Mini 3 Axis CNC scratch built

[Seraph]

Hi all.

I have finally decided that its not worth buying a mini mill just to spend an absolute fortune upgrading it to CNC, so after haunting this forum for a few months I finally got planning.

Plans so far include:
- Linear Rails
- Ground Ball screws (Ebay is my friend)
- A Sieg X2 spindle assembly
- Lots and lots of aluminium extrusion
- Servo driven (its going to be a pain I know)

I also plan to enclose it in polycarbonate sheet fixed to the frame (since its going inside my house and I like my walls and floor free from swarf )

Am I missing anything glaringly obvious? and does anyone have any idea where to get cheap aluminium extrusions from? spending £300+ on extrusions gives me a headache.

Nick

[Seraph]

Just starting out with the calculations to see if this concept will actually work in practice.

Its mainly going to be for machining Delrin and Aluminium, the spindle is a 1/2 HP affair that I hope to be able to run at about 2500 RPM (the belt drive conversion lets you go up to 4000 RPM but will probably have very little torque up there)

Work Area: X=320mm Y=150mm Z=200mm
Weight: works out at around 70kg excluding the servos (since i haven't got hold of them yet)

the X and Z axis works out at 24kg in moving parts

The linear Rails I have lying around are THK HSR20 (overkill I know but thats ebay for you) and the ballscrews I have around are 20mm Dia 5mm pitch C7 grade with a double ballnut (not the ground ones I originally thought of using but I just looked at the prices of those things new ) again probably overkill but they are sat doing nothing.

Also just had a small panic attack about the rigidity of the Z axis at its lowest point, 200mm of free floating rail and aluminium jig plate with a whacking great spindle on the end .

Nick

[Oldmanandhistoy]

Hi Nick,

Have you seen this thread http://www.cnczone.com/forums/showthread.php?t=34954 ?
Sounds to me that you and digits have a common goal and a quick post in his thread with a couple of questions may help you on your way.

Just out of interest have you considered buying a cheap mill like this one http://cgi.ebay.co.uk/XJ20-Milling-M...QQcmdZViewItem using the cast iron frame to mount linear slides, your ball screws and servo system? This could work out to be a better bet but I don’t know for sure with out costing out both possibilities.

John

[Seraph]

Whew, that thread made for some interesting reading I am trying for a much smaller machine but the principles seem pretty similar.

I hadn't considered building on a cast iron frame stripped from a mill although as a framework it doesn't really allow me the flexibility of design I wanted, still, time for some research.

You have my thanks Oldmanandhistoy some interesting ideas for me to ponder.

Nick

[digits]

Originally Posted by Seraph Just starting out with the calculations to see if this concept will actually work in practice. Its mainly going to be for machining Delrin and Aluminium, the spindle is a 1/2 HP affair that I hope to be able to run at about 2500 RPM (the belt drive conversion lets you go up to 4000 RPM but will probably have very little torque up there) Work Area: X=320mm Y=150mm Z=200mm Weight: works out at around 70kg excluding the servos (since i haven't got hold of them yet) the X and Z axis works out at 24kg in moving parts The linear Rails I have lying around are THK HSR20 (overkill I know but thats ebay for you) and the ballscrews I have around are 20mm Dia 5mm pitch C7 grade with a double ballnut (not the ground ones I originally thought of using but I just looked at the prices of those things new ) again probably overkill but they are sat doing nothing. Also just had a small panic attack about the rigidity of the Z axis at its lowest point, 200mm of free floating rail and aluminium jig plate with a whacking great spindle on the end . Nick

Hi and welcome to the mad house!

I'm no expert, and given that I'm nearly one year into my build, I wouldn't hold myself up as a guide to quick and easy builds, but I do have a couple of questions and/or suggestions for you.

What sort of performance are you looking for from this machine of yours? You have ballscrews and linear slides, and you say you are going for servos, all of which imply that you want to be cutting fairly rapidly. If this is the case, you probably want a 25,000 rpm spindle rather than 2,500!

I avoided a traditional gantry after scaring myself that it would be too flimsy - I then created a mad design with the whole X-axis moving up a pair of columns which was a complete disaster! A long way into my build, I happened on a web page which reminded me that in the optimal machine, the deflection of the tool tip is the limiting factor. So, if I were you, I'd work out what the machining force at your desired performace (depth/width of cut and feed rate) will be, and then calculate how much the tool will deflect. From that you can work out how little your want your machine to bend under that cutting force, and then how strong you need to build everything.

