Guide rod diameter to length and weight of head?

[JestingAngler]

Does anyone know if there is an equation, chart, or rule of thumb on the gantry guide rod diameter depending on length and head weight. I haven't been able to find any information on this. I'm building from scratch and am planning to use solid t.g.p. Any help welcome. Thanks

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[coherent]

Do you mean linear bearing/rail? If so the load ratings and specs etc would be available from the manufacturer or distributor of the rail and bearing assembly. It varies greatly depending on each specific design, bearing type, and so on and there is no "universal" chart.

[JestingAngler]

I wasn't planning to purchase rails. Instead I intend to use two rods on the gantry and maybe oil impregnated bronze bushings in the head block. I have access to 3' drops of 1.25" t.g.p and 1/2" Thompson shaft, was hoping to use some of that if it's sufficient. Thanks for the reply

[ger21]

I don't know what tgp is, but 1/2" unsupported rails are woefully inadequate. I know someone that built a machine like that years ago, and could move the tool about 1/2" with 1 finger.

[JestingAngler]

T.g.p is turned ground polished. If memory serves me 1045 steel.

[gregoose]

T.g.p is turned ground polished. If memory serves me 1045 steel.

Found this, ENGINEERING.com | Beam Deflection Calculators probably for mild steel so you could estimate from there. A solid shaft 1.25" dia, 36" long with 100 lb load deflects nearly half an inch. I would guess about half that for tgp, then half it again since most machines use two shafts. So maybe 1/8" deflection. I'm not an engineer, just estimating. Any supported shaft will have far less deflection and be the ideal route for any milling/routing configuration. You could always drill and tap holes in the shaft and make your own supports too.

[JestingAngler]

That's a really good idea, drilling and tapping the shaft would be a simple way to add a lot of rigidity. Thanks for the good idea.

[wizard]

That's a really good idea, drilling and tapping the shaft would be a simple way to add a lot of rigidity. Thanks for the good idea.

When you say TGP what exactly are you talking about? Normally when i think of TGP stock I think of softer shafting commonly used in power transmission. This shafting isnt suitable for linear bearing use. On the other hand if you are talking Thomson like shafting that has a hard exterior shell then that is something different. The problem is drilling and tapping through that hard case is not at all easy.

So to answer you original question the 1/2" shafting is virtuall useless for a router. You can support 1/2" shafting at both ends for an extremely short stroke axis (a couple of inches) but most people wouldn't bother no mater how good the shaft. The larger shafting would certainly be stiff enough for some designs but if it is not hard your shaft life span will be extremely short.

As for mounting the shafting, you really need to have your linear bearings fully supported no mater the type. With large diametter rod you can get away with unsupportted for short stroke axis. In the end you are better of supporting linear guide with a stiff frame.

In any event if that TGPis what i think it is, (relatively soft) id seriously consider selling or trading it for components dsigned for the purpose.

[JestingAngler]

Thanks for the input. You are correct about the tgp being a softer material. I also have access to 1" Thompson, just in 10" pieces. That may work for the Z. I have drilled and tapped through the side of Thompson shaft before. You just have to use a carbide end mill to get through the case and not bottom the tap on the other side of casing. I appreciate all the information, this is the first time I've ever considered building a machine like this.

[wizard]

That was my fear, soft shafting just wont last very long.

The thompson rod would make for a nice short stroke Z axis. While i dont recommend unsupported rods 1" dia rods over a short distance wont be too bad. If you design in a center support block the stiffness will be fairly good.

Thanks for the input. You are correct about the tgp being a softer material. I also have access to 1" Thompson, just in 10" pieces. That may work for the Z. I have drilled and tapped through the side of Thompson shaft before. You just have to use a carbide end mill to get through the case and not bottom the tap on the other side of casing. I appreciate all the information, this is the first time I've ever considered building a machine like this.