[HorridHenry]

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?

[foam27]

Not missing a thing, and I totally agree

[datac]

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.

[HorridHenry]

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.

[HorridHenry]

Here's a prime example of working above the bed.
CNC router - badog cnc

And here's a better solution to go down into the bed,far better design.
CNC Router Plans for Sale

[langwadt]

Here's a prime example of working above the bed.
CNC router - badog cnc

And here's a better solution to go down into the bed,far better design.
CNC Router Plans for Sale

The second needs dual drive, on for each side of the gantry, the first can probably do with one central drive under the table

and as far as I can tell, how the cutting forces will work on the linear bearings
will be quite different

So both have pros and cons

[ger21]

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.

[txcncman]

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.

[jharvey407]

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.

This is true. I built my machine with 6" of travel. I am in the process of moving the table up 1 1/2" to make the machine easier to work with and increase the Y travel just a bit.

[ihavenofish]

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.

in my experience, travel and clearance are clearly separated.

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.

[jharvey407]

in my experience, travel and clearance are clearly separated.

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.

This is a very good point. I've never really measured the clearance/travel of my machine. I'll take some measurements on Monday and post them when I get a chance, as well as a typical job work envelope for comparison.

[ftkalcevic]

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.

[compositesgal]

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.

Thanks,

CC

[ger21]

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.

Thanks,

CC

I think you'd be much better off with 8-12" of travel, cutting your plugs in thinner layers then assembling them. You'll still have some handwork from joining the seam though. But I think you'll run into serious clearance issues with a taller gantry, especially on steep vertical areas. Router bits are only so long.

[txcncman]

How about 48" Z stroke?

http://www.dmscncrouters.com/images/...g%20Tables.pdf

[jharvey407]

I have seen some CNC machines that are built for mold making that have very long, 12" or more Z travels.

It all comes down to the design.

Start by defining your work envelope. Then calculate the cutting force required for your work. Then design a gantry and Z axis that is stiff enough to withstand the cutting force.

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.

Thanks,

CC

[compositesgal]

Thanks to all for the feedback, pro and con.

Gerry does have a point, though. Router bits are relatively short and the more vertical the surface, the more difficult it becomes.

Sounds like I need more than 3 axes to achieve my overall objective.

CC

[datac]

Thanks to all for the feedback, pro and con.

Gerry does have a point, though. Router bits are relatively short and the more vertical the surface, the more difficult it becomes.

Sounds like I need more than 3 axes to achieve my overall objective.

CC

It takes a whole lot of "machine' to avoid excessive flex when you start pushing beyond 12" of z stroke. If I were attempting to make a deep cutting axis like that FOR WHAT YOU need to do, I would be building a spindle that IS the Z axis. It wont be simple, but I have seen it done.

How do I explain this... Your goal has to be as narrow top view "footprint" as possible so you can do typical 3d deep plunge toolpaths without having anything like a conventional router hanging out in the way. If you just take a typical 6" long bit in a traditional router, the achievable angle of approach for a deep cut is very limited.

So, make the sliding portion of the Z axis out of very thick wall ROUND tubing, and put a machined and balanced shaft with a collet system on the bottom, right thru the middle of it with high end bearings properly mounted on each end. This round tube can slide up and down on properly sized, conventional THK style bearings and rails, with the rails mounted on the tube for maximum travel. The new spindle shaft is driven by toothed belt and pulley from the very top, where whatever motor you pick to drive it is also mounted or clamped to the side of the tube.

Now if you have access to a really good machine shop, you could have the bottom foot or so tapered down, or slimmed down to narrower tube to minimize the footprint near the cutting tool. The linear rails start a bit higher than this narrow portion.

Some calculations have to be made or some good seat of the pants understanding applied when it comes to the size of this tube so it does not flex. If you were cutting foam plugs, this could be lighter duty. Again, you really need a thin profile of the spindle in order to get your longest bits to run along the vertical sides of what you are cutting without a router in the way.

[ftkalcevic]

Then calculate the cutting force required for your work.

Any links on how to do this? The only information I've ever found was a reference to a very expensive book - Which I can't afford, so I've never tried.