SIEG X2 Mini Mill CNC Build - Component Selection

Monday, May 1, 2017
Set of castings for SIEG X2 CNC Mill

In the last post I outlined my plan for building a CNC mini mill from a set of SIEG X2 castings. As the first step I wanted to order the parts for the mill itself, ignoring the steppers, controllers, etc. and make sure that I can get it up to snuff. There is no set budget for this project, so the choice of the parts is mostly based on availability (and common sense). The two areas I wanted to concentrate on are spindle accuracy and overall repeatability. With that in mind, in this post I will explain what parts I chose to buy and why.

The Castings

As mentioned before, I already have the table, and at some point I bought a replacement R8 spindle, drawbar and the spindle nut. To complete the mill I'll need the column, spindle housing and spindle housing mount.

I also decided to skip the stock column pivot and make a bracket from a piece of steel. This means that I'm missing only three other castings: column, spindle housing and spindle housing column.

Little Machine Shop sells solid column conversion kits, as well as just the column (as a replacement part). I briefly thought about getting a solid column and adapt it to the base I already have, but then decided to get the standard column to stay true to what one would need to do in order to rebuild a standard mini mill. At the time of this writing the cost of those castings, including shipping, is $226 (US Dollars) from Little Machine Shop, so I went ahead and ordered those right away.

If I didn't have the parts already I'd just get a mill from Harbor Freight, which is currently listed for $700 + shipping but I believe HF honors their %20 coupons, and the mill comes with a motor, control electronics, switches etc. Cost for all of the needed castings is close to $500 + shipping, so getting a mill makes more sense.

Ball Screws

The next thing to sort out are the lead screws. Several people suggested re-using the stock lead screw but at the end I decided to go for ballscrews. Chinese "C7" ballscrews are relatively inexpensive and while they have some minimal backlash, I know how to deal with it. In theory they have tolerance of +/- 50 micrometers per 300mm of travel, or a bit under 0.002 over foot of travel. This sounds pretty bad, but I will be able to map the screw in LinuxCNC. One of the perks of TouchDRO project is that I have several glass DRO scales, so I can mount one, have the stepper advance the axis and record the output from TouchDRO to a file.

After a few minutes of searching I found a set of 16 mm, 5mm per turn ballscrews from a seller with decent rating. Total cost, including shipping was $105. The downside is that I will need to turn the ends myself, but that shouldn't pose a huge challenge. Add four axial bearings (two for Y and two Z axis; X already has a pair) and for $130 I'll have much less backlash than the stock acme screws.

Spindle Bearings

SIEG Mini Mill uses basic [cheap] radial ball bearings. Those bearings are designed to support radial loads and are inadequate for this application. A set of tapered roller bearings or angular contact bearings is more appropriate. Spindles in industrial CNC machines use matched pairs of A/RBEC 7 or 9 bearings (ISO Class 4 or Class 2), but those are insanely expensive.

A set of ABEC 3 angular name brand bearings will be a huge improvement on the stock ones. They have the same dimensions as their radial counterparts. Moreover, compared to the roller bearings, they run cooler and generally have higher RPM rating.

Part numbers for Mini Mill's stock bearings are 6007 and 6206 for bottom and top bearings respectively. The first digit designates the series. In this case "6" is for radial bearings; angular contact bearings are series "7", so I need 7007 and 7206.

After some searching I found a new old stock SKF 7206 ABEC 3 [top] bearing; I got lucky with the bottom bearing and found "new old stock" KOYO 7007 ABEC 7 [bottom] bearing for only $41. For $96 shipped for both I'm going to get a huge upgrade in spindle accuracy. One minor inconvenience is that those are open bearings so I will need to make a shield for them.

"New old stock" angular contact bearings will be a huge improvement

List price for a set of name brand ABEC 3 bearings is closer to $200. While it's difficult to justify paying that much for two bearings, it is still well worth it. Accurately running spindle is crucial for the machine, and with proper lubrication and maintenance those bearings will last for many-many years.

Spindle Motor

Since I've been hoarding motors for years now, I already had a lightly used treadmill motor rated at 2.5 horsepower and 6700 RPM. The 2.65 HP claim is obviously worthless - it [optimistically] describes the amount of power the motor would draw when stalled, right before it melts down. In practice it's probably safe to assume that it can reliably deliver around 1 HP, which is more than enough for this little mill.

Treadmill motors with KB Electronics speed controller

The donor treadmill came with a working controller, but that controller has one annoying safety feature. Namely, it won't start until the speed is reduced all the way to 0, and then its spins up rather slowly. Therefore I decided to use a KB Electronics industrial speed controller that can deliver up to 12A to the motor. Since I already have both, my out of pocket cost is effectively $0; if I recall right, I paid $75 for the donor treadmill and $56 for the speed controller several years ago.

Miscellaneous Parts

There is a lot of parts I can make myself, such as motor mounts, belt drive components, bearing retainers, brackets and so on. I already have some aluminum that will work; rest of the metal will come from my local metal distributor that has good selection of small remnants at very reasonable prices. As I'm building the mill I will keep track of things that I end up buying/using.

Conclusion

This covers most of the parts needed to build the mill, except the stepper motors, drivers, power supply and the controller. I intentionally decided to punt on this while doing some more research. My total out of pocket cost so far is as follows:

  • Missing cast iron parts - $226
  • Ball screws and axial bearings - $130
  • Spindle bearings - $96

If I recall correctly, the cost of the parts I already had is as follows:

  • X2 table assembly - free (left of from mill upgrade)
  • Treadmill motor - $75 paid for the whole treadmill two years ago.
  • Motor speed control - $56 off ebay in 2013
  • R8 spindle, drawbar and nut - $64 in 2013

The grand total out of pocket is $452; grand total, including the parts I already had is $602. I expect that the cost will double by the time I factor in stepper motors, drivers, power supply, enclosure, etc.

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