Another PM940-CNC-VS Thread...

[phazertwo]

I had two big steel parts that a friend had given me over a year ago to fix, and I was feeling bad about still having them and having done nothing to fix 'em! So I decided that this would be a good test for the coolant, because I would need to build an aluminum fixture that would need a fair bit of material removed.

I programmed it a little more aggressively than any cut I had run so far. I used my favorite roughing end mill, and I knew for a fact that the flutes would gum up far before this with out constant WD-40 spritzing... Game on!

Tool path:

And the details:

That's 0.003 ipt, at 38.4 ipm, with all the RPMs the 940 can muster. 0.1125 step over (30% of cutter Ø) and I'm leaving 0.020" to clean up with a 6mm carbide, but I'm bringing it right down to the deck. Fixtures don't need to be pretty.

Finished fixture:

I like to pretend I'm a tough guy, but let me tell you... I giggled like a 3rd grade school girl at a sleep over... for the ENTIRE TIME it was cutting. Watching the mill chew up that material with out me needing to do ANYTHING but watch, freakin' amazing. Mist coolant is THE BOMB!

However... the setup needs work to be perfect. When the head is down it siphons pretty bad, and the ball valve on the tank cannot meter the coolant at all. I was putting out way too much coolant with the thing barely cracked. On the next few parts I manually opened and closed the valve to conserve coolant.

Fixture worked perfect. Look close at the closest hole, it's an oval, that's one of the things I needed to correct. One of the parts that needs fixed:

Here is the other plate... the water jet that cut the part crapped out before finishing:

After machining:

Again I pushed the mill pretty hard one this one. 0.002 ipt 0.075" WOC, 0.500" DOC. Everything seems to work really well... until the tool started pulling out. Backed it off to 0.250" DOC and it cut just fine, next time I will likely bump the WOC.

That pretty much brings us up to date, which means things will slow down a little bit, and I will include far more of my research planning, and questions.

My next step will be spindle upgrades...

PZ

 

[phazertwo]

After months of going back and forth and back and forth... I finally decided on AC bearings for the spindle. I was looking at P5 tapered roller bearings, but wasn't real excited about the limiting RPM and that P5 is about the highest grade you can ever get...

I ended up going with some NSK AC bearings:
7206CTRSUMP3 - NSK Precision Angular Contact - 30x62x16mm
7207CTRSUMP3 - NSK Precision Angular Contact - 35x72x17mm

It was about $240 for the two. These should be very high precision bearings, and are rated to 18k RPMs greased. I don't intend to get anywhere near that kinda of speed, but it will at least allow me to hit my goal spindle speed range of 100 - 7500 RPM with two speeds.

I have not yet figured out exactly what I'm going to do for "gears" besides that I want to use J profile ribbed belt/pulleys. I would REALLY like to come up with a nifty way to change pulleys automatically, so if you've seen anything cool feel free to drop a line or a link here so I can take a look. To start I am just going to make a sliding motor plate and run just the high set of pulleys, as I will need to get the mill back up and running to make what ever parts I will need for an automatic gear changer... thingy...

PZ

 

[WyoGreen]

My wood lathe uses varible pitch pulleys (with a V belt) to vary the speed with the twist of a lever. My wifes hybrid uses the same setup for it's transmission, using a metal belt. The same setup is used in Snowmobiles, again with a heavy rubber V belt. So the setup can take considerable torque.

It might be something to check into, Steve

 

[phazertwo]

I'm actually well versed in the world of snowmobile clutches, and the idea of a change on the fly infinitely variable setup sounds awesome. Due to the nature of my controller, I would have to know the gear ratio that it's operating at, and have a way to change it to pre-set "gears" if that makes any sense. I know the CVT's used in cars these days are computer controlled, unlike a snowmobile/4wheeler that uses springs and weights to actuate the clutch as it spins up. If I could figure out how to control one like a car does... that It would be the ticket... Honestly though, it sounds like a whole bunch of parts and complexity.

Good idea though! Keep em coming!

