I want to drive a table on a mini mill with a stepper motor.
I have been advised a pololu 1474 motor with a Tic 36v4 controller would be suitable. I also need a pot for speed control. A pair of limit switches an enclosure and a coupling. Advice please.
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The starting point would be to set out some details of your application. For instance:
Is this for milling and high speed, or is it an additional high-speed only drive?
What is the drive mechanism (belt, screw, rack, etc)?
What distance is required for table travel?
What is your estimate of the torque requirement?
Is position feedback required (in addition to the end stops)?
What power supply is available?
Are you prepared to write the code for the controller, or will you use a package solution?
If this is a commercial mini mill a link to the product details would be useful.
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Milling and high speed
Screw drive 2 mill pitch
800 travel
Not much
No
Haven’t got one yet
Optimum bf 20 lv
This one? M650 - BF-20LV Opti-Mill Drill - Geared & Tilting Head
According to the specs the table travel is 480mm by 175mm, so it’s not clear what you will be driving to 800.
To determine the motor required you will need to get an idea of the torque. You can attach an arm to the appropriate drive wheel, set it at horizontal, and hang a container of some type from the arm. Fill the container with water until it reliably pulls the arm to vertical. Then measure the weight of water and container, measure the distance from the centre of the shaft to the attachment point of the container, and use a formula that you can find here to calculate the torque. Then add a percentage for headroom.
If you are driving the X axis then it looks to me like shaft is already set up for a motor drive by replacing the position indicator with a gear. You should disassemble the fittings at one end and see what would be required to fit a gear wheel. Or perhaps you can simply replace the handle with the gear. The motor drive is likely mounted on the rear of the table with a belt from the motor to the gear. Then you can use the torque figure with some typical maximum speed data from various motors to work out what combination of motor and relative gearing between the motor gear and the screw gear will give you the required speed while still generating sufficient torque. That will then give you the size of power supply needed as a minimum.
Y-axis is more difficult, as there is no convenient place to mount a motor alongside the existing handle. You should start by checking whether the screw is accessible from the rear. If not, the usual solution is to replace the front screw mount with a completely new part that incorporates a mount for a motor that drives the screw directly. If you can’t find a motor with the right specs to drive the shaft directly then you could incorporate a planetary gear into the drive train. This option is much more complex than the X-axis, but hey, you have a mill to make the required parts.
The controller/driver you have mentioned is much more complex than you require. Any common MCU combined with a generic stepper driver (matched for current and voltage capability) would be simpler and cheaper. Pot control is supported with ADC input for most common MCU’s.
Limit switches are usually mounted on the pedestal, with small brackets screwed to the table at each end to trigger the switch. Or you might be able to use the adjustable stops to trigger the switch.
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Hi Kim
Since we don’t know which part of our planet you live I think this would be a valid question.
Are you measuring and quoting in metric or imperil.
In some circles this would be 0.002 inch.
800 what.
I would assume millimetres (mm) although Murphys law says that could be wrong and it could be inches.
In the interest of getting accurate and useful answers I would suggest you quote your unit of measure when posing a problem such as this as this Forum gets queries and problems world wide such as UK and USA who do use the imperial measurement system.
There’s lots of information out there re stepper driven milling tables and I think there are quite a few on this Forum with fairly extensive experience (not me) so wish you all the best in your venture.
Cheers Bob
I’m in Australia and for the most part work in metric. travel is 600 mm. I didn’t think it would matter as it is just more or less turns of the motor.
The torque according to the calculator is 9.75 lbf -in if I got it correct . It was 2.6 kilograms at 45 mm from the centre of the hand wheel to the hand wheel handle
I would like to use direct drive, no gears or pulleys
Kim
I’m not doing CNC
Kim
Hi Kim
It was really unlikely to be 800 inches. I was just pointing our that as this Forum seems to be global to an extent it does pay to be specific mostly. I was not trying to be smart but it does make life easier when trying to help if one thinks in mm and somewhere down the track you find it is inches or mils (0/001 inch). Just makes it easier if any grey areas are specified early.
From the above and
I would assume you just want to operate the X axis with a variable and constant speed that you normally try to do manually. This would release one hand and allow you to concentrate on other aspects of the job. As you surmised this is usually done with a stepper. As I see it your main problem will be mounting a suitable motor. There are so many out there you would have to find a suitable unit. You might have a study of both ends of the table. There might be provision for mounting a stepper already. It may just be in the form of 4 threaded holes which would suit a particular size (physical size) stepper. It might be the opposite end to the handle. This might be the best end as it would allow the handle to be used for manual movement when required.
I am not personally conversant with the nitty gritty of steppers but as you have given a fair indication of the torque required someone (hopefully) will come up with a solution for you.
Cheers Bob.
I think mounting the motor is the easy part. What motor to mount is my problem. I’m totally useless with electronics.
Kim
Hi Kim
This might do your job
Bipolar Stepper Motor with Planet Gear Box (18kg.cm)
Core stock FIT0349.
You might check with Core or someone else though as there seems to be a conflict between specifications. The stated specs are 200 steps/rev and 18kG/cm at the gear box output. Now I think that 200 steps/rev is the actual motor. If so the actual gear box output will be 5.18 times that or 1036 steps/rev. This might not be fast enough in RPM for you so don’t take my word, it is only a suggestion. Please check with Core or someone else to ascertain suitability.
Cheers Bob
If you know how you are going to mount it and you have the minimum torque requirement then all you need to find the right motor is the steps per revolution (with a 2mm screw that’s obviously going to have to be 200, or some multiple) and how much you want to pay for it. It’s not an electrical problem.
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Hi Kim,
I think the three requirements that drive a choice for a controller and motors are the torque, precision and speed required. In your case since you’re just doing speed control, precision isn’t too important.
The Tic you mentioned is a good pick, as it has limit switch inputs, a potentiometer input, and can drive the largest stepper motors you’ll see in the maker space.
If you’d like to find out more about what’s involved setting it up, Pololu have an excellent guide:
As for motor choice, just make sure it has a torque output higher than your requirement with some headroom. The gearbox model Bob suggested is a good pick if you know you’ll be moving slowly, or you could go with a larger unreduced model if you need more speed:
A higher supply voltage will allow you to reach higher speeds, and allow the stepper to change directions more quickly.
As for limit switches and potentiometers, they are very electrically simple so any should work, some examples:
Extra content if you're interested in steppers in general
While you mentioned you weren’t doing CNC, Here’s a video that goes over the basics of steppers in the context of machining if you’re interested in learning more about them, as a plus ToT one of my favourite content creators, he’s got a nice style IMO:
Build Your Own CNC! (Part 1) - Hardware - YouTube
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Thank you James. Very helpful.
Kim
Hi Kim
That stepper that James suggested has almost as much torque as the one I suggested but with no gear box so will be much faster.
Ocean Controls here
Also have a selection of steppers with a bit of grunt if you can’t find what you need at Core.
Cheers Bob
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