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2013-12-26 15:16

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CNC - BT30 Spindle - Assembly

So, finally it's time for some assembly.

I packed the bearings with a grease from SKF called LGLT2/0.2 which is a low temperature, high speed grease. Then I put the bearings in the oven at low temperature (80-100C) and the shaft in the freezer and left them there for an hour or so.

With a difference in temperaure of around 100C and with help from a aluminium tube and a plastic hammer the beraings was quite easy to fit to the shaft.

 

 

The KM-nut that holds the bearings inner races together is tightened and secured with normal strength LOCTITE.

Then I sprayed some WD40 in the spindle shaft and on the drawbar tube and pushed it in. It took quite some force get it all the way in there but that's not bad.

 

 

 

 

 

With the lower bearings secured to the shaft I put the shaft the in the freezer and the housing in the oven for a while. After that they went together nicely. Sorry for the blurry photo.

 

 

 

The top bearing was a little harder to fit. I used the aluminium pipe and plastic hammer and finally got it all the way down.

At first the spindle turned very easily, I could give it a quick spin with my fingers and it kept rotating for a another revoultion or two. Then I mounted the flange (without seal, for now) and therefore preloading the bearings it got a little harder to turn. It still turns REALLY smooth though, just not that easily. It will be intersting to see how it performs when it gets powered up. I suspect there will be some heat in the lower bearing zone. The bearing however can stand quite some heat and since the aluminium housing expands faster than the bearings when getting hot I think and hope that it will be just fine.

 

Here's some more random photos of the spindle:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The drawbar is actuated by a 100mm pneumatic cylinder. It originally had a stroke of 50mm but I cut it off, reducing the stroke to ~15mm. Then I fabricated four stand-offs so that the cylinder could be mounted on top of the spindle with a little adapterplate. When I write this the belleville washers in the drawbar are mounted in stacks of two and preloaded so that it takes around 1 bar of pressure to start releasing the tool. That means that the clamping force is around 750-800N. It takes 2.5 to 3 bar to push the drawbar down enough for the tool to come out. The belleville washers will later on be reconfigured in stacks of 3 and the clamping force should then be somewhere between 2500 and 3000N.

 

Here's a video (14Mb, 52s) showing some initial testing of the pneumatic drawbar. The same video is also available on YouTube for streaming if you prefer.

 

Update 2007-10-30.

After assembly I had around 0.02mm runout inside the taper of the spindle. Although not very good it's acceptable. However inserting a toolholder with a short tool and measuring at the tool the runout was close to 0.2mm - not acceptable. After dissasembling the spindle and checking as much as I could I went back to the grinding shop with the spindle shaft and together we determined that they messed up. The taper has a good fit to the holder but it was not parallel to the OD of the shaft. They've promissed to fix it though and work to do so are in progress.

 

Update 2013-12-26.

Wow, 6 years down the road....

The issue with the taper was eventually sorted out but then an issue with the gripper presented itself. Basically the gripper was pushing, laterally, on the pullstud, pushing the holder out of alignment, again causing runout. I made a couple of attempts at correcting that but didn't succeed. Then, I got the bigger knee-mill so the focus shifted to that project instead and at the same time someone approached me asking if I'd sell them the BT30 spindle, which I did. He then made a new floating gripper which fixed the issue and it's been running since - at least from what I've heard.

 

 

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