PZ

 

[phazertwo]

The head of the mill is torn down, and cleaned. The oil that came out of the head looked/smelled/felt just like the stuff in the automatic oiler. It was full of grime and crap, after I got the top popped off I found that the gear case was full of gunck including metal shavings and other small particles. It could be sand from the casting process, but I doubt it because the rest of the casting is free of sand. Anyway, the top bearing on the spindle was in really rough shape for a bearing that has >20hrs on it (almost all of those hrs at max speed). Here are some pics.
Here you can see the rings of "stuff" on the edges of the oil:

Under the cap:

A look at the "stuff" left over in the bottom:

Another look at the "stuff" and the weird grease that was in the bearings, and here you can see that the rollers actually don't have much grease packed around them... turns out when I got them out the bearings didn't have any grease "packed" into them:

And the bottom bearing, again you can see not much grease "packed" in the rollers:

Upper race, you can see serious scaring near the middle of it... If I had to guess I would say the pre-load was too loose:

And the upper bearing itself, scared to match the race:

Lower race and bearing looked perfect, so I didn't take any pics. Also didn't take any pics of it clean, but you should be able to see it in the re-assembly pics.

All and all, the fit and finish on all of the machined parts in the head were quite nice. The spindle and quill fit together nicely and the press fit on the bearings and races were good. A little more cleaning before oil, some better bearing packing, a touch more pre-load on the bearings and it would be about perfect... I still think that bang for buck you couldn't beat this mill with a club, considering that all of the heavy metal (un-replaceable parts) seems to be good quality.

I've also started getting the head in Fusion... it's a bear of a job trying to get everything dimensioned properly, but it's coming along nice.

PZ

 

[phazertwo]

Brown Santa left me a nice little gift this afternoon.

I never would have guessed that I would think bearings are sexy... I was wrong. These are marked to show the max run out with an o, and the outer race has a V that tells you which way to install it, and where the max run out is. SUPER cool, though I'm not sure it's going to help me because I don't know where the max run out is on the spindle and quill. Also the -2 indicates the deviation in bore size in microns... that 0.002mm... or 0.000079" freaking cool!

Getting excited to put this thing back together.

PZ

 

[navav2002]

I am replacing the bearings in my PM932m mill. I'm interested in what grease you are intending to use on the roller bearings??

 

[TomS]

I am replacing the bearings in my PM932m mill. I'm interested in what grease you are intending to use on the roller bearings??

When I converted my PM-932 to AC bearings I used Kluber Isoflex NBU 15. Not cheap but highly rated. My mill has 8800 rpm capability and I've run at that speed for more than an hour on numerous occasions with no problems.

 

[phazertwo]

I am replacing the bearings in my PM932m mill. I'm interested in what grease you are intending to use on the roller bearings??

Like Tom did, I'm going to use Kluber Isoflex NBU 15. It certainly is expensive, however the precision AC bearings were a lot more so it's not worth it to cut corners IMO. I was able to by a small amount of it from The CNC Specialty Store so it wasn't too bad.

https://cnc-specialty-store.com/gre...m-tube?zenid=c288e5e92d51aa4139e3195aff46151a

PZ

 

[phazertwo]

Well I chickened out big time today. I was going to install my new bearings, but realized a few things that held me up. First was that the upper bearing will just be open to the world, anything can fall down in to it.

Even if I put the seals back in, the seals are a Ø35mm ID and the spindle is only a Ø28mm, not only that, there will still be an opening at the splines that will allow crud through.

Second, I was looking through all the gizmos I keep around the press for tooling, and I have nothing that will work well to pushing these things in. I'm sure I can get them in there, but I don't want to push on these beautiful bearings anything but the right way...

Unfortunately I need the mill to make the proper press tooling and a new seal adapter to plug the opening on the top... Kind of a catch 22. So I ordered a set of VXB ABEC-5 AC bearings to run in the mean time. They were about $90 for the set instead of $240, so I figure these can be a practice run, and buy me some time to get other parts made...

Today wasn't a total loss though! When I first started this, I ran the motor while it was chucked in the vice to figure out how many poles the motor has. It didn't sound so good, and was actually vibrating, so I ordered new bearings for it at the same time I ordered the spindle bearings. I DID have all the press tolling I needed to get that done (pretty easy). Now she sings, and is smooth all the way up to 150hz. And in case anyone was wondering, it's a 4 pole motor (1800RPM at 60Hz). before I put the motor back together, I took the time to tap the holes on the cap that hold the fan assembly to 10-32.

I had tightened these screws so many times trying to keep the thing from rattling that they were all stripped out. With the larger threads, and some blue Loctite I don't think I will have to listen to it rattle ever again... but only time will tell.

PZ