CNC Design and advice

[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)


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.