HF Mill drill conversion question

[109jb]

Hi all. Been a lurker for a while now and am getting serious about converting my HF round column mill drill. It is an older one that is a little smaller than the current HF offering. Table size is 6.25" x 20.25". I have tried searching for the answer I need but haven't had a whole lot of success. I plan to convert the machine to ballscrews and have a good handle on how I am going to do that, but am not sure about some of the details on driving the machine. At first I plan to get the machine running in just the X and Y axes and I plan to use stepper motors for the conversion. I would like to do a direct drive conversion to make things a bit easier. My budget will allow me to get 880 oz-in NEMA 34 size steppers. I would like to microstep the motors (about 4 microsteps), which I understand reduces the running torque somewhat. Would this size motor be sufficient for my plans?

Thanks in advance.

John B.

[Robin Hewitt]

880 oz-in onto a 1/4" pitch screw would give you a theoretical 1380 pounds force on the nuts, that should move it

More importantly, try and find a graph of torque against speed for the motors so you know how many volts you will need to get a credible rapid transit on a G0. A smaller motor might hold torque better at higher revs and still move it okay.

Microstepping is springy, use it to smooth the motor rather than to improve resolution.

A NEMA34 direct drive will rise above the top of the bed. Might you ever want to machine anything that overhangs the bed?

Only XY control? Sounds like you want to drill rather than mill?

[109jb]

I'm only starting with XY and then I will use the XY with manual positioning of the Z to make the pieces I need for the Z axis. I can think of lots of thing that can be milled in this way.

I have an old NEMA 34 motor and the X motor will be about 3/4" below table height in a direct drive setup. Y isn't an issue. This isn't the tiny little mini mill.

As I understand it, microstepping is for 2 purposes, 1-resolution, and 2-smooting the motor out. I do want microstepping for both of these reasons. With the direct drive I would only have 0.001" resolution without mircostepping. As I see it that would lead to 2 problems. First the obvious limitations on accuracy, although for 99.9% of the stuff I would ever do 0.001" is close enough. However, I can see on shallow angle taper cuts or on circular milling being able to see the steps created in the finish cut.

I guess my concern is that the 880 oz-in rating is holding torque and running torque, especially microstepping is lower. I have a graph of running torque vs pulses per second for the motor I am looking at. It is for 1/2 step @ 30V @ 2A. It shows the following torque ratings. I put in the equivalent feed rate for the pulses per second with a 0.2" lead ballscrew.

540 oz-in at 300 PPS (9 IPM)
400 oz-in at 600 PPS (18IPM)
300 oz-in at 900 PPS (27 IPM)
250 oz-in at 1200 PPS (36 IPM)
200 oz-in at 1500 PPS (45 IPM)

I guess its probably sufficient. If not I can alway revert to a 2:1 drive system if I find it necessary.

I'm not real concerned about having lightning fast rapids, but I also don't want them to be at a crawl.

[kensbey]

depending on what drivers you get, you can make the most of microstepping. A lot of drivers, such as the geckodrive stepper drivers, run at 10 microstepping for low speed and then move up to full steps at higher speed. This means that you arent losing torque at high speed where it is alreadly lowest.

i agree with hewitt, use your microstepping mostly for smoothness at low speed. Also remember to take into account the step accuracy of the stepper itself, most are around 5%. This means that there really isnt any point microstepping more than 2000 ppr, except for smoothness.

[SCzEngrgGroup]

880 oz-in onto a 1/4" pitch screw would give you a theoretical 1380 pounds force on the nuts, that should move it

More importantly, try and find a graph of torque against speed for the motors so you know how many volts you will need to get a credible rapid transit on a G0. A smaller motor might hold torque better at higher revs and still move it okay.

Microstepping is springy, use it to smooth the motor rather than to improve resolution.

A NEMA34 direct drive will rise above the top of the bed. Might you ever want to machine anything that overhangs the bed?

Only XY control? Sounds like you want to drill rather than mill?

