Connecting linear rails to steel tubes with screw threads and required wall thickness

[Goemon]

My plan for the long axis on my CNC router build is to mount my linear rails on some steel rectangle tubes using screws.

Now that I have started drilling and attempting to tap my steel rectangle tubes, I am wondering if I have enough wall thickness to add sufficient threading to hold my screws with any stability.

I should have asked this this before I bought my steel components but how are you guys mounting your rails? Are you tapping your screw holes (doesn't metal work sound filthy...) or are you using pressure and smaller holes to keep your screws firmly in place?

Is there a minimum wall thickness required for drilling and tapping steel tubes?

My steel rectangle tube walls are a mix of 1/8" and 1/4" thick.

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[Qicker306]

A general rule of thumb for minimum thread engagement would be to have it equal to your fastener diameter, i.e 1/4" fastener requires 1/4" of thread engagement and therefore minimum 1/4" thick material to tap through. This is assuming the fastener and the material in which the hole is tapped are of equal strength, i.e. steel fastener in tapped steel hole. This rule more or less plays on the side of caution if you consider your tapped hole to at least be equivalent of a standard nut on the end of your fastener, i.e. 1/4" fastener uses a nut that is 7/32" tall which will have a total thread engagement slightly less than that due to the chamfers on both sides of the nut. So as long as you aren't using fasteners bigger than the thickness of your material you should be fine.

[Goemon]

A general rule of thumb for minimum thread engagement would be to have it equal to your fastener diameter, i.e 1/4" fastener requires 1/4" of thread engagement and therefore minimum 1/4" thick material to tap through. This is assuming the fastener and the material in which the hole is tapped are of equal strength, i.e. steel fastener in tapped steel hole. This rule more or less plays on the side of caution if you consider your tapped hole to at least be equivalent of a standard nut on the end of your fastener, i.e. 1/4" fastener uses a nut that is 7/32" tall which will have a total thread engagement slightly less than that due to the chamfers on both sides of the nut. So as long as you aren't using fasteners bigger than the thickness of your material you should be fine.

Do you think that using threads is better than using pressure for this?

I am most worried about the screws getting pulled loose under load if there isn't enough thread to create a strong grip or if the steel is not thick enough and the hole gets widened after repeated stress.

I saw this happen almost immediately when I tried to drill and tap an aluminum extrusions early on. The metal was too thin and just tore like a tin can when I pulled on a screw...

I was thinking that I might have to buy some thicker steel bars to mount on the inside face of the tube behind the rails to give them something extra to grip onto...

[Qicker306]

I'm not sure exactly what you mean by 'pressure'? Do you plan on inserting fasteners that are larger than the thickness of the material that you are tapping? The 20mm linear rails on my machine use M6 (0.236") screws every ~3". I would be more than comfortable attaching my rails to 1/4" thick tubing by simply tapping holes in the tubing. Aluminum is much softer than steel and therefore a fastener would require more thread engagement to achieve a suitable connection. Drilling through holes in your tubing and then tapping the holes in a piece of flat bar that goes in the tubing is an option but only worthwhile if you're not following the rule of thumb I mentioned above.

[LeeWay]

I would use something like this.
https://www.mcmaster.com/#captive-nuts/=18sqkzc
Especially if you have thin wall stuff for some of it.
You won't need to waste a bunch of money on taps either.

[mactec54]

My plan for the long axis on my CNC router build is to mount my linear rails on some steel rectangle tubes using screws.

Now that I have started drilling and attempting to tap my steel rectangle tubes, I am wondering if I have enough wall thickness to add sufficient threading to hold my screws with any stability.

I should have asked this this before I bought my steel components but how are you guys mounting your rails? Are you tapping your screw holes (doesn't metal work sound filthy...) or are you using pressure and smaller holes to keep your screws firmly in place?

Is there a minimum wall thickness required for drilling and tapping steel tubes?

My steel rectangle tube walls are a mix of 1/8" and 1/4" thick.

You need to have 1-1/2 times the diameter of the capscrew (SHCS) you are using, so would need to have 9mm for a 6mm capscrew, (SHCS), most who are doing this weld a flat bar of steel on the side or inside the Rectangle Tube, the surface is then machined, for the rail to mount to

Either way you do it you need to machine the surface that the Rail will mount to, if welding to the out side, stitch weld the bar where the mounting holes for the rails will be, the bar needs to be wider than the rail so when it is machined you can machine a shoulder on it for the rail to sit on, if putting the bar inside the Tube you only need to tap the bar and weld it in place at the ends

[Goemon]

I'm not sure exactly what you mean by 'pressure'? Do you plan on inserting fasteners that are larger than the thickness of the material that you are tapping? The 20mm linear rails on my machine use M6 (0.236") screws every ~3". I would be more than comfortable attaching my rails to 1/4" thick tubing by simply tapping holes in the tubing. Aluminum is much softer than steel and therefore a fastener would require more thread engagement to achieve a suitable connection. Drilling through holes in your tubing and then tapping the holes in a piece of flat bar that goes in the tubing is an option but only worthwhile if you're not following the rule of thumb I mentioned above.

