[Mike F]

I started a couple of threads on the 'DIY CNC Wood Routers' forum some time ago but now I've started building I realised I should be posting on this forum and Paul (CNCAdmin) suggested I start a new thread.

Those wishing to read the history of the project can look at:

http://www.cnczone.com/forums/showthread.php?t=2849
http://www.cnczone.com/forums/showthread.php?t=2971

To recap: I teach Design Technology in the UK and was responsible for introducing CNC into the school's curriculum some six years ago. We have a small (300mm x 250mm x 90mm) Isel Automation machine which is OK for small items and as an introduction tool for key stage 3 (11-14), but when it comes to GCSE and A level work it is somewhat lacking in the work area department - some of our students think big!

It was apparent we needed something bigger. However, the phrase " I want to spend big sums of money" and the word 'Bursar' do not sit comfortably together and the cash could not be made available to finance a machine outright and by the time the department had saved enough, I would be looking for early retirement. (Don't want to be teaching much past 55, as there are plenty of things I want to do with my time)

The solution is that I will build the machine and finance the major items and the school will have use of it till I retire at which time the machine retires with me to my workshop at home. Bursar's happy - I'm happy.

At this point, it must be said that I would not have contemplated this had it not been for CNCZone where I had lurked for some time before realising that it was possible to think the unthinkable. I will be leaning heavily on you guys over the coming months, particularly when it comes to the electrics.

Below is an unfinished CAD rendering of the proposed design and I will follow this with progress photos. This could be a long journey.


Mike

[Mike F]

Items bought to date:

2200mm THK Linear guides and blocks
700mm THK Linear guides and blocks
300mm THK Linear guides and blocks
2000mm THK C7 Precision rolled 25mm ballscrew & preloaded nut
600mm THK C7 Precision rolled 20mm ballscrew & preloaded nut
300mm THK C7 Precision rolled 16mm ballscrew & preloaded nut
Fixed and supported end bearings for above ballscrews.

That little lot from THK at a supposed educational discount was £2,500.00

Welded, 60mm x 40mm x 4mm box section, steel frame made by local firm £160
Facing, drilling and tapping of frame for linear guides £250 (essential for getting faces in the same plane and guides parallel)

Photos of the THK goodies and frame are on the other threads and will be updated here as and when progress is made. Next job is to mount the frame on the wall and get it trued.

[turmite]

Hi Mike,

My first question would be the possible cantilever problem when the axis that carries the z reaches it's greatest distance from the long axis. I am by no means a design person as far as machines go but I currently have a machine that quite a bit of overhang and has much more vibration than I care about.

Mike

[ynneb]

Sorry if i missed it in your writing, but is this to be mounted to a wall ?
If so, why?

I also agree with Turmite, you may get a sort of gate effect happening. I would recomend some serious bracing.

Good luck with your unique project Mike

[Mike F]

Termite & Ynneb,

Thanks for the posts.

Yes, the whole thing is attached to a wall. The idea is that, with limited space, the table can be removed, gantry parked at one end and the floor area used for other purposes. I know it's not an ideal arrangement but the need for a larger machine outweighed the other details. A bonus is that any size of material can be placed under the machine - I even had thoughts of students driving their cars under it and doing some serious artwork on the bonnets (trunks?)

I appreciate that the worst case is with the Y axis at its furthest point and that there might be cause for concern with movement but there are a couple of design details that minimise the problem. One, the Y axis motor, and hence its mass, is placed as close to the X axis as possible giving very little moment about the X axis. Two, the intention is to make the whole gantry out of carbon fibre. Composites is one of my interests and it struck me as an ideal material for this type of work as it has very low thermal expansion and is far more rigid than aluminium. Another plus is that the expansion ratios of steel (ballscrews and linear guides) and carbon are closer than steel and aluminium, the two materials most people are using.

I have built a computer controlled oven for baking pre-preg carbon and am confident I can make this work. My initial thoughts are going to be tested out using MDF but will then be replaced as the design gets verified. I know I will have problems with the MDF but at least I can test the structure.

If you have read the other threads you will be aware I have already got the frame made and X axis rails mounted. Just waiting to screw it to the wall now.

Mike

[buscht]

Mike, we have a commercial router at work. A Routech 220, made in Italy that is designed almost exactly like your machine. Its not mounted to a wall, but the concept is similar.
They have a wider spread on the X axis bearings for stability (horizontally, not vertically) and use a large tubular frame for the for cantilevered part.
I'm sure that your idea will work, just allow double, maybe triple your anticipated build and debug time.
I hope that you are going to add some support for your X axis ball screw. That thing will whip like crazy. If you can do it, maybe mount it stationary and spin the nut.

Keep us posted, as this is going to be really spectacular.
Trent

[Mike F]

Buscht,

Distance between rails on the X axis is 640mm and the distance between the blocks is 350mm. THK assure me that at this spacing, there should be no skew at all and so far, with the MDF board in place, there is no discernable movement.

I'm a bit concerned about the build time too, especially the electrics as this is definitely not my forte. Will have to lean heavily on you guys for info. I want to use servos but have no idea of size needed for this machine. Any thoughts?

Working volume should be about 1800mm x 600mm x 100mm.

As for the whip on the X axis ballscrew, if only I could find a way to mount everything in fresh air it would save an inordinate amount of time :-) Seriously, THK claim that with both ends supported the 25mm screw should be stiff enough.

Mike

[Mike F]

Things are progressing slowly. Problem with teaching is that pupils tend to get in the way of personal projects and progress can only be made while I am in school.

