Awesome New Mill- Is it worth it?

[T Bredehoft]

I agree with Eddyde, above, I do a lot with aluminum, I miss being able to tap below 200 rpm, I can and do mill above 2200, 1/8 four flute, feeding at 8 inches a minute. I lube with rubbing alcohol, it lubes the flutes and leaves no residue.

 

[brino]

What a great discussion and variety of opinions!
I am glad the question was asked.

Thanks all!
-brino

 

[JetMech]

I don't know how to advise you but my mill has speeds from 50 to 1,400. There were times when I would have liked to go even slower than 50 rpm but I don't recall ever regretting not being able to go faster than 1,400 rpm

Well, it's clear that the low end is important and is more important than high end.

I appreciate the responses, all of you.

Since I posted this, I talked with our machinist at work, and he basically said the same and described feeds and speeds of various types of work. He made it sound like it would not be a particular challenge to work within the 6 speeds offered. This doesn't replace me looking it up and learning feeds/speeds myself, however. . . And he operates a full-size knee mill, so imay need to take his advice with a grain of salt.

What occurs to me now, though, is that the real answer may be to buy the non-variable mill and convert it, thus, for the same or possibly less money, having the same or possibly better variable speed setup.

 

[JetMech]

If you are going to be milling aluminum with smaller tools, I recommend you get the variable speed as you will want the higher rpm.
This hobby always costs more...

Right, I'm seeing the high end is important also- it's clear there is a good case for the variable speed. I agree with those who recommend the variable speed and wider range, that is clearly preferable.

My issue now is that I can get the regular mill much, much cheaper and maybe should just convert that myself. But do I want the project?

 

[JetMech]

The way to look at speed is to consider the material and cutter size and then look at the recommended cutting speeds for that material. For example, aluminum is rather soft and requires pretty high cutting speeds. A 1/4" finishing end mill taking a shallow profiling cut of 0.02" deep would have a cutting speed of 800 sfm. This translates to a speed of about 12,000 rpm.

To make this useful, remember that you calculate the rpm required by finding the listed cutting speed of the material and using this formula: RPM = SFM X 3.82 / D, where SFM is the recommended cutting speed for the material in sfm and D = diameter of the cutter. You can look up cutting speed charts on the net.

You will need lower speeds when cutting hard materials with larger diameter cutters. If you plan to do smaller work primarily in aluminum then lower speeds is not as important as the higher speed range. This is especially true if you ever use carbide tooling that requires higher speeds to cut well.

Personally, I would go for the variable speed with the higher range.

What an awesomely insightful post, thank you.

I admit I hoped you'd made a mistake with your math- 12,000 rpm is well outside the range of either the normal or variable speed mill, which tops out at 3000 rpm!

I checked an online calculator and indeed, to get 800 sfm out of 1/4 diameter you'd need that rpm. So what now? Use larger mills for finishing? Feed slower?

This continues to push me towards getting the regular mill and doing my own conversion, so I could perhaps get more speed out of it. But then, I have to ask- how can I determine how fast I can run the gearbox? How would one determine a safe rpm limit for such a thing? Even if I had the motor and drive for it, could the gearbox take 12,000 rpm?

Questions, Questions. . .

 

[JetMech]

I agree with Eddyde, above, I do a lot with aluminum, I miss being able to tap below 200 rpm, I can and do mill above 2200, 1/8 four flute, feeding at 8 inches a minute. I lube with rubbing alcohol, it lubes the flutes and leaves no residue.

Cool, thanks for the input. It's clear that even if I can't get 12,000 rpm, it would still be much better to have even 3,000 at my disposal.

Good tip with the rubbing alcohol, will be sure to try that. Hear what you're saying on the tapping thing.

 

[JetMech]

What a great discussion and variety of opinions!
I am glad the question was asked.

Thanks all!
-brino

These guys are amazing! I'm in the final stages of settling my deal with the vendor, and when it goes down I'll be sure to post details.

I'm leaning towards getting the cheaper mill and doing my own conversion. Talks with the vendor have indicated that the variable speed model is merely the same model of mill exactly, just with the motor swapped for what I'd guess is DC with a controller, but I suppose could be AC with a VFD, but I doubt it.


I can get the non-variable mill for around $500 less, and normally the difference between the two is $150.

So, that implies that the parts to convert cost the vendor about $150 or so. With a $500 discount, I could probably upgrade it myself with possibly a better motor and drive, and get better results. I've done that sort of conversion before.

Ahh, decisions, decisions.

 

[brino]

the variable speed model is merely the same model of mill exactly, just with the motor swapped for what I'd guess is DC with a controller

That's probably true.

With a $500 discount, I could probably upgrade it myself with possibly a better motor and drive, and get better results. I've done that sort of conversion before.

If you haven't see the conversions done using DC treadmill motors, dig around here for some.
I bet you could find a suitable treadmill for the price of hauling it away in your local Craig's List ads.
Though you may need to replace the controller.

Please do share what ever way you go!
-brino

 

[JetMech]

I have done a few treadmill conversions, it works great. For this size motor and my desired results, I'd probably buy the components but the cool thing is that i have some experience with it now from my treadmills and know what to get.

Right now I'm researching whether a 3 phase motor and VFD might be the better choice. You could find success either way I'd bet.

I expect if my mill purchase goes through, I will check out the motor, observe the ratings, and then shop from there, to see if DC/Controller or AC/VFD is the way to go.

 

[Bob Korves]

Variable speed over a wide range is a wonderful thing, but the dirty little secret is that a wide RPM range by itself is not always so useful. There also needs to be substantial torque at the lower speeds, and indeed good torque across the entire range of speeds, and there needs to be adequate motor cooling, and the motor, spindle, and all the drive components need to not fly apart or otherwise fail at higher speeds. A lot of amateur and plenty of commercial variable speed setups do not pass all those tests, and some suffer from mechanical and/or electronic problems and failures. Make sure to do your homework. Find a bunch of favorable reviews of any system you are contemplating going with, especially long term. Reviews given the day of first motor run are not all that useful...