From what I've learnt about working with extrusion, the 200mm length of your spindle 'ram' is unlikely to deflect much, but it will cause a massive torsion on your X-axis. The THK rails won't bat an eyelid, and the force will be transferred straight to the tops of your X-axis supports. You will need to spend a fortune on brackets for your extrusion - you need multiple brackets per joint to make any kind of rigidity, and you are probably best off getting your extrusion supplied cut to size.

Hope that helps - feel free to tell me to butt out or to ask any more questions!

I do quite like John's suggestion of converting a big manual mill with ballscrews and rails, but where's the challenge in that

Good luck!

[Oldmanandhistoy]

Originally Posted by digits I do quite like John's suggestion of converting a big manual mill with ballscrews and rails, but where's the challenge in that

I suppose it’s what ever floats your boat ; if it’s a challenge you’re after then building from scratch is the way to go. If on the other hand you want the machine up and running to get cutting part asap then the upgrade of a cheap Chinese mill seems the logical approach.

Digits knows far better than me how much time, work and cost goes into his preferred approach and can tell you what you are getting yourself into.

One advantage of starting with a basic machine is that you have the machine to cut the parts needed to convert to CNC.

John

[digits]

Originally Posted by Oldmanandhistoy I suppose it’s what ever floats your boat ; if it’s a challenge you’re after then building from scratch is the way to go. If on the other hand you want the machine up and running to get cutting part asap then the upgrade of a cheap Chinese mill seems the logical approach. Digits knows far better than me how much time, work and cost goes into his preferred approach and can tell you what you are getting yourself into. One advantage of starting with a basic machine is that you have the machine to cut the parts needed to convert to CNC. John

Coming from a computing background, rather than a mechanical one, I would have been useless with a manual mill, unless it had a DRO - I can do coordinates, but I can't remember how much backlash a handwheel has, or how many times I've spun a handwheel in the last of 3,000 passes

With the benefit of hindsight, I might suggest buying 2 of those cheap but capable manual mills - use one to modify the other - (you will need a mill to build a mill IMHO) and then flog the manual one on eBay once your CNC one works

That said, I couldn't see any manual mills that did >200mm of Y-travel but weighed less than 300kg when I looked. This is the main reason I went DIY - but I'm not sure my mill will be under 300kg either! A year ago I had a dream about making my own CNC-milled parts - somehow I'm still making parts of a CNC mill!

I hope at least I can help Nick avoid making some of my mistakes!

[LeeWay]

I built a mill using basically the same type parts you have. Little bigger on the rails. Overkill. You betcha, but it sure does a nice job. No vibrations or anything like I thought I was going to have. I had intended to fill the extrusions, but that really wasn't necessary. I have just been taking fairly mild cuts in steel. maybe 1.5 mm deep. My spindle is running @ 3740 rpm.
You can take a look at the build thread here.
http://www.cnczone.com/forums/showthread.php?t=32609

Now it's not a gantry type like you are intending, but it is a pretty sturdy little beast.

[Seraph]

Originally Posted by Oldmanandhistoy I suppose it’s what ever floats your boat ; if it’s a challenge you’re after then building from scratch is the way to go. John

That was kind of the idea

I already own a Sieg X3 Super mill and a Sieg C3 Lathe for building the parts with.

Nice Machine LeeWay that spindle mod of yours has given me more ideas.

I was planning on starting off with a Sieg X2 mill head and using it to build parts for a spindle (this would be rather slow at 20 ipm max cutting speeds).

I did a bit more maths and worked out a 100ipm max for my wishlist (full Y travel in 3-4 seconds ) that gives me a 15,000+ rpm spindle for the sort of small tooling I want to use but that's quite a way in the future and I am terrible for doing that, thinking about step 10 while working on step 1 .

After even more maths I worked out that 45x45 gusset plates and 90x90mm plates for the X axis should give plenty of strength against flex according to Bosch the extrusions of the size and length I'm using are good enough for about 20 times the forces the mill should be capable of generating, however the rotational forces worry me a bit.

