This is just one of several personal manufacturing projects that is work in progress.

Computer Numerical Controlled Mill/Lathe

Introduction

Sometime back in the mid 1970's, I purchased a small Craftsman^® lathe with vertical milling column from Sears^®. While Sears has long since stopped selling this particlar product, the original manufacturer, Sherline, continues to market and support this product. While there have been numerous product improvements over the years, the new products continue to interoperate with the orignals. Frankly, I was pretty lucky.

Back when I originally bought the mill and lathe, I always had a dream that one day I would convert them over to CNC (Computer Numerical Control.) These days there are a number of companies that specialize in CNC Conversion of Sherline Mills and Lathes. Indeed, Sherline now sells CNC ready mills and lathes to make the whole process easier.

While I could just by one of the more popular CNC conversion kits from an outfit like FlashCut or Hobby CNC, that is not my style. Instead, I tend to do things my own way.

What I intend to do is to start with a a modular stepper motor controller that I developed a couple of years ago. While I am not too thrilled with how that particular project turned out, the stuff does work, so I will work with it for now. Next, I will take some 20 year old 65 oz.-in., 5V, 200 steps per revolution stepper motors and attach them to the lathe/mill along with some limit switches. Finally, I will modify my HobECAD hobbiest electronic computer aided design software so that it can drive the mill to do printed circuit board milling.

Printed circuit board milling is the process were a blank copper cladded printed circuit board is attached to mill table and the software drills all the component holes and mills out the outlines of each circuit trace. While PCB milling is not as good as shipping the board design off to a rapid turn-around printed circuit board manufacturer (like Alberta Printed Circuits), PCB milling has faster turn around and costs less. The main draw back with PCB milling is that the holes are not plated through. An example of how far PCB milling has progressed can be found at LPKF where they have developed technology that is capable of putting up to five signal traces between component pins separated by .1 inches!!! I will be quite happy if I can get a single signal trace between pins that are separated by .1 inches.

The remaining sections of this document describe the steps taken perform the conversion.

Setting up the Mill

My Sherline lathe is currently closest to the Sherline Model 4000 lathe shown immediately below:

Most people eventually break down and purchase some sort of base for their vertical milling column such as the Sherline XY Base shown below:

Again, I like to be different. Instead, of buying an XY base, I will just use the lathe bed instead. However, there is one major problem with using the lathe bed; namely, the crosslide table is only about 3.5 inches long which is very restrictive. The XY table has an `X' table length that is is 9 inches long. My solution to this problem is to remove a the crosslide table and replace it with a 9 inch table from the XY table via individual replacement parts. The parts I need are:

Part No.	Fig. No.	Description	Price ($US)
50180	95	Mill Table	$55.00
50170	89	X Leadscrew (English)	$12.00

Another issue with the Sherline mill is that it does not have a great deal of overhang (or throat.) The standard vertical milling column only provides 2.25 inches of overhang. The standard solution to this problem is to get a mill spacer block (part number 1297) that adds an additional 1.25 inches to the overhang. It may be possible to stack a two or three of these things on top of one another, but for starters I will only use one. According to the accessory price list, the mill spacer block sells for $40. With one mill spacer block, I will be able to fairly easily mill PCB boards that are around 3.5 inches wide. If I am willing to do some board flipping, I might be able to successfully mill boards that are 7 inches wide. Since most of my projects can easily be fit on a 3.5 inch wide board, I am not anticipating any real problems in the overhang department.

Attaching the Stepper Motors

The stepper motors I am using were purchased on the surplus market in the early 1980's. Luckily, stepper motors can be purchased new from outfits such as JameCo or Digikey. Surplus stepper motors are available from numerous surplus outfits like Herbach and Redeman and All Electronics.

When purchasing a stepper motor there are a number of issues to consider:

Step Angle

The step angle is the resolution at which the stepper motor naturally steps at. 100 steps/revolution works out to 1.8 degress per step; similarly 200 steps per revolution works out to .9 degrees per step. Multiply the lead screw pitch by the steps per revolution gives you the basic accuracy of the system. For example, the Sherline has a lead screw of 20 revolutions per inch and my stepper motors are 200 steps per revolution, yielding 4000 steps per inch (or .00025 inch/step.) There are some tricks called half stepping and micro stepping that can increase the positional accuracy of a stepper motor.

Single/Dual Shaft

A dual shaft stepper motor has a shaft coming out of both ends. Obviously, a single shaft stepper motor has only one shaft coming out of it. Dual shaft stepper motors allow you to attach your hand wheels to the other shaft so that you can operate your mill/lathe manually as well as under computer control. The single shaft stepper motors will mandate computer only operation (unless you make them easy to remove.)

