I just got done doing up my mill to go faster and thought I'd just drop a thread to help others. My mill is a 325J. It has the 3500RPM spindle, with the Yaskawa spindle motor and formerly yaskawa drive (swapped it out for a Teco MA7200 last summer when the Yaskawa poo'd the bed). The mill was in near mint condition when I got it and spent most of it's life cutting plastic so I kind of lucked out. Spindle bearings are good as was just about everything else.

Parts required are:
5.2" 8L taperlock Pulley
Appropriate Taper lock Bushing
63.5" 8L Belt (635L8
New 8mm key for the pulley (mine was 8mm YMMV)
Maybe new Precision Matched bearings for the spindle pulley.
a short jumper wire
Possibly a torch
Possibly a bolt type wheel puller
Some soap for your mouth so that when its done you can feel better about kissing your wife or mom

Getting it done:

So first you need make with ordering the parts. I got this pulley. Note that the screws are British Whitworth. This doesn't really matter as long as you don't lose the screws, which do come with it. And don't let the 10 groove scare you away. The pulley is an L pitch so you will just have 2 unused grooves, the distance/pitch between the V's is the same. Big whoop. The thing is, this is probably the only pulley readily available to make this happen otherwise it is a 10-16 week wait for them to make one (in china by the way, not too happy about this but beggars can't be choosers). Next you will need the appropriate taper lock bushing. This is the one I got. The spindle on my motor is 28mm and this bushing is also British Whitworth to match the pulley. I ordered on Wed and they parts showed up on friday. Very happy with that. For the belt I hit up amazon as it seemed like the best deal and the only place I could really find what I was looking for. I grabbed this one which is a 635L8. After that you might need a new Key for the motor. Mine was stepped. The motor shaft was an 8mm but the stock pulley was a 7mm. New pulley is an 8mm so I ran to the local ACE and grabbed a new 8mmx40mm key. I had to shave about 0.5mm off of it to make it fit though. I also was told that new pulley bearings will be needed for this. Since I am not very affluent at the moment, I am holding off until I can swing it. I can always run at lower speeds until the funds roll through to get some sweet new bearings. I am 90% sure the bearings that are in there are a precision matched set, but I am not 100% sure. But they work at 3500rpm for now so I can run there if need be.

Once the parts show up, you need to get the belt off and the old pulley off of the motor. This was a problem for me. The factory motor pulley had a taper lock bushing as well. Normally you just remove the screws holding the taper lock bushing into the pulley and put them into the other holes in the bushing so it becomes self extracting. Well I did this and the old cast iron bushing was wedged in there too tight and all the flanges broke off. So the next morning I grabbed a map gas torch and a bolt type wheel puller. I also opted to get some 10.9 metric screws of the appropriate length to fasten to the 3 holes that were used to affix the taper bushing to the pulley. This seemed to work well. I heated the crap out of the pulley with the torch and slowly got it off with the puller a quarter turn at a time. Took the better part of 45min to get it done. So beware there. You have to hold the pulley assembly still. To do this I shoved an 8mm Allen wrench into the unused slot in the puller and got one of the better upper body work outs I have had in a long time. Also if you destroyed the bushing like I did, you might be able to get a replacement from Ametric, assuming you ever want to go back. Or you can just get a new pulley and bushing. To swap belts, you have to pull the shaft cover off the top of the mill, then remove the 4 M10 (I think thats what they are) socket caps so that you can get the 1" thick top plate off. There are 2 taper pins on either side as well. You can thread in a M6 bolt to use to pull the pins out, but the pins on mine were stuck so I had to do some very careful prying to dislodge the cap. Applying heat might help but could also fry some of the paint Its your mill act accordingly. Once the cap is off you can remove the belt. Also, if you have a lot of knocking noise coming from the top end when the mill is running, there is a way to fix that assuming it is not a bearing. There is a little urethane piece on the bottom of the spindle pulley that you can adjust, maybe that will help take up some of the slack. Mine was shot so I made a new one very craftily. But that is a whole different can of worms for another post. Once belts are swapped you will have to tweak the motor pulley height a little then you are done there. Just make sure you put the Key in the keyway or bad things will happen. I had a manual mill where some idiot did that and it wasn't fun to correct that. To get the RPM's to change you go into the control cabinet and on the bus look for a terminal marked 238. One side of the 238 terminal should be empty. Take a short jumper and go to the terminal for wire 200. This will get you the 24v input needed to change the max speed from 3500 to 6000. But I will say, bust out the voltmeter and verify that you have 24v there. Always be safe. On mine, the 200 wire terminal (there were actually 3 of them in a row) were right next to the 238 terminal so this was easy peasy. The relays that control the spindle on signal, the drawbar and a few other things all run off of the 24v supply if my memory serves. If you have a wire diagram this will aid immensely if you have a non-standard arrangement in your control cabinet. All that is left is to reassemble and check everything.

To test, I got an inexpensive optical tachometer from HF. It was like $40 and it seems to work well. I measured the speeds on the before and the after. Before, at around 3350rpm I saw about a 13% difference in the programmed speed and the actual. The actual speed was 3842. It was a pretty consistent 13-14% from 800 rpm up to 3500. After the swap the swing was much less. At 2k it was at around 2042. At 5250 it was running at around 5428, so it is roughly a consistent 3-5% or so off. I can deal with that. Not like I will be tapping at those speeds. I think the variation is partly due to the Spindle Drive swap I did. I got the factory parameters for the old drive and plugged them into the new drive. I think that they might have done the 3500rpm machine's volt hertz curve a little different than the 6k, maybe? I don't know. I may play with the curve a little bit, but really 3-5% difference ain't gonna kill me. Most of the tapping I do is under 500rpm so an extra 25rpm will be easily comped with the tension compression tap holders I use. But it will be interesting to see how well it taps now with the larger pulley mass adding a bit of inertia. I had to adjust the accel and decel times on the drive as i had tuned them a bit for the 3500rpm so that I could get it up to speed and stopped as quickly as the drive would allow. With the bigger, heavier pulley and the higher spindle speeds it takes longer to get to speed and a bit longer to slow done. So the longer stop/braking time could effect tapping some. We shall see.

Hope this helps those that are looking at going down this path. I had trouble getting all of the information for this swap and put if off for way too long. Can't wait to see what this does to the cycle times of the parts that are run on it.