Got it. That makes sense and it makes me feel a little better about using threads on the 1/4" wall tubes. I need to experiment with the 1/8" wall tubes.

I am mounting the tubes on an epoxy granite / carbon fiber machine base. In theory I could fill the void in the tubes with the epoxy granite mix to give the screws something extra to grip. Epoxy doesn't hold threads well on it's own but maybe it will be enough as an addition to the steel.

By "using pressure" I was referring to a method of just drilling slightly undersized holes and adding the screws directly without tapping threads. I.e. The screws are held in place by just the wall of the hole instead of threads and the tight fit stops it moving. I often need to use a power tool to get the screw in when I do this with steel but it can produce a firm grip sometimes

- - - Updated - - -

I'm not sure exactly what you mean by 'pressure'? Do you plan on inserting fasteners that are larger than the thickness of the material that you are tapping? The 20mm linear rails on my machine use M6 (0.236") screws every ~3". I would be more than comfortable attaching my rails to 1/4" thick tubing by simply tapping holes in the tubing. Aluminum is much softer than steel and therefore a fastener would require more thread engagement to achieve a suitable connection. Drilling through holes in your tubing and then tapping the holes in a piece of flat bar that goes in the tubing is an option but only worthwhile if you're not following the rule of thumb I mentioned above.

Got it. That makes sense and it makes me feel a little better about using threads on the 1/4" wall tubes. I need to experiment with the 1/8" wall tubes.

I am mounting the tubes on an epoxy granite / carbon fiber machine base. In theory I could fill the void in the tubes with the epoxy granite mix to give the screws something extra to grip. Epoxy doesn't hold threads well on it's own but maybe it will be enough as an addition to the steel.

By "using pressure" I was referring to a method of just drilling slightly undersized holes and adding the screws directly without tapping threads. I.e. The screws are held in place by just the wall of the hole instead of threads and the tight fit stops it moving. I often need to use a power tool to get the screw in when I do this with steel but it can produce a firm grip sometimes

[Goemon]

You need to have 1-1/2 times the diameter of the capscrew (SHCS) you are using, so would need to have 9mm for a 6mm capscrew, (SHCS), most who are doing this weld a flat bar of steel on the side or inside the Rectangle Tube, the surface is then machined, for the rail to mount to

Either way you do it you need to machine the surface that the Rail will mount to, if welding to the out side, stitch weld the bar where the mounting holes for the rails will be, the bar needs to be wider than the rail so when it is machined you can machine a shoulder on it for the rail to sit on, if putting the bar inside the Tube you only need to tap the bar and weld it in place at the ends

I have no welding skills or equipment here. If I use an additional bar inside the tube, it will be held in place with the screws from the rails on the outside of the tube. It would be like a clamp or vice holding the tube.

If I submerge the tube and bar in my epoxy granite mix and make it part of the machine base, that would perform the same function as welding in terms of preventing any movement.

I have thought about renting welding equiptment a few times for this project. I'm just a little hesitant about learning on the job with this. seems like a good way to waste all my materials when I screw up the weld.... maybe I need to stop being a baby a learn how to weld already!

[mactec54]

Got it. That makes sense and it makes me feel a little better about using threads on the 1/4" wall tubes. I need to experiment with the 1/8" wall tubes.

I am mounting the tubes on an epoxy granite / carbon fiber machine base. In theory I could fill the void in the tubes with the epoxy granite mix to give the screws something extra to grip. Epoxy doesn't hold threads well on it's own but maybe it will be enough as an addition to the steel.

By "using pressure" I was referring to a method of just drilling slightly undersized holes and adding the screws directly without tapping threads. I.e. The screws are held in place by just the wall of the hole instead of threads and the tight fit stops it moving. I often need to use a power tool to get the screw in when I do this with steel but it can produce a firm grip sometimes

- - - Updated - - -



Got it. That makes sense and it makes me feel a little better about using threads on the 1/4" wall tubes. I need to experiment with the 1/8" wall tubes.