Torque

Torque is typically measured in units of oz-in (ounce-inch) or gm-cm (gram-centimeter). Technically, gm-cm should be written as gf-cm (grams-force-centimeter), but you will see it written on the specification sheets as gm-cm. In general, the more torque your stepper motor has the better. My stepper motors are 65 oz-in; hopefully they will be powerful enough. There is this trick called half-stepping that can be used to almost double your torque.

Unipolar/Bipolar

Unipolar stepper motors have four distinct electical coils per motor and bipolar stepper motors have two electrical coils per motor. In general, unipolar motors are easier to drive electrically than bipolar motors which require something called an H-bridge for each coil. However, the ready availability of integrated H-Bridge integrated circuits such as the L293 and the L298 have made this issue much less important than it used to be.

Current, Voltage, Resistance, & Inductance

The relation between current, voltage, resisteance and inductance can get pretty complicated. For stepper motors, the maximum current is probably what you have to worry the most about. The maximum holding voltage is computed by mulitiplying the coil resistance by the maximum current. Please note that you can let the voltage exceed the maximum holding voltage as long as you have some circuitry in place to ensure that the maxmium current is not exceeded.

Stepper motors can be operated in three modes:

Single Step Mode

In single step mode, one coil at a time is energized. This is the simplest mode to use.

Half Step Mode

In half step mode, two coils at a time are engergized simultaneiously to cause the motor shaft to position itself between the two steps. In half stepping mode, the angular resolution is increased by a factor two. First one coil is energize, then two, then one, then two, etc.

Double Torque Mode

In double torque mode, two coils are always energized to ensure that twice as much torque is being applied to the motor shaft. The additional torque comes at the expense of requiring twice the power over single step mode.

After much experimentation, I discovered that my Z axis motor needed to be run in double torque mode and the X and Y axes worked fine in single step mode.

Converting the Mill

As I was converting my mill over to CNC I took pictures of most of the steps. The mill was converted to CNC via the following steps:

