Math check-New build

1. ## Math check-New build

Ok guys, I have been reading this forum for years and am finally at a point to start designing my plasma table. I have read and re-read many posts. I need someone to check my thinking on the following:

I plan on using two motors to drive the X-axis. I am looking at a set of Nema 34 motors that generate 300in-oz at 450RPM according to the motor graphs. This is also peak torque. If I reduce this by 3:1 using a .196" pitch belt and 60 tooth drive gear, I'm getting about 588 IPM. The RPM at the drive will be 150RPM X 3.92"/Rev=588IPM. The torque will be 900 in-oz per motor.

Resolution ends up at about .00196"/step using Resolution=(Distance/rev)/#steps*Gear Ratio.

Of course accelleration and decelleration plays into this, but I figure the pair of motors should cover this. The estimated Gantry weight at this time is around 57 Lbs.

Does this all look right? It's been 10 plus years since having to work through some of this stuff and I'm not sure I'm getting it all.

Thanks,
Mike

2. ## math questions

Sir,

There are several questions:
1. Will you use microstepping for your stepper motors?
2. What pitch of gear rack are you planning on?
3. What gear pressure angle will you use, 14-1/2 or 20 degrees?
4. Your carriage weight seems awfully light; does this include the torch lifter,
torch, cross axis drive, popwertrack etc.?
5. Will you do plasma or oxyfuel cutting?
6. If plasma, what torch power and what is the thinnest you plan to cut?
7. What max overall size will the workpiece have?

Regards,
Jack C.

3. Microstepping will be in use. 2000 steps/rev.
Driving the gantry with timing belts instead of rack and pinion. Total gear reduction is 3:1
The gantry is only estimated at this time. I still need to add a few more components.
This will be plasma only.
I have not decided on which plasma unit I will be using.
I based needed IPM rates on the HYpertherm recommended cut speeds. At 40 amps a cut speed of 415 IPM is recommended for 26 Gage, so this was the min rate I was looking for. The thinnest material I will cut will probably not be any thinner then 22G.
Cut area will be 61"x61"

Thanks

4. Originally Posted by mag409
Microstepping will be in use. 2000 steps/rev.
Driving the gantry with timing belts instead of rack and pinion. Total gear reduction is 3:1
The speed of the timing belt drive is PI times the diameter of the drive pulley. To get decent belt wrap (more teeth involved will reduce backlash and tendency of the belt to walk out of the pulley) you need at least a 2" dia pulley. That says you will get 6.28 inches of travel per rev of the drive pulley. If you directly connected it to the motor you would have rapids up in the 3600 IPM range with a corresponding drop in resolution and torque. You would need at least a 6:1 belt reduction between the motor and drive pulley to get back to ground zero (motor base torque and resolution).

Microstepping should not be used to calculate the true resolution/accuracy of a table. Microstepping effects go away at higher RPM and some drives (like the newer Gecko's) actually "morph" to full step at higher RPM to gain back the loss of torque microstepping causes (nothing is free (:-). So resolution changes across the RPM spectrum. Microstepping is to smooth out the motor motion and reduce the harmonics (resonance) not to increase the accuracy.

I guess I am not understanding your mechanical drive setup. You stated an overall 3:1 reduction but does that included the increase posed by the final drive pulley? I'm not visualizing how that is going to work. Maybe smaller drive pulleys?

Instead of high speed for thin material consider using smaller nozzle sizes (like the Hypertherm Fine Cut or the TD 25A nozzles) and lower current that will let you cut 26 ga in the sub-300 IPM range. Given a choice between speed and acceleration you should lean toward acceleration. Your light gantry will help you out on that end but proper gearing gets you optimized acceleration and good rapids.

The 34 frame motors @ 300 oz-in are pretty wimpy by todays standards. You can get 23 frame with torque ratings up to 620 oz-in and single stack 23's (really small motors physically) with 300 oz-in ratings. The 34's we currently use start at 740 oz-in and go to 960 oz-in as an upgrade. You can get bigger ones than that but they have a diminishing set of returns (have a sharper high end torque drop)

Tom Caudle
www.CandCNC.com
Totally Modular CNC Electronics
New Rotary Plasma Cutting Solutions

5. great reply tom - lots of good info there.

I keep reading the recommendations not to use high microstepping values for resolution - but was is reasonable? 1/4? 1/8?

What is a decent value in Mach for acceleration? (as a target for a good machine)

6. Originally Posted by multiplex
great reply tom - lots of good info there.

I keep reading the recommendations not to use high microstepping values for resolution - but was is reasonable? 1/4? 1/8?

What is a decent value in Mach for acceleration? (as a target for a good machine)

For smoothness and reduction of vibration/resonance a value between 8 and 16 works the best over the RPM range of the stepper. Resolution is defined as "the smallest distance you can move per step". The problem with microstepping is you are using the opposing coils in the motor to "hover" between steps (the motor has hard steps (poles) every 1.8 deg of rotation causing it to have 200 distinct "detents". As the PRM increases and the inertia of the motor increases it becomes more difficult to force the hover and the hard poles tend to dominate. So to answer it's a moving number for resolution. You CAN guarantee the 1.8 deg and get something better SOMETIMES. From a marketing perspective 10 times the motor resolution looks impressive.

So....just because you use 2000 steps per revolution in the calculations does not translate into that level of position accuracy when running.

For good plamsa cutting your acceleration should be in the 10 ips/sec range or higher. That gets harder and harder to do for every pound you add to the gantry. If you start out with reduced torque because of improper motor to drive gearing you fight and uphill battle. I geed an average 3 or 5 e-mails/calls a week from builders that are getting rounded corners cutting with plasma. While the Constant Velocity (CV) settings in MACH can be adjusted to an extent, you can't compensate for lack of acceleration.

Tom Caudle
www.CandCNC.com
New BladeRunner 5 axis AIO All-In-One CNC Controller

I guess I am not understanding your mechanical drive setup. You stated an overall 3:1 reduction but does that included the increase posed by the final drive pulley? I'm not visualizing how that is going to work. Maybe smaller drive pulleys?

The 34 frame motors @ 300 oz-in are pretty wimpy by todays standards. You can get 23 frame with torque ratings up to 620 oz-in and single stack 23's (really small motors physically) with 300 oz-in ratings. The 34's we currently use start at 740 oz-in and go to 960 oz-in as an upgrade. You can get bigger ones than that but they have a diminishing set of returns (have a sharper high end torque drop)
The motor will have a 16 tooth T5 pitch pulley running to a 48 tooth T5 pulley. The 48 tooth pulley will be on the same shaft as the drive pulley and will also be 48 tooth. At least this is how I have it drawn at this time.

Are the torque figures you quoted "Holding Torque" or torque from the graphs. All of the graphs I have seen show a lower generated torque then the stated maximum holding torque, so I have been looking at the graphs to determine the useable torque the motor can generate.

I am starting to look for the acceleration equations and finalizing the gantry weight. My plan is to design the system, verify the numbers and make any necessary changes to the design to get the machine into the proper range.

Thanks for the input Tom and others.

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