New Machine Build- Designing an 8' by 4' machine - need some wisdom!

[elitechicken]

Hello Chaps,

I've lurked here a long time, and have tried my best to soak up the knowledge which is freely shared. I don't normally post on web forums but this forum comes across as very supportive and well meaning.

Since late last year, having read the excellent MadVac webpage and being thoroughly inspired, I have set out trying to acquire the skills needed to build a capable 8x4 machine for cutting MDF and ply sheet. I had a 6 month false start where I tried to do it on the cheap, using 10' scaffold rails as my linear rails and a frame made from scaffold, rolling on small bearings. I got as close as moving the various axes with Mach3 before realising that I would never be satisfied with such a machine and immediately went back to the drawing board.

Attached are some work-in-progress images of my design so far. The steel tube is 120x60x4mm on the long axis, 60x60x3 on the "end assemblies" and 80x40x3 on the gantry. The Zaxis is two sheets of 15mm aluminium. The X and Y linear bearings are 25mm profile type rails, and the Z axis uses unsupported 25mm linear rails. The CAD model uses parts modelled from the dimensions in the datasheets.

The motors used will Nema34 frame stepper motors: 12Nm on the long axis and 8Nm on the other two.

The ballscrews will be 25mm diameter 10mm lead on the two longer axes and 16mm diameter 5mm lead on the Z.

The questions I'd like to ask you are:

* Will my frame assembly be rigid enough using the flanged and bolted together system shown?

* Is having the cutting table and the long rails on different steel frames going to cause me problems with sag in the middle? What is the optimal frame arrangement? I see a lot of machines such as the shopbots using the rails on the same structure which supports the cutting table and wonder if this is perhaps better, or is this just a better arrangement for rank and pinion systems?

* Can the accuracy required for such a build be achieve using a very agricultural pillar drill, a cut-off band saw and a file? I've struggled thus far to drill holes to within 0.5mm of my intended mark and would really welcome some tips on "best practices" when it comes to drilling and marking.

* Do you think my long axis ballscrew is substantial enough at 25mm D?

* What is the best way of attaching the profile rail bearings to the gantry? Does some interface piece as shown have to be used to get around the fact they bolt in from the top, but are going on the underside of the gantry..

* On the Z axis, I'm using unsupported round rails mostly for cost reasons, but also because I read somewhere that supported round rails shouldn't be used with forces as they are on a Z axis. Will 25mm rails be meaty enough? I'm also concerned that the whole weight of the spindle and Zaxis will be supported by 8 M6 bolts going into tapped aluminium, if I use the SK style mountings as shown.

* In the MadVac machine, Vaclav goes to extraordinary lengths to ensure the flatness of his steel before mounting his profile rails to it. As I'll be using rolled steel, I was simply going to "fill the valley" with epoxy. What approach would you recommend?

I have a lot of other questions, but I think that is enough to be going on with for now! Please bear in mind that I don't have any great mechanical experience being quite young and an electronics graduate in a software job, but I am very eager to learn.

Thanks a lot
Joe

[RGeo]

My first thoughts are: looks like a fairly nice machine, why not use servos instead of steppers? Also, I would opt to hide the linear guides underneath the framework, this helps eliminate buildup. I am assuming this machine is a router? One other thing, you can use floating rails to support your ball-screws intermittently, along the way. It’s really nothing more than a bushing attached to a rail that your axis drags along. Actually, there may be a better solution, although I have no personal experience with them, I have seen articles about roller pinions, I’ve included a link. I’m sure there are some articles on this site that have discussed them as well. Just looks like something to look into. Long ball screws doing work over long open spans tend to deflect a bit.

http://news.thomasnet.com/fullstory/...rnative-460795

[ger21]

First, I think your in way over your head.

I think you'd be better off building a Mechmate, but I'll give some advice here.

For the Z axis, I'd recommend 15mmprofile rails. Much less weight, more compact, and far more rigid.

To answer your questions?

Rigid enough? Probably depends on your fabrication skills.

Will it sag? It'll definitely have some flex to it. I wouldn't want an unsupported table that long.

Accuracy of the build? Again, depends on your skills. Some really skilled can do much better work with bad tools than someone with no skill and good tools.

Ballscrew? Depends how fast you want to spin it, and how it's mounted.

Attaching blocks. You can get flanged blocks that will bolt from the bottom.

Fill the valley with epoxy? Your assuming they are straight. They probably will have some twist, and filling with epoxy is probably not as easy as you think.

[Benonymous]

It would be useful to know the type of work you intend to do with this machine. When we know that, we can frame our advice to fit.

Honestly, I'd suggest a smaller machine for your first time but that said, here's my advice.

I'd probably go along with the other responses. That length screw will whip after you get bored with running the machine at very low feed rates. The gantry looks to have a nice wide base so that might prevent racking but with a big whippy screw, that's still a possibility. As far as the flange/bolt construction goes, I'd say if you can jig and weld the flanges accurately it would be OK. I'm planning to use bolts and brackets for all my frame and gantry joints, no welding at all.

The main improvement I'd make would be a rack and pinion system for the X drive.

[elitechicken]

Thanks for your replies. I agree that I'm probably biting off more than I can chew, but I like a challenge.

I agree with the suggestions to use rack and pinion on the X axis. I've designed it so far to use a ballscrew because its simple, but theres no point going to this length to build a machine like this to then cripple it with a low max speed. However, it would appear than rack and pinion systems such as the ones from Marchant Dice will cost me a lot more than the ballscrew. Do you need to have a motor/rack/pinion on both sides of the gantry?

