New Mill build log, and looking for help

[jalessi]

Hedgehog23

Many industrial CNC machines are factory fitted with drive belts.

Backlash is not a issue as long as the belts are tight.

The KL23H284-35-4B will be fine for the X and Y axis, the Z may be a issue depending on the weight of the finished axis with spindle assembly.

Jeff...

[bad bearings]

You could get yourself an air spring for the z axis to reduce the gravitational load. SPC or an auto supply store would have one--for a trunk.

[veteq]

Hey,

in my opinion direct drive with the 380 steppers will do and is in most cases overkill, but offcourse it all depends on your application and design.

imagine you want a perfect system, no tolerances and backlash, a belt always brings backlash in time and it gives an extra travel error to the system
(same as c3 or c5 or c7 ballscrews, the tooth profiles are not ground and precision parts)

Big vmc`s cann use them because they use a servosystem with encoders on the motor and also a linear encoder (scale) next to the table, the computer is constant calculating and adjusting the servo to keep the table in the right position. (thus canceling the backlash and travel error in the mechanical parts.

If you look at the x and y axis, gravity isn`t a problem, since the axis of freedom is in a different direction (in z axis the gravity works in the same direction as the axis off freedom, so the stepper continiusly has to overcome the gravity and less torque cann be used to accelerate and deliver force to mill).

For x and y axis,
the stepper will give torque, this is by the ballscrew converted to a force.
- This force can be used to accelerate (F = m * a)
- Overcome the force generated by milling (difficult formula, but always low force)
- Overcome the resistance to rotate the ballscrew, accelerate.
( moment off inertia http://en.wikipedia.org/wiki/Moment_of_inertia )

imagine, you`re table weights 20kg and you want 10 m/s^2 acceleration,
(This acc. is the same as a fast expensive vmc!!!)
force=mass*acceleration so, 20*10= 200N off force (nothing)

The force needed for the real milling is very low and lets say, never higher then 200N.

Then the force for rotational acceleration off your ballscrew, offcourse it takes more torque to accelerate a 40mm ballscrew then a 14mm diameter.
Most force gets in rot. acc. off the ballscrew.

A ballscrew with a lead off 5mm will generate with your 380 steppers around 2800N off force on the table. so, 2800 - 400 is 2400N left, or 2,4Nm for overcoming momentoff inertia.

This 3 parameters off force cann be easely calculated after a weekend off reading some lecture. This are the main calculations needed for a mill with linear bearings, ballscrews etc. (for the z axis gravity comes in to play f=m*a
a = 9,81m/s^2).

So it will all depend on the weight of your moving parts, needed acceleration and diameter off the ballscrews. For highest speed look at the point in the torque curve off the stepper where it is the same as the needed total torque you calculated. then take a FOS.

Remember that resistence off the linear bearings and ballscrews are almost 0.

regards,

Roy B.

[hedgehog23]

interesting read , thanks Roy. anyone else think the 380's will cut the mustard?

[LeeWay]

I used the keling old style 495's. They are better than the newer ones I've seen there. I used 203V's and a 72 VDC PS. Everything else I used is much the same as yours. I found these are great for X and Y, but was lacking on Z. I wound up using a timing belt and gears from https://sdp-si.com/eStore
Then I was satisfied with it all. Plenty strong now. The other day I was using a 3/4" end mill and found that I bogged the spindle motor before any missed steps. It was bad coding on my part and shouldn't have used it the way I did. Making a 7/8" hole in 1 3/8" thick CRS bar. Smaller EM's work much better.
I was hogging .05" and running it at 3200 RPM @ 25 IPM. Wayyy too much for my little motor.

[hedgehog23]

a few update pictures. im on holidays for a bit so i should get some real work done.





need to pick up some metric hardware , and make up a block for the ballscrew nut.

[hedgehog23]

just looking for some feedback on some spindle considerations.

http://littlemachineshop.com/product...ory=-269978449



combined with a Tormach tooling system basic kit



http://www.tormach.com/Product_TTS_sets.html

then a homemade air actuated draw bar.

top it all off with a belt drive , dc drive vari speed motor.

comments welcomed

[hedgehog23]

im a little concerned over the two speed plastic gears in the housing. id like to use something off of the shelf to save build time but i have my doubts on this one.

[tootalew]

LMS Has metal replacment gears available.


http://www.littlemachineshop.com/pro...3456&category=

http://www.littlemachineshop.com/pro...3457&category=

[LeeWay]

All that is what I used as well. It works great for me.
I would ditch the gears though.
They will give you some spindle torque, but thats really only useful if you intend to hog off material.
The spindle itself would then be the limiting factor. I would likely just buy the head casting and spindle parts, less the gears and bearings. I would install new bearings from somewhere else.
I think mine came from Mcmaster Carr or VXB. Much better performance than the OEMs.

Go with a belt drive and after market motor as well.
I was able to get 6600 RPM. That speed works nice on aluminum.
I still get lots of torque at slower speeds with a single belt.
I use coolant. My slowest speed that I use is about 2500, but do use 2000 with a 1.25" facemill for some things.

[hedgehog23]

All that is what I used as well. It works great for me.
I would ditch the gears though.
They will give you some spindle torque, but thats really only useful if you intend to hog off material.
The spindle itself would then be the limiting factor. I would likely just buy the head casting and spindle parts, less the gears and bearings. I would install new bearings from somewhere else.
I think mine came from Mcmaster Carr or VXB. Much better performance than the OEMs.

Go with a belt drive and after market motor as well.
I was able to get 6600 RPM. That speed works nice on aluminum.
I still get lots of torque at slower speeds with a single belt.
I use coolant. My slowest speed that I use is about 2500, but do use 2000 with a 1.25" facemill for some things.

Sounds great, any pictures of your spindle , or a build log link?

[RICHARD ZASTROW]

Once synchronous belts are "run-in" and re-tensioned they stretch very little. The new carbon fiber reinforced belts are very good. We have used these belts on (6) axis, servo powered, gear hobbers to produce class 10 and better spur and helical gears.

Gears have more backlash than belts, which in theory, have no backlash.

$.02 Dick Z

add: Our hobbers have air cylinder counterbalance built in on the vertical axis.