Mill: Angular Contact Bearings

Please read the complete article:
http://benchtopmachineshop.blogspot.com/2017/04/mill-spindle-bearings.html


It turns out the new spindle I installed in my mill was ground at an angle. Fortunately, Grizzly sent me a new spindle without questions. Since you cannot remove the bearings in a X2 mini mill without destroying the bearings, I decided to take this opportunity to upgrade to angular contact bearings. I already had a pair of SKF ABEC-3 bearings on hand which I'd gotten for a good price.

The advantage of angular contact over deep groove bearings is the angular contact can take a much higher axial load, and are in fact designed for it. This also allows you to preload them much higher than deep groove bearings, and higher preload mean a stiffer spindle. Angular contact bearings are directional, meaning they can only take axial load in one direction. In fact, there is play between the bearing races until preload is applied.

Stock deep groove radial bearing on the left and angular contact bearing on right.

Before starting, I chucked the spindle in the lathe and sanded the top bearing seat with 400 sandpaper until the bearing was a light press fit. This will making setting the preload easier later on, and apply less static force on the bearing while doing so.

Instead of thermal fitting the bearings like last time, I decided to create a press for the bearing so I wouldn't have to remove the mill head. Since both the outer and inner races are a press fit, I needed to make adapters for the press which would push both inner and outer races equally. You never press a bearing through the rolling element or you'll destroy it! The adapters were made using two 3" diameter by 3" long pieces of aluminum rod, and a 2" diameter by 1" long steel rod. The press itself is a 5/8" long piece of threaded rod with a nut red Loctited onto one end.

The aluminum was faced on both sides, and then a 5mm long section was turned down to 60mm . The center was then drilled and bored out to 31mm. This allowed it to slip over the top of the spindle and press evenly on both inside and outside bearing race. It also allowed it to center over the top bore of the spindle head when pressing in the bottom bearing. The bottom adapter was again faced on both sides and the end of the ID turned down to 60mm. The center was drilled and bored out to 43mm, which is wide enough to not touch the inner bearing race. The steel rod was faced on both sides, and had the center drilled and bored out to 16.5mm, wide enough to clear the 5/8" threaded rod.

I then thermal fit the bottom bearing onto the spindle. Make sure the wide part of the outer race faces the top of the spindle, and the wider part of the inner race faces the bottom of the spindle. Remember that the angular contact bearings have play between the races until preloaded. With the aluminum bottom adapter in place I wanted roughly the midpoint of the play to put the spindle's nose flush with the adapter's surface. I kept taking facing cuts on the adapter until I reached that point. So with the play removed one direction the nose sat slightly under the adapter's surface's level and with the play removed the other direction it stood slightly proud.

To to press the bottom bearing and spindle into the mill head I put the 5/8" threaded rod through the center of the spindle, put the top adapter in place (the top bearing is installed later), put the bottom adapter in place over the spindle and bearing, placed the steel adapter over the bottom adapter, and ran the nut down on the threaded rod, holding everything together. The steel adapter supports the spindle's nose, which in turn supports the bearing's inner race. Since we adjusted the bottom adapter so the inner race would sit in the middle of its play, we can now press the bearing into place without applying any force to the inner race. I lightly oiled the bearing and the bore and tightened the 5/8" rod until the bearing pulled and seated into it's bore.

I then disassembled the top of the press, lightly oiled the top bearing and bore, and pressed it into place as well. On the top I needed additional clearance for the spindle, so I used a PVC pipe fitting; it worked just fine.

Bearing press assembled using old spindle and bearing.

Any high quality 7206 bearing is going to be open, unless you spend $300 for sealed ones. Initially I was worried that open bearings would quickly lose their lubrication, but in practice that hasn't been the case. To help retain the lubrication I took an old milk jug and using a compass cutter cut out seals which would sandwich underneath the plastic bearing covers. The ID of the seal was large enough that it would not rub in the inner race. In addition, I created a rubber shield between the plastic bearing cover and the spindle. I made it from EPDM sheet again using the compass cutter, and hot glued it into the plastic bearing cover's bore; the bore has a great little lip which makes it very easy. The seal is very close to, but does not touch the spindle. All in all, this setup keeps the lube in place nicely, and does a good job of dirt out. It also comes apart pretty easily in order to lube the bearings. For grease I used Lucas Red N Tacky which does a good job of staying put, is easy to get, and works well enough for the low spindle speeds I run (max 3,500 RPM).

