Diy Brushless Spindle

[Jonathan]

Unless I'm missing something obvious the collet chuck shaft isn't actually fixed axially to the angular contact bearings?
The outer rings of the angular contact bearings are fully constrained by the housing and axial force from the Belleville washers, however neither of the inner rings are attached to the shaft, so the two angular contact bearings are effectively acting as just a standard radial bearing with zero clearance. Currently when the shaft is subjected to an axial force it would move, except for the grubscrew and bearings in the motor - so it is those standard bearings in the motor that are actually resisting the axial force which defeats the object of using angular contact bearings.

You need to somehow fix the inner ring of the angular contact bearings to the shaft - best way is probably to machine a fine thread on the shaft and use a locknut (rather like on ballscrew mounts). You then have both bearings facing the other way round and place the Belleville washers between the nut and bearings and use the nut to apply a known preload by measuring the compression distance of the springs. This is how I've done it on a ballscrew... same principle applies. The problem is the shaft will be hard to machine so someone will suggest you simply locktight the lower bearing to the shaft, but that's rather crude.

The preload required must be greater than the maximum axial cutting force and greater than the minimum load specified by the following formula:
http://www.skf.com/portal/skf/home/p...newlink=1_3_10
This avoids sliding friction within the bearing which will lead to excessive power dissipation and thus heat build-up. However equally you don't want to use more preload than necessary as clearly that will also cause the bearing to heat more. You could machine slots into the housing with a parting tool to make fins so the greater surface area increases heat dissipation. Even better, make a channel in the housing (or mount) and pump water through it.

[njhussey]

Unless I'm missing something obvious the collet chuck shaft isn't actually fixed axially to the angular contact bearings?

If the fit of the bearings on the ER11 shaft aren't a press fit then there would be a problem. I've just discovered my bearings slide up and down the shaft!

Isn't the spindle up pressing up against the inner race of the lower bearing? So wouldn't this mean that any axial load is applied to the inner race of the lower bearing and the Bellville washers would transmit the load to the upper bearing?

[Jonathan]

If the fit of the bearings on the ER11 shaft aren't a press fit then there would be a problem. I've just discovered my bearings slide up and down the shaft!

If the bearings were a press fit the belleville washers wouldn't do anything - you must have a sliding fit so that the springs can apply a preload.

Isn't the spindle up pressing up against the inner race of the lower bearing? So wouldn't this mean that any axial load is applied to the inner race of the lower bearing and the Bellville washers would transmit the load to the upper bearing?

Yes for an 'upwards' axial force it's constrained - but not downwards. If it wasn't for the normal bearings and grubscrew the shaft could just fall out.

[Villacherman]

Thanks for the replies...

How about if I add a shaft coupling to the stop with a couple of belleville washers? Then using the collet head pressed up against the lower bearing inner race and the shaft coupling, you would get your axial constraint.

I would also load the angular contact bearing in the same orientation (both loaded with "upward" force). This would make the bearings strong in plunge operations, but weaker in operations where the bit is moving up out of the work piece. Would this be a problem though? Most operations are pressing into (down) the work piece.

[blackburn mark]

I would also load the angular contact bearing in the same orientation (both loaded with "upward" force). This would make the bearings strong in plunge operations, but weaker in operations where the bit is moving up out of the work piece. Would this be a problem though? Most operations are pressing into (down) the work piece.

on most occasions your tool would be pulling down as you are cutting but as i have said above, get some tension in those balls and and loctight your nose bearing and you should be fine

bearing heat may be an issue if your running above 10000rpm (keep an eye on the temp though as it may take a while for your bearings to settle in) if your cutting plastics and pcb's and your not in a crazy arsed rush then 10000rpm is a tad fast... i cut acetal at between 2000rpm and 4000rpm (6mm 3mm tooling) im sure i read somewhere that anything up to 110 degree C and your ok .... mine dont get anything like that hot

the small engraving spindle i made gets pretty hot when i run it above 30000rpm so i tend to run it closer to 20000rpm , i dont run that one very often so its hard to say how long the bearings will last?

