First Build - Rack and Pinion On Small Machine For Experience

[xxtoni]

So I've definitely decided on building a router for our business.

I have certified welders in my company, electricians as well, rest is on me.

I am thinking steel base (I have some 30 tons of steel tubing lying around) though I'm not sure how to level it as of yet.

What I'd really like advice on is how to drive the machine. My real ambition is building a 3x2m router that can run at 20-30m/min but I already have a router (which is painfully slow, hence why I want to build another one) and I'd like to have a CNC plasma cutter in the company so I've decided to build that first.

Not sure on the size yet, probably around 1,5x1,5m or something like that but I have come across a bit of a dilemma. From what I have read for this size using a ballscrew "driven" system would be best but for a bigger machine like the 3x2m one I want to do in the future a rack and pinion would be better.

My dilemma here is that if I use the ballscrew one for the plasma I still won't have a clue about doing a rack and pinion machine so I'll have to start from scratch, to some degree, with the bigger machine which is also about to have a lot more issues considering the greater forces at work there compared to a plasma cutter which is smaller, lighter, has less resistance...


Any advice ?

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[awerby]

If you use ball screws in your plasma cutter, make sure to protect them with bellows, or they won't last long. The stuff that plasma-cutting puts into the air will destroy them in short order, if they are exposed to it. Rack and pinion is a better bet in general; you don't really need the precision of good ball screws for plasma.

[xxtoni]

If you use ball screws in your plasma cutter, make sure to protect them with bellows, or they won't last long. The stuff that plasma-cutting puts into the air will destroy them in short order, if they are exposed to it. Rack and pinion is a better bet in general; you don't really need the precision of good ball screws for plasma.

First of thank you very much for the reply, much appreciated.

I'm definitely going rack and pinion.

I have been reading up extensively on cnczone about rack and pinion systems and watching youtube videos and I understand the general premise but I have some major concerns which I haven't been able to answer.

- How do I determine the size of the various components needed ?

For example I am looking to build a 1.5x1.5m plasma, what size rack and pinion would one use for that ? Is there some conclusive math to determine this ?

How do people usually determine the size of their motors, so far I have guesstimated that NEMA23 motors would probably be a good fit for a machine this size but I'd like a more scientific way to determine these things.

- Gantry weight

Since I will be making my plasma from steel tubing I am concerned about the weight of the gantry. My current CNC router, which we bought, shakes violently when if I increase the speed and acceleration, I am assuming that this is due to the weight of the gantry because the machine is pretty massive so rigidity shouldn't be an issue.


Rails or a cheaper alternative and sizing ?

As of yet I haven't figured out on what the all of this will move. My router uses rails and carriages and from what I can see that's what most people go with and if that is the best option I will probably go with that as well. However I have no idea how to choose the correct size of the rails ?

Any advice in this regard ?

Also another major concern for me is the flatness and general accuracy of the base. I have read that people make them as flat as 0.0015mm which as of right now I have no idea how to achieve because it is after all welded steel tubing.



Thanks in advance!

[C0matoes]

1/2" rack with anywhere from a 20T to 30T gears should work fine for your project. Mine uses 25 tooth gear and moves pretty fast. I would upsize the drive motors to Nema34 though for not much more money. Mine uses nema34 3.6:1 gear reduction motors. I'm not sure they are necessary for a plasma but it's what my shopbots came with so I reused them so you may need to consider using belt to reduce the load on the motor. My gantry is 72" wide and made from steel so its pretty heavy. For the gantry cross bar you may consider going aluminum to cut the weight down. There's a place called Faztech that sells extruded. As far as rails go, in hindsight I would have used Steel base frame rails with aluminum T bar on the sides. This would allow for top and bottom V-rollers on the rails, in case you ever wanted to do some drilling. There are a lot of options for how you can make it roll but this struck me as the cheapest while still giving good, bind free rolling of the gantry and torch head. Sure you could spend more but you won't notice much improvement with linear bearings and rails. The original ShopBot I have sits on 6" C channel for side rails with an angle rail sitting on top for the rollers to ride on. That's where I would switch to the T-bar. As far as flat goes, consider what you're cutting and what you're cutting with. A plasma has what I would call a lot of play in this arena considering cut height can vary up to 1/8" on smaller torches and produce a good cut. Depending on the gauge thickness of the steel you're cutting coupled with heat transfer given a specific speed, you'll soon be hunting a THC device and/or a magnetic or breakaway mount for your torch.
Since we are talking about steel, getting it flat and true depends on the fabricator and his ability. Notice I did not use the term "welder". Not all good welders are good fabricators and not all good fabricators are good welders, but it helps. Start with your legs and side rails, buy a good, true level and get them level and square. No floor is flat so consider adjustable swivel pad feet to help with getting the assembly level. Throw the .0015MM out the window. Someone lied if they claimed this tolerance with steel over very much distance. I got mine to around .025" across 20' x 6' so I can attest to that tolerance being achievable but reaching for more was just beating our heads against the wall and just not necessary IMO. You'll find it handy to have an aluminum straight edge of some sort. Mill steel does not make for a good straight edge due to the process and handling. We use a good extruded piece of rectangular aluminum tubing for ours. My bots electronics didn't care for the interference from the pilot arc of my torch so I ended up building the table with electrical isolators to keep the cut portion of table and the bot portion of the table from "talking" electrically. I've attached a couple of pictures for reference. This is based off the original ShopBot design with modifications to suit our needs. Missing is the cut surface so you can see the isolators. And Yes, my shop is a bloody red mess.