Driveshaft Sizing

[Down&Dirty]

I am currently building my second table due to some problems with the first. The first was a small 2x3 table that used sliding glass door bearings on 1/4" round bar for linear guides. I used ACME threaded rod to provide the power. The problem was that getting a straight threaded rod that didn't whip and induce noise into the machine was impossible. So this lead me to timing belts as they should not have the same problems. Of course the switch meant that I had to rework most of my gantry. The original design used dual steppers on the X and a single on the Y. Turns out that my cutter needs to lift off to start the arc so I needed to power my Z axis also.

All of these problems have led to me just biting the bullet and building a 4x5 cutting surface table. To open up my controler to be able to handle the Z axis I am going to use one stepper to drive the X axis. This means I need to do a driveshaft across the table to turn the two pulleys that will power the X. The plan is to have the gantry walk back and forth on a stationary belt. I am doing this so that upgrading to a longer table later is not a problem and also to make it easier to break the table down and move it.

I have the option of making this driveshaft out of either 1/4" or 5/16" solid rod. The problem is that 1/4" is really common and 5/16" is harder for me to get locally. To be honest I am not sure that either one will not twist along its length under the torque required to move the gantry. My entire gantry is going to be around 15-20lbs. Plus the force required to drag the plasma supply hose around.

Which size should I go with? Do I need to go bigger? Where do the pulleys come from for larger rod? I can get 1/4", 3/8", and 1/2" or larger easily locally. The 16th inch sizes are special order.

[jcc3inc]

Sir,

I have never liked leadscrews in a plasma cutting environment. Not only do you have the problem of critical speed (whip), but keeping the screw threads clean is difficult. Although many other drive types, chain, toothed belt, and friction can be used, I prefer gear rack and pinion. R & P is tolerant of moderate amounts of dirt, and can be setup with spring loading so as to yield zero backlash. We built one 4' wide machine, the Model 610A, where we used a single X axis drive motor with a 4+ foot 3/8 inch shaft having a pinion on the left side and the second drive pinion on the right. We used 3/8 gear rack placed teeth-up so that the spring laded pinions meshed with the rack on left and right sides. While not the ideal in terms of keeping it clean, it was simple.

With a decent "delta" carriage design you can operate quite successfully with single side drive on a plasma machine.

Getting a 3/8 or 1/2 inch shaft should be easy from your near steel service center. For gear rack, an excellent source is Standard Steel Specialties [www.stdsteel.com/gear.htm]. We used 1/2" x 1/2", 24 dp, 20 degree pressure angle rack and pinions. In choosing pinion number of teeth, be sure to have at lease 18 teeth on each pinion for smooth operation.

I'd use rack for the cross axis as well, and it can be easily used for the Z axis.

There are lots more aspects to a good design.

Regards,
Jack C.

[jimcolt]

The PlasmaCam machines, and there are thousands of them in the field, use a 1/2" cross shaft to drive the gantry on both sides. The drive is with servos, not steppers, and acceleration is pretty high for an entry level machine, good acceleration is important to good plasma cut quality. These gantries are about 50" wide, and likely weight about 60 to 70 pounds with torch, carriege, z axis, and all hardware. They are pretty much trouble free, relatively accurate, and stay square.

Jim

[Down&Dirty]

I would love to use a rack and pinion but they are way out of my budget from every source I have talked to. I have seen a couple machines using the toothed belts.

I just completed the modeling of my entire table in Solidworks so that I can make the parts faster. I am borrowing time at a friends machine shop to build the table and I have been wasting time trying to design and build the table at the same time. With the appropriate stepper motors and parts the end gantry weight is at 28 lbs. I am hoping that because of this I can get away with some smaller parts.

The current plan is to use a 5/16 shaft to couple the two sides. Honestly the table linear guides should provide more than enough rigidity that would eliminate the need to drive both sides, but it is just a little more money and a couple extra holes to add right now in the initial build stage. If I wait and need it later it will be a lot harder and more complicated to add.

My gear reduction of the steppers is just over 3:1. The final gear walking the belt has a 0.897 inch pitch diameter. This is going to leave me with approximatly .3 inches per motor rev, and a resolution of just under 1.5 thou per step. It should still leave me with plenty of torque to accelerate quickly. My last table used a 10TPI acme thread so this is all new to me. The old table couldn't move fast enough so I knew I wanted to use less gearing in this setup. It would be nice to be at half a thou per step, but I would have to step down to a smaller reduction belt size.

For belting I am going to use a 3/8" XL series belt. This will be for both the reduction side and the actual drive side. Hopefully it won't stretch under acceleration leading to problems. I am hoping that this machine will be able to plasma, oxy/act., and also use a plate marker to engrave labels on parts.

The next big step is to finsih figuring out the Z axis. I definitly want a floating head so that I don't have to reset for different materials and to also touch off to start the arc.

[Diane-MooreGear]

Have you checked with CNCRouterParts? Also, I have gear rack at VERY Competitive pricing and we accept Credit Cards which makes that an altogether easier transaction than the other guys.

Diane@mooregear.com

[Down&Dirty]

Originally Posted by Diane-MooreGear Have you checked with CNCRouterParts? Also, I have gear rack at VERY Competitive pricing and we accept Credit Cards which makes that an altogether easier transaction than the other guys. Diane@mooregear.com

I have looked at those. That is a great price, but the flat belt drive for the entire machine is going to cost less than putting a rack on just the Y. I wish I had the money to go all out with this one, but I am 5 months from graduation so money is really tight.