[nash]

Hi All,

This is a rather ambitious project to build a gantry mill with a 2.2mx1.7mx0.9m of work envelope. Why the odd size? Cause I have space in the house that would accomodate it . The accuracy that I'm after is better than 0.05mm (Y and Z) for and better than 0.1mm for (in X) within the entire envelope. The repeatibility should be better than 0.03mm over the entire envelope. I know most of you seers there must think that you'd like to have a bit of what I'm smoking ... LOL. But shooting for the stars and hoping to land at-least on the moon. But I have help: my closest friend runs a tool room. So when the going will go tough (and it will), I've got a fall back, mainly for alignment (instruments) and for milling+grinding of alinment parts.

Currently, I'm scrounging for different elements now that the design is partially underway. The most iimportant items are:

1. Get your critique on design elements.
2. Find a stress releiveing facility to do(preferably donate) me the services for my humungous frame.
3. Order mechanical elements (guides+carriages+screws+nuts)
4. Get a final critique on design elements...
5. Get the steel tubes chopped, welded and stress relieved.
6. Get the structures delivered at my home.
7. Moglice for alignment.
8. Alignment jigs (taut piano wire)
9. Drilling+reaming and drilling+tapping
10. Mechanical installation
11. Electrical installation
12. Exhaust installation (dust collection etc)
13. Making first parts (mainly dust protectors and guards etc.)
14. Mill my first part.

What do you think? Please add your bits to this list....

Anyway, here's the progress on the design. I had a meet a friend of mine who designs cranes for a living and he said that to get a structure stress-relieved is going to be really difficult. I wonder if I use a moglice type of solution for rail-bed-alignment and dont stress relieve it, since its going to be used at room temperature (around 22-25degC), how much would the structure warp?

Tomorrow, I've got a meeting with a small workshop that has sheet metal bending machines. After looking at what they do, I may change the design to a sheet metal one if that would reduce the number of welds needed. I'm thinking of 5 or 6mm thick structural steel. If that happens, I'll update you all.

In the figure, you see only the welded structure and the positioning elements (rails +guides). The rest still needs to be done. In another post: http://www.cnczone.com/forums/showthread.php?t=27495
I've got a problem in selection of rails (comptop/hiwin) what should I order? Are there any folks here from Italy (Milan/Parma region) who are on the zone?

Eager for your critique.

Best regards,
N

[svenakela]

0,05 mm is very difficult to get, let's impossible with a welded frame. You need to stress it and mill or fine tune the surfaces for the rails in one way or another.

[nash]

Hi Svenakela,

Thanks for the response!! Thanks for reminding me... well, there are two options: Either get an annealing place (extremely expensive for this size) or think of a different design using epoxied and bolted structure. If however, I do all the alignment and potting of the railbed at the temperature I'll use the machine, I should be able to get the accuracy. I'm still debating on the path to take. Tomorrow, I'll have a meeting with some friends who're going to do the welding for me (these are professionals). I'll have more to about the change in design.

Another question, suppose I can get steel sheet of 5mm cut using plasma and bent into a monocoque structure and then do all the joining using epoxy + bolts, will it also have the same problem of the welded frame or can I get rid of the problems?

Thanks, please keep your suggestions coming.

[ad_bfl]

I am in the process of building a welded steel frame over 10' long for my catilevered mill.

The basis is a Steel I-beam 24" between webs, 5/8 thick flanges 10' wide, with lots of 4x4x.25 wall A36 steel tubing welded in a torsion box design on bot sides between the flanges.

The cost for stress relieving something this big and massive in Northern CA is going to be between $1000 to $1200 dollars.

Blanchard grinding the long axis is going to cost another $750.

When I look at the overall cost and affect on accuracy, it seemed like a good tradeoff vs other construction techniques.

If the frame isn't straight and flat, it will be useless, hence the stress relief and grinding.

Al

[nash]

Al,

I just came back after a meeting with a fabrication shop that would do the job for me (the basic frame etc.), they proposed to use induction heating to get it heat treated. Its not as even as proper furnace and may also be less expensive, but not a problem.

I'm not going to go into a major redesign now, and use the welded frame provided the induction heating shop costs are within reach. The problem is that these are different shops. (more $$$).

Do you have a sketch of your design?

I would also prefer to get the rail-beds machined and the rail holes drilled in the frame on an accurate mill, but am still looking around.

N

[ad_bfl]

Hi N

Don't have a sketch I can send electronically, but my design is not a gantry design, it's a cantilevered arm like the Biesse routers so probably of little use.

My reason for going in that direction is that for my use cases, workpiece holding and alignment is a harder problem, so having a open design is advantagous, assuming the assembly is rigid and resonances are controlled.

Check around on shops. I called three different shops and got very large differences in price depending on their scheduled workload.

One thing to consider is to buy a good MIG welder and do that yourself. MIG is easy to learn.

Drilling is going to be a different story. Alignment using the tight wire method and some optics for precise positioning with a mag drill and patience is my approach.

My bigger anxiety is if I break a tap in one of the holes. That would be a nightmare.

