[Slackdaddy]

I am back on my design phase of making a CNC router that will be a dedicated gun stock router.(hobby grade).
I have access to a full metal shop so I was going to build a heavy/rigid metal frame.
This will have a permanent 4th axis indexing built in.
The Y axis (front to back) tool travel would be 40-42"
The X axis (Left to right) tool travel would be 12"
Z would be 8-10"

This machine would be routing gun stocks that would that would still be sanded and finished after the routing,, so I do not need .001" precision here,, but I do want dependability and longevity .

Couple questions:

The y rails/rods,, I was looking at 2 unsupported, parallel Rods on each side about 4-5" apart that the gantry sides would ride on. Reason is I could have the rods mounted to an Adjustable plate on each end, as opposed to directly to the steel frame. What dia rods would effectively span 48" without giving me longevity issues.
I was shying away from continually supported rods or tracks for fear the steal frame would not be 1000% in spec for flatness.,, Thoughts??

I do not want to "re-invent the wheel" for every bracket and piece, What is a good supplier for Z axis/spindle units, brackets, etc ?

Thanks,
Slack

[joeavaerage]

Hi,

What dia rods would effectively span 48" without giving me longevity issues.

Any diameter rod would be fine, I doubt you'd ever wear out the rod, but may wear out the bearing if dust and other contaminants get in there on a regular basis.
The real problem is stiffness, or rather the lack of it. If you take steel bar, say 1" in diameter and a yard long, support it at each end, how much force would it take
to deflect the centre of the bar by 20 thou?. Not bloody much I'll be bound. This will happen when your cutting, the cutting forces will deflect the rails and all accuracy
is lost. If your rails were 12" or less I'd say you would be OK....but 42" forget it.

Your rails, either square OR round must be mounted along their length or they WILL deflect. If that means you have to get the steel table machined flat, that's whats you have to do.
There is a reason that many people on this forum talk about methods to reduce or eliminate the requirement for machining, or by design reduce the cost and complexity of that machining.

Square rails are a factor of ten stiffer than round rails and much preferred. Stiffness is the absolute minimum requirement for accuracy, if the machine is not stiff it will flex and vibrate like hell
and any semblance of accuracy or surface finish is lost.

Craig

[Slackdaddy]

Craig,
Thank you for your response.

I had thought about having the metal frame "decked" (milled) at a local machine shop after welding, IF I were going to mount a "track rail" to it.
My concern with a track mounted to the top of the table is debris build up from the router.

Hard to explain, but I was going to extend the table frame 6-8" above the table on the sides (y axis), so basically I would have the legs extending up 6-8" in each corner, and a heavy steel member running front to back on each side (welded),, then mounting a continuously supported rod track to the underside of the side members. Then adding the 2nd unsupported linear rod 4" below the "track rod".
That way I would have 4 linear bearings on 2 parallel rods for the gantry sides. With the top rod being continuously supported it would stop any deflection. But the problem would be that this design would be nearly impossible to have milled after welding together.

I am not yet set on any type of Rod/Rail system for the gantry to ride on,, But I do want a HEAVY/STIFF welded table/frame as a basis of the design.
Just have to figure on a Rod/Rail system that can be "adjusted" when mounting to an imperfect steel frame.

Hi,


Any diameter rod would be fine, I doubt you'd ever wear out the rod, but may wear out the bearing if dust and other contaminants get in there on a regular basis.
The real problem is stiffness, or rather the lack of it. If you take steel bar, say 1" in diameter and a yard long, support it at each end, how much force would it take
to deflect the centre of the bar by 20 thou?. Not bloody much I'll be bound. This will happen when your cutting, the cutting forces will deflect the rails and all accuracy
is lost. If your rails were 12" or less I'd say you would be OK....but 42" forget it.

Your rails, either square OR round must be mounted along their length or they WILL deflect. If that means you have to get the steel table machined flat, that's whats you have to do.
There is a reason that many people on this forum talk about methods to reduce or eliminate the requirement for machining, or by design reduce the cost and complexity of that machining.

Square rails are a factor of ten stiffer than round rails and much preferred. Stiffness is the absolute minimum requirement for accuracy, if the machine is not stiff it will flex and vibrate like hell
and any semblance of accuracy or surface finish is lost.

Craig

[joeavaerage]

Hi,

My concern with a track mounted to the top of the table is debris build up from the router.

I understand the concern, on the other hand if the machine is not rigid enough, then you'll never use it because it fails to produce the standard of work you require, and in
absence of debris it will last, not productively however!

