View Full Version : Black pipe questions

11-10-2003, 12:47 PM
I am considering drawing some plans for a homemade Cnc wood router. The cutting area is 18" x 24".

My question is would 1" diameter pipe be sufficient for this span?

Would it need to be braced to keep it from bowing in the middle?


11-10-2003, 01:39 PM
1" black pipe is pretty crappy stuff to use for your rails. Its not smooth, and there isn't anything about it that is 1" :)

11-10-2003, 02:42 PM
I'd go 1.5 or better, you don't want any flex.

11-10-2003, 03:33 PM
I'll second Hu and CNCAdmin's comments. Having used "Black Pipe" myself. Drill Rod is affordable and has excellent tolerance. I'd recommend using it instead. Your 1" drill rod will flex much less than the 1" pipe will. still if you can swing a bigger Diameter you'll be better off in the long run, unless you enjoy building a second machine to overcome the weaknesses of the first.

11-10-2003, 04:07 PM
This guy has used pipes for his with a fairly large span.
He has added a support in the middle.

I came across an electrical Conduit pipe which is made from stainless steel and has a ground finish, - nice stuff.


11-10-2003, 07:29 PM
Black pipe is widely used on Crankey's machines and many of the ones seen on this Forum.
The size and weight does definately have an effect on the black pipe. And if you are building one larger than Crankey's designs you possible will need larger or different material.
In this Forum Chuckknigh is making a pretty good sized machine using black pipe and he has addressed the problem of pipe flex by installing a piece of wood that has been chamfered at an angle so as to contact the pipe and minimize the flexing of the pipe.
Here is a photo I borrowed from his photo showing his approach the the flexing problem.

11-10-2003, 08:39 PM
This raises a question or two for me....................(OK, I'm a total newbie and still in design stage), but enquiring minds need to know:D

I'm planning on having a total X-axis length of between 30" - 36" and will be using solid steel (not stainless) for the rails. Motors weigh approx 2 lb each, gantry will be aluminium and initially I will be using a Dremel...........the main use for the machine is to engrave lettering in aluminium plate (front panels for audio equipment).

Armed with this knowledge, would anyone care to give some advice on rail diameter or any other factors I need to consider.


11-10-2003, 08:55 PM
I'm using 2" EMT tubing. It's only $10 for 10 ft. A 3 foot section is pretty stiff. Mine is not suspended, though. The 2 pieces are held in compression to a wooden framework. Here is a link:

Like I said, if you only need a 3' span, It's pretty rigid.


11-10-2003, 09:04 PM
has anyone considered using angle iron for the rails?

11-10-2003, 10:21 PM
Keep in mind, that many home made machines are built to cut stuff like model airplanes and toys out. Black pipe works great for this type of machine. The loads are light and the accuracy will be such that you need a vernier to see that it's the wrong size...Its all about what you will use the machine for.

If you wan't to cut metal, buy a milling machine and convert it. If your cutting balsa or plywood, use what works and what you can afford.


11-10-2003, 10:43 PM
I'd look more closely at using any of many C1018 cold drawn round or square steel products before I would waste my efforts with black pipe. For a small machine, the difference in price isn't going to break you. There is little point in sabotaging your own efforts with an inferior foundation (the rails) to begin with. That's just my opinion. When building your own machine, you're saving enough money anyway, that you should be able to sink a bit more into better materials. That is how I look at it when I build something.

11-11-2003, 12:18 AM
Here is a photo I borrowed from his photo showing his approach the the flexing problem.

I like that little work-up you did, Hager. Thank you! May I borrow the picture? It makes the forces pretty obvious, I think.

OK, explanation time. The gantry will force the rails outwards, but that's not the only force vector present. You also have to address the gantry's weight, i.e. gravity, with a force vector that would be drawn "downwards."

Think back to vectors in high school or college -- add the two of them. You end up with a combined vector pushing outwards diagonally, both outwards and down, at the same time... So, as you can see from the picture, my rails are below, and the chamfer pushes them "in." Simple, but not necessarily obvious.

-- Chuck Knight

11-11-2003, 12:31 AM
Go for it Chuck.
What goes around comes around.


11-12-2003, 09:34 AM
On the machine I am messing with, I bought hardened 4140 ground shafting from our local steel supplier. It was amazingly affordable, and available in all diameters. Mike.

11-12-2003, 08:38 PM
tell me about the 4140 groulnd shafting.

11-13-2003, 09:49 AM
It is sold as hydraulic shafting, ground and semi polished. 4140 is still machinable with the right tools, yet is much harder than cold rolled or pipe for that matter. Perhaps Hu or another experienced machinist could elaborate. I bought two 1.5" x 42" pieces and it was less than $50.00. Mike

11-13-2003, 10:08 AM
The product that MikeA is referring to, in my experience, is called C4140 Heat treated, stress relieved, centerless ground shafting. This is an alloy steel, made for heavy duty usage.

It is no where near full hardness (which is 60 Rockwell, like a file would be), but is more around 35 Rockwell. It is heat treated for strength more than to provide a wear resistant surface, although it will be somewhat better than C1018 (cold drawn shafting) for wear.

If the product Mike purchased is chrome plated, then that is truly a good choice for wear. Hard chrome is very hard, about 72 Rockwell if memory serves me correctly. Chrome plating is typically only .0005 thick, so you can still drill through it to put a bolt or thread a hole in it with HSS tools. Up in Canada, chrome plated shafting is typically made with C1045 shafting as a base. This is a regular medium carbon steel and is readily machineable.

Full Induction hardened chrome plated shafting (also C1045 base metal) is also available, with a fully hardened skin about 1/8 to 1/4" deep. This stuff, you cannot drill and tap threads through the surface, although you can drill and tap the ends, because the core is soft. This material is typically used for hydraulic cylinder rods that are subject to bruising, because a direct hammer blow hardly marks it, if at all. For typical machining purposes, I usually "spoil the hardness" near the ends where I might be turning and threading it. I do this with a torch, heating the part up quickly to 1000 degrees F or so, taking care not to allow the heat to go to far up the shaft and soften the sliding area. This is a low alloy shaft, and it is easy to spoil the hardness by "over-tempering" it this way.

11-13-2003, 06:49 PM
Just wondering why it always seems to be 3/4 " MDF is used why not thicker 1" or 1 1/4" bound to be more ridged especially for the base.

11-13-2003, 08:51 PM
Weight, cost, availability. Take your pick. It's extremely easy to find 3/4" MDF at Home Depot, Lowes, and almost any small town lumberyard. It's very difficult to find anything thicker.

Now, some of us could have glued up thicker pieces, but a more practical use of that material would be to cut it up, and build a torsion box or a U-channel (my approach) which would be the equivalent of a 4" thick piece of MDF, at 1/4 the weight...it'll sag under its own weight, too, so building "light" can be important.

-- Chuck Knight

11-13-2003, 09:16 PM
1" (25mm) would be fairly common here, 30mm is also available.

Seen 60mm used for a guys workbench, now that's heavy stuff.

02-21-2004, 04:24 PM
I'm the guy that built the machine shown at MachineCnc.com. This machine didn't work out and is being redesigned. The gas pipe would sag in the middle, so I put in the support. The two parallel gas pipes wouldn't stay at a fixed distance from one another. Driving the x axis with two steppers caused one side to get ahead of the other. The two steppers have to be connected to turn or move together in some way. Rack and pinion on each side that move together with a shaft or belts I think would be best.

The machine shown was my first effort. I cut it in half, put wheels on it and now I'm building a fixed gantry with moving table on the x axis. Hope to have much more accuracy, stiffness, and speed.