Al

[nash]

Al,

Many thanks for the tips. I'll keep you updated with what I decide and how.

all the best,
N

[svenakela]

I have a big MIG-welded table, and it's pretty tough making it straight and then you need to calculate for the heat bending.
I'm making a new table now and I'm using a good epoxy instead, it's a lot easier to make it good and I don't need the stress relieving.

[nash]

Svenakela,

All the solutions come with their own caveats!! MIG is doable, but I'd rather that a fabricator does that due to the weight, size and space needed (they all have cranes to swing a 600kg frame while I don't. I do agree with the residual-thermal deformation that will most definitely progressively distort the frame. However, heat treatment using either induction, full blown furnace, or a vibration based stress relieving can take care of it.

The usage of epoxy is also a very interesting option. Here the biggest issue is getting the surfaces rough enough and super-clean enough. Usage of HCl or other etching agent, while ok for the machine will be disastrous for the local waste water. I'd think a zillion times before doing that. Each joint will take its own sweet time. BTW anyone who thinks that cold-rolled steel tubes are free of residual stresses maybe needs to heat the tubes once and see the distortion.

Another Idea I had was to use compressed concrete prefab as the base foundation. Machined railmounts with embedded nuts and the whole thing potted using steel loaded epoxy on the concrete prefab. This would be simple no need for welded frames, and can be done by one person. Shrinkage? Warpage? Any ideas?

[nash]

Folks, Good news is that a friend of mine just gave me the name of a specialist heat treatment shop in northern Italy (which is one hour drive for me). They can do the stress-relieving of the structure, which involves soaking the structure in a temperature of 580degC, then letting the temperature drop slowly (in the oven itself). Sort of like switching off the oven and taking the part out the next day. I've asked for an offer donno when and if they will send me one... tinkerers are usually the last priority of such specialized shops

[nash]

Guess I was too thrilled to begin with, after finding the heat treatment shop. They want Euro 1800, that is over US$ 2300.

While awaiting another response from another place, if that too turns out to be this high, I'm going back to the drawing board and redesigning the YAG-M with epoxy and hex-screws.

Another quick question: If I drive the gantry with two servo's and two screws, how difficult will it be to maintain synchronization between the two? All those with two screw's driving the gantry, please own-up and speak-up, now is the time...

[MotorMaister]

Hi There!

I have a gantry type machine and I use two leadscrews to drive the Y axis.
My obsevation so far it is relaible .But one time a drive chain sprocket became loose and nearly destroyed my gentry because the other motor was still driving the Y axis.

Also I am using stepp&direction type servo drivers which prove to be relaiablefor this application.

If you are looking for such a high level of accuracy your structure has to be extrimely rigid.Even with rigid structure you may have considerable tool deflection unles you are machining something soft like foam or balsa wood.
At the time I started looking into designing my own cnc router was considering using conrete tables as the base for the machine .So one day went to a used building material yard .By sheer accident I saw this outdoor picnic table made out of concrete and best of all the surface was treated with some kind epoxy resin making it glass smooth.Even though never made use of the idea I still belive buying two of these concrete picnic tables would make about the size of machine you are about to build.That is my view.

[nash]

Cool idea. I was thinking on the same lines, going out to get a prestressed and factory cured concrete slabs for each of X axis rails (these are available from prefab companies), then laying of each rail using granite-epoxy or moglice on top of the concrete slab...

[JerryFlyGuy]

Guess I was too thrilled to begin with, after finding the heat treatment shop. They want Euro 1800, that is over US$ 2300.

While awaiting another response from another place, if that too turns out to be this high, I'm going back to the drawing board and redesigning the YAG-M with epoxy and hex-screws.

Another quick question: If I drive the gantry with two servo's and two screws, how difficult will it be to maintain synchronization between the two? All those with two screw's driving the gantry, please own-up and speak-up, now is the time...

Nash, the best way isn't always the cheapest.. thats for sure.. but then.. thats why its the best way

My system of screws and epoxy has worked great, I haven't put the epoxy in yet but the rest of it worked w/out a hitch. The epoxy can be expensive so just be prepared.. I've bought $320 of the stuff and thats just enough for one rail on my machine. There is a cost to making 'subplates' for the rails to mount to as well so look at the whole picture before making a decision.

Typcially w/ long machines of a gantry style. You drive either end of the gantry [usually w/ two motors, but it can be made to work w/ one depending on the size of your machine etc..], this can be done w/ most Machine control software sets [ Mach3 for example has a 'slave' function which makes both motors move together] You'll find that the longer the axis travel length the higher the cost is going to be, ie. a ballscrew that is 10' long is going to be VERY VERY expensive. You'll have to spec it for resistance to whip more than linear movement/control. If you are going this long I would sugest you check out a rack and pinion type system over a ball screw, they may not be as accurate initially but the variances can be tuned out of the system once its up and running by adjusting it and controling it in the machine control software. [ You teach it where its out and then in the future it will adjust at those locations by taking into consideration the error's in the rack]

You'll find that everything in machine design [well any type of design] is always, ALWAYS a compromise, and usually one side of that equation is cost vs ___??

Hope that helps..

Jerry [I recommend you take this part of your design process very slowly and methodically, you'll be glad you did once you get to the build process ]