[njhussey]

I'll reply to the rest when you've done a drawing...

Well I've had a look at the design and tewaked it a bit. Will probably do a MK2 spindle using this method or I might tweak this one I've already machined. this one is only really going to be cutting up to 6mm balsa and 3mm ply so I can't imagine the forces as being huge. I might stretch to some delrin/acetal and even try some Ali for a laugh



This shows the top collet (but I haven't bothered showing the grub screws) on the 8mm shaft which is pre-loading theinner races against the bottom collet. I'm guessing I could get away with not securing the bottom collet (locktite) as it will be trapped between the bottom bearing inner housing and the ER16 collet.



This shows the assembled unit (minus the bolts but we all know what they look like!!)



The bearings in the above are not spaced the correct distance, they will be a further 10mm apart so I can get some Bellville washers in between.

This design has the inner races locked to the 8mm shaft and the outer races are constrained by the endcaps...............comments?

[njhussey]

If the bearings were a press fit the belleville washers wouldn't do anything - you must have a sliding fit so that the springs can apply a preload.

Ok, yes I can see that.

Yes for an 'upwards' axial force it's constrained - but not downwards. If it wasn't for the normal bearings and grubscrew the shaft could just fall out.

How does the motor work with a prop on it in a plane then? There is a circlip to stop the shaft pulling out if the motor is used as a pusher or reverse mounted but nothing to stop it being pulled out apart from the bearings and the grub screw as you say. This seems to be enough for a model so I'd have thought it would have been enough?

Maybe if I machine 2 steel collars 8mm ID and 10mm OD and secure them with grub screws (locktited in of course!!) one above the top bearing and one below the bottom then that would stop the movement? Would the grub screws be strong enough?

I'll do a drawing later as on the mobile...

[Jonathan]

[QUOTE=njhussey;30976
How does the motor work with a prop on it in a plane then? There is a circlip to stop the shaft pulling out if the motor is used as a pusher or reverse mounted but nothing to stop it being pulled out apart from the bearings and the grub screw as you say. This seems to be enough for a model so I'd have thought it would have been enough?[/QUOTE]

On a plane the magnitude axial force (thrust) from the motor is much smaller than in our application, plus accuracy (end-float, radial clearance etc) is pretty much irrelevant for just spinning a propeller. The bearings will wear out much faster than if they used angular contact beraings, but the cost isn't justified. If sufficient axial force is applied to a deep groove bearing the balls will ride up the sides of the rings outside of the superfinished races on to the rougher (relatively speaking) area of the ring, causing increased wear.

I'll reply to the rest when you've done a drawing...

[blackburn mark]

i personaly wouldnt get to bogged down in all the rocket science... make sure you get some tension in your balls and make sure your shaft cant easily rotate in the bearing bore (loctight the nose bearing if its a tad slack) and all should be fine

any slack will result in balls skidding or a sypathetic rolling rotation/vibration between your shaft and your bearing bores
(a very short life span for either and poor finishes)
iv used super cheap skate bearing on my er11 with aprox 2kg of tension between the two and its still going strong
if you get any high pitched squeals/squeaks etc when the spindle runs up or down you need to fettle

the beauty of these spindle is they are cheap n simple
iv seen threads on diy spindles that are works of art and that have followed the theory to the letter, however, i get good finishes in acetal and alli so seen no point in getting all posh if the aim of the game is to get yourself up and cutting :)

PS: your bellville washers are bagged n ready neil, i just need to get off my arse and post them :(

[njhussey]

PS: your bellville washers are bagged n ready neil, i just need to get off my arse and post them :(

Cheers Mark, no huge rush as I've still got lots to do on the router (everything apart from the base!!) which I'm hoping to break the back of over the weekend.