CNC Design and advice

[Mad Professor]

Good day all.

Due to lack of funds, I have decided to look at making my own smallish cnc unit mainly from MDF, unto such point that I can afford otherwise.

The unit is going to be mainly used for milling and drilling PCB boards, but I would also be using it for Acrylic, and maybe small alu jobs.

I already have 4x Astrosyn MY103H702 stepper motors, 1x 4 Axis TB6560 stepper motor driver board, 1x 24volt 12amp switch mode power supply.

I have been trying to come up with a design using solidworks, and here is the current design.


http://www.mad-professor.co.uk/Misc/DIY%20CNC%20Mill.jpg
I have only been using solidworks for just over a week now, so please bear with me.

The design is by no means finished, and is subject to change depending on what feedback I get.

When I 1st started designing this unit, it was meant to be quite small, but the more I design the bigger it has got.

Any advice you can give me to help finalize the design would be grate.

Thanks for your time.

Best Regards.

[Jonathan]

How much travel are you hoping to have on the Z-axis? I think the spacing of the linear bearings on the Z axis looks a bit close at the moment. I'd extend those triangular pieces on the side of the Z-axis up to the top of that sheet - it should stiffen it a lot. If you don't then I think that the sheet of MDF will flex too much, resulting in poor accuracy.

What pitch screws are you intending to use? You'll struggle to get a good feedrate with those motors on only 24v. Their inductance is too high. They work well on 70V though...

Here's a drawing in solid edge of the motor you're using...might find it useful! You should be able to open it in solid works, if not let me know and I'll convert it.

3119

[blackburn mark]

looks a bit tall and flimsy on the gantry sides... if your making a moving bed you can afford to beef up those gantry sides
also id be tempted to look at puting the spindle over the centre of the X axis so the bed is only needing to travel half its own lenght either way (if that makes any sense) besides that it looks like a classic design and that seems to be the way to make a winner

[blackburn mark]

bearings on the Z axis looks a bit close

not sure i agree jonathan,,, they look about right to me... to far out and the middle will sag under pressure


edit: sorry jonathan i thought you meant the X axis,,, im with you on the Z axis

[ecat]

As far as small and simple goes this is the best I think I've ever seen...
http://www.cnczone.com/forums/showthread.php?t=109390

probably not up to Ali but maybe you can borrow some ideas :)

[Jonathan]

That looks nice. I was about to say it looks like a very sturdy machine...then scrolled down and realised the sides are nicely painted MDF not aluminium!
I bet that would be good for milling aluminium if the MDF was replaced with aluminium/steel and shouldn't cost too much more.

[irving2008]

I would suggest. for a first machine and primarily PCB milling/drilling to keep the z-motion quite small... no more than 50 - 75mm .. This design looks a tad top-heavy. What are the overall dimensions?

[Mad Professor]

Jonathan: thank you for the stepper motor files, I will use that to update my next rev of the design.

As I am going to be using the TB6560 stepper motor driver board, <30volts is a must otherwise you end up smoking the board and or chips, so I have settled for 24volts.

Speed would be nice, but for now just a working unit will be grate.

If can afford it I will be using balls screws and nuts from Gary, but otherwise it would have to be threaded bar M10 or M12.

There is an error with the Z-Axis in that 1st design, The Z-Axis is meant to be 300mm with 150mm of travel.

Here is the updated version (Rev1.1)
http://www.mad-professor.co.uk/Misc/DIY%20CNC%20PCB%20Mill%20(Rev1.1).jpg

[irving2008]

rather than use M10 or M12 threaded rod, use TR12x3 trapezoidal. its not much more expensive and will give you 2.5x performance as M12 threaded is only a 1.25mm pitch. Still think the Z travel too much..

What are you proposing for the table to run on? and how bigs the table?

[Jonathan]

rather than use M10 or M12 threaded rod, use TR12x3 trapezoidal. its not much more expensive and will give you 2.5x performance as M12 threaded is only a 1.25mm pitch. Still think the Z travel too much..


M12 is 1.75mm pitch...still I agree that's too small if you're not using pulleys to compensate.

Agreed on the Z travel being too much. Maybe you could make just the Z axis using aluminium plate? It wouldn't cost too much more....What do you need to make with 150mm of travel.

This reminds me of everyone trying to persuade me that 400mm is too much travel on my Z-axis! But I'm using 20mm aluminium plate and supported rails now so it should be ok....

[irving2008]

with these motors he'll need to keep the loads down... MDF is fine, just dont expect too much, and limiting the z-travel to say 75mm is still good enough for a wide variety of jobs. PCB milling/drilling only needs 10 - 20mm of travel... just enough to get the bit into the collett....

[GeorgeD]

Hi MP

Take a look here,you can't go far wrong getting some ideas off this design,even though its in mild steelbox section,its a similair design to your own and is worth considering changing the material for its construction.
A welder will or might charge you somewhere in the region of £50 to £70 to weld it together? providing allthe cutting is done by yourself and a drawing plan submitted.
http://www.cooperman.talktalk.net/

Irving,if I was to buy the trapozoidal TR16x4? what pitch and threads per inch would I have to set on the mini lathe? I have not mastered this side of the lathe yet in terms of screw threading but will have a go.

Sorry forgot to mention....its for the ballnuts internal threading.

[Jonathan]

Is a steel frame really necessary. Since it's a moving bed design you can make the frame with as much MDF/wood as you want really and compensate for the strength that way. £50-70 buys you a lot of timber.



Irving,if I was to buy the trapozoidal TR16x4? what pitch and threads per inch would I have to set on the mini lathe? I have not mastered this side of the lathe yet in terms of screw threading but will have a go.

