choosing the right electromotor, please help.

[Giel]

I am building a concave cutting machine. (or I am trying to build one  )
But I have trouble choosing an electromotor for the cutter.
I am buying a reversible DC motor but I dont really know how much torque I need.
I have been looking at torque conversion tables but I am kind of stuck.

I was thinking to buy this motor (24 volt, 36W, 3000 RPM):
https://www.ebay.com/itm/36W-Permanent-Magnet-DC-Motor-2000-4000RPM-High-Speed-Large-Torque-Motor-12V-24V/302564825509?hash=item467244d9a5:m:m5uIJ9PnJ47Tm0HqWOMKUSQ:rk:36:pf:0

Would this be sufficient or would it be overkill?
thanx in advance for the advice!

[MakkyBrown]

Hi Giel,
I be pretty temped to use a cnc spindle with the collet already attached to it. Have you read FlashGP thread, he was having trouble with run out.Maybe belt driving the collet shaft might not be a bad option.

http://aussielapidaryforum.com/forum/index.php?topic=6456.msg56588;topicseen#new

Plenty of cnc spindles on ebay. Interested in how you go with it.

CheersAndrew

[Giel]

Thanx for the reply Makky.
Yeah I read the other post about the concave cutter, didn't know if I should have asked on that thread or create a new one.
I have already been looking for a cnc spindle on ebay, but most seem to run 12000 RPM and up.
They are also just the motor with a separate colletholder. (the affordable ones)

I also considered to make a separate collet shaft driven by a small belt, but I need extra room on the machine for that, meaning that option makes the whole setup bigger, thus creating other problems.
For now I cant makeup my mind....

Just considering the torque of the motor, do you think its sufficient, to little or overkill?

[MakkyBrown]

From memory might have been Ultratech or Polymetric spindles speeds weren't slow might have been 8000rpm.

Not sure on torque.  Also worth making the machine really rigid so it doesn't flex much when you push the stone on the mandrel. I suppose the more rigid the machine the more torque you could use when cutting.

[MakkyBrown]

polymetric 5000rpm
http://cyberrockhound.com/polymetric.htm
Ultratech 3000rpm
http://ultratec-facet.com/Fantasy-Main.htm

[MakkyBrown]

Regarding the torque I'd be tempted to go with more if I built one. I'll have a think over the next day or so and compare it to some motors I have.

[Giel]

Regarding the torque I'd be tempted to go with more if I built one

yeah I know, the project is already spiraling out of control! Might as well get good linear bearings...precision machine parts....etc etc.
Someone sold me a bunch of 20mm linear bearings and thats where the problem started.....
The bearings are very good quality and very tight but big and heavy, so I need 20mm linear rods and the whole machine gets bigger that way.


I prefer a robust machine, but it needs to fit on a workbench.
I want to make it so you can turn the spindle 90degrees but that takes a lot of space on the workbench. (difficult to explain wihout drawings)

I am drawing the machine in 3d in tinkercad, I m used to drawing in tinkecad because I am into 3d printing.
But I am not familiar with solid works or autocad. So I cant show the drawings...unless you can open an stl or obj file.

thanx anyways for willing to help out!

[Giel]

would you like something like this? 

https://www.ebay.com/itm/ER16-Spindle-Unit-Power-Head-3000rpm-5Bearings-for-CNC-Drilling-Milling-Grinding/192021811919?hash=item2cb56452cf:m:mH1qSUiLxV4Yx9NfIJCBoFQ:rk:2:pf:0

big and burly heavy duty

[MakkyBrown]

Instead of the linear rods, I purchased some on these for my new faceting machine, they seem pretty nice. I reckon they'd be pretty good. Or this style in better quality but they are expensive.

https://www.aliexpress.com/item/2pcs-12mm-Linear-Guide-MGN12-150-200-250-300-350-400-450-500-550-600-mm/32912475672.html?spm=a2g0s.9042311.0.0.27424c4duhiDIh
Yeah that power head looks nice, but you only probably need ER11
Try Fusion 360 it is pretty good and you can get a free license. I've been using it for awhile but I'm still a bit of a novice with it. My sons pretty good with it so I can get help

[Rusted]

I don't think you would need all that much torque, look at the area of contact, just a few square mm max, there's no way you are going to be leaning on a stone enough to slow the shaft.

RPM, a bit of maths could give you what your expected RPM should be, look at the surface feet per minute (dropping back to imperial) on a normal lap then calculate what RPM you would need to get similar on whatever diameter you are using in your concave cutter.

I think the biggest consideration for this motor considering how rapidly it's spinning is noise, you don't want something screaming at you while you are faceting, it would get tiring very quickly.

[MakkyBrown]

Rusted what about for roughing/coarse cutting on a big stone. If your doing that on the machine I'm thinking you could potentially use quite a bit of torque. 
Good point about noise.

With motors if I did one I'd probably go for a motor with an 8mm shaft.I was just looking at these, not sure though.

https://www.omc-stepperonline.com/brushless-dc-motor/24v-3500rpm-047nm-172w-104a-%D1%8457x69mm-brushless-dc-motor-57blr70-24-02.html
I purchased a fair few steppers from them and always really good motors.

[MakkyBrown]

I've got one of this out of a huge HP printer.Model number is 1.13.044.246.50
Says 15vdc
Shaft is only 6mm but.
Dead quiet and very hard on the fingers trying to stop it grabbing the pulley. I think If I did one I'd might be tempted to use this motor due to the quality.
http://mugul.com/wp-content/uploads/2017/02/DC-Motor_51x88_1.13.044.2XX.pdf
Thoughts?, you can get them on ebay salvaged from printers. I might do some more tests on it.
Search  "buhler 3140-1081" on ebay

[Rusted]

Makky would you even use a concave machine for doing roughing?

I thought the idea was to cut a tier or two concave not the whole stone.
I don't know, I think I would be roughing the stone as normal then swapping the mast to the concave machine to finish the concave facets.

That looks like a decent motor, I have a couple of nice Siemens motors saved from old Teleprinter machines stowed away for this project.

[MakkyBrown]

Rusted, I don't know mate, I've never even seen concave faceting done  Be tricky polishing a table on it so your probably right. After feeling the torque on that motor I'm thinking it would probably be enough. I just found another motor the same brand with an encoder on it with a worm drive, maybe I could use that for the oscillator.

Have you got your encoder going yet?, offer still stands send me the bits and I'll see if I can get something basic going for you to get you started.  Did you ever get your Arduino board going?? it might be missing the boot loader as from memory you bought the cheap Chinese ones.

[Rusted]

Thanks Makky. No same with me, when I see concave faceting done it will be a first.

The arduino is fine,library for the display is the problem, but not really a problem I can just fall back to a very basic display and it will be a goer.