Looking for information to help buy my first CNC machine

[JAZZCNC]

Perhaps the panel would like to express their view on how these machines would handle Sika modelling board. I'll be more than happy to exchange notes with the original poster as I'm seriously thinking about one of these machines.

It won't have any trouble at all handling modeling board from a mechanical strength point. Where it will struggle is with obtaining the correct feed rate to give best results IE Finish and tool life with some materials.

With only a 5mm pitch screws and I would hazard guess an under sized power supply along with crappy drives mixed and compounded with other aspects of the design and build quality then you'll be under 4mtr/min of usable cutting speed (Not to be confused with rapid feed). Which for most plastics and softer materials, even most woods, would be considered on the low side for feed rate. Cutting too slow will give a poor finish resulting in edge burning on some materials like balsa wood or thin ply plus greatly shorten the tool life.

This is where so meny new folks fall foul and don't see the potential problems with machines like this.! They think because they only want to cutt soft materials it won't be a problem.?
The reallity is that soft materials often need high feed rates for reasons stated earlier and these machines struggle to run at these high feed rates.
If pushed to run at there max they wear out very quickly drasticly reducing the machines life. The quality and accurecy is reduced because the poor gantry design can't handle the high lateral inertial loads without flexing and basicly they are just not built or spec'd to do the high feeds required.

My experience is that those that buy this type of machine while at first think they are ok but soon start to realise there down sides resutling in several different out comes.
1: Struggle with it, working around it's limits and bad points untill it becomes so annoying or restrictive they build/buy there dream machine.
2: Struggle with it, working around it's limits and bad points untill it becomes so annoying or restrictive they try to upgrade it and blow even more money trying to turn a sows ear into silk purse. (never achieved)
3: Instantly see it's bad points and get rid (Often they've had a little cnc experience before but got caught trying to take the quick cheap upgrade route)
4: Don't realise and just think cnc rubbish and either go back to the old way of doing it or give up on it all together.
5: Don't realise blissfully unaware producing crappy work and burning cutters up thinking thats just the way it is.!! . . .Untill one day they see a good machine strut it's stuff and then penny drops or they realise it's not them doing something wrong.

In most all cases they are well pissed off in the end.!!

[boldford]

Your obviously unimpressed with these far eastern machines. What would you recommend for a similar sized machine where the budget is limited

[JAZZCNC]

Your obviously unimpressed with these far eastern machines. What would you recommend for a similar sized machine where the budget is limited

Argh this is the problem and why these chinese machines have appeal.? . . .It's very difficult, infact I'd say not possible, in this country or EC to buy a machine off the shelf that's specd the same but built properly without the weak area's for the same amount.

This is why so meny build there own machines, DIY is about the only way you'll get near a correctly built machine with a similiar spec for around the same money, even then it will be close call sub £1500 if you want all the trimmings like 4th Axis.!

[Jonathan]

This is why so meny build there own machines, DIY is about the only way you'll get near a correctly built machine with a similiar spec for around the same money, even then it will be close call sub £1500 if you want all the trimmings like 4th Axis.!

That's what I was about to say. The only way is to either do the research and DIY, or give someone who knows what they're a clear specification and get them to design and make it for you. I think DIY is the best option as in making the machine you'll learn more about how it behaves and what to look out for.

Down sides is that it requires very accurate cutting (but that is a common to all materials); it is required to be coated with something to stop oxidation from getting a start;

Cutting accuracy depends on how you're joining it. If it's just welded then you can make sure it's clamped accurately and rigidly whilst you weld and small gaps can be filled. For my frame I started off milling all the ends nice and square and milling a chamfer on. Then when I started welding it I realised it's a bit of a waste of time so I just cut it with my metal bandsaw (same as Jazz's), which at the time was cutting quite square and nipped the burrs off. Either way it helps a lot to have a milling machine and metal bandsaw. Frame cost me about the same as steppers and drivers.

[m.marino]

Options for lower budget Machines include:

Steel box section as while weight increases which makes moving the beastie a problem the rigidity also increases which aids in accuracy.
Down sides is that it requires very accurate cutting (but that is a common to all materials); it is required to be coated with something to stop oxidation from getting a start; may require welding which then brings it's own problems.
Up sides is the weight it brings for a base helps decrease vibration massively; has a low expansion rate to temperature changes (not non just low); With a little bit of looking and shopping around can be gotten for a lot less then profile or Ali' box section.

Recycled Profile, IF you can find some one or some where that deals in break down of machines that are past their useful lifespan you can get good profile on a discount. Upside it is profile which is light compared to steel and yet retains the rigidity needed for repeatability/accuracy.
Down side is that it will most likely have some work hardening in the section and the will be drill marks and such. Which mean you have to be a bit careful in choosing what you want to work with.

Also starting with an air cooled spindle like a Kress or something a bit better (again look around for folks selling on equipment in good condition). That or save up and get a water cooled unit though realize a good a good VFD is not cheap nor the proper wiring and in line filters to have it work at it's best.

Depending on what you are doing with it Trapezoid Screws with spring loaded anti backlash nuts can be as accurate as you might need. This is where a serious sit down and looking at what you are going to be doing in real world here and now terms can help massively. It is why my second machine that Jazz is building is custom and built towards a goal of specific usage. The frame and structure of a machine is from what I have experienced only about 40 to 50% of the overall cost. Steppers/Servos, drivers, BOB, computer, spindle, vacuum for cleaning, and many more add up quickly and from personal experience are equally important.

Another option entirely is start haunting the equipment auctions and chatting with a few folks here who know where there are auctions of equipment. Go and look a few times and get a feel of what you are looking for. See if one the folks here is heading to one that you can go with. You would be surprised what you can get used industrial machines for at times. Depending on what you plan on doing, refurbishing an older machine might give you an affordable answer to your goal. Yes most of them are heavy as heck and will need work on them. Look around and talk to folks here and at model engineer swap meets and other places the similar minded people meet (robot war groups come to mind) and you might fins what you want exactly or so close that it does not matter.

Good luck and keep asking questions.

Michael