880 oz-in is serious over-kill for a machine that size. Motors that size are often used on full-sized Bridgeport knee mills. If driving through the typical 1/4" ballscrew shaft ends, I'd be concerned about twisting the ends right off in a crash. I think you could easily get by, and would be safer, with NEMA23 motors with barely half that torque.

Regards,
Ray L.

[kensbey]

880 oz-in is serious over-kill for a machine that size. Motors that size are often used on full-sized Bridgeport knee mills. If driving through the typical 1/4" ballscrew shaft ends, I'd be concerned about twisting the ends right off in a crash. I think you could easily get by, and would be safer, with NEMA23 motors with barely half that torque.

Remember that that torque is only present at standstill. As he said, that torque rapidly drops of with speed. Thus, if you want to be able to make faster cuts, you need to oversize the motors. With steppers its all about minimizing the chance of lost steps.

[109jb]

depending on what drivers you get, you can make the most of microstepping. A lot of drivers, such as the geckodrive stepper drivers, run at 10 microstepping for low speed and then move up to full steps at higher speed. This means that you arent losing torque at high speed where it is alreadly lowest.

i agree with hewitt, use your microstepping mostly for smoothness at low speed. Also remember to take into account the step accuracy of the stepper itself, most are around 5%. This means that there really isnt any point microstepping more than 2000 ppr, except for smoothness.

Thanks for the tip on the drivers. I wasn't aware that there were some sophisticated enough to go to full step at higher speeds. I guess they must take the step pulses from the computer and depending on the pulse rate modify what is sent to the motor. Correct? I guess my question on this is if it is all handled in the driver, or do you have to do something special in the controller on the computer?

880 oz-in is serious over-kill for a machine that size. Motors that size are often used on full-sized Bridgeport knee mills. If driving through the typical 1/4" ballscrew shaft ends, I'd be concerned about twisting the ends right off in a crash. I think you could easily get by, and would be safer, with NEMA23 motors with barely half that torque.

Regards,
Ray L.

I appreciate your opinion, but from things that I have read, I don't think the 880 oz-in is too much motor. Maybe more than is necessary, but what I have read indicates that you can't really have too much motor when it comes to steppers.

As kensbey mentioned (below), I am concerned about losing steps. When my father had his shop he bought a new CNC mill that used steppers and it lost steps constantly. It was in the shop for a month while we tried to work with the machinery supplier and manufacturer to work out the bugs. He eventually sent it back and bought a used Bridgeport with CNC controls on it (servo drives with encoders). The stepper controlled machine was very nice but those lost steps totally ruined the experience. Incidentally he thinks I'm nuts for even thinking about using steppers. I guess they left a bad taste in his mouth.

As for twisting the end of the ballscrew shaft off in a crash, Unless it happens at full speed I doubt it will happen. 880 oz-in is only 4.6 ft-lbs, so the torque alone isn't going to snap the shaft. It will need motor inertia to do it, and I really don't think it will do it then. If I'm wrong and I have a crash, I guess I'll have to make a new end for my ballscrew. Also remember that I am planning direct drive, so I won't have the torque muliplying effect of a reduction drive. This means that I have to get enough torque straight out of the motor. I have seen examples of similar size machines fitted with smaller motors, but I tend to always see them with a 2:1 reduction drive which means a 440 oz-in motor is supplying as much torque to the ballscrew as a 880 oz in direct drive motor. On top of that, I have found that I can buy the 880 oz-in motor for just a few dollars more than say a 440 oz-in motor. I'd rather buy too big once than buy twice.

Remember that that torque is only present at standstill. As he said, that torque rapidly drops of with speed. Thus, if you want to be able to make faster cuts, you need to oversize the motors. With steppers its all about minimizing the chance of lost steps.

You understand my concern. I dread the thought of lost steps, and I will certainly test the machine out to determine its capabilities. I don't think there will be a problem direct driving this motor on the mill I have now, just wanted some re-assurance. I also plan to some day get a larger square column mill drill. The one I am looking at has a 9" x 32" table which is approaching bridgeport size. I would transfer the stepper motors to the new machine and sell the one I have now.

