tube as shown here:

http://www.mechmate.com/Forum/messages/3347/3470.jpg

So, if putting one cable in the gantry tube is OK, what's wrong with a few more? Plus, there should be no burrs on the inside of the tube from drilling if they are following your plans which indicate to debur and use sharp drill bits. I think if there is any danger to the cables, it is when they are exposed "outside" of the machine where they can be damaged by operators, tools or dropped materials. Also, without the conduit attached to the side of the gantry, there is less places for dust to build up on.

So, in conclusion, I personally feel that for my build, moving the cables to the gantry tube was cleaner, safer, quicker (maybe maybe not...) and cheaper. I'm not trying to infer to anyone that they should abandoned the current "approved" method of using the conduit, just providing an option that someone may have not already considered.

Maybe the titles of all the posts should have something like "APPROVED METHOD:" if the posts in that thread are of a plan matching, approved process - everything else doesn't get the "APPROVED METHOD" stamp.

I'm not questioning your design - your design is, as I have said before, without a doubt, top notch.

[Gerald D]

The original title you put on the thread stated that your method was the "solution". I am not convinced there is a problem that needs a solution.

Drilling big holes in the middle of the gantry is a bad idea - period.

David, I have had enough of this. You waste too much of my time and so I ask you to move along please.