Summer hit hard

Summer in Florida is off to a hard start. My window ac just wasn't doing the job. It was running 24/7 trying to snuff out 5000 btus coming into my workshop.

It was time for an upgrade:

12,000 btus of glorious ac for $499 out of the box. It was a solid weekend of work and another $100 of materials since my lines ran longer than the stock 15ft tubes they gave me but when it's set to 74F it stays 74F.

First Chips!

Last CNC post was mostly the physical build. This post is system integration, which turned out to be much more tedious. You can see it running at: First run on wood

First I had to gather the last of the parts. I bought a Dell Precision mid-tower on Ebay for $90 free up my. Laptop. I added 8 more GB ram and a spare solid state drive I had.

I tried a lot of different limit switches. Stay away from the hall effect sensors. The state of switching those proving isn't really discrete enough to give the smooth stepper board a definitive on/off. After about a month of trying to get those to work, I gave up and bought some high quality spring rocker switches. For now, I am just running them for homing with soft limits on the other side.

I trammed the solid column using brass shims to 1.5 thousandths from left to right across the x axis and when I push on the head it will only wobble about +-1/2 thou. I calibrated each axis using the step calculator wizard to well within a thousandth repeatably. I discovered that you need to run your test in the same direction till you nail it. Then your next incremental move in the revered direction. The error here is your backlash. I found XYZ had 1.25/3/3 thou respectively. The software compensation corrected this.

I know its always best to eliminate the source of backlash so only compensated after tearing the entire machine apart. I discovered that ~1 thou will be inherent in each axis because the thrust bearings are not preloaded more than the very well machine housing provides. I accept this slop because they are so well done I can count on the software to fix it. 

Now z axis had about 3/4 thus more backlash reversing to move up than it switching down. It took me some thinking to realize it is because it's the only axis with a belt. It is being stretched dragging the head up. That combined with a head weighing 3x more than stock, created the discrepancy. So I psudeosplit the difference and added compensation of 3.5 thousandth because Id rather have the to be too shallow than too deep. I'll add the 0.25 in my g code.

The y axis has a problem I don't know how to fix. The ball screw is pinned to the slider base with a single set screw. Physically it doesn't move linearly but when you reverse directions the nuts swings about a quarter turn before being engaged. I've got to fix this out later. It's hard to get a set screw or something in there because it's in the very heart of the machine with at least 2 inches of iron surrounding in either direction. The software seems to work great for this as well because the source is predictable.

Going for my first metal chips in 6061 the next dragon reared its ugly head. I tried running a 6mm carbide 4 flute at 10ipm, max rpm (~2700) in a 50 thou depth of cut. At first, it sounded great but it slowly started stalling because the belt was slipping. It is because my spring motor tensioner isn't enough. Back to ths drawing back. I'm going to be replacing it with a 1/2 nylon stainless steel bolt with a lock nut for some springyness. I'm going to be adding a secondary drive pulley as well to bump up the 3.7:1 ratio I'm running. The bearings and grease can handle 8k so I'm going to shoot for 2:1 to 1.75:1.  

Shop Upgrades

I spent the weekend rebuilding this 1950s vise I found in a pawn shop for $25.

Going full blown CNC

Finally with a makerspace of my own, I can get to one thing that has been on the back burner for a while: Mini-mill CNC conversion.
But of course, I can't just grab a ready to install, set-it-and-forget-it kit. I have to go way over the top...so I did.

I ordered a CNC conversion kit from mbbilici. He is a Turkish guy who makes his kits to order in monthly batches. I ordered on January 1st and got the kit in the second week of march. I chose his design over CNC fusion or heavy metal CNC because of the Z-axis. I like the ball screw not being under the head of the mill where it could gather chips. Ball screws are fairly unforgiving so a few chips develop a knock and knocks add up to scuffs and scuffs turn into binding and binding turns into seizure. Mbbilici's design avoids the exposure by having a driven captured  nut at the top that rotates with a pulley to draw the ball screw up and down. Price cost between the 3 kits is about the same ~$700 no matter which one you go with.

The motors were sourced from China. The ones I am using are 3 phase NEMA 23, 3.5 N-m servo motors, which are just stepper motors with encoders installed and drivers that can accept the feedback automatically. I built my own unregulated power supply because regulated or switching power supplies are known to have issues with triggering safety shutdowns. The drivers can accept up to 50 VDC.

I knew I would be rectifying an AC signal to get the DC voltage I needed for the steppers so I grabbed a 500VA 32VAC toroidal transformer to drop from the 110V outlet to something more manageable.