I am mounting the tubes on an epoxy granite / carbon fiber machine base. In theory I could fill the void in the tubes with the epoxy granite mix to give the screws something extra to grip. Epoxy doesn't hold threads well on it's own but maybe it will be enough as an addition to the steel.

By "using pressure" I was referring to a method of just drilling slightly undersized holes and adding the screws directly without tapping threads. I.e. The screws are held in place by just the wall of the hole instead of threads and the tight fit stops it moving. I often need to use a power tool to get the screw in when I do this with steel but it can produce a firm grip sometimes

The information Qicker306 has given you is incorrect

You can not use a self tapping screw to mount your Rails, , you are dealing with a precision ground linear Rail, which need to be mounted correctly, or they will bind / jam up if not installed correctly

[Qicker306]

The information Qicker306 has given you is incorrect

You can not use a self tapping screw to mount your Rails, , you are dealing with a precision ground linear Rail, which need to be mounted correctly, or they will bind / jam up if not installed correctly

The only information I have given is to have a minimum of 1x the diameter of your fastener for thread engagement/tapped hole wall thickness whereas you say 1.5x. At 1.5x this would make a standard nut and bolt connection severely undersized from a thread engagement point of view.

Do not attempt to thread a fastener into an undersized untapped hole, if you're worried about screws backing out use Loctite.

Alignment of the rails is an entirely different subject that I have made no attempt to comment on.

[wizard]

There are a number of rules of thumbs for the question of thread engagement. All of them have qualifications such as the screw holding ability of different materials, environmental conditions, class of fit and other factors.

One of he more interesting pieces I've seen on the web is the result of some Finite Element code a guy ran on gun barrel threads. He found 5 threads pretty much gave you all the holding ability you would get from thread with a fast taper off to no advantage after 7 threads. Interestingly this works out very well and supports the idea of one diameter in many cases. For example 5 - 20ths of a 1/4-20 bolt ends up being a 1/4". Obviously this doesn't work in every case as we have fine thread screws that are generally considered to be stronger than the course threaded bolts of the same diameter.

Generally I hope for at least 1.25 diameters for bolted structural assemblies. This deals with a couple of things: You really should countersink, chamfer or counter bore your threads a little bit to prevent the rolling of the threaded above your machined surfaces. Structural steel is somewhat soft compared to the steels we see in the tool and die industry. Lastly if things do strip out you have enough meat for a thread insert.

My plan for the long axis on my CNC router build is to mount my linear rails on some steel rectangle tubes using screws.

Now that I have started drilling and attempting to tap my steel rectangle tubes, I am wondering if I have enough wall thickness to add sufficient threading to hold my screws with any stability.

You should mention the was thickness here. Here is the reality you can get by with less than 1 diameter thickness (not too much less) it is just that longevity will suffer especially if you assemble and disassemble the structure a lot. If the material is really thin you can always weld nuts to the back side. Another option is a piece of steel bar drilled and tapped to match the rail and slipped inside the steel tube.

I should have asked this this before I bought my steel components but how are you guys mounting your rails? Are you tapping your screw holes (doesn't metal work sound filthy...)

I prefer a good tapping because I'm filthy. Seriously tapping is like the easiest way to mount your rails but the alternatives aren't that difficult. The good question here is; have you purchased your linear rails yet? You may not know this but the rails can be ordered drilled and tapped instead of having clearance holes.

Also in classical machine building the linear rails are often registered against a shoulder milled into the mounting surface. In many cases only a master rail is so mounted.

I've seen engineers use fixture clamp hex clamps to locate linear rails in none critical applications. This isn't as useful in my mind as milled locating surface but it does offer a registration point for reassembly.

or are you using pressure and smaller holes to keep your screws firmly in place?

Well technically you are using pressure no matter what you do. The only thing resisting movement of the rail is the applied bolt preload.

Is there a minimum wall thickness required for drilling and tapping steel tubes?

It depends upon the bolts you are using, the material the tube is made from, and other factors but I'd shoot for at least 1 diameter.

My steel rectangle tube walls are a mix of 1/8" and 1/4" thick.

So what size rials are you choosing? If the rails are mounted with 10-32 or 5 mm screws you are likely to be OK but not optimal.

[wizard]

Do you think that using threads is better than using pressure for this?

I am most worried about the screws getting pulled loose under load if there isn't enough thread to create a strong grip or if the steel is not thick enough and the hole gets widened after repeated stress.

These are reasonable concerns.

I saw this happen almost immediately when I tried to drill and tap an aluminum extrusions early on. The metal was too thin and just tore like a tin can when I pulled on a screw...

