Thread: Making a new CNC machine

Hi Bob,
If you are getting 3 to 4m/min on good acceleration I would consider it a done deal. If you want speed from a large heavy gantry then use pulleys or R&P to get a gear advantage. A 25mm ballscrew has a lot more mass than a 16mm screw so there is that to consider as well. I am not saying 25mm was a wrong choice as you would need the stiffer screw for long unsupported travel but it does introduce more mass for the motors to turn.

Steppers have a massive torque drop off as soon as they move - strongest torque is at zero. As Daniel said 800RPM is it - I actually work on 600RPM.
At 600 RPM on a 5mm pitch screw you get 3m/min but with a bit of tweaking with the acceleration you can improve that. Personally I would settle for 3m/min with fast acceleration over 4m/min with slower acceleration.
The higher torque at lower speeds is why I suggest gearing your machine up to take advantage of the lower speed torque. Trade off is you loose some resolution but this is small in real measurements.

Once you get to your size machine on ballscrews then servos will deliver the best performance as they increase in torque the faster they go.

Smoothsteppers deliver a much cleaner signal and can increase the speed of a machine particularly if the PC is under rated. If you are using an old clunker computer perhaps that is a problem. Might be worth trying a better computer if that is the case.

Failing all else then consider gearing up your drives with pulleys.

Bob,

I'd have to agree with rod. I would settle for the 3-5m/min with a fast acceleration. Doing 3d work, the machines never really go over about 1-2m/min due to alot of variations in the milling process. Running CV mode will up this but again this only works over larger radius/3d moves. Mulitcam ,i think it was, took the market quite a few years back as they were the first to figure out that for 3d routers that faster acceleration will get the job done better than those with a high end top speed that are suited to heafty spindles and cabinet machines where 2d cutting is primary. Industry standards are servo's for the reasons rod stated.

Rods suggestion of pulleys is a great idea. A simple 2:1 ratio would see you loose some resolution but unless your milling mating moulds the resolution difference i doubt would be seen. Pulleys would also reduce alot of stepper resonance.

Daniel

Rod and Daniel
Thanks for both of your inputs and maths.
I was not previously aware that the stepper motors had a maximum rotation speed of 800 RPM. Had I been, I would have been quite happy with the speeds that I was getting with the setup that I had. So that is 400 steps per @ 1000 acceleration @ 4 mpm on a 5mm pitch.
It would appear that if I want to get a higher speed of say 12metres a minute then I will have to go to a geared something. With the larger motor I now have, this should be quite doable.
Sean should have his mill finished soon, and when he has, I will make sure that he makes one of his first jobs the manufacture of such a geared device.
Any one else who wants one of these should make yourselves known now so that he can do a batch run. He will let you know the price when it is done.

I managed to work out a way of putting both PSUs in the control box. Not an ideal solution as I had to mount the smaller PSU over the BOB. This covers over the wires and doesn't allow me to see the LED and it also introduces an unwanted extra possibility of interference with the signals. Luckily, the connecting signal wires are relatively short and there are few floating signals. Actually, none that I can think of right now.

I also had to remove the emergency stop switch from its position on the control box as the lid would not close with the new arrangement, but this is actually a positive thing as I have now mounted it right next to the computer so that it and the other power switches are all within arms reach.

Hi Bob,
Just for the sake of others steppers will spin faster but the problem is they will have very little torque. Do a search for the torque curve on a stepper motor and it will be obvious how the torque is affected by speed.

Bob 12m/min will be like playing tennis with a medicine ball. The inertia of your large gantry will be scary stuff as it comes towards you at that speed. I ran Morphy at 10m/min for a couple of hours and coundn't get used to it so detuned to 4m/min. My reflexes are not quick enough to stop damage if something goes wrong at that speed.

XL or T5 belts and pulleys will be fine to get some gearing happening.

Sound like you are getting on top of things with the electrics and you have a plan for gearing.

"Bob 12m/min will be like playing tennis with a medicine ball"...Nice call. I'll think that next time i have my cnc running. lol.

Bob, mine is capable of 16m/min with everything going well, but it is detuned to 8m/min, this is because getting from one side to other on a large cnc sheet part is time consuming. Detuning makes it very reliable, and like rod say reflexes must be quick or you have to have some serious safety controls like warning proximity switches etc and brakes on motors etc. Funniest thing is people telling you they have these high max speeds, ask them what cutting speeds they get, unless they have huge motors and huge spindle HP, or they tell you about foam cutting, they will be cutting at less than 4m/min anyway.

Like rod said you seem to be on track. For belts, especially with the dollar so strong, you can get them from econobelt america for alot less. I would also recommend AT5 belts as they are rated higher and are designed to be higher accuracy and less slippage.

Daniel

Hi Rod and Dan

Thanks for the info on the belts and gearing.

The 12 metres a minute holds no fears for me as I was going to get Sean to stand there rather than me. I just need to get him off his bum and making a couple of mounts.
Daniel is getting 16 metres a minute so I have just decided that Sean will have to make the mounts a 5:1. This should give me about 20 metres a minute and with Sean at the end of the table I can run it flat out without any worries at all.

lol Bob aren't you on holidays yet?

I think Chris has the pulleys and belts

Yep if someone has a Dxf that I can pilfer to make these up for him it would be good if not I will draw something up but be warned bob it may be real fast looking .



Sean

The gearboxes will need to be drawn and made to suit the space/design of where they sit. It would be lucky if someone provided drawings and it suited. Here you will find a program which will help with pulley sizes and belt lengths. If you have issues like me with firefox opening the app, run it in explorer.

Also remember though, increasing your gearbox ratio on stepper side reduces torque but increases speed, and on the ballscrew will increase torque and lower speed. Damn tradeoffs

Thanks for the link Daniel.

Did you do any research on the advantages/disadvantages of using gears instead of pulleys?

Bob

ps. the app runs fine in Firefox. It just takes a while to download the script

Originally Posted by WillyInBris

lol Bob aren't you on holidays yet?Sean

Start Monday. Be back in about two weeks.

Originally Posted by WillyInBris

if not I will draw something up but be warned bob it may be real fast looking .Sean

If by "Fast looking" you mean sleek and aerodynamic then OK. But, apart from that, I expect nothing but the highest of standards from you considering how much I am paying you.

Gear meshes are alot harder to work with as they have to be precise. Also metal to metal mating causes wear amongst parts so it wouldn't be long till you get backlash in the system, most gear to gear setups are in non critical setups.

Pulleys however are much cleaner and the belts are about the only thing that wear. Good thing as replacement costs are cheap and minimal break down to replace it.

Daniel