A supported ballscrew seems attractive as I can stay in the ballscrew "comfort zone". I may try to come up with some homebrew sprung system which retracts when the gantry comes past.

I'm using steppers because I have a couple of suitable ones already, and have some geckos knocking about, but should steppers be unsuitable, hopefully the NEMA34 mounting will let me use servos without too much work.

The machine is going to be used to cut stations for building boats, a bit of furniture out of MDF, and also for prototyping for ideas I get. Ideally I'd like to be able to machine aluminium too. My budget is about £4000-5000.

This evening I shall refactor the design to use the rack and pinion as listed here http://www.worldofcnc.com/main.asp?c...ack+%26+Pinion. I'll also try to support the table a bit better and I'll post the results later.

Do you think for the aluminium pieces in the Z axis it would be worthwhile outsourcing this to be made on CNC for me? Would it work out cheaper in the long run to invest in a manual mill and learn how to use it?

[rowbare]

You might want to look at this thread for inspiration: http://www.cnczone.com/forums/showthread.php?t=30751

bob

[RGeo]

When cutting aluminum you will have more tool pressure then if you are cutting say MDF, so given the size of your machine you will deflect too much to do machined aluminum parts any real justice.
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As for dual rack and pinions, you can usually use one motor driving a shaft which is common to both rack and pinions. One pinion will need to be rotationally, adjustable for alignment purposes.
/
Concerning the location of your rails, I would bottom mount them. It simply helps reduce contamination.
/
Also, the firm and proper mounting of linear guides is paramount to proper operation and accurate finished parts. The structural members used for your frame will not be straight enough to use as a reference. Without the proper alignment equipment, you are at the mercy of the materials, unless you know some tricks. I used to build / reassemble large precision machines in the field and build new machines in the factory and I could likely come up with a modified procedure to help you come up with a straight machine. I would begin by mounting the rails to cold roll steel and bolting the cold roll to the structural member, add some threaded holes to put some set screws and you have a surface that you can straighten mechanically.
/
Additionally, I know that the high end controllers, i.e. Fanuc, Siemens, Mitsubishi, etc have dual axis compensation, however, I am not familiar enough with the lower end controls to say which ones do. Dual axis compensation allows the machine to actively offset the position of one axis, in response to the position of another axis. In other words, if my X,Y is skewed it can be straightened by parameter rather than in the physical world. It very helpful, but it’s not a magic bullet. There is no real substitute for a geometrically aligned machine. Anyway, consider this feature when purchasing a control or software for your PC control.

[elitechicken]

rowbare - thank you for that link. I've been trawling this forum for build threads such as that and didn't read that one but it is exceptionally informative and has forced me to rethink a little bit.

RGeo - I hadn't considered a method like your idea for mechanically straightening the long rolled steel box section but if i continue down this route with this machine, I will employ a system like that. I'd be very grateful for any further ideas you have! Also, what job did you have where you were building precision equipment in the field? That sounds quite an interesting career.

I think before going off full speed down the full size 8x4 machine, I'm going to make an MDF machine as tight as I can so that I can mill out some alu parts for this one. I've been considering how I can get a precise Z axis assembly and have come to the conclusion I would either have to outsource it or acquire some significant tooling to do the job.

[Al_The_Man]

Originally Posted by RGeo Additionally, I know that the high end controllers, i.e. Fanuc, Siemens, Mitsubishi, etc have dual axis compensation, however, I am not familiar enough with the lower end controls to say which ones do.

With Mach/Galil control and servo's, the electronic gearing command can be used to synchronize two X axis servo motors, one as a master the other slaved off it.
Al.

[RGeo]

Al, what I'm talking about is more on the control software side; the dual axis compensation function / procedure creates a matrix map of the plane you are working with and offsets the axes based on this data. For example, even when you are simply making a move in X, all by itself, and you were watching you position page, Y would obviously stay stationary; however, with this function, if you looked at the servo page, Y would be making minute corrections, depending where X is, thereby correcting for deviations in geometry relative to XY.

[ger21]

Mach3 can do that, but the correction would be linear. For instance, Y can increase .01mm for each 100mm that X travels.

It can also square a gantry by homing each side independently to separate switches.

[RGeo]

ger21, that's pretty much what I'm talking about, although Mazak, Mitsubishi, Funuc, and I believe Siemens all have something that allows you to correct in + and - directions. There is basically a password hidden area that has a spread sheet that you fill in, if you want to do just a linear adjustment, like you are talking about, you need only three rows of data, consisting of an X position and a Y compensation which is either + or -. For correcting a not so linear, linear axis, you would compensate every so often, most people use the ball screw thread lead, so maybe every 10mm. You can do all of the axes against each other and have the machine read as perfect as you want. However, I am a strong believer in getting the actual geometry correct first, then using this function just to tweak a few tenths, but you can much further than that if you want to cheat. The problem I have with relying on this function is issues with tool angle. So I see no serious negatives on machines with really short Z-axes, such as plasma, laser, punch press, and even some routers, but bad geometries on something with substantial travels in all axes results in a volumetrically bad machine, even with this type of advanced compensation. I guess my point being, I would want my control to be capable of this, but I’m still going to build and adjust for proper alignment for days, if necessary, before I resort to using the function.