Not missing a thing, and I totally agree
Didn't know where to put this so I thought this might be an appropriate forum to put it in.
The gantry machines that have been built past and present are a bit of a mystery to me in terms of height,even now today people are building their home based machines with ludicrus height gantrys.
I mean I cannot understand why they make it so high in reality as to how much the spindle axis will move over the job and that is about 1" to 2" lift and then descend down onto the job anything above this height is just waste area of the component build form,also take into the fact of swaying of side motion with the height and the Z axis cutting from left to right has a bearing on this.
So am I missing something here or is it a case of follow that camel?
Not missing a thing, and I totally agree
My first machine was built with exactly 6.25" of clearance under the gantry and Z axis plate when raised, and the Z axis ran a plate with the rails attached that traveled exactly 6.125", leaving exactly .125" between the Z axis main plate bottom and the table. I used it that way for years with your typical routing/engraving projects.
If you take note, by having the linear rails on the z axis moving plate and the bearings mounted to the Y axis carriage, I couldn't raise the plate any higher than the exact 6.25" clearance on the gantry. I happen to prefer this method, but this method also means that any tooling had to stick out below the clearance of the gantry.
But then one day, a friend approached me to cut a 3d foam plug for an RC airplane. While his model would fit under the gantry, it would not fit under my mounted tooling. So, I lifted the gantry 4" to make a 10.25" clearance under the gantry. This worked well for his model and 4" long bits, though it also means that 4" long tooling is always hanging out under the overall gantry clearance. This is not all bad though, as I have found that when using a 4th axis for lathe style turning and spindle operations, it can come in handy because you have additional plate space to mount side cutting fixed tooling.
My friend has gone on to build his own machine with a very radical 24" travel Z axis that only has a 2" diameter top view footprint (let that sink in.... a 2" diameter, 24" travel Z axis). Deep plunging into foam plugs for things like RC aircraft needs that specific type of work envelope. So, in the end, it all comes down to what you plan on doing with a machine,,... what your specific NEEDS happen to be.
But there is more to the issue than just basic gantry height selection. It also depends on your particular Z axis slide DESIGN.
While I used and actually prefer the Z axis to end up being a literal sliding 1/2" thick plate that has the slide portion of the linear rails mounted to it, the alternative is to have a Z axis plate where the bearings are mounted on its backside and they rode on rails fixed to the Y carriage. Generally, this design would let you pull your tooling up higher than the other design, such that even long tooling can be raised high enough to be flush with your gantry clearance.
In the end, whatever the design, you have to live with the tradeoffs each method provides. On the one hand, the first method I mentioned does allow you move your Z axis plate right down to the table top whereas the second I described does not because the linear rails themselves end at the bottom edge of the Y axis carriage.
I always say that you have to design a machine according to the needs of the specific work you desire to do. These two Z axis design variables are important to consider when planning final gantry height.
You can do away with excessive height of the gantry merely by lowering the bed,like this(crude drawing I know).....
With a little bit of ingenuity the bed can be made adjustable for future depth.
I just don't think that height plays a major part in the design.
I personally would never want to use a machine with a lowered bed, as it makes loading and unloading much more difficult, especially as the machine gets larger.
But to answer your first question, yes, everyone thinks they need 8" of Z axis travel, but very few ever actually use it.
Mach3 2010 Screenset
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)
I would think this is an individual thing, but it is based on several common factors. As mentioned, for standard sheet size materials, having a dropped table, lower than the frame, bad idea. Then there is material thickness to be considered and spoil board thickness (possibly 1"). Even though sheet thickness will rarely if ever be more than 6/4 (1-1/2"), on a multi-use machine, I can envision material thicknesses up to 4" or more. Then there is the option to have a tool changer and where to place it. If an umbrella type changer, it almost has to be lower than the beam of the gantry. Once you add that factor in, then you must consider the maximum tool and tool holder length that might be needed. If you are machining 4" thick material on top of a 1" spoil board and want to cut through, you would need a minimum tool length of 4". So, 4" + 1" + 4" = 9". Then also add in the tool holder length. End mill holders (not collets) and drill chucks are usually 2" or more from the spindle face. Now you are up to 11" and have not accounted for clearance over the 4" material or any potential clamps. Potentially, you could gain some of this clearance back with enough Z travel above the tool changer position, except, with the tool changer, the idea is to have other tools hanging down.
a guitar for example may be 3" thick max, plus 1" for a vaccum jig. so you need only a little ove 4" of clearance.
however, to drill a hole clean through the part, you need over 3" of motion, PLUS 3 more inchres for the tool clearance, plus the drill chuck, plus some safety. seems like only 3-4" travel needed at first, but consider your next tool to use might be a stubby 1/8" end mill with no chuck to register at jig level. you need alot more travel. i found 5" nowhere near enough. 8-10" would be ideal.
so i think you want the lowest amount of clearance you can get away with for the best rigidity, and the largest amount of z travel possible.
Buy A Kitten!!!
I built my machine as a general purpose CNC machine that I could experiment with and extend.
Sometimes I machine small aluminium parts mounted in a vise that is already 3" tall.
With only 2-3" of clearance, my 4th axis wont fit.
I make fiberglass parts for vintage racing motorcycles. The effort of making plugs and molds by hand has gotten real old real fast.
So my machine needs to be large enough to accommodate bodywork that could conceivably be 3 ft. wide x 4 ft. long x 3 ft. tall (racing sidecar fairing for example). But if I'm going that big, it would be nice to be able to make bodywork molds for my mid-engined car project (roughly the size of a Miata).
My material of choice is MDF and although my glued up blocks are frequently hollow, they may still weigh several hundred pounds.
Due to weight considerations, I had intended to move the cutting tool in X, Y, and Z, keeping the material in fixed position rather than move the table.
Is this a totally stupid way to go? I'm not getting a clear signal from the group on the advisability of long Z travel from the gantry.