Reduction gearing for mill head drive?

[daved20319b]

This is a follow on to my earlier question re: how much torque does is take. I posted basically the same question in the general forum and got no responses, but figured someone that's done a CNC conversion might have some answers.

So I have a Grizzly G0619, and as everyone probably knows, cranking that Z axis handwheel is stroke inducing. I've already done a table power feed using a stepper motor, came out great, and now I want to do the same to my Z axis. I found pics on another forum that look perfect, I just need to machine some motor mount brackets that will bolt down with the column bolts, add a mounting plate, then replace the gear on the bottom of the lead screw with a cogged gear for a timing belt. The stock setup has a 2:1 gear reduction, but I selected a motor that should have plenty of torque to run this at 1:1. Assuming it will, is there any reason to still run a reduction? And is the timing belt the best option, or should I look at something else? Thanks!

Dave

 

[Reddinr]

I would do some investigating and make a decision on min/max travel speed you are looking for.

What is the turns per inch for the lead screw?
Figure out the min and max speed you want the travel to be. Is one inch per second enough? Is 5 inches per second too much?
Look at the motor RPM and divide by 60 to get RPS.
Will your motor be on/off or will there be a speed control? What is the slowest you want to run the motor so enough torque is still available?

Then, based on the min/max speed of the motor and the min/max speed of travel you want and the gearing ratio, figure out if that works.

Travel speed - inches/second = (Motor RPM / 60) * gearing ratio * inches/crank

Example:
1800 RPM = 30 RPS
Say the Z axis screw is 6 turns per inch
Say the pulley ratio you are using is 1/2 (One motor turn results in 1/2 screw turn)
Say you only want to run the motor down to 70% of it's rated RPM.

Max travel speed is 2.5 inches / second (RPM / 60) * (1/2) * (1/6)
Min travel speed in this case = 2.5 inches / second * 70% = 1.75 inches / second

For the 1:1 case, in this example, max/min travel speed would be 5.0 and 3.5 inches / second

I think I got the math right.

 

[daved20319b]

I would do some investigating and make a decision on min/max travel speed you are looking for.

What is the turns per inch for the lead screw?
Figure out the min and max speed you want the travel to be. Is one inch per second enough? Is 5 inches per second too much?
Look at the motor RPM and divide by 60 to get RPS.
Will your motor be on/off or will there be a speed control? What is the slowest you want to run the motor so enough torque is still available?

Then, based on the min/max speed of the motor and the min/max speed of travel you want and the gearing ratio, figure out if that works.

Travel speed - inches/second = (Motor RPM / 60) * gearing ratio * inches/crank

Example:
1800 RPM = 30 RPS
Say the Z axis screw is 6 turns per inch
Say the pulley ratio you are using is 1/2 (One motor turn results in 1/2 screw turn)
Say you only want to run the motor down to 70% of it's rated RPM.

Max travel speed is 2.5 inches / second (RPM / 60) * (1/2) * (1/6)
Min travel speed in this case = 2.5 inches / second * 70% = 1.75 inches / second

For the 1:1 case, in this example, max/min travel speed would be 5.0 and 3.5 inches / second

I think I got the math right.

I think you're thinking DC motor, I'm using a stepper motor, but yes, I've done all the above. Lead screw is 10 TPI, which makes the math dirt simple. The beauty of stepper motors is that the slower they go, the more torque they make, and this thing will probably never see even 500 RPM, which would be 50 IPM. Yes, I'll have speed control, and initially I'll be using the same controller I have on my table feed. I have it set so it reads out in IPM, I can go as fast as I want, but usually 30 IPM is as fast as I go, and that's just to get things out of my way. I anticipate the same sort of speed for the Z axis. I can go as low as 1 IPM on the table, I may setup the head controller to do 1/2 IPM steps, I can see a need for going slower when doing stuff like boring operations. Mostly concerned with getting the head out of my way quickly, as this is a small/light machine, I default to using the head as much as possible vs. the quill, just to maximize rigidity, but cranking this thing up is a pain, literally. The downside to what I'm planning is that I lose the Z axis handwheel, no way to make it work with both unless I do a lot more work and spend a lot more money. So precise vertical movement will still need to be done with the quill fine feed.