Managed to machine the stand-offs to go between the wall and the frame. This will allow me to finely tune the parallelism and trueness of the rails once mounted to the wall. Below are pictures of the stand-offs and CAD rendering of how they will be positioned and used. The nuts either side of the frame will allow very fine adjustment and although the whole leveling and truing process is likely to be lengthy, at least I know I can get it right.

[Mike F]

Well, by the skin of my teeth I have made my first deadline and that was to have the frame mounted on the wall by the end of this week - don't forget, I can only work on this project during school time, when not teaching.

The photos show the wall mountings and the frame in place. Not yet got it perfectly true but have thought of a good way of doing it. I am going to mount a laser line to the X axis plate and place an 8' x 4' board of MDF some 20' infront of the machine on which is drawn a perfectly straight line. As the X slide is moved along the rails the laser line should follow the line. With the board being so far infront of the machine, it will exagerate any misalignment of the rails. If the laser deviates from the line, the rails can be adjusted to bring it back on track. This method should true the rails up for parallelism and twist.

I'll report back as to how successful I am with this method.

Mike

Hope to start on the MDF test version of the gantry next week.

[buscht]

Mike, this is a fantastic project. Great job and thanks for sharing.
Trent

[ynneb]

I love alternative thinking like this. Thats how new inventions are formed.
Keep up the great work and the pictures.

[Mike F]

Buscht & Ynneb,

thanks for the encouragement, I am going to need plenty before this little beast is up and running. Still unsure about motor sizes for this machine. Any help will be most welcome. The intention is to use servos and with having linear guides and ballscrews all round, I am assuming I can get away with smaller motors i.e. less friction to overcome. I would like it to be capable of machining aluminium in light cuts - time not really an issue.

I keep hearing of 100, 200, 300 - 800 oz in motors but have no way of getting an idea of what this means in actual cutting and operating terms. For example, would the cheap servos, often mentioned on this site, be suitable? I believe Jeff Davies of HomeCNC still sells them.

Hope someone can help.

Thanks,

Mike

[arvidb]

Mike,

check the article I wrote (PDF) here: http://www.cnczone.com/forums/showthread.php?t=5866

It tells you how to calculate what linear force you can expect to get from your motors. Then there's another matter to know if that will be enough or not, but at least it should be easier to get a "feel" for than motor torque.

Arvid

[Mike F]

Arvid,

Thanks for that. I've read through your article and printed it for further digestion- bed time reading, I'm at that age now

It's interesting that you use the same screw pitch and speeds I am considering for my project so it was pretty easy to translate. However, I don't know what a cutting force of 200N equates to in terms of cutting speed, depth of cut and material. I appreciate there are many other variables too like spindle speed/power, tool diameter etc.

I am really quite worried about the electrics as it is something I have always fought shy of and I don't want to go to all the trouble of making a good, accurate machine only to make an expensive mistake with the motors/drivers and all. Having said that, I do have some local expertise and I am sure a call to the zoners will bring the right results.

Many thanks again,

Mike

[sol]

For ballpark comparison only ....
My table uses Jeff's -Homecnc servos, the 430 oz in jobbers, with Gecko 320s on a 2700 mm x 1270 mm table, they work fine; the rack and pinion X axis with a 80 lb (36 kg) gantry does 2500 mm/min rapids; the Y with cheesy Allthread (temporary ) will do about half that speed. Still testing the limits for cutting speeds, but relatively aggressive cutting in wood can move at 700 mm/min.
Ninewgt sells plans for a 4' x 8' table he made that uses 470 oz in steppers, I think ...Says it is all he needs...
Today I was rewiring the control box into something less rat's nesty and was struck by how long it took to do it the first time and how straight forward it seems the second time. So anyway no need to worry about the electrics...lots of helpful souls here.

Nice project you have going there!

[Mike F]

Sol,

Thanks very much for the info - quite reassuring. It is apparent that the quality of the drive system plays a large part in motor selection. Your reply has definitely given me a clearer picture of what I should be looking for - I am more of a hands-on type, empirical designer/worker and sometimes have difficulty working purely theoretically. Once I can visualise something, the numbers become much more meaningful.

[Mike F]

Managed to get round to trying the laser line technique for truing the rails. The photos show how it was done. The close up of the pencil marks on the board show how far out the top rail was to begin with - there was some 10mm difference from top to bottom. By placing the board well away from the rail, it exaggerated any discrepancy. In fact the amount the rail was out was immeasurable by any other means.

The rail was then adjusted till there was no discernible deviation of the laser line. The full board was then placed on the rails and was further tested and everything was spot on. By using just the one block to begin with, I was able to find larger errors, as the distance between two blocks tends to even out some of the minor errors.

The next job is to check the two rails for perpendicularity and I will do this with the laser pointing away, perpendicular to the frame, aimed at a board some 6 metres away.

More anon,

Mike

[InventIt]

Good use of the laser line. Smart thinkin'

Frame work looks good, nice "factory" looking paint job too. No one will know you built this yourself

[Mike F]

What a bummer! Once I had trued the top rail, I began checking the bottom one and to my horror the two top surfaces of the rails were not in the same plane - this despite them being supposedly trued on a large CNC machine, for which I paid a not inconsiderable sum! Placing a straight edge on the top rail I measured a 3mm gap between the straight edge and the bottom rail surface, with the rails being 640mm between centres.

Problem, how do I fix this? I decided the error was too small to shim behind the rails so eventually opted to shim the blocks. A few calculations later and it was realised that I needed shims that tapered 0.115mm over the 50mm block width. I pressed our little Isel machine into service and produced the four shims as seen in the photos. Now when the board is mounted, it is true - job sorted!

Shame it had to be done though.

[mícheál]

What are the dimensions of the THK rails you bought? I am looking at a set of rails 1300 mm long, x 20 mm x 15 mm.

Mícheál.