Honestly, the system the X2 uses for preload adjustment isn't all that great or accurate. The absolute first thing I did was face both side of the adjustment nut. From the factory it's pretty far off. This will allow even pressure to be applied to the bearing. Next I took the set screw, cut the point off, and faced it. This would still provide enough holding power to keep the nut from turning, but wouldn't damage the spindle threads. To make adjusting the nut easier, I took a 32mm socket and ground it down until I had four teeth to interface with the nut; it's much easier than using a lock ring wrench.

32mm socket ground down to fit the spindle nut. Regarding the finish, I did it with an angle grinder, so what do you expect?

Adjusting the preload was a little tricky, since I wanted to get between 100 and 125 pounds of preload. I measured how much torque was required to remove the play from the spindle, then I added 20 in/lbs of torque to that, and tightened down the but. It was roughly 55 in/lbs of torque. Using a DTI attached to the spindle, I checked for play in the spindle; you shouldn't see any play with properly preloaded bearings. Then I ran the mill at high speed for 30 minutes while monitoring the temperature of the mill head right next to the bearing. I used a IR thermometer which I pressed against the side of the mill head right at the bearing. I've checked the temperature at that location versus the temperature right at the bearing's outer race using a thermocouple and there is only a couple degrees difference. If the temperature stays under 60* C then you're fine on preload. On mine the temperature barely even reached 45* C. I then made sure to put a witness mark on the nut so I could tighten it to the exact same point every time.

UPDATE: Well, my sealing measures weren't enough and the lower bearing was contaminated. To make sure this doesn't happen again, I'm spending the money on SKF sealed 7206 angular contact bearings (7206 BE-2RZP). While Arc Euro Trade offers sealed angular contact bearings, they're P0/ABEC-1 precision which means a runout of 0.0005". The SKF bearings, on the other hand, offer a runout of between 0.00015" and 0.0003". On a mill low runout is especially important, as it effects tool chip load, especially on small diameter tools. http://benchtopmachineshop.blogspot.com/2017/01/mill-angular-contact-bearings-ii.html

Lathe: Chip Guard

I wanted a way to keep chips off the cross slide digital scale and the cross slide dovetails. Since I had several pieces of 2mm thick aluminum I cut two sections to fit and drilled 1.25mm holes which I epoxied neodymium magnets in. I now have two covers which stick to the top of the cross slide and do a pretty good job of keeping the chips away.

The two halves of the chip guard with the magnets epoxied in place.

The two halves of the chip guard in place on the cross slide.
The left side guard is longer to better protect the left dovetail.

UPDATE: I've since installed an extension on the back to better protect the cross slide dovetails and leadscrew.

Mill: Stacking Tolerances

I know my spindle has 0.0008" runout, so I marked the "high point" on the side of the spindle. In turn I've checked all my tooling and marked the "low point" on them. Then I always match up the marks when I mount the in the spindle, so the spindle's and tool's runout cancel each other out to some extent. For example, my ER25 collet chuck has 0.0005" runout (it's a cheap one from China), but when mounted in the spindle with the high and low points aligned I end up with with only 0.0003" runout total.

Mill: MT3 Spindle Release Improved

A while ago I modified an 8" C clamp to make releasing tooling from my MT3 spindle easier: http://benchtopmachineshop.blogspot.com/2013/06/mill-mt3-spindle-release.html. It worked OK, but the clamp would frequently slip off the drawbar or the clamp would slide to one side.

To fix it I removed the pad from the clamp, leaving just the ball. I then turned a spherical dimple in the top of the drawbar roughly the same size as the ball on the clamp. Now the ball mates to the drawbar and keeps it from sliding and directs all the force downward.

It's made the tool work much better and it's a breeze to release my MT3 tooling now.

Clamp with the pad removed from the screw, exposing the ball.

Dimple in the top of the drawbar which captures the clamp's ball.

Mill: Bearing Replacement

Please read the complete article:
http://benchtopmachineshop.blogspot.com/2017/04/mill-spindle-bearings.html


After boogering up my spindle I was forced to replace it. While I was at it I decided to replace the cheap bearings with high quality Japanese bearings. The procedures you see for mini mill bearing replacements will work, but the methods used put unneeded stress on the bearings and require some specific tools. I decided to do it an easier and better way.