A very rough calculation of the force trying to rotate the X axis (with the super spindle) gave me 1500N, too much force for 4 Bosch gusset plates at 90x90mm but 100x100mm plates would give me 1920N resistance not much of a margin there, time for some reinforcement I think.

I definitely like the idea of getting extrusions cut to size digits, I didn't fancy trimming them down on my mill to size them properly Giant Mecchano

Phew, loads of info for me to digest, cheers guys.

Nick

[digits]

Originally Posted by Seraph That was kind of the idea I already own a Sieg X3 Super mill and a Sieg C3 Lathe for building the parts with. Nice Machine LeeWay that spindle mod of yours has given me more ideas. I was planning on starting off with a Sieg X2 mill head and using it to build parts for a spindle (this would be rather slow at 20 ipm max cutting speeds). I did a bit more maths and worked out a 100ipm max for my wishlist (full Y travel in 3-4 seconds ) that gives me a 15,000+ rpm spindle for the sort of small tooling I want to use but that's quite a way in the future and I am terrible for doing that, thinking about step 10 while working on step 1 . After even more maths I worked out that 45x45 gusset plates and 90x90mm plates for the X axis should give plenty of strength against flex according to Bosch the extrusions of the size and length I'm using are good enough for about 20 times the forces the mill should be capable of generating, however the rotational forces worry me a bit. A very rough calculation of the force trying to rotate the X axis (with the super spindle) gave me 1500N, too much force for 4 Bosch gusset plates at 90x90mm but 100x100mm plates would give me 1920N resistance not much of a margin there, time for some reinforcement I think. I definitely like the idea of getting extrusions cut to size digits, I didn't fancy trimming them down on my mill to size them properly Giant Mecchano Phew, loads of info for me to digest, cheers guys. Nick

What did you estimate your cutting force at to get 1500N, and do you really mean Nm if you're talking about a torque?

I estimated my cutting force to be about 200N - 6mm endmill 2mm deep in ally at 1200mm/min. Even with your 200mm spindle ram, that'd only be 200N * 0.2m = 40Nm.

Have you got a link to the Bosch bits you're talking about - are you buying from KJN? I deliberately have very,very few butt-joints in my design - the ends of the precut extrusion are flat to about +/- 0.5mm which is good, but not perfect. You can have them milled at extra cost - but I foolishly never enquired about that! Having broken a tapping tool and pulled my back trying to tap the 12mm holes in the end of the extrusion, I would stongly recommend you get them done for you when the extrusion is cut, or buy the magic self tapping fixings they sell.

There are some very strong brackets available for the extrusion - the ones I've used are supposedly good for 6000N each - i.e. you could hang 600kg off them, and they're only 80x80mm!

One other thing to keep in mind is that the forces generated by your screws and servos may be far larger than your cutting forces!

I'm not trying to scare you, they're just things to bear in mind.

Cheers.

[Seraph]

Ah, hence why I decided to post these things. Google clearly didn't help with its formulas, I just tried it again and got rather more conservative figures of 95Nm (yep I did mean Nm).

probably didn't help that I got my moments calculation for the ram with incorrect units in it *hangs head in shame*

OK so it looks like it works on paper, thanks to your help digits.

The Bosch extrusion figures I got where from their catalog http://www13.boschrexroth-us.com/alu...ingcatalog.asp

I have been looking at various places for extrusion, Marchant Dice being one of them.

Nick

[digits]

Originally Posted by Seraph Ah, hence why I decided to post these things. Google clearly didn't help with its formulas, I just tried it again and got rather more conservative figures of 95Nm (yep I did mean Nm). probably didn't help that I got my moments calculation for the ram with incorrect units in it *hangs head in shame* OK so it looks like it works on paper, thanks to your help digits. The Bosch extrusion figures I got where from their catalog http://www13.boschrexroth-us.com/alu...ingcatalog.asp I have been looking at various places for extrusion, Marchant Dice being one of them. Nick

I don't think Marchant Dice use Bosch extrusion - I think their supplier is Hepco, who let me down rather badly, stretching a 5 day delivery time to nearly a month! KJN Ltd use Bosch and were pretty efficient, even sending me on parts they didn't have in stock for free on a delivery date of my choosing.

The two profiles are compatible, but are different. I prefer the Hepco profile, but the Bosch fittings! You can ofcourse mix and match to some degree.

Cheers.