Arvid's first CNC machine!

[arvidb]

Hi all!

This is my first message to this list and I'd like to start with saying that this really is a nice place! I've been checking the board for new messages something like three times a day or so since I found it (Thanks balsaman for the link!)

I've had the thought of building some kind of CNC router or milling machine for some years now, but haven't had neither the space nor knowledge enough to feel ready to begin until now. During the last year or so I've been doing some construction (CAD) work at work, and also some manufacturing of parts on a small desktop manual mill/drill, and I've learnt a lot. Enough to feel ready to begin building!

This is what I had in mind:

* work area 600 mm x 420 mm x 100 (150?) mm (This is the same size as an A2 paper, nice if I want to plot large drawings or stuff like that. Also big enough for loudspeaker building
* repeatability better than 0.1 mm (preferrably <0.05 mm)
* ability to cut aluminium, wood, delrin etc

I've done quite some research and this is what I have found out (feel free to comment on this, i's all "hearsay", I haven't tried it):

The demand to cut aluminium is the controlling factor. It requires a machine with high rigidity and no backlash, even for lighter cuts. Wood and aluminium also requires different types of spindles, where the aluminium needs lower speed/higher torque unless the machine has the power and rigidity to sustain a really high feed rate.

Ok, so the machine has to be rigid. That doesn't really help :-/ ... I decided (by guessing) to aim for a machine that can take cutting forces of about 1000 N. This is what the linear guides, leadscrews and motors have to be able to handle. I have no idea of how much the MDF machine frame will flex under this load, or what kind of cuts I will be able to make, but we'll see

I don't have access to a machine shop, so the machine will be built out of MDF. To get the machine as rigid as possible, it will be a fixed bridge moving table machine.
After looking at different choices of linear guides, leadscrews, and motors, I think a reasonable price to aim for would be $1000 for these parts and the MDF.

Parts:

* X axis (600 mm) linear guides: I will use 20 mm dia, 1100 mm long guide shafts and linear ball bearings from INA for this one. The guide shafts are unsupported which is not good, but since I already have them I'll use them. I will support them at the middle in addition to the ends, which will limit my travel to about 500 mm, but I can live with that for now.
* Y axis (420 mm) linear guides: I bought real linear rails and slides (THK HSR25) from ebay for this one. Quite a lot of money, but very much fun I got them just a few days ago, and they're beautiful!
* Z axis (100 mm) linear guides: haven't decided yet, want to compare the linear shafts and the linear rails first.
* Leadscrews: I first thought I would use ACME lead screws with anti-backlash Acetal nuts from Ball Screws and Actuators (BS&A). The 16 mm version have a design load of 175 lbs (about 775 N), and the 20 mm ones are rated for 250 lbs (about 1110 N). I can buy these locally. However, it would be nice to find a used ball screw to play around with, at least for one of the axes. Probably the Y one, since it'll also have the nice linear rails I will use axial bearings and radial bushings (both from FAG) with these. This is the cheapest solution I've found that's backlash free. I found a local source for these as well.
* Motors: one design goal is 1000 N of cutting force. I want to be able to cut at 0.01 m/s at this force (guessing again). Lets also say I want a top speed of 0.05 m/s (about 120 IPM). 1000 N and 0.01 m/s gives 10 W of power at the table (P [W] = F [N] * v [m/s]).
To figure out what speed and torque I need, I used the following equations:

* n = v/s, where

n = motor speed [r/s, revs per second]
v = feed rate [m/s]
s = lead of screw [m/r, meter per rev]

and

* M = F * s/(2*pi) * 1/Eff, where

M = motor torque [Nm]
F = force at table [N]
Eff = efficiency of screw, about 0.4-0.5 for the BS&A ACME screws, and about 0.9 for a ball screw.

The lead of the BS&A screws are 4 mm/turn, or 0.004 meters/turn. This gives: n = 0.05/0.004 = 12.5 r/s = 750 RPM, and M = 1000 * 0.004/(2*pi) * 1/0.4 = about 1.6 Nm. Now, with a belt reduction of 2.5:1, this becomes 1875 RPM and 0.64 Nm.
(Hmm... 1875 RPM = angular velocity of 1875/60 * 2*pi = about 196 [rad/s]. 196 [rad/s] * 0.64 [Nm] = almost 126 watts! (Angular velocity [rad/s] * Torque [Nm] = Power [W].) But only 10 watts at the table! Where did all the power go? The answer is that I don't need maximum speed and maximum cutting force at the same time. I do however need a motor that can supply 0.64 Nm at low speeds, and have a top speed of at least 1875 RPM while still supplying some torque.)
After looking around a bit I found a source of used, 200 watt Yaskawa servo motors with encoders. These are WAY overkill (stepper motors would be better suited to this project, I think), but I have fallen in love with them and they're not too expensive. They will be very difficult to drive however since they are 3 phase AC motors. I will have to build a servo driver board for them, but I think I can do this (and if I succeed, perhaps I can start selling the drivers and actually make a profit out of this! :-D). They have a rated/peak torque of 0.637/1.91 Nm, and a rated/peak speed of 3000/4500 RPM. I ordered one motor a few days ago, and it'll be here in about a week if everything works out fine. I long to hold it, caress it ... *ahem* ... let's just say I'm eagerly awaiting it

I think that's it for now (wow, that's a lot of text! ) Attached is a CAD drawing of the machine frame. It's preliminary since I don't have all the parts yet, but should give you an idea. I used Pro/DESKTOP to draw it, which can be downloaded for FREE from www.ptc.com. You have to register (free) to get the activation key, though. The program has some annoying bugs and quirks but is really easy to work with. And did I mention it's free? :-D

Please feel free to comment.

