[men8ifr]

I have a choice whether to spec my build with 4 or 8Nm steppers

From a calculation sheet I found it appears max speed with 4Nm steppers would 5300mm/min

and 8Nm steppers 3800mm/min

I believe both have the torque to still perform the cuts I need it's the rapid speed that brings the limitations.

I've heard max speed on a dovetail machine will be ~3000mm/min - but I don't understand why.. does the machine get damaged moving faster than this? Does the friction increase at higher speeds, or more likely does the oil get too hot or temperature changes in the slideway oil which causes problems?

I would like to understand this limit - if 3000 is definitely the max I'll get 8Nm motors as the extra torque at low speed *may* be useful..

I will get closed loop steppers if that makes any difference.

[109jb]

I'm not sure where you heard that the limit for a dovetail machine is 3000 mm/min (120 IPM), but it isn't so. Many machines dovetail machines are capable of much higher speeds. Here is a RF-45 machine that is running 300 IPM 7600 mm/min) on x and Y.

https://youtu.be/dpO42wHA9kY

Here is his build thread

https://www.cnczone.com/forums/rf-45...gineering.html

He is using 8 N-m closed loop steppers on the X and Y

[arizonavideo]

The real question with a RF-45 type mill is the cutting speed.

The PM-30 does have a 3000 RPM spindle so you can cut faster than most RF-45 that have 1800 RPM but you will be around 50 IPM with a 1/2" cutter.

As far as rapids for smaller mills you see little time savings over lets say 150 IPM compared to the cutting time, set up time and tool change time.

On thing I did find when my mill was new was a ball screw that is tight and going 300 IPM gets burning hot.

https://www.youtube.com/watch?v=M299KWBsuBs&t=6s

I think for long term use at 300 IPM you would want to have forced oil cooling for a ball screw to get good life span.

[men8ifr]

300ipm is 7620 mm/min so much faster than I've heard before. Interesting the ballscrews temp is the limit, I wonder how linear machines get away with 24m/min..

Seems like ballscrews preload is a compromise between temp and max rapid speeds (if your motors will allow)

[dazp1976]

300ipm is 7620 mm/min so much faster than I've heard before. Interesting the ballscrews temp is the limit, I wonder how linear machines get away with 24m/min..

Seems like ballscrews preload is a compromise between temp and max rapid speeds (if your motors will allow)

You'll get high dovetail & Gib wear the faster you go.
Get the 8nm motors and live with 3000mm/min.

[arizonavideo]

Preload is a comprise like so many other things. A ground screw with lets say 4 rows of balls that fit a conforming contour like a C4 Steinmeyer ball screw that I have in my hand right now cost $1100 for a 25mm screw. It is a faster screw and with some 1000 watt servo will go 1000 IPM just fine with forced oil and not over heat.

With the better fit of the balls and much greater contact area the Steinmeyer is both more ridged and has lower internal preload and friction than a rolled screw will be having near the same amount of backlash.

If you want to trade off some speed and efficiency for lower backlash then a nice tight rolled double nut is a good trade off.

You can have everything but it will cost more. I have been vary happy the rolled double nut HIWIN on the IH mill in the video. After over 7 years of almost daily use I had to re shim the X screw and I'm back to zero backlash on the X screw. I guess its OK to do that every 5 years or so.

[109jb]

You'll get high dovetail & Gib wear the faster you go.
Get the 8nm motors and live with 3000mm/min.

While it is true that increased speed can accelerate wear a bit it is more dependent on proper lubrication. 300 IPM on sliding surfaces isn't all that high considering. I used to work on Cincinnati Milacron VMC's with about 700 IPM rapids and box ways that ran 24 hours a day for years with no issues with slide wear. Granted dovetails are a bit different, but for a PM30 converted mill, I would say you are talking years of frequent use before any wear issues would ever present themselves no matter what rapids you choose..The rapid traverse moves only happen infrequently on typical jobs anyway.

As an example of the importance of lubrication, consider a car engine. I have a Honda civic with over 300,000 miles on the original bottom end. With about 2" diameter journals and an estimated 1500 rpm at 55 mph and 300,00 miles of use the sliding distance between the bearings and journals is about 3 billion inches. At 1500 rpm the sliding speed is 9,400 IPM. This is with proper lubrication. Take the oil out and run the engine and the life of the engine/bearings even at idle speeds would be measured in minutes.

Bottom line is that I would not worry about 300 IPM causing undue wear on a converted PM30