1. First I started with some 3" × 1/8" flat aluminum stock.
2. Next, I cut some 3" × 3" squares off the stock.
3. Next, I found the center of each square by drawing two diagonal lines from each corner to create an `X' that indicates the center of the squares. I used an awl a hammer to punch a starter mark.
4. Using a 1/4" drill, I drilled a hole in the center of each square for the stepper motor shaft.
5. For, fun I just took a picture of the stepper motors. The tape measure is there just to give an idea of how big the motors are.
6. Next, I stuck the shaft of the stepper motor through the square plate. Then I stuck a sharp drill bit that just barely fit through one of the mounting holes. Then, I twisted the whole motor around by 360 °ree; to scribe a circle where the mounting holes intersect the diagonal lines. The square on left has been scribed and the one on right is about to be scribed. You can see the drill bit sticking out of the mounting hole. (Yup, that's my finger partially obscuring the picture; nothing but the highest quality pictures here.)
7. Next, I took my hammer and awl and punched four starter marks where the scribed circle crossed the diagonal lines. Then I mounted them in my drill vise and drilled four holes in each square. I used the same drill that I used to scribe the circles in the previous step. (Normally, my drill vise is just sitting on top of the drill press table, but my previous operation had the vise bolted down, so I just left it bolted to the drill press table.)
8. I removed the squares from the vise and deburred the holes. The resulting squares with five holes in them are visible in the picture (along with my fat finger over the lens again.)
9. The Z axis table comes with two holes predrilled in for attaching CNC adaptors. The two holes were threaded with a #8/24 threading tap. (I think it was #8/24.)
10. Using a random piece of paper, I attached the thrust plate using a hex hollow head screw. I used the end of the Allen wrench to poke two holes to form a drill template. I should have also drawn a circle in the middle where the stepper motor shaft would go. (You can also see that my work bench is starting to get a little cluttered.)
11. Using the paper template from the previous step and a hammer and awl, I marked the positions for the 2 holes to drill.
12. Next, I drilled the two holes and mounted the X axis plate with a couple of #8/24 round head machine screws.
13. The next step is I got one of those bimetal circular hole cutting drill bits that are rated for cutting through nails. Using a 1" diameter cutting bit and plenty of cutting fluid (I use WD-40), I cut out a 1 inch circular hole. (That was fun.)
14. Since the thrust plate is thicker than my 1/8" square plate, I needed some shims to get the surface of the square plate up above the the surface of the thrust plate. The shim are just a couple of flat washers that I ground the ends off using a bench grinder and a pair of pliers. The picture shows the X axis before the adaptor plate is attached.
15. Next, I just attached the adaptor plate on top of the shims using a couple of round head screws. Later on, I discovered that I couldn't access the screws, so I switched over to hex hollow head screws that could be tightened using an Allen wrench.
16. Now, I switched over to the Y and Z axes. Since, the Y and Z axes are basically the same, I'm only showing the Y axis being worked on. (Frankly, I forgot to take pictures of the Z axis modification.) Using the bimetal circular hole cutting attachment (and plenty of cutting fluid), I cut a 1" diameter hole. in the Y axis adaptor plate.
17. Next, I drilled two mounting holes into the Y axis mounting plate right near the edge of the 1" circular hole. I decided to drill the mounting holes to take #6 machine screws.
18. Using the Y axis adaptor plate as a template I marked both holes with a hammer and awl. Mounting the Y axis (i.e. the lathe bed) in the drill press vise, I drilled two screw holes in the Y axis.
19. Using a thread cutting tap, I threaded both holes to take #6/32 machine screws.
20. Next, I mounted the Y adaptor plate. Initially, I used round head machine screws, but later on I switched over to hex hollow head screws so I could access them with an Allen wrench.
21. Now I needed a 1/4" shaft collar. I visited all the local hardware stores and there were no 1/4" shaft collars to be found. Eventually, I got the bright idea of converting a shaft coupler to a shaft collar. This is delicate operation is performed by taking a hack saw and cutting off the two coupler sleeves.
22. Using a bench grinder, I ground a flat into the X axis lead screw. The flat provides a nice flat surface for the collar set screw to attach to.
23. Next, I took the Y-axis and ground a flat into it as well.
24. Now, I used a counter sink bit and the drill press to drill out four countersunk holes in the adaptor plates. Note I had to detach the adaptor plates from the X and Y axes in order to do this operation.
25. Using some 2-1/2" #8 flat head machine screws, some washsers, lock washers, and hex nuts, I installed the standoff screws.
26. Now the X adaptor plate is reinstalled on the X axis. In these pictures, I'm still using the round head machine screws; they get changed to hex hollow head screws later on. In addition, one smooth side of the 1/4" shaft collar is greased up. Notice the huge can of grease; it was the smallest amount I could buy; I suspect that it will last several life times.
27. The 1/4" shaft collar is installed and tightened using a small Allen wrench.
28. The 1/4" shaft coupler is installed next and tightened using a small Allen wrench. That black cross like thing is the torque adaptor the fits between the two shaft couplers.
29. Next, I ground a flat into the stepper motor shafts using a bench grinder. A 1/4" shaft coupler is installed on the stepper motor shaft and tightened using a small Allen wrench.
30. The torque adaptor is inserted and some nuts, lock washers, and regular washers are twirled onto the standoff srews.
31. Finally, the stepper motor is slipped on top of all of this and the two shaft couplers are mated. some additional hex nuts are twirled down and everything is tightened up using a hex wrench. The X axis is now ready for some CNC machining action.
32. Switching over to the Y axis, the 1/4" shaft collar is greased up.
33. Now, the 1/4" shaft collar is installed and tightened with a small Allen wrench.
34. Next, a 1/4" shaft coupler is installed and tightened with a small Allen wrench.
35. Now, a 1/4" shaft coupler is installed on the stepper motor shaft and tightened with a small Allen wrench.
36. {Adaptor plate is installed.}
37. {Stepper motor is attached. Done.}

The Floating Z Head

I have taken some pictures of my current floating Z head technology:

• The overview picture shows the entire CNC mill with the floating Z head attached. Sorry, about all the additional clutter in the picture.
• The next picture is a little closer and shows the spacer block attached to the vertical column. Two flexible pieces of sheet metal are attached from the spacer block to the spindle assembly. The triangular thing on top is use to ensure that the hole spindle goes up when the Z-axis goes up. The screw on the left adjusts when the Z-axis up movement will engage. A Dremel ^® provides the spindle.
• A round head screw that has been ground slightly concave provides the footpad where the floating Z head touches the printed circuit board. A few nuts and lock washers keep the footpad from vibrating loose. Note that a collar has been attached to the mechanical etching bit to help remember how far the shank goes into the collet.
• The depth of cut adjustment device is held together with a rubber band (it works!) The end of the screw has been ground into a point, that keeps in in a small depression. Since the screw has a fine pitch and it is quite a distance from the pivot, a small movement by the screw causes an even smaller movement by the footpad. A bunch of nuts held together by lock washers provide a more comfortable grip to twist the adjustment screw. There is washed out image from a different view point and another.
• A washed out image of the angle aluminum attach points and one that is less washed out show how the metal flexors attach to the spindle body. Some radiator hose clamps are used to attach the Dremel ^®.
• The triangular Z stop is used to force the spindle to move upwards when the Z axis is moved up on computer control. The screw on the left is used to control when the spindle is forced up.