So bottom line is, I don't THINK I need any reduction from a torque or speed standpoint, I just don't know if there are other reasons that it might still be a good idea. Thanks for your response.

Dave

 

[Reddinr]

What are you using to drive the stepper? What I'm thinking is that there is likely a fairly cheap and "easy" way to have a speed control as well as an MPG control of the stepper. It wouldn't give you the feel of the crank for feedback but it could give you incremental control. Might need a little electronics to do it but not too much I think.

If your stepper driver has a step/direction input you can hook up an MPG to it's input with a small interface between. You could have a "speed/MPG switch" to select which input to use to drive the input of the stepper driver. You might gain/lose a step or two in the switch-over but I bet it won't matter to you. There is a chip called an LS7084 that can make an MPG input into a step and direction output. About $6 plus shipping. You can get an MPG dial for about $50-60 I think. Or there are several other ways too.

If you are interested, I can help you out with the electronics. I have so much electronics "c**p" lying around I would be glad to donate.

 

[daved20319b]

What are you using to drive the stepper?

A DM860T digital driver, with a ZM-SMC01 controller. They have 2 versions of this controller, the ones I'm using which are just controllers, and another version (SMC02) that also includes a driver board. It works surprisingly well with smaller motors, but it can't provide enough current or voltage for larger motors, so I'm using a separate driver. Finding these controllers is what finally made my table feed a reality, that's something I've been tinkering with on and off almost since I got this mill back in 2011. I have an adjustable voltage power supply, right now it's running my table at 30 v., but will go up to 48. That said, I'm thinking I'll order a 60 v. supply for the big motor, I don't really need the extra voltage in my application, but it would be nice to have some extra wiggle room. My long range goal is a central control for both motors using an Arduino (actually an ESP32), but I'm still a newb at programming, so I'm learning as I go, and building the code piece by piece. It'll be fairly complex when it's done, not sure it's worth it, but it gives me something to keep my hands and mind busy when I don't want to be in my shop.

So any particular reason for the question, or was it just idle curiosity? Fine either way, later.

Dave

 

[Reddinr]

I edited my comment with more info. Also, I can help with programming questions.

 

[daved20319b]

I know I've seen the term MPG before but I'm drawing a blank. Is that like a jog dial? I've considered adding something along those lines, just not sure it'll really be particularly useful in my application. What I have on the table is working really well, the down the road central control will just be adding some bells and whistles, and give me a concrete project to practice my coding on. But I'm always interested in electronics help, this is all still pretty new to me, I have NO "muscle memory" when it comes to this stuff. Later, and thanks.

Dave

 

[Reddinr]

Sorry. It is annoying when people throw out acronyms thinking everyone knows.... Manual Pulse Generator, a jog dial. Could also be an input to your ESP based system. I have an MPG that drives my RF45 mill clone which I converted to full CNC quite a while back. I use it for positioning and for constant velocity moves when I don't want to create a G-code file to do simple stuff. It is extremely useful for doing things like drilling a number of holes at exact locations or facing.

Will be glad to help. I retired from electronics engineering a short while ago. Machining is only a hobby for me. Have done countless projects with small microcontrollers. There are several others on the board that have experience in these fields too.

 

[DAT510]

I have a G0619 that had the Syil CNC conversion done to it. The Z-axis ballscrew is direct driven by the stepper, but the conversion did add a pneumatic piston to the Z axis. I'll see if i can find the specs on the Z axis stepper. It's buried inside the column, so not easy to directly access.

 

[DAT510]

Found it... the Z axis uses a NEMA 34 Stepper (with the help of the pneumatic cylinder).

From the Syil manual:
"The Z Axis uses the NEMA 34 stepper. They have a holding torque of 460 ounces per inch. And consumes 3 amps per phase."