Even run of the mill structural steel is not aluminum. Aluminum is extraordinarily bad at screw holding so you will be better off with steel right away. However you can have similar issues with steel if the screw threads are overloaded.

I was thinking that I might have to buy some thicker steel bars to mount on the inside face of the tube behind the rails to give them something extra to grip onto...

In many cases it is better to mount such bars on the outside. The reason here is that it provides for an extra bit of clearance for lead screws. Beyond that you can easily machine the surfaces after welding to get your rail ready flat surfaces. Right now I can think of several dozen examples of such an approach at work in the automation industry. This includes custom machinery and machinery purchased form vendors all over the world, it is pretty common practice.

[wizard]

Got it. That makes sense and it makes me feel a little better about using threads on the 1/4" wall tubes. I need to experiment with the 1/8" wall tubes.

Just design such that you are only running threads into the thicker tubing. In any event remember that the rails dictate screw size. With 1/4" thick walls you are really pushing it to be using 6 mm screws. Generally you want more than one screw diameter and by the time any. machining is done you will be just barely covering 6mm at one diameter

I am mounting the tubes on an epoxy granite / carbon fiber machine base. In theory I could fill the void in the tubes with the epoxy granite mix to give the screws something extra to grip. Epoxy doesn't hold threads well on it's own but maybe it will be enough as an addition to the steel.

I can't see the epoxy doing anything positive for you in this instance. What do you do if you have to remove the screws at some point in the future?

By "using pressure" I was referring to a method of just drilling slightly undersized holes and adding the screws directly without tapping threads.

We have a term for such half assed workmanship at work which I will not repeat here. In any event do yourself a favor and put this idea out of your head.

I.e. The screws are held in place by just the wall of the hole instead of threads and the tight fit stops it moving. I often need to use a power tool to get the screw in when I do this with steel but it can produce a firm grip sometimes

- - - Updated - - -

Well I believe the sometimes part of this.

Got it. That makes sense and it makes me feel a little better about using threads on the 1/4" wall tubes. I need to experiment with the 1/8" wall tubes.

I am mounting the tubes on an epoxy granite / carbon fiber machine base. In theory I could fill the void in the tubes with the epoxy granite mix to give the screws something extra to grip. Epoxy doesn't hold threads well on it's own but maybe it will be enough as an addition to the steel.

By "using pressure" I was referring to a method of just drilling slightly undersized holes and adding the screws directly without tapping threads. I.e. The screws are held in place by just the wall of the hole instead of threads and the tight fit stops it moving. I often need to use a power tool to get the screw in when I do this with steel but it can produce a firm grip sometimes

[wizard]

I have no welding skills or equipment here. If I use an additional bar inside the tube, it will be held in place with the screws from the rails on the outside of the tube. It would be like a clamp or vice holding the tube.

That is entirely doable and frankly isn't much different than trying to bolt to extruded aluminum T-Slot. The tricky part is will this cost you in morning position flexibility.

If I submerge the tube and bar in my epoxy granite mix and make it part of the machine base, that would perform the same function as welding in terms of preventing any movement.

I have thought about renting welding equiptment a few times for this project. I'm just a little hesitant about learning on the job with this. seems like a good way to waste all my materials when I screw up the weld.... maybe I need to stop being a baby a learn how to weld already!

Hey now everybody has had to learn a new process at one time or another in their life. As for screwing up a weld, it is pretty hard to do so in such a way that it can't be corrected with a 4.5" grinder and more welding.

The biggest problem with welding isn't so much the actual welding but rather the need to machine afterwards and possibly stress relieve the weldment. Welding by definition distorts metal, it will happen to some extent no matter what you do. A skilled welder, with the proper equipment can greatly lower that distortion but even a master welder can not completely eliminate the distortion.

Now given that this is a new to you process you really should consider engaging a professional welder ideally one with some machine shop equipment. The cost to you will likely be cheaper than a decent welder.

[wizard]

The only information I have given is to have a minimum of 1x the diameter of your fastener for thread engagement/tapped hole wall thickness whereas you say 1.5x. At 1.5x this would make a standard nut and bolt connection severely undersized from a thread engagement point of view.

There are so many rules of thumbs when it comes to thread depth that it doesn't even pay to worry too much about the practices in any one location. I generally prefer to have more than one diameter of thread depth as that deals with factors other than ultimate engagement.

Do not attempt to thread a fastener into an undersized untapped hole, if you're worried about screws backing out use Loctite.

Alignment of the rails is an entirely different subject that I have made no attempt to comment on.