Centroid Community CNC Support Forum

Along with my spindle encoder modification viewtopic.php?f=20&t=1944 , I'm converting my 2J variable speed head into a single speed belt drive for use with a VFD. This isn't that complicated, but I thought I go ahead and document it here to save others a little trouble.

First some design criteria. I wanted a 5000rpm top speed, and the motor on my mill was rated at 1800rpm@60Hz. Figuring I could safely run this motor at 120Hz, that meant that I needed a 5000/3600 = 1.389:1 speed increase on my new pulleys. For this "3" horse motor, (amperage draw says it's really a 2Hp motor), I decided to use a double band 3VX pulley/belt combination. Looking at available pulleys from Martin sprocket, it looks like a 3.65" OD driven and a 5.00" OD driver gets pretty close at 1.37:1. That means I'll need to run the VFD at 121.7Hz to get my 5K spindle speed. Perfectly do-able.

With a 9.75" C-C distance between the motor and the spindle, I need a 33.127" belt that will be running at 4782SFM at top speed. That speed is well below the recommended 6500SFM max for the 3VX belt. Closest stock size is 33.5" in the banded belt. A quick search on Amazon turned up both pulleys and 3VX335/02 belt for less than $45 total, all with free shipping.

After disassembly (see https://www.youtube.com/watch?v=B56XhVq1_bw), the first step was to slot the motor to allow for belt tensioning. When mounting a tapered QD bushing, the QD bushing maintains is position on the shaft, and the pulley moves along the bushing taper to squeeze it onto the shaft. This can cause problems aligning the pulleys on the shafts because the starting position is not where the pulleys end up when tightened. I'm going to mount the pulleys and hubs so that when tightened, both will move in the same direction and hopefully end up still aligned.

The motor shaft is 30mm and the QD hub is mounted in the standard method. When the hub screws are tightened the pulley is pulled down closer to the motor flange. The spindle pulley will be mounted in the reverse configuration on it's 35mm shaft so that when the hub screws are tightened the pulley will move away from the VS sheave and up towards the motor mounting flange.

On the motor, you'll want to mount the pulley as close to the face as possible to reduce the overhanging load, so go ahead and remove the variable speed sheave completely from the shaft. The pulley will need to be high enough to clear any ribs inside the aluminum cover. On the spindle, it doesn't matter because the pulley is between the bearings and the lower half of the sheave incorporates cooling fins for the motor/belt area, as well as the brake drum so you'll want to leave it alone. You'll need to make some stepped keys for both of the pulleys as the VS sheaves have non-standard keyways and won't match the QD hubs. I mounted the motor pulley first. On my motor, there was a step in the shaft, so I needed to make sure the hub wasn't straddling this step. I then snugged the screws down so the hub was a tight slip fit on the shaft. I then measured the distance from the BOTTOM of the pulley to the motor face. This distance was subtracted from the cover height to give the required spindle mounting distance. The spindle pulley is then snugged on the shaft and the distance from the TOP of the pulley is adjusted to equal the cover height minus the motor pulley distance (don't forget to subtract the thickness of the parallel from the measurement). Go ahead and torque the mounting screws on both pulleys and then re-check your measurements. Mine was out by 0.020" which was probably fine, but I went ahead and measured the bushing height from the end of the spindle shaft and zeroed my calipers there. I then loosened the spindle pulley, moved it (the hub actually) the 0.020" was off, and then re-torqued the hub screws. A quick test assembly to verify everything lined up and I had enough slot length to tighten the belt.

Thanks for documenting this Tom. Also by fixing the pullies, does it not give you room on the spindle for a timing.pulley so as to mount an encoder?
Marty

It does, but it would only be valid in high gear. When in back gear, the face spline on the bull gear disengages with that pulley shaft, turning the pulley into an intermediate shaft in the drive train.

To get to a spot in the drivetrain that is always turning at spindle speed you have to go much deeper!

There is one unfortunate side effect from this conversion. The variable speed sheaves are where the balancing adjustments were made for both the spindle and motor. Both will need to be rebalanced after the conversion. I took some standard split shaft collars and profiled them on the mill to turn them into balance weights. You can do a fairly good balance without expensive balancing equipment if you want. The keyways are probably going to be the light side. Start with the balance weights 90º to the keyway and slowly move them towards the keyway symetrically while checking the balance. At some point the vibration will decrease to a minimum and then begin to increase again. Move the weights back to the minimum point. We now have a good magnitude for the imbalance amount. While keeping the angular relation between the two weights the same, slowly change their angular relationship to the keyway. Once again, there will be some point where the vibration is minimized. This is now the correct angular position.

Another great post, Tom.
I did essentially the same thing, and wanted to reach 5k also I decided to go with a poly-vee belt instead though, and did not use split taper hub pulleys. the thing that I did NOT do was balance the motor and spindle shafts afterwards. It didn't occur to me to do so, although I would have probably realized it once I actually start using the machine (I've not quite gotten the servos tuned and finalized the axis calibrations). I'll be able to find this post again quickly when the time comes to balance the drivetrain now that I've replied.

I do some balancing for a living and have balanced or checked various pullies over the years.
Most from reputable mfgs come already balanced. Some to a tighter tolerance than others.
Granted, in place balancing can trim out any extra vibes induced by off center or outside influence.
I was just wondering if going to this trouble in this case was being over cautious.

Tom,
Since you are using V belts, did you find a source for a matched set?

Dave,

Instead of a matched set, it's a banded v-belt - two belts connected by the same backing. Much better than matched belts.

https://www.amazon.com/gp/product/B00CJ ... UTF8&psc=1

Although it was only $15.95 when I purchased it back in July!

As far as the balancing went, I balanced the motor and spindle separately and got them both down in the 0.1mm/s range, and it did make a noticeable difference. While the pulleys I added were made by Martin and were balanced as designed, the variable speed sheaves I took off were where the original motor and spindle balance was done. Balancing of the new assembly was needed to counteract the keyed shafts that remained from the old setup.

Geeze I have not seen that style of belt before.
Now I know what to look for as I have a lathe getting close to needing replacements.
Cool.

Just an update to this thread. Performance of the stock Chineseum motor was not very good at 120Hz causing overcurrent trips. I ended up replacing it with a Baldor Inverter Drive Motor (IDNM3661T) that is actually rated for 6000 rpm (240Hz). This let me underdrive the system when keeping my 5K max spindle RPM and up my low end torque even more.