Based on the rule of thumb that a rectified voltage will be ~1.414*32VAC = 45VDC. Once I ran the 32 VAC through a 50 amp bridge rectifier and into a 22,000 uF capacitor, I got a reading of 48VDC which is even better than predicted. The capacitor was only ~$3 on ebay but it did take a month and a half to get here. The transformer was $55 and I had the rectifier left over.

Since I was already modifying my mill, I upgraded the spindle drive motor as well. I tried to build my own motor controller. I went through a dozen different PWM designs that included power MOSFETS and IGBTs. I cooked them, fried them, locked them, blew fuses, added zenier diodes, RC filters, inductors...everything. I broke down and bought an MC-60 for ~$60 on ebay and it works like a charm. The analog circuit used in the treadmill controllers are really the best way to handle the power these motors need.

I installed the 3 hp treadmill motor by welding a bracket out of 3/8" thick steel plate that I bolted onto the top of the mill. The drive belt is a standard 4A v-groove power belt. On spindle is a steel pulley that I bought on ebay and bored out to fit on the spindle. I tacked it down to the spacer which has a key way groove that meshes into the spindle. I took the flywheel off of the treadmill at first but I noticed that despite tripling the power I was able to bog down 1/2" end mills. It turns out that the treadmill's power rating includes the flywheel weight. I put turned the grooved section down to accept the 4A v-belt. After putting the flywheel back on it now runs like it should. Smooth and strong.

With the upgrade to the power came a change for me to upgrade my spindle bearings to tapered roller bearings. There is a lot of debate out there about which bearings work best and why (angular contact versus deep groove verses tapered...) I choose tapered rollers because they work so well in my lathe. The preload is not as sensitive or critical. Tighten it down and as long as the bearing doesn't over heat, its fine. It doesn't run as high of speeds as other configurations but if I need to run 10k-25k+ rpms, I'll add a separate high speed spindle attachment. I plan on milling aluminum and steel which is more of a 3-5k rpm range though my grease is good up to 8k.

While I had my mill apart already, I figured I would fill the frame with epoxy granite and the head with lead adding about 65 lbs to the mill and significantly more damped. The head along went from about 8 lbs to almost 30 lbs.

Also, in the pursuit of rigidity and minimizing chattering, I welded up a T out of 3/8" x 2" steel plate 2 ft long. That was held parallel to the back of the column and welded down to the 3/4" steel plate that serves as the as for the milling machine. I have two 1/2" bolts that are going to be tapped into the back of the column. I ran a FEA on this configuration and with a 500lb  side load the deflection of the column cuts down from 14 thousandths to 2 thousandths (0.014" to 0.002")

Shiny new technique: Metal Spinning!

So I have decided to complete readdress my approach to regenerative cooling. After seeing the BPM-5 build by Copenhagen Suborbitals, where they used metal spinning to spin their chambers out of steel, I looked into what it would take to do something similar by hand on my mini lathe.

In my typical fashion after seeing a cool video then reading most of an article online, I decide to commit fully to spinning an aluminum combustion chamber. I modified my matlab rocket design scripts to given me the right dimensions for a 250 lbf chamber and get to work on the mandrel that I will need so that I can smoosh the aluminum into shape.

Shop press needs reinforcement

I tried to press out the inner race of a deep groove ball bearing and those things can take a lot more force than I anticipated. That is 3"x 1/2" thick plate/ I heated it up, straightened it in the press and tacked 2 more pieces of 3/8" thick x 3" plates in place. Then followed up with 200A full penetration 6011 welds to make sure it  stays in place. The cross beam alone has to weigh close to 50 lbs now.

Paradigm shift

Now that I have a welding machine that can weld aluminum my paradigm has shifted. I see the designs in my head from a whole new perspective. For injectors for instance, typically I went from idea to what seals can I buy to how I can make sure that it all bolts together to making sure there was enough rooms for flanges and a way for me to get a pressure transducer in there...

Now, I have a design in my head and as long as I can nest the components I can weld it in place. If I need a way to mount it to a load cell, I can just tack a bracket on after. There is so much more flexibility in modularity compared to the integrated unit conceptual approach I have grown accustomed to.

While my brain is reprogramming itself, I continue to invest in my capabilities. My milling machine on a rolling cart seemed like a good idea at the time but it turns out the cart is a giant resonance chamber. I had to move the mill so why not build myself a crane! Great welding project to keep honing my skills on. While I was at it I figured I'd build myself a shop press as well.

The boom is 2x2x1/4" square tube with a nested 1.5x1.5x1/4 extensible boom and a 4 ton HF jack to actuate the arm.

The shop press part is a 20 ton low profile jack and 2 3" wide by 1/2" thick steel flat bar pieces 30" long with 1" thick press plates.