Er, are you thinking of threadcutting a nut for that on the lathe? If so that's 4mm pitch, so 25.4/4=6.35TPI ... awkward. If your lathe can do that then youll need to either use a tool with 4 points, or use the compound slide to advance the tool 1/4 of the lead (i.e. 1mm) across to do the next thread etc. That assumes the compound slide is at 0°, if not you'll need to work out the right amount to advance it.

Can't you just make a tap from a bit of spare trapezoidal rod? Standard way with milling a few slots, turning a taper and harden it? Maybe I've missed the point completely!

[Jonathan]

Argh sorry I completely misread the 'TR16x4'. It's just a single start thread. In that case you just need to cut it as normal with 4mm pitch (6.35TIP).

[John S]

George,
As Jonathan has said 4mm pitch but depending on what gears your lathe came with and leadscrew you may have to get some conversion gears for the screwcutting train.

You mention Trapezoidal thread TR16 x 4 then mention it's for the ball nuts internal threading, two different animals.

[Mad Professor]

I have never heard of trapezoidal thread before.

Just had a quick look on eBay the trapezoidal threaded rod prices seem good, but there seems to be only one person sellings the trapezoidal nuts, the ones he is selling are made from bronze, and to me seem quite pricey being £40 for three 12x3, I don't know if that is the going rate.

Besides eBay does anyone know any websites for where I can buy trapezoidal threaded rod & nuts?

The PCB drill bits i use are 40mm long from end to end, and the working end is 10mm long.

I went for 150mm of travel on the Z-Axis as I did not want to have to rebuild it again if I needed to work on a bigger job.

I know irving2008 keeps saying 75mm or there abouts for the z-axis travel, so how about going to 100mm of travel sound?

[irving2008]

MP,

Dont buy ready made nuts, they are expensive. buy an extra 100mm of the rod and make a tap as Jonathan has suggested and cut the threads in some delrin (engineering plastic). It will do for what you want. You don't need to harden the tap for plastic.

As to the steel frame, its massive overkill for a first tryout machine. Also those motors/driver wont be up to the job. My recommendation is keep it small and simple, you can do a lot on a small simple machine as long as you plan carefully and dont expect miracles.

[GeorgeD]

As to the steel frame, its massive overkill for a first tryout machine.

I don't understand the logicinthat statement,Iriving.

I'm assuming you're stating.....build it in MDF then find out the pitfalls and gather some experience from the MDF build then apply it to a more sturdy material in the future?

Isn't that a more expensive way of looking at it than doing it in a more sturdy material from the start? I myself looked and thought about the material I was going to use first,MDF didn't even get a look in and so it was either Perspex,Aluminium or MS box section.

Perspex was not suitable so that left Ally or MS,Ally was far too expensive on a budget so MS was the deciding material for me.

[irving2008]

I don't understand the logicinthat statement,Iriving.

I'm assuming you're stating.....build it in MDF then find out the pitfalls and gather some experience from the MDF build then apply it to a more sturdy material in the future?

Isn't that a more expensive way of looking at it than doing it in a more sturdy material from the start? I myself looked and thought about the material I was going to use first,MDF didn't even get a look in and so it was either Perspex,Aluminium or MS box section.

Perspex was not suitable so that left Ally or MS,Ally was far too expensive on a budget so MS was the deciding material for me.

George, you're missing the point... he's already got small motors, and the overall size of that machine is maybe 400 x 400mm. Its too small to justify/need a steel construction and MDF will be quite sturdy enough for his needs of PCB milling/drilling. Plus to do a steel frame needs much more complex equipment than MDF and MP already discussed with me elsewhere about the difficulty of accurately cutting MDF with the tools to hand so I am making a fair assumption that steel is beyond his capabilities right now. Unlike you George it would appear MP isnt in a position to go out an buy a lathe, welder, etc. as you did, so lets get him up and running as simply as possible.... so lets get this back on topic as regards MPs design for an MDF-based machine OK?

[Mad Professor]

I have done a few little tweeks to the design.

(Rev 1.3)
http://www.mad-professor.co.uk/Misc/DIY%20CNC%20PCB%20Mill%20(Rev1.3).jpg

It is no longer quite so top heavy.

I would be nice to build a cnc unit out of steel or alu strate from the word go, but this is not possible for me, this is why I am going down the MDF route.

I am still very new to the world of CNC, so I have alot of learning to do.

I have been out of a job for allmost two years now, so very little money can be spent on projects.

I don't have a workshop full of tools, like lathes ect.

The only sort of tools I own at this point in time, are power drills, piller drill, small belt sander, jigsaw, and hand saws.

I know making the cnc unit out of MDF is not going to be that strong.

I am not looking at a super fast unit or anything like this, I just want to get on the 1st step of the ladder, and go from there.

So unless someone is going to offer to make my design out of steel or alu for next to nothing, I think I will stick with MDF for now.

[John S]

No problem with that.
I have seen some very nice work done on MDF routers and once you have a working machine it's far easier to make the MKII

[Mad Professor]

My current design is based around 12mm thick MDF.

I have just been around to my local Sydenhams, and they stock MDF in the following thickness 9, 12, 15, 18, 22mm, they can also order in 25 and 30mm.

What thickness would you advice me to build the unit out of?

Thanks for your time.

[John S]

18mm, it's only half as thick again and twice as strong.

[Robin Hewitt]

Confidence is that feeling you get in the design stage. Suggest you get a bit of MDF, stick it in the vice and try bending it, you will find it goes easily one way and hard in the other. Right angle joints are your friends.

You started out designing a machine to cut PCB's and then, with an eye to the future, you enlarged it so you could make saucepans. You now have something that will do neither.

Strongly suggest you scale it back to cut chocolate box sizes. It should look chunky, the only thing on it that should look spindly is the spindle :beer: