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booski
27-07-2013, 04:47 PM
Hi all,

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?

Jonathan
28-07-2013, 12:54 AM
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

Neither as since you have a mini lathe you should be able to bore the pulley to the correct size. Take small cuts towards the end and try to insert the shaft after each and you'll get the proper fit. The drill will be nowhere near and reamers are a bit hit and miss.

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.

booski
28-07-2013, 10:21 AM
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.

Thank you!

John S
28-07-2013, 12:19 PM
The drill will be nowhere near and reamers are a bit hit and miss.



Well get some decent reamers numbnuts.
Reamers come in all tolerance sizes and also diameters.

It's not answering the OP question but I always mill a keyway in the motor shaft and broach a keyway in the pulley, one grubscew on top of the key sees a good job

booski
28-07-2013, 08:03 PM
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?

m_c
29-07-2013, 12:18 AM
If you want reamers, Tracy Tools do them for reasonable money - REAMERS (http://www.tracytools.com/straight-taper-reamers)

The adjustable ones are only listed in imperial sizes though.

John S
29-07-2013, 12:47 AM
The adjustable ones are only listed in imperial sizes though.

That's gotta be a joke yes ?

Like the adjustable spanner marked out in inches one side and mm the other ?

ARC's reamers are far cheaper, Tracy wants 12 for a 3/8" reamer, ARC wants 5.70
Tracy only do metric as a set.

Clive S
29-07-2013, 01:05 AM
That's gotta be a joke yes ?

Like the adjustable spanner marked out in inches one side and mm the other ?

ARC's reamers are far cheaper, Tracy wants 12 for a 3/8" reamer, ARC wants 5.70
Tracy only do metric as a set.

Yes ARC do a good service but for the record Tracy does Metric here :-http://www.tracytools.com/metric-reamers

m_c
29-07-2013, 11:13 PM
That's gotta be a joke yes ?

Like the adjustable spanner marked out in inches one side and mm the other ?

ARC's reamers are far cheaper, Tracy wants 12 for a 3/8" reamer, ARC wants 5.70
Tracy only do metric as a set.

I could of maybe worded that better, but I'm sure you know what I meant.


I normally use ARC, but Tracy is handy for the more oddball stuff that ARC doesn't do, like the adjustable reamers I got last week for a bit head stock tweaking.

booski
30-07-2013, 12:22 AM
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?

Jonathan
30-07-2013, 01:56 PM
And any more info on balancing with a scope?

Something like this:
Balancing Tool (http://www.turbinemuseum.de/Gasturbines/Balancing_Tool/balancing_tool.html)
Or you could use accelerometers...


I assume when turning shop bought pulleys its better to chuck up by the boss and not flanges?

Never hold them by the flanges - they're only pressed on and not very strong. Hold the pulley by the boss and it's sensible to check with an indicator that it's on center.