Arvid

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[arvidb]

... and here's a pic of the linear guides. Aren't they beautiful?

Arvid

[Mr.Chips]

Arvidb,
Welcome to the CNC Zone. I'm a newbie too, and in the process of building mine. I'll let the resident experts give you advise, just wanted to say you have a good start and it looks pretty rigid, what with the box construction and bracing.
I purchased Turbocad but haven’t begun to use it. Will build mine from sketches.
Regards,
Hager

[machine ed]

Hi,
I'm a newbie to CNC also, some of the machines for "first timers" are REALLY making me humble!
ARVIDB, looks good!
I plan on starting as simply and cheaply as I can. I'm collecting linear bearings, shafting, motors, etc. but have a question. Has anyone used UHMWPE or Delrin for the linear bearing? (drilled and reamed for class 2 running fit?)
I'm drafting a 8-1/2" x 11" cutting area machine for my first and will post for critiquing when ready.
Thanks for the input in advance.
This is a wonderful place to learn, I'm thankful I found the Zone!
Take care,
Ed

[Mr.Ed]

Hi there, welcome and join the club.

What i was wondering is, you desided to use Linear slides on your Y-axis. Great idea, i'm still working on my first machine (without slides) and planning on using them on my second machine.

So why not using them on your x-axis as well?

Ed. (It's crowded with Ed's overhere)

[arvidb]

Thanks for the welcomes, folks!

Originally posted by Mr.Ed
Hi there, welcome and join the club.

What i was wondering is, you desided to use Linear slides on your Y-axis. Great idea, i'm still working on my first machine (without slides) and planning on using them on my second machine.

So why not using them on your x-axis as well?

Ed. (It's crowded with Ed's overhere)

The reason is cost - linear slides are expensive! I bought mine on eBay, and the shipping to Sweden was 30% of the total cost. Then add duty (some 4%) and VAT (25% in Sweden) and they get REALLY expensive! (Actually I haven't had to pay any duty or VAT yet, but I expect an invoice will show up soner or later...)

I have tried to find these kinds of things (ballscrews, linear slides, servo motors) here in Sweden, but it's not very easy, unfortunately. Actually, no luck in the whole of EU! Perhaps one would need to search in German, which I don't speak... Anyone's got any good European sources?

Arvid

[Mr.Ed]

Thats's my point !

I just bought one set of linear slides on ebay as well, and i have to pay 95 us dollars for shipment alone.

I opted for quality so there is nobody to blame but me, but still...

I have allso searched everywhere, and keep on ending up at the other side of the ocean.

I think this will encourage me even more to perform better and build to the best of my ability, trying to make something out of this.

Anyway, balsaman made me addicted to these slides, i will just blaim him.

Ed.

[balsaman]

Ahh, it's all my fault again is it? I was just minding my own business....



Eric

[arvidb]

After two weeks in transit, two used ballscrews landed on my doorstep (well, actually at my postal office, where I collected them )

I will use these for my Y and Z axes. Still have to order ACME lead screw for the X.

[arvidb]

Closeup of the Z axis one (about 130 mm of travel):

[arvidb]

... and a closeup of the angular contact ball bearings of the Y axis one. The nut that keeps them in place have been loosened and the bearings separated somewhat. I'm actually not really sure how to mount these, so if anyone have an idea...

[cncadmin]

You are not playing around with this bullet proof set-up are you?

[arvidb]

Bullet proof? What's bullet proof?

No, seriously, I paid less for these used ballscrews than I would have had to pay for new ACME screws with AB nuts. And they came complete with machined ends and bearings, too! One of those rare win-win situations .

// Arvid

[Mr.Chips]

How would the resolution on these screws be, since the TPI is fairly low?

[Bloy2004]

If you have a high ratio from motor to shaft would that keep the motor rpms up and increase resolution?

[balsaman]

Yep.

E

[arvidb]

Originally posted by Mr.Chips
How would the resolution on these screws be, since the TPI is fairly low?

Well, the Z axis one is 10 mm/turn and the Y axis is 15 mm/turn. The manual for the motors I hope I'll be able to use specifies the encoders as "2048 P/R", which should mean that there's either 4096 or 8192 counts/rev. And I'm going to use a timing belt with a ratio of between 2:1 and 3:1. So worst case resolution for the Y axis is below 2 microns (15 mm/(2*4096)), which should be enough, I think . Of course, with servos, I have to take into account that they might be off a certain number of counts, but hopefully they'll stay within +/- 10 counts.

Actually I had much rather used 5 mm/turn screws, not so much for resolution, but because of the higher cutting force. The servos would still have plenty of speed. But I guess I'll take what I can find...

// Arvid

[balsaman]

Your speed will be limited to what your pc can pump out. I think 35 khz is max for mach 2. Turbocnc maxes out around 25khz. 250-500 cpr encoders would be better. I think the G340 drive has a pulse devider for high count encoders.

Eric

[turmite]

Eric Mach2 can output 45k hrtz if the computer is up to it. I have finally got mine running and I cannot believe how much smoother the machine is than what it used to be with the full /half step system.

turmite

[arvidb]

Hi everybody! I hope you all are having a nice christmas!

I need some advice about timing pulleys. My motors have 14 mm axles, the screws have 10 mm axles. I want about a 2.5:1 reduction. Also, I need a type with low backlash.

Do you guys drill/turn your own bores in the pulleys? If so, how to do that without a lathe? And will a setscrew be enough to keep the pulleys from skidding on the axles? The axles are all smooth, no flats.

I haven't been able to find anything that would fit my axle dimensions and reduction ratio .

// Arvid