Cnc plasma

[Traian]

Hi guys i would like some advice for building my cnc plasma
My idea is to use steel L unequal profile 150x90mm, 1700x1250mm size frame with downdraft,
rails hgr25 in the side of base with direct drive motor with helically rack and pinion,
gantry i was think to use rectangular box section with one rail in front bottom and second top gantry hgr25 with hgr20 and same helically rack and pinion. I know the gantry has to be as light as possible but in same time I want to be stable, as many projects see it many of them are aluminium extruded box section but not reali sure about it
Now my questions are:
what closed loop motor should i choose because there on the market are plenty like nema 23,34,hibrid motor servo, servo motor.... because of direct drive i understand is better but probably with planetary gearbox low backslash ? i presume you won't put gear on motor shaft and drive?
Rack and pinion is any particular module to use ,i was think to use 1.5 M but how many tooth to chose helically gear, or how to calculate with this idea i want direct drive,
reason telling you guys i want to make a stable gantry, is in the future I'm planning to found information for bevel head to attached, i think they have servo motor inside the head to angle head when plasma cutting.
i will be happy you could help me please or any anything I left behind unmentioned
Thanks

 

[Traian]

They seems affordable but not sure if are right or good

 

[JimDawson]

You might look here for some ideas https://www.hobby-machinist.com/threads/cnc-plasma.86436/

Regarding motors, Delta servos look to be pretty good. Like anything else, you get what you pay for.

1.5 Mod is OK for the rack & pinion drive. most machines use a straight cut gear. You need about a 4:1 reduction, this can be a planetary gear box or a belt reduction.

 

[Traian]

I’ve been searching for delta servo, different watt speed,any specific? they are priceless
Looking on Alibaba.com found cheaper but I think freight will kill
Any trusted suppliers around Europe?
or any other option on motors? Does have to be left right about helically rac? Thanks

 

[Traian]

If I look for delta what NM, kw,speed ....or how do i determine what motor power using by you determine with reduction 4:1 ?

 

[JimDawson]

750W seems about right for that machine. Should have enough power and speed. Most of those small servos are rated at 5000 RPM, more than enough.

I don't know of any suppliers in Europe. I like DMM Technologies servos, but they are in Canada.

 

[Traian]

Right im looking around to buy servos delta or dmm but far way for my budget now, what about china one like this
Should i be aware about anything about cheap servos or china one . What i mean like mybe not be compatible with any software i will use or with thc.... ??? Don’t know what exactly to look for, as a beginner

 

[spumco]

If I could make a few suggestions for you to consider as this is your first time build...

1. Don't use HGR25 linear rails. Unless you have some way to straighten and machine the L-profile steel angles, the linear guide bearings will bind. You need a guide system that is much more forgiving of misalignment. The round-shaft rails with built-in supports (SBR-20?) are more forgiving of misalignment than the high-precision type profile rails, and wheels/bearings rolling on plates is even more so.

2. Stick with closed-loop stepper motors for your first build. Plasma tables do need reasonably high speed, but very aggressive acelleration is key. Stepper motors have lots of torque at low RPM, which means a 3:1 or 4:1 belt reduction is usually sufficient (assuming similar size stepper vs servo). You will lose top speed as the steppers cant spin as fast as the servos, but how fast do you need to cut? 400ipm maybe for thin stuff?

The nice thing about closed-loop steppers is that virtually all of the ones I've seen have an alarm output when they lose steps. That's critical for a dual-motor gantry system to keep it from racking.

3. Drive system... Getting a rack system aligned can be tricky, especially a helical-tooth version. I suggest you consider a belt-drive system. For a short table, a moving belt system works fine (<1m length). For longer (>1m) I would use a stationary belt and mount the motors on the gantry.

I've built a few tables - huge and small - and a belt drive has worked exceptionally well in all cases. You just need to use a very wide belt to make sure it's stiff, and stick with a polyurethane/steel cord belt.

The easiest and cheapest linear guide system I've seen is one for a smaller table I built. Flat steel plate, bolted vertically to a box tube forms the guide. V-profile roller bearings run on the top and bottom plate edge. If you use plenty of bearings the uneven plate edge gets averaged and there's no binding and essentially no 'slop'. Or at least not enough to care about on a plasma cutter.

The V-bearings are hardened and will eventually wear out the plate edge, but if you have lots of wheels the load is low per bearing. The first table I built is in daily use, over 100k pierces, and hasn't required adjustment yet. And neither has the belt drive - still on the original belt & pulleys. Lightweight gantry, and NEMA23 closed-loop steppers drive it to 1000ipm no problem. Cuts like a dream.

If I did another one like it I would have used steel tube for the stationary rails and a simple aluminum tube for the gantry to save some money over the 80/20 extrusions. But it worked out fine and the cost difference wasn't a big deal in the overall budget.

(ignore the fiddly lightening holes on all the motor mounts/drive assemblies. That was me just having fun on the CNC mill trying to 'hot-rod' the thing to save weight. It helped, but wasn't strictly necessary.)

My most recent build runs on profile rails, has DMM servos, is gigantic, and unbelievelably precise... but it was expensive and fussy to get aligned well.

The really important parts of a plasma table are the torch height control, and the downdraft (or water) system. Building a plasma table is fairly easy, but having it cut nice and not turn your shop in to a disgusting cavern of soot and filth means a happy long-term relationship with it.

Keep it simple for your first build. Steppers, rollers (of some sort), and belt drive.

-Ralph

 

[JimDawson]

Right im looking around to buy servos delta or dmm but far way for my budget now, what about china one like this
Should i be aware about anything about cheap servos or china one . What i mean like mybe not be compatible with any software i will use or with thc.... ??? Don’t know what exactly to look for, as a beginner

The China servos will work OK with your software, but normally have poor documentation and no tech support if you have problems.

What @spumco said above is good, closed loop steppers are a good option. I'm going to start with steppers on my build to see how well they work, and I have them on the shelf. If they don't perform the way I want I will install DMM servos.

 

[pontiac428]

I think you'll be happy with steppers. There is no cutting load for plasma, so all the movements are just tracing without contact. Steppers have to work much harder on a router.