Hard to explain, but I was going to extend the table frame 6-8" above the table on the sides (y axis), so basically I would have the legs extending up 6-8" in each corner, and a heavy steel member running front to back on each side (welded),, then mounting a continuously supported rod track to the underside of the side members. Then adding the 2nd unsupported linear rod 4" below the "track rod".

I get the general idea. There is a point that you are overlooking, that is stress relieving. Once a steel frame is welded it will have major stress built up int it. Even if you machine it flat it
will move over time rendering your expensive machining null and void. You MUST stress relieve THEN machine.

You might be saying' even more expense' and the answer is yes it is more expense. For all the expense however a welded steel frame, assuming good design and section choice,
is the cheapest way , including stress relieving and machining, to achieve a rigid frame. You may have seen people using concrete, epoxy granite, granite and the likes, but they
are still more expensive solutions than a well designed and made steel frame, relieved and machined.

When I built my new mill I had three axis beds cast in grey cast iron, 115kg each, they cost $3500NZD ($2450USD)....and I thought, 'I'm glad I've got that cost out of the way', but then I took
them to be machined and stress relieved and that cost $6000NZD ($4200USD)....I nearly s...t my pants! The upside is that they are absolutely superb, I have exactly what I wanted and
paid for, no compromise and will stay that way for decades to come.

If you want a good result be prepared to pay up for a good and rigid frame.....its the one thing you cannot up-grade at a later date, you get it right first time or your stuck
with in inaccurate, vibrating piece of rubbish that you will hardly ever use.

I note that you are still considering unsupported rails. I think that is a mistake, certainly the long rails. May I suggest an experiment. Find yourself a steel bar, mild steel reinforcing
bar say 25mm in diameter and 1m long. You should be able to pick up a piece as scrap for next to nothing. Support it at either end and measure how much force it takes to deflect
the centre by 1mm, you might be surprised, even shocked at how little it takes. No doubt you'll say 'but they use good steel for linear rails', and yes they do use hardened steel,
but hardened steel will probably be twice to maybe as much as three times stiffer than mild steel, but hardened steel is not magic, it still bends.

Craig

[peteeng]

Hi Slack - You will be very disappointed with unsupported rails at 48" long. They will deflect and vibrate, issues that then can't be solved as they are built in. For gun stocks as you say micron accuracy is not needed so go with round supported rails. They are tolerant of "poor" foundation geometry. Make the long rails plane vertical and the cutter horizontal like a lathe then the muck will fall away easily. The long rails can even be upside down so they are out of the way, then the spindle could be vertical above the billet. The drive can be above the stock, again muck can't get at it easily.

"Heavy" is not necessarily stiff. Steel is a good starting point for stiffness but don't make the sections thin. Consider heavy structural channels as the basic building block. Easy to bolt to as they are open. In terms of machine design they can be the same bending stiffness as a closed section. Just not as good in torsion which can be fixed using a good machine frame layout. Plus being thick they can be machine dressed easier then tube and tapped if needed... cheers Peter

[elfrench]

Definitely use supported rods. You can also use self leveling epoxy to get it perfectly (within tolerances :halloween) flat. There are quite a few threads on it.

[joeavaerage]

Hi,

You can also use self leveling epoxy to get it perfectly (within tolerances ) flat.

Better than nowt I guess, but epoxy is squishy by comparison to steel. Welded steel frame, stress relieve, machine, mount linear rails.

Craig

[peteeng]

Hi Slack - You mention access to machining shop so machine lands flat, that's the better approach then epoxy. Lots of people do it but its not a good solution IMO. Peter

[elfrench]

Welded steel frames vibrate, you should make a cast iron frame :banana: I don't think you'll have any squishy issues using epoxy on a machine only cutting wood. You may also find it to be less expensive.

[peteeng]

Hi Elf - There are 10,000's (probably more 100,000s) of steel mills and routers that use welded steel frames and they don't vibrate. If the machine is stiff enough in any material it won't vibrate. I'm sure someone can design a poor frame in cast iron and it will vibrate. CI is not a wonder material when it comes to vibration. Slack has access to a machine shop so epoxy is not needed. Peter

[Slackdaddy]

I have access to an engine machine shop,, the mill that they "deck" a head, and a mill they deck a block could be used to mill OUTSIDE surfaces.
I have a pretty full metal shop at home (cold saws, mig and tig),, I do not have a line on someone with a furnace to post treat/stress relief. That is something I have to figure out.

