[fpworks]

Hello,
I have been reading for a while, but this is my first post. I have a question with regards to some parts made with my Sherline 5410 series mill.

I seemed to hit the limit of my operations the other day while cutting 7075-T6 aluminum.

DOC: 0.030
Spindle speed: ~5000 rpm (no tach, but theoretical 5500 rpm no-load)
E-mill: 1/8", 2-flute cobalt, standard-ish LOC.
Feed: 10 ipm
Chip load: .001 ipt
Cooling: Trico (Tri-Cool I think) synthetic mist

There appeared to be some "following errors" on the small radius curves. (as if the cutter was not accurately following the expected tool path) The larger radius corners looked fine but the overall finish was poor. (expected) There were no abnormal vibrations and the machine seemed happy, but the "following errors" on the parts were unacceptable. I do not get these errors when I reduce the feed rate.

Where could my problem be? Too high chip load? Cutter deflection? Have I made an error in my feed/speed calculations? Steppers can't keep up?

I ask this because I am concerned that my steppers may be at fault, and I have no reservation on upgrading the steppers or converting to servos. However, I understand that the feed limitations of Sherline spindle may make upgrading the steppers a waste. Is upgrading the steppers a waste on this machine?

I have some short LOC carbide 3 & 4 flute endmills on the way to hopefully rule out cutter deflection.

Any advice is highly appreciated!

Justin

[fpworks]

Well,
I pretty much ruled out cutter deflection. Ran the same program with a carbide 4 flute stub endmill. Same speed/feed, and same problem.

If I back off the feed, this error goes away. My only guess is that the stepper/driver is the culprit. I am a bit stumped because I am not loosing any steps as the rest of the part is fine and the machine is running very smooth.

Am I overlooking something else here?

[Ken_Shea]

Justin,
Your rpm, chip load and feed was high for the HSS but OK for the carbide.
I do not pretend to have a answer for you and am just thinking out loud here, my guess is you have nothing wrong with the stepper's but it seems that your equipment may be running ahead of or behind the software, or perhaps a interpolation setting in your controller, just something to think about.

Ken

[fpworks]

Ken,
Thanks for the reply. I knew the 2-flute HSS endmill was pretty much maxed out, but the error was repeated identically over 30 times.

With the carbide endmill, I tried cut depths from .010 to .050 with no change in error. Reducing feeds is the only apparent way to reduce the error...5 inches/min produced no visual error, but that is SLOW!

I think you may be on to something in terms of the hardware/software interface/or controller settings. I am running Sherline's turn-key setup, with Linux and EMC...but I have been running the G-code straight from the floppy disk (not copying it to the harddrive). I believed that the entire g-code file is loaded in the computer's RAM when running the steppers...someone let me know if I am mistaken.

Thanks,
Justin

[fpworks]

I believe I have located the problem. Excessive backlash. At higher feedrates, it appears the steppers cannot "take up" the backlash fast or smooth enough when carving fast/tight curves (radii from 0.04" to 0.4").

I measured 'X' backlash at 0.004" and 'Y' backlash at 0.003". Unacceptable. I have new brass leadscrew nuts on the way and I will update with my results after this weekend.

I'm starting the search for a ballscrew equipped mill for my CNC duties.

Justin

[fpworks]

Severe backlash was the issue for sure. I have found a ballscrew equipped mini-mill that will allow me to use my Sherline spindle and tools.

I will be upgrading to servos, but I need some advice on how fast of a system I can effectively use with the Sherline spindle. There are large variations on how much money I can spend on a servo system.

In my limited experience, the Sherline motor and spindle has far more ability than the Sherline mill and/or stepper system (my steppers run out of speed before anything else complains). Having said that, I don't know the load limits of the Sherline spindle.

How fast/deep can I expect to feasibly cut with this spindle? This will enable me to select a proper servo setup without going overkill ($$$).

(with 1/8" and 3/16" carbide tools, 7075-T6 aluminum, I have 2800, 5500, and 10,000 rpm pulleys)

Again, I appreciate any advice.

Justin

[Swami]

Got an update to this? I have a Sherline, and have done my first tune-up. I am not getting unacceptable errors, but then again, I am making low tolerance parts.

Did you get something else? How is it doing?

Swami

[fpworks]

I ordered a Minitech Mill/3 shortly after I wrote this last post...and STILL haven't gotten it. Apparently x-axis ballscrew is on backorder.

My Sherline has been retired (and it is fairly new)...it just isn't cut out for production CNC work. I will take that a bit further:

<START VENTING>
The Sherline was originally designed for small job, low use MANUAL machine work. CNC + coolant + carbide tooling allows you to easily run the Sherline's feed rates at full blast in aluminum (plus some) and the little mill's lead screws/brass nuts wear too quickly for anything other than the occasional hobbist. </STOP VENTING>

I'm in the middle of moving right now, so my machine work is on hold until I get my new shop set up. (~2 months from now) I'll post how the Minitech mill is working out when I get it running. I have seen these mills in industrial applications and seem very well suited to my application.

[Swami]

And you had to pay how much for the minitech?

I didn't THINK I wanted to do production work, but I am having too much fun. I dont want to outsource anything, lol.

I can dial in my sherline to about .002 X backlash and .004 Y backlash, which is still acceptable to me for now, but I have only been using it one month. I actually cannot even detect it in my parts, but I try to compensate by how I approach and by using the software settings.

Thanks,
Swami

[fpworks]

I believe I paid ~$6500 for the Minitech, but I ordered a different control system (10 amp DenverCNC with Geckodrives), asked them to delete the spindle/motor/controller and no software. This knocks off ~$2000 off the price.

I was getting similar numbers as you for backlash...which isn't good enough for me. Especially since I'm doing curved surfaces, the software compensation can't take up the backlash quick enough unless I run impossibly slow feed rates.

Take care,
Justin

[bernielim]

What is the max feedrate for the sherline mill