Designing a Metal milling bushless Spindle

Hi all,
im new to Diy CNC'ing in general & finally bit the bullet & started ordering parts for my own 3-axis router. i started ordering the parts looking at the Target'ed work envelope & pretty soon i realized that it would be better if i start building the machine around the Spindle. therein lies my problem as i would like to mill metal & i would appreciate some help from you guys collective experiences.

at the moment im researching brushless motor choices against:

1. power requirements(W)
2. Suitable Kv(RPM/V)
3. power source (coupling matched pairs in series &/or parrellel to reach the target motor operating range- not necessarily motor max)
4. shaft diameter & coupling method(ie:- replacing the motor shaft with the collet shank / end-to-end-lovejoy type / threading the motor shaft & screwing it into the collet shank) ect.

as a compromise for spindle stability vs bearing RPM Max vs target RPM for metal, i figure a 12mm shank dia was a good starting point so i went ahead & bought a "C12 ER16 100mm collet chuck shank" along with 2 matching single row a/c bearings & a radial bearing(good for 20k on oil).

im hoping i'm on the right track so far,

im really hoping you guys can point me in the right direction on the points listed, and any others i need to be aware of. how would you go about tackling this build?
hopefully this will turn out into a dedicated build log.
Thanks,
Kevin

http://www.mycncuk.com/forums/showthread.php?t=3793

That thread may interest you!

Many of the larger motors are around 500kv, these motors allow you to replace the motor shaft with the collet system.

You can use higher kv spindles but their diameter reduces and the vibration obviously becomes worse and needs balancing.

Another good spindle totally re made to test your engineering skills,

http://www.dieselrc.com/projects/cncspindle/

I have my 450kv motor running on a 24v power souce using a 100a speed controller, i rarely go over the limit of the power supply and trip it, but when i do its usualy an error!

There will be more experience people along soon to help with the bits i cant :) and of course welcome!

thanks totts,
i read both of them, the dieselRc build is impressive, but lacking in detail as to how & why regarding motor choice vs power requiements to cut metal. ie, typical spindle rpm/feed rate/depth of cut ect...
yours is approx 10k rpm, what kind of feed rates & depths is it comfortable with? im assuming you have an ER collet series?

Hi, yes it is a bit vage! My cnc isnt highly robust so roughing cuts are usually a 6mm slot drill 0.5 deep at around 6000rpm, usually at 200mm/m. I do a lot of engraving so its usually running flat out, but finding i need higher rpm, so currently finding other solutions. There are calculations that can be used to find the apropriate speeds and feeds, but machine stability limits these.
I currently have a er20 set up as this will cope upto 13mm shanks which is ideal for 1/2" shanks.

Hope this helps.

finally recieved all the parts to start designing the spindle, pretty much whats been seen on the forum. Attachment 8129. the motor is a NTM Prodrive 35-42 1000kv unit from HK. Im targeting 12k RPM as its my bearings upper limit, on a yet to be converted ATX PSU.
.

Im guessing it should be 1000kv? On a 12v atx power supply?

Have you got a link for that motor?

What size shaft has it got?

I may have to make another spindle for the higher rpm ..... Look what you have done haha.

my mistake, it is 1000kv, on a 12v atx PSU. NTM Prop Drive 35-42 Series 1000KV / 700W shaft is 4mm, but im using a coupler bolted to the motor housing & clamped at the ER shaft end.

Attachment 8140

Ahhh i seee now! Hopefully you wont get any vibration / miss alignment from the joint, keep us posted!

well, i got the spindle done!.. Attachment 8756

a shot of the coupler, i had to redesign it to clamp the shaft using grub-screws. the entire construction is using EN19.

Attachment 8760

Looking smart! Have you had it running?

How did the converted atx controller go?
I have still yet to design another spindle :(

thanks, i hav'nt yet converted the ATX so no test run yet, im also anxious to check the runout.

what are you looking to cut with the new design? will it be an ER collet?

much too early, but im looking to order a NTM 5060 & design a unit to hold an Arbour. i really want face milling capability, & ER collets dont seem to have that option.

Thats a big motor, will get some nice grunt from that but will lack rpm, would you be gearing it up a bit?

Being able to use a shiney new cnc lathe that i can get my hands on, im going to be trying to design and make a bt30 atc spindle. May be a bit optimistic. But time will tell.

very nice indeed. what size bearings did you use?

How have you pre-loaded the bearings? For machining metal you would ideally have a decent amount of fixed pre-load but for high speed they are normally spring preloaded. Having said that most of the spindles on ebay etc are spring preloaded and I believe people use them milling metal ok, as with all design its a case of compromise you have to decide how fast you want to cut metal as in reality you will just have to take slower and shallower cuts. This is the same problem I came up against when I was looking at a spindle capable of milling meatal and routing wood. I have put mine on hold as I have decided to go down the adjustable hydraulic pre-load and air/oil lubrication route, which means a lot more work and probably a few trial runs.

With all steel construction how are you going to deal with the heat dissipation at high speed? would a finned Ali housing for the motor be better? or are you using the top as the mount to the machine?

Look forward seeing how you get on with this.

@ Ross, im using 2 12mm ID AC bearings up front, Nachi #7201B i think they were. Rear is a 12mm radial bearing.

well, on the preload issue, i've got the bearing pocket slightly undersized in depth & im using a shim between the bearings. preload is by tightening down on the bearing retainer cover. its not very high tech, but i figure im going to run this as is & see how long it lasts. not very scientific i know.

as to the Heat dissapation, ive got one big vent hole to draw in air on the front of the square block with 4 smaller vent holes on the top of the block, where the motor mounts. the square block will bolt into the Z-axis moving plate with 4 bolts.

one thing im worried about is the weight. its at 5kg already, i designed my z axis on 4 THK RSR 9WN's... :dread:

Quote:

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Originally Posted by totts

Thats a big motor, will get some nice grunt from that but will lack rpm, would you be gearing it up a bit?

Being able to use a shiney new cnc lathe that i can get my hands on, im going to be trying to design and make a bt30 atc spindle. May be a bit optimistic. But time will tell.

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man would i love to work on a CNC of my own =P.... oh well, guess thats why im here right? yeah thats a pretty decent looking motor. i figure i could convert 2 ATX's & pump out 24V @ 20A. like i said, its gonna be a future project.

Yeh luckly our work bought one :), so by converting an atx what is involved?

Quote:

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Originally Posted by totts

Yeh luckly our work bought one :), so by converting an atx what is involved?

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lots of good tutes on youtube but i plan to follow this; Converting Computer ATX Power Supply to Lab Bench Power Supply - YouTube
my atx throws out 20A on the 12V lines.

Quote:

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Originally Posted by Kevman

@ Ross, im using 2 12mm ID AC bearings up front, Nachi #7201B i think they were. Rear is a 12mm radial bearing.

well, on the preload issue, i've got the bearing pocket slightly undersized in depth & im using a shim between the bearings. preload is by tightening down on the bearing retainer cover. its not very high tech, but i figure im going to run this as is & see how long it lasts. not very scientific i know.

as to the Heat dissapation, ive got one big vent hole to draw in air on the front of the square block with 4 smaller vent holes on the top of the block, where the motor mounts. the square block will bolt into the Z-axis moving plate with 4 bolts.

one thing im worried about is the weight. its at 5kg already, i designed my z axis on 4 THK RSR 9WN's... :dread:

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That's the best way to do the pre-load. Most of the high end ones are done that way as it keeps the pressure equal, the draw back is that it is not easy to adjust.