Yes round rails are more forgiving of tolerances than are square rails. I WAS looking at 2 parallel unsupported rails (4=5" apart on the long (Y) axis,, but would have to go with 50mm or better,, and the cost goes crazy. Cutting tolerances on a dedicated gun stock machine do not dictate square rails. So continuous supported round rails it appears is my choice.
My next design issue is protecting the "split" linear bearings that run on that rail. Your suggestion of running the long 42" rail in the vert is interesting, but would force me to stand the entire machine on end?
Remember this machine will be a "comapct" machine dedicated to routing gun stocks, the outside of the frame will be roughly 50" long (Y), 18" wide (X), 18" high (Z,, excluding the gantry).

Once again, thanks for the help.

Hi Slack - You mention access to machining shop so machine lands flat, that's the better approach then epoxy. Lots of people do it but its not a good solution IMO. Peter

[Slackdaddy]

Question:
Is there continuously supported round rail, that the mounting leg of the rail is stout enough to only contact the frame metal at the fastener locations (every 3-6" ??)
I ask this, because I am thinking the continuous rail could be mounted to an unmilled steel frame using shims to take out the unevenness of an unmilled frame.

[elfrench]

You could put the same size shim under all of the fastener locations, then adjust from there. I would be tempted to use a 48" long granite surface plate as the base instead of welded steel. They're guaranteed to be flat

I use felt pads cut to fit around the supported shaft to keep the dust out of the carriages on my rose engine. I haven't added accordion bellows yet as the felt pads are working well. The bellows are inexpensive on AliExpress.

[peteeng]

Hi Slack - Here's a photo of my router. Instead of having the Y axis moving you could have fixed columns under the gantry. Then the dust is all below the machine. For stocks I imagine you will need a tall Z to accommodate their size. Not much dust gets on top of Scoot. Then mount the rotary under the gantry. Scoot uses square rail but next machine will use round on the gantry. Time to get into CAD? Peter

[Slackdaddy]

I had planned on something similar, The sides would "frames" made of roughly 2" x 3" x 1/4" tube. So they could be welded up higher to avoid chips.

Hi Slack - Here's a photo of my router. Instead of having the Y axis moving you could have fixed columns under the gantry. Then the dust is all below the machine. For stocks I imagine you will need a tall Z to accommodate their size. Not much dust gets on top of Scoot. Then mount the rotary under the gantry. Scoot uses square rail but next machine will use round on the gantry. Time to get into CAD? Peter

[joeavaerage]

Hi,
steel is relatively cheap so I would use the heaviest sections you can get and weld properly. The emphasis here is on the thickness of the section.
As peteeng points out that channels have the advantage you can get to the inside of the section for bolting and similar, if you have a thick walled
square or rectangular section you can drill and tap with good result.

I have found that 100 x 100 x 10 is readily available and can be welded to full depth in one pass with dual shielded MIG welding (CO₂ plus flux)
with 1.8mm or 2.4mm wire at around 300A. Dual shield makes a beautiful weld.

Another possibility is to use the 100 x 100 x 9 as the upper components on which you mount your rails, round or rectangular, and bolt the table, sort of suspended below it.
This would mean that the main steel components have never been welded and may obviate the need for machining. Even if you did machine them two
lengths of steel say 48 inches long machined flat on one surface only would be as cheap an operation as you could devise.

Many years ago I made what I call a 'trammel mill' for making model aircraft wing molds. It had a 50 x 50 x 6mm gantry on which I had a bearing loaded car to which
the router was mounted. Naturally I was concerned that the gantry needed to be straight otherwise any curve in it would be reflected in my wing molds. On getting the
material I found that it was straight to within about 0.1mm over 1.5m which I determined was adequate for my purpose.

I am not aware of any specification that controls 'straightness' but none the less was quite happy that the material as supplied was good enough. It may well be that
you would find the same.

Craig

[peteeng]

Hi Slack - Is your machine an X & Z axis with a rotary or an XYZ with rotary? Peter

[Slackdaddy]

X, Y, Z, PLUS rotary

Hi Slack - Is your machine an X & Z axis with a rotary or an XYZ with rotary? Peter

[peteeng]

Ah - I was imagining a XZ+R so its a 3D router with rotary, very good. I highly recommend a high wall design. I've built 3 machines with steel moving columns and then Scoot with timber walls is stiffer. It contains dust very well and the X drives are up out of the dust as well. I'm really happy with this config. In your case if you can get to the back and front of the machine I'd put the rotary at the back so the front is a normal machine. If stocks are the main thing then reverse that concept or put the side to the wall so you have side access at both sides of that makes sense.... Some machines have the rotary on the side going fwd and back with the gantry overhanging the side for the rotary. I take back putting the rotary at the back as this would make the machine too long due to the gantry dead space in that direction. So the rotary is at the front an could take advantage of the spindle overhang fwd....Peter