[Dongle]

This is going to sound really dense, but I'm struggling with visualising how the bit between the computer and the machine works, having yet to do any at all, so was looking for some explanations please!

Basically I understand that the software that generates the paths says to get from this point to this point, I need to go x this far, y that far, z that far - so 3 axis movement I can picture OK - I think (am I wrong?!).

What I really cant understand is when you bring in extra axis. As an example, I am just starting out building a large foam and wood cutting router using something along the traditional gantry style approach. If I decided to mount the motor and cutter on a pivot so it could swing at an angle to the bed, giving a forth axis, how does the software know things like distance of the cutter head from the point of rotation, or more importantely WHEN to use that ability over just stepping the other 3 axis to get the same result?!?!

Same with a rotational table in the middle of the bed?!

I suppose some of this will become apparent when I start using it, but I'd like to be able to plan ahead slightly - I already know of one use for my router where the ability to rotate the head to an angle could be necessary (or at least highly adventageous)

Many Thanks!

[HuFlungDung]

On a cnc machine, with the spindle Z axis above the table, you would generally not have an extra axis facing upwards towards the spindle, because the controller is able to mimic rotary action by what we call circular interpolation of X and Y axis. There are special gcodes (G02, G03) for this common operation for interpolation of arcs. So your linear moves are made in X and/or Y linear interpolation mode, called G01 and arcs are made by the G02 and G03 (CW or CCW arc).

So a 4th axis would typically be set with its rotational axis at right angles to the spindle Z axis. By means of rotation of the part mounted on the 4th axis, you are then able to bring other surfaces of the part into the "visibility of the spindle".

A simple example would be drilling two cross holes in a shaft, at right angles to each other and to the axis of the shaft. This operation is impossible to do with a single spindle and a single clamping of the workpiece. But the 4th axis makes this easy to do, plus increases the accuracy of the relationship between the two holes, because the part maintains a perfectly well defined location, because it does not have to be unclamped from the 4th axis chuck.

[Jarwalcot]

Dongle,

It is a joy seeing someone trying to learn something new.

Let's leave the "4th" axis question out of this and focus on the angled mounted head design.

Most tool paths are generated based on a specific tools design. I suggest you research the following topic: Cutter Compensation (Cutter Comp). I would offer more but I don't want my point of view to hamper your efforts to learn.

Keep it up,

[Dongle]

Thanks for the responses guys. LOL I think "trying to learn" was a very apt turn of phrase Jarwalcot, this CNC is quite a steep learning curve!! I'll certainly do some reading on Cutter Compensation - I think half the battle with learning something like this is knowing exactly WHAT to search for, the terminology just isn't in my vocab' yet to know what I'm looking for.

HuFlungDung - I understand what you are saying, my reason for asking was due to a piece I know I'm going to need to cut (the CNC project has been born out of necessity!) and am not sure how it would be approached - as I'm so new to this I'm trying to picture different parts I'm going to need to cut and visualize how the cutter will have to work. The part in question is a large piece of foam, about 2"6' by 4"6', In one place there is an undercut. Due to the size it wouldn't be possible to cut the undercut from an overhead gantry unless I either made the gantry greater than 2"6' high, or rotated the table or cutter to an angle where it could reach inside?

[HuFlungDung]

If you can show us a rough sketch of what kind of part you would be cutting, some of the guys may be able to give you an idea of how to approach it.

While it is fine to dream of a nutating 5th axis (where the spindle swivels around), you don't want to end up with something that you cannot find software to write the gcode with, unless you have some major funding available, at least

[Dongle]

LOL - the words most likely to put me off a plan - "Major funding"!!!

OK here goes... you said rough!!!! once you've wiped the tears of laughter away I'd like to point out I'm at my "9 to 5" work and have nothing other than MS paint and a broken mouse to work with at the moment

this is a cross section view - the piece has basically this sort of profile (2"6' wide) for a length of about 4"6' - it's not quite that simplistic but you can get the jist of the undercut which does run most of the length.

[ger21]

If that's 30 inches wide, and 54 inches long, and you want to cut that in 1 operation, then major funding will be needed. From the looks of it, it doesn't appear that even a 5 axis machine would be able to reach in there.

[HuFlungDung]

Hint: don't design shapes that are re-entrant!

How about a two piece design with a horizontal part line in the middle of the re-entrant pocket? Then, glue them together.

Consider that your proposed nutating spindle is going to have to get really chummy with the table to machine down that close to the top. Most likely, you need to block the part up just to obtain clearance. If you have to do that, then you might as well just rotate the part underneath the gantry to start with, so the slot faces upwards. All you need to do is expand the Z travel of your proposed machine.

[Dongle]

LOL unfortunately this bit of the car sort of designed itself. I did think I'd probably end up machining as more than one part and joining, was just hoping to avoid it and trying to use it as an exercise to learn more on the machines at the same time.

In all honesty it's not really as deep as that, quite a bit shallower in fact, but trying to draw it with the tools to hand it looked more like a straight line so I emphasized it a weenie bit

[WhiteTiger]

If the drawing is anywhere close to scale, then it should be doable as a simple linear profile with a 4th axis that is an articulation of the Z. Be a pretty intricate and pricey build though and I have no clue what software would be required as a minimum.

It could also be done on a 3 axis with a beefy slow speed spindle swinging a large diameter cutter with radiused peripheral teeth, but the Y axis would have to be long enough to account for the clearance for your 8 or 10 inch cutter on both sides.

Too many unknowns to say much of anything for sure, but the profile itself doesn't put the thing out of reach of DIY. Material and tolerance specs could easily do so though.



Tiger