To answer some questions some may have, I have posted a few pictures. One is of my current mill drill to give an idea of what I'm working with. The next is a picture of a NEMA 34 motor and the end of the table showing that a NEMA 34 will not stilck up above the table surface. The last is of one of the square column mills I have my eye on. I'm not sure if this is the brand I will buy, but it is the size I would like.

As an aside, I got spoiled when my dad had his shop with access to some real nice equipment. I worked for him for a few years, but working with family didn't suit me. He eventually sold his shop and retired. Unfortunately the buyer wanted everything so that the shop would be turn-key, so I wasn't able to snag any of the machines he had. I then worked as a CNC programmer and operator on Cincinnati Millacron 5VC and 10VC vertical machining centers for about 5 years. After that I married and moved and worked for an inventor machining parts for his new inventions. He passed away and I decided to get an engineering degree. Since then, my machining has been limited to my own projects. So as you can see, I'm not exactly new to CNC, but I am new to converting a machine to CNC. I welcome help from those that have been there.

[109jb]

Well I was cruising around the internet looking for information and came across these items in the pictures.

The first is a new machine conversion being sold on e-bay (e-bay link). Of note is that it appears to be about the same rough size as my mill, maybe a little bigger, but the ad states that it is using 1600 oz-in steppers. You can see in the pictures that the motors are direct drive on the X and Y axes. I have included a picture for when the link dies.

The second is also a similar sized mill being sold new. (link) This one uses 595 oz-in steppers, but appears to have a belt reduction on x and y. If it is 2:1 (just a guess), that would give 1190 oz-in to the ballscrew.

The third one is also a new machine that has 1200 oz-in direct drive steppers on the X and Y. (link)

Now these have me second guessing myself regarding whether 880 oz-in will suffice for direct drive. I know it will be big enough for a reduction drive setup, but what about for direct drive. Pleas give me your reasoning, or working examples.

[arizonavideo]

Your over thinking this a bit.

If you have the quill un locked on a RF-31 you will have more than .001" play right their. You don't have to worry about the stepper resolution at that point.

The machine cant really hold sub .001 tolerances.

Some of machines you posted had stock lead screws and steppers which is a bad idea with the 31 because the stock nuts wear so fast. Stock screws also need at least twice the power to turn them compared to a ballscrew's.

With a 5 TPI ballscrew a 500 to 600 Oz stepper will be fine.

One of the best steppers out there is the 570 Oz from kelinginc.net

The KL23H2100-50-4B is all the stepper you will ever need. Get two direct drive and be done.

If they work fine for the Z on a G704 then they will work fine on the X an Y of your machine.

[kensbey]

I hate to disagree, but i do. i am using 1600 oz-in steppers to convert a HM-46 mill. playing with the steppers on my workbench the other day, it is so easily apparent how quickly torque falls off. I wouldnt consider using anything less, especially seeing the cost doesnt decrease much as you reduce size.

the other advice i would give is this. get some good drivers, as they make all the world of difference. splurge for some more expensive reputable drivers, you won't regret it.

[109jb]

...
Some of machines you posted had stock lead screws and steppers which is a bad idea with the 31 because the stock nuts wear so fast. Stock screws also need at least twice the power to turn them compared to a ballscrew's.
...

All of the machines I listed say they have ballscrews on the x and y axes.

[Robin Hewitt]

i am using 1600 oz-in steppers to convert a HM-46 mill. playing with the steppers on my workbench the other day, it is so easily apparent how quickly torque falls off.

You don't need bigger motors, you need more Volts. I fitted 425 oz-in NEMA34's and the torque disappeared PDQ at 30V. I then fitted high Voltage drivers and ran the X screw up to 300 rpm without problem. 300 looked a bit frantic, especially on short transits, so I eased off.

I did the measurement and a NEMA34 direct drive would sit half an inch below my bed. Heaven only knows where I got the notion it would overhang

Micro stepping is great for smoothing curves but it is springy and you don't know if you are on a microstep when you make that long straight finishing cut. I quarter step 5 microns, approx .0002"