Possible new machine - thoughts?

[Mike F]

I am just reseaching the building of a CNC router/light milling machine for use in my school's Design Technology department. Appreciating that the mechanics have to be right I am intending to buy THK linear guides and ballscrews The rep has been to visit and is confident he can come up with something within our buget - one up for THK.

Regarding the construction, the machine will be some 2000mm in X, 500mm in Y and 150mm in Z and I am after the highest resolution and repeatability possible - hence the expensive mechanical bits. Having been lurking here for some time, absorbing all the wonderful projects, I wondered if someone may care to comment on a possible arrangement shown below.

The idea is to mount one of the X rails on a wall, the other on a very solid bench. This may alleviate one of the problems associated with less than robust gantries i.e. the gantry will not be able to move in the Y direction. The rough drawing is just a general arrangement with nothing to scale.

What do you think?

[Mr.Chips]

Hey Mike,
The object always is to make a machine as rigid as possible. How about a little more information?

What is the construction material of the wall, wood, concrete, etc?

Likewise what is the construction material of the bench and how is it secured to the wall and floor?

Answers to these questions will help people understand the the things that might influence the performance.

I haven't seen this type of construction it's a good shot at an alternative methoid of building in rigidity.
Hager

[balsaman]

I like it. It will be hard to move around..

Eric

[Mike F]

Hi Mr Chips,

Wall is concrete blocks - pretty rigid. Bench will be of welded steel construction, possibly utilising extruded aluminium mounted on it for attaching rails etc. and the whole caboodle will be bolted to the floor. I appreciate that one of the major problems in this set up is one of parallelism of the rails, but assuming this is possible, is there anything I have overlooked in essence?

[Mike F]

Balsaman,

Not a problem - the school has been there for 160 odd years and there is no intention to move it :-)

[duluthboat]

Mike,

I love your creative thinking. The question for me is how will you drive X, one or two motors? With one, torque or twist may be a problem.

Gary

[Hobbiest]

Make sure that the table is absolutely square to the wall. If the gantry is not too wide (I'd say not over 24"...especially using THK), there shouldn't be too much yaw in the movement of the gantry...especially if built out of metal, and solid.

[NeoMoses]

One potential problem I see is thermal expansion. Aluminum, metal, and concrete block will all have signifigantly different thermal expansion coefficients. Using the wall as the main support for one side of the machine will likely limit the accuracy/repeatability of the machine. If you're spending all the money for precision ballscrews and THK rails, you should probably build the machine as a freestanding machine, and have 1 or 2 places to anchor it. I would recommend anchoring to the floor if you must, but the wall might work OK, too.

[sbrpollock]

Great Thinking!

The geometry that you propose is very sound. It has been used for many many years in the design of heavy production cranes in the steel industry and I'm sure many other heavy industies. (The blue prints for the last one I worked on were last REVISED in 1936.) It is called a "Semi-Gantry".

They are generally designed with one rail on the floor (or at grade level) and the rail on the other side of its work area hanging from the ceiling or building superstructure. The rail on the floor is rigidly located and the one hanging is free to swing some small amount. This actually provides a suprisingly stable machine. The rail that I refer to as the upper, swinging rail would be the equivalent of the one you have mounted solidly on the wall. I would think this should be very stable and rigid.

One point I'd make is this:
On the cranes I'm talking about, they are very frequently driven from one side. The side with the rail on the floor has all the drive motors and mechanical equipment and is where the majority of the weight is concentrated. (I would guess eighty percent) This part is quit wide. The part that hangs on the upper rail is not driven and is very narrow. (I'm talking about along the x-axis) This upper part is not driven, it just coasts along the rail as the other end does the driving.

I would think that maybe you could make the upper part significantly narrower than the lower part (along the x-axis), but somehow I think that no matter how you do it you will be committing to driving the gantry along the x-axis at both the top and bottom. (It would have two lead screws)

I will look for links to some pictures of what I'm describing.

[Mike F]

Duluthboat,

Drive might be an issue. I had intended using only one ballscrew as recommended by THK - they do not like two drives with their screws and guides. I had wanted to use two before speaking with THK and their considered opinion is that there should not be a problem with twist/yaw.

Hobbiest,

Y axis will only be some 400mm working, approx 600mm overall. I am assured by THK that yaw should not be a problem with their guides and I was thinking of a fairly generous spacing of the blocks.

NeoMoses,

If you notice, on the drawing, I have mounted the upper guide on an aluminium extrusion, just like the bottom one and this will, hopefully reduce the differential expansion problem.

Patrick,

Thanks for your comments.

Taking all the above into account, I am beginning to think that conventional thinking might be the way to go. After all, convention became just that because it works! However, I will pursue this approach a little further as it would enable building the machine without massive gantry supports to limit distortion in Y when under load.

I am comitted to building this machine and will no doubt be posting many more questions before the end of the project. Incidentally, I will not be producing this machine in a weekend as some others here can apparently do !!!!

[Hobbiest]

I don't think yaw will be a problem with that small of a gantry. Remember this though...You may want to move this machine sometime for cleaning, painting of the wall, demonstration, etc. Also, material introduction could become a pain in the rear with one side mounted to a wall. Not shooting down the idea at all, I think it is great, but just some things to think about. Also, a lot of the time, convention is dictated by tradition, not by what works best.

[High Seas]

Mike - I also applaud your thinking out of the box and using all available resources! Who needs a table if ya got a wall!
You might consider mounting your linear rails on the TOP of the AL extrusions.
My story here - I built matching sets of drilled and tapped bar stock that align perfectly with the holes in the linear rails. The whole assembly slides into place, rails on the extrusion and mounting stuff inside the channel.. Mind you I can adjust the rails to ensure they remain in parallel - but moved them to the top of the extrusion -vice the sides (as per your figure). My concern was the while bit "slipping" downa dn out of parallel.
I've hung a few photos in the members gallery - more to come in a bit showing the reconfigure as System2. System1 shows the rails "sidemounted".
Jim