Thread: Pulley grub screw location
I'm currently building a CNC machine for a friend which is an re-worked version of my own.
For the re-designed spindle, I'm going to use a brushless motor and pulleys.
I know that the likes of T5 and T2.5 pulleys are rated for 500w and 40,000rpm so speed and power isn't an issue.
The issue is how to mount them to the shafts as I have limited tooling For example, the spindle itself has got a 10mm shaft so do I
1. Drill the pulley to 9.9mm using my mini lathe and hope that the runout is enough to make the hole 9.95ish for interference fit
2. Ream the pulley to 10mm and use a grub screw
These options are both troublesome as the spindle is intended for high speed use, about 20,000rpm and the motor for about the same, probably more and I don't really know how grub screws will affect the balance unless I use 2 opposing screws.
Simply put, I would like some words of wisdom on this matter?
Don't use two opposing screws as that wont support the pulley properly. It's normal to use two at 90 degrees, but maybe 3 equally spaced since you're concerned about balance.
20,000rpm at 500W is 0.24Nm, so I'd expect grubscrews to hold. It would be better to machine the pulley such that it clamps on to the shaft, however then you would almost certainly have to balance it. Have you got an oscilloscope, or access to one? If so then it shouldn't be too difficult to dynamically balance the spindle shaft with pulleys yourself.
Thanks for that, very helpful.
I haven't got any tool steel small enough to bore a 10mm hole so may have to get some.
What about the pulley for the motor? It will only be a small motor with perhaps a 3.175mm shaft.
And yes, I do have an oscilloscope, a V2 DSO nano but how would I use a scope for balancing? I've seen people balance spindles before using a pair of accelerometers before which I don't really have access too so Im very interested where a scope fits in.
Unfortunately, I don't have access to a mill or broach set and we are talking of very small diameters here. I have found online (from china however) adjustable reamers.
I understand that a cheap reamer may not be wholly accurate whilst an adjustable one you can step it up until you get the right size. The trouble still arises when dealing with small bore like 1/8" for the motor.
Obviously, I could simply turn a length of aluminium down to a reasonable size, drill to 3.1mm and press onto motor shaft. Then with the motor shaft in the lathe, turn the aluminium down to the correct size.
There are many methods, I know, but I'm not really a machinist. I've completed basic machinist courses to the best in class and done various pieces of machining in my old job of which I was frequently called upon to perform but nothing really to this degree of accuracy.
Just to throw another spanner in the works, I have a reasonable size piece of aluminium available to me, what if I were to machine a V-pulley to be used with o-rings instead of buying T2.5 / T5 pulleys and machining them in the hope I don't ruin them?
Last edited by booski; 28-07-2013 at 06:04 PM.
That's all fairly reasonable. Not seen a 1/8 adjustable reamer yet but tempted with the 10mm.
What would be preferred though, interference fit or snug fit with grub screws?
And any more info on balancing with a scope?
My alu tube turns up soon so I can start building the spindle and once its done, ill get some pulleys.
I assume when turning shop bought pulleys its better to chuck up by the boss and not flanges?
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