New Machine Build- Beginnings of a 3 axis machine

[razzledog]

Hi Guys... I have decided to construct a small 1 meter by 600mm 3 axis CNC. So far I have organised the bed frame from 125mm x 8mm aluminium channel and settled on 16mm hi tensile Samson rods for driving (what I call) the 'bridge' supported by 6202 ball bearings in plates at each end. I have set a goal to have this machine up and going by Christmas this year

A couple of questions I have....
1. Should I use a stepper for each lead screw or connect them via a toothed belt to a single motor?
2. Will a decent motor have enough grunt to turn the screws?
Thought I'd get the mechanics sorted out first and worry about the electronics later.

I realise it's a steep learning curve, although I have successfully turned a wood lathe into a metal lathe and made a small mill by fitting a cheap Ozito router to my drill press, although usually I only build cameras.

I posted this on the OZ clubhouse without much reponse.......

[CarveOne]

I don't see any reason why the C channel wouldn't make a rigid frame. Are you planning to TIG weld the corners or bolt them together with right angle pieces?

Belts have some minor issues with tension and routing path, and I suppose that two steppers can get out of sequence if there is binding on one side versus the other. If one side starts missing steps it will cause the gantry to get out of alignment and bind. I have a dual leadscrew with belt (Solsylva) and may try the dual stepper drive in the future if the belt presents too much effort to keep it running right. I haven't seen any real complaints about belts so far, just mostly questions as to which way to go with new builds.

For that size machine an approximately 270 oz/in should be fine. You can find the formulas to calculate it on this site or just go with what equivalent size machine builds have used.

Maybe they were too jealous of your C channel pieces to comment.

[razzledog]

I stumbled on the C channel sections at the local scrap yard....an ideal place to find most things! Only a hundred bucks for the lot, it looks new and is well under half price, the 'metalo's' is a common source for materials. Maybe the big section is overkill on such a small machine, but I really want to avoid any flexing. I was going to weld the corners, but worried about twisting, so I may bolt the corners instead as I figure that with a hefty lump of thick plate to serve as a bed, nothing should move. Hi tensile 16mm samson rods were fairly cheap too....$90 for four with long nuts included. I plan to machine the ends down to a half inch and run 6201 bearings on each corner, mounted into the 8mm fixing plates. I figure it's best if I deal with one axis at a time, or it will seem too much computation. So far the machine's plans exist inside my head with nothing on paper It's more fun that way.

[razzledog]

Something I now need to consider is the size of a stepper motors.....How do I gauge whether a 270 oz in with a 1/4 inch shaft or a 1200 oz in motor with a half inch shaft is best suited? This is all new and very interesting for sure. Perhaps there's a way to calculate the ideal, I combed this great site but couldn't find the help I need. Does 1200 oz in mean 75 in lbs of torque? that's a fair turning effort......are steppers really that powerful? Another question is do all three steppers have to be identical? Too many questions, so my apologies for being such a dummy, this is all new and very interesting for sure.
Cheers.

[harryn]

In the end, many people build machine number 2 - and bigger, then need bigger steppers and drives, which is a large % of the total build cost, so buying oversize is not a bad idea in case you build number 2.

The actual torque is less than spec because steppers are really a constant power device - higher rpm results in less torque, and the torque is specified at zero rpm. There are further losses of torque from how they are driven - a chopper circuit, which most use, turns the power on / off very fast, which results in good control, and loss of another 1/2 of the spec torque. An approximation is that your actual torque will be 1/4th of the spec rating of the motor.

This info is all just from reading the forum and motor info - I actually am still very much a beginner, so don't get your hopes up for my post to be exactly true. My plan is to use a nema 34 size motor with at least 500 oz in and maybe more. You don't need to use identical motors everywhere, but I would if they are on the same axis. If you get into higher torque rated stepper motors, it sometimes is cheaper to buy one size larger frame size as this leaves more room for the magnets and windings. There is a world wide shortage of Ga, which is used in very high end magnets - resulting in crazy pricing. A big motor has room for more affordable magnets of similar power.

In the used market, it is hard to find size 34 motors, at least around here.

Edit - PS - partly because I am a contrarian, and partly due to cost, I am soldering up a stepper driver kit from linistepper - it is linear and unipolar - so entirely different than the more common chopper and bipolar method. There is info on it in the electronics / stepper section - just run a search for linistepper. These are used by others, but mine are still unproven.

[LeeWay]

I think Harry is right about the torque labeling. It is holding torque rather than driving torque. As a fer instance on my mill, I initially used 270 0z motors with a Xylotex driver. They did drive the axes well. I was using linear bearings and ground ball screws. They really lacked speed and holding torque though. Especially in the Z axis.
Ultimately I have went with 495 oz motors with Gecko 203V's. Now I have plenty of speed and holding torque, but I find I need a little more on the Z to be able to peck drill faster. I have a belt system that I am going to install on it. The screws I am using have a high helix too, so these motors would work better with a lower pitch. The pitch on these is 10 mm. Makes for a pretty fast and accurate machine, but could be even better with either larger motors or a belt drive on all three axes. I am satisfied with the other two though. It can rapid @ 300 IPM but that is crazy fast for a mill this small. I rapid @ 200 IPM on X and Y and 125 on Z.
Back driving is what its called I think when the motors don't have enough holding torque to keep the still axis from moving away from the cut. Bigger motors or gearing are the fix. Drivers have a lot to do with getting the most out of the motors too. Gecko's out preform Xylotex simply by supplying more amps and voltage to the motor. Other reasons too, but you pay for that output. Very well worth it though.

[razzledog]

I had my 81 year old machinist Father step the ends of the leadscrews to one half inch on his lathe, then cut the channel sections the correct length to provide a little preload to the bearings. I managed to mill out the end plates to suit the bearing's outer diameter and assemble it.....shaft turns smoothly with no backlash! I've done one side of the bed, now to replicate it for the other....this sure is fun.