The mill uses deep groove radial bearings. They're not ideal for a mill spindle, but they'll work adequately. Since I have a MT3 spindle I use two 6206 bearings, and you can get them either sealed or shielded. Shielded actually have a tiny gap between the shield and the inner race, so they don't protect as well, but they also produce less heat and have a higher top speed. Sealed protect the bearing better, but they'll produce noticeably more heat and, once they're installed and preloaded, their top speed will be about the same as the top speed of a belt driven mini mill.

Most Japanese bearings, even if they're listed as ABEC-1 will actually be ABEC-3. Really, either is good enough for the tolerances of the mini mill. A lot of replacement 6206 bearings you see are C3 tolerance. The C3 indicates they have greater clearance inside the bearings. On the mini mill they should be ok to use since the bearings are preloaded. You can also use CN (normal internal clearance) or C2 (reduced internal clearance). I ended up with SKF sealed C2 bearings since that's what was readily available to me.

Getting the bearings out wasn't hard. As I hammered the spindle out using a rubber mallet it pushed the bottom bearing out. The top bearing was removed using a piece of pipe and a mallet. The bearing bores were thoroughly cleaned and lightly oiled.

First I needed to install the new bearings in the mill head. To do so I removed the mill head (you'll need to remove the motor to do so) and put it in the oven at 220* F for 1.5 hours. The previous evening I put the bearings in the freezer. After using my IR thermometer to make sure the mill head was up to temperature, I took it out of the oven and pushed the two bearings into place. You have to be quickly because the heat iron will quickly heat up and expand the bearings.

If you can't do it, or couldn't get the bearing all the way in before getting stuck, you'll need to press it the rest of the way in. The key is you can only press the bearing on the outer race. The way I do this is take the old bearing, remove the shield from one side, and grind the inner race down. Now you can use the old bearing to push the new bearing in without applying pressure to the inner race. I then use a 1/2" threaded rod, a couple nuts, and several thick washers to push it into place.

Installing the spindle is a similar procedure. I put the spindle in the freezer overnight and the mill head, with the bearings in it, went back in the oven at 200* F. I put a very light mineral oil on the spindle to help lube it during assembly. Once the mill head was up to temperature I quickly removed it and seated the spindle in place. I had to tap it once or twice with a rubber mallet when it jammed during insertion. However, I never had to hit it very hard so it didn't damage the bearings.

Once everything was together I reassemble the mill, head, and motor. I then tightened the reverse thread nut on top of the spindle down tight to remove the play from the bearings, backed it off slightly, and tightened down the set screw.

Once back together I ran in the bearings using the SKF recommended procedure, basically running the mill in increments of 500 rpm until the bearing housing temperature stabilized.

UPDATE: Well, the new spindle's bore was ground at a slight angle, so I replaced the spindle for a second time. This time using angular contact bearings: http://benchtopmachineshop.blogspot.com/2015/05/mill-angular-contact-bearings.html.

Lathe: QCTP Tool Holders

The inside bottom corner of the 0XA tool holder (and I suspect pretty much any other QCTP tool holder) has a slight radius. This helps prevent a stress riser from forming in the corner and a crack forming. However, this doesn't allow tool bits to rest against both the bottom and side of the tool holder.

Since I'm not going to mill a sharp corner in the tool holder for the reason above, this left remove the corner from the tool bit instead. Since I mostly use indexed tools I just clamped the tool in my angle vise at 45* and milled just a little amount off the edge. This gives enough clearance for the radius in the tool holder and allows the tool to sit flush against the bottom and side.

For my non indexed tool bits I just run the edge along the grinding wheel just enough to knock the corner off.

Edge milled off tool bit holder and sanded smooth.

Lathe: Setting Tool Bit Height

I had a hard time quickly and accurately setting my tool bit height, and finally found a solution using an old inch -only dial caliper I didn't use anymore. After chucking a piece of aluminum in the lathe I took a couple cuts. I then measured from both the top and bottom of the turned portion to the top of the cross slide. Averaging those measurements gave me the distance from the top of the cross slide to the center of the work. I then set my old dial calipers on end on top of the top slide and adjusted the outside movable jaw until it was exactly that distance from the cross slide.

Now to adjust tool height I set the caliper on end on the cross slide and adjust the tool bit until it just clears the outside movable jaw on the caliper.

Setting tool bit height. I want the bit to just graze the caliper as it slides under the jaw.