Monday, September 25, 2017

Decisions, Decisions

After living with my CNC G0704 since last spring, I'm pretty happy with it.  I can see a few things to improve it and reasons not to do anything.  Lately I've been asking myself what's worth doing.  I thought I'd update on what's going on in the shop. 

The three things that would make life a bit better are - in no order at all:
  • Improve the enclosure
  • Implement a standard way of handling and changing tools, probably the Tormach Tooling System
  • Swap the motor - or change the way it's implemented - to double or triple the spindle RPMs.  Also, add turning it on or off and setting RPMs from the computer. 
The short story on the enclosure is that it wasn't very well documented and it's almost a bit of an afterthought.  It's not very robust and isn't the way I think I'd do one if I were starting from scratch.  I've seen better looking, and doubtlessly more expensive enclosures since then. 

The enclosure was based on one that Hoss built for his mill, and like many folks, my conversion was based on his DVD.  Of the many CNC-converted Grizzly G0704 mill/drills out there, it seems that the majority use some version of his approach.  It's not really shown on the DVD I bought, but rather there's a handful of videos on YouTube and many details get talked about in his CNCZone Forum.  

On the other hand, the purpose of the enclosure is to keep the chips and overspray from my misting cooling system inside, and it definitely does that and does it pretty well.  With the exception of the top rail being low so that when I lean inside I bang my head on it about 25% of the time (I am learning to reflexively bend over), it works.  It rattles if you bang it, but the latches hold it down, so this is a pretty minor "nice to have" upgrade. 

The Tormach Tooling System would be an absolutely necessary addition if I was a professional shop trying to recover my costs of doing things.  But I'm not.  For those folks not familiar, the way tools are mounted in mill like this (like most) is with a tool holder that goes inside the spindle.  That holder will hold the cutter, a drill chuck, or other tools.  The G0704 uses an industry standard spindle size, called R8, and R8 collets for all sorts of tool sizes, dedicated tool holders like drill chucks or other things can be found.  The way I change tools now frequently requires me to change the collet and the cutter, then find the new position of the tool.  Back in June, I told about a "touch plate" that uses a piece of software to set the Z = 0.000 inch point. 

What Tormach does is use one slightly-modified R8 collet that is permanently mounted in the spindle.  This is sized for a 3/4" diameter tool, and they have a series of holders that are a 3/4 bar on one end and a custom holder of some sort on the other end.  The feature here is that if the collet is slightly loosened, the tool and holder combination can be swapped with another combination and the system registers the position of the new cutter very precisely in all three dimensions. 
The holders are intended to be used with an automatic tool changer, but can be mounted by hand.  The R8 holder system uses a large bolt (7/16-20) that draws the collet up and tightens the hold on the tool; that task is frequently automated with a power draw bar that releases one tool and then grabs its replacement once the tool is mounted.  The act of tightening the draw bar is what precisely positions the cutter.

In principle, once you find zero for your axes, you always know where X=0 and Y=0 are, and the only axis that might change is just where Z=0 is, if the tool is longer or shorter.  My touch plate can give me that.  What this means is that it might take me extra time to change the collet, change the cutter or drill bit and re-zero, but if I'm not trying to minimize my cycle time in order to keep my price down, do I care?   Except for the obvious neatness factors, I'm not sure I do.

The big drawback to having a separate holder for every tool is that I'd need a separate holder for every tool.  Right now, my tool box has a lot of different size cutters in it.  Many of them need the same size holder, 3/8", but it would be extreme to get one for every cutter in the drawer.  There's a small number that I use the most often.  They would get a separate holder and a permanent place in a software tool table, so that my controller software knows the diameter and length of the tool. 

How much does all this neatness cost?  It could easily run several hundred bucks.  I haven't actually run up a total cost, but I've seen an eBay seller with "clones" of the TTS holders at about $18 each, including shipping (from guess where), if I buy 10.  In addition to those, I'll need collets for the tool end of the tool holder.  There's a few series of standard collets, called ER and then two digits to designate the series.  ER-16, ER-20 and ER-32 are commonly used and I'm likely to need a bunch of those.  In the case of the 3/8 shank end mills prices on Little Machine Shop tell me I'd need an ER-16-9 collet at $9.66 (based on being one of a set of six different sizes), and a TTS ER collet holder at $34.75.  That's $44.41 to hold one tool - and those are "real" TTS tools, not the cheap clones.  As a WAG, I could cut that to about $25 each.  Still not insignificant if I have 10 or 15 tool holders. 

Yet it seems all it really does for me is to make getting a precise Z-axis position easier, a task I've already automated.  Since I'm not trying to shave seconds of a mass production product, it just doesn't seem to matter. 

Finally, the motor.  Here I know the least amount of hard details, but the issue I'm trying to fix is that the max spindle RPM I can get is around 2200.  Another thing to upgrade is that I currently turn on the motor manually by pushing a button on the mill and set the speed with a knob next to it, but both of those operations are simple, standard CNC code.  I just don't have a control that works over the interface yet. 

Virtually every time I'm cutting aluminum, my speeds and feeds calculator tells me to run higher RPMs than 2200.  What's wrong with going 2200 when the software would rather have me go 3600 or 4400?  It wears out the tools faster.  Tool life is a balance of how fast it rotates and how fast it cuts into new metal.  On the other hand, whenever I cut steel, the recommended spindle RPM drops down well below the 2200 limit.  The current motor is specified at 1 HP (750W) and while I'm not sure it even does that (I know... running downhill with the wind at its back), I don't think I'd want to go much above that.  I understand that 3 phase motors are quieter but that will require an electronic box called a Variable Frequency Drive.  A motor like this has been recommended to me.  Whatever motor I'd choose would end up needing an encoder to send speed information back to the controller program, and would need to accept control inputs. 
(Automation Direct GS3-21P0 VFD)

Again, I'm looking at around $500 to implement a better motor.  I might only need the higher RPMs sometimes, but it would be quieter all the time, and I could control it with my CNC control box. 

Since I'm not Elmer J Fudd, millionaire, this raises the usual questions everyone has about the budget only being so big.  Of the three, the motor upgrade might well be the most expensive, but it seems to  make the most sense. 


  1. If you speed up your spindle, you may need to change spindle bearings. The lower spindle bearings on my Grizzly G0755 mill get hot enough I stop for them to cool after 10-15 minutes at the 1970 RPM high speed setting. I don't think this is a flaw, I think this is a design tradeoff to get stiffer bearings at slower speeds.

    1. Yeah, I've seen lots of guys on the forums change out their spindle bearings. That's probably down the new-motor road, too.

      This is kind of a theme of mine. These machines are systems, and that means that every part was chosen in consideration of the rest of the parts. The 2200 RPM motor is fine when it's manually cranked, because humans can't crank fast enough to take advantage of a faster spindle. The closest-sized Tormach mills, which are designed as CNC out of the box, have 5000 or 10,000 RPM motors.

  2. Paste a small picture of a duck on the door. Eventually, you will learn...

    1. Say the secret woid and win a prize!

      Viaduct. Viaduct? Vy not a chicken?

  3. Using as large a cutter as will do the task will add SFM without increasing rpm.
    In my shop I have occasionally spent large sums on machinery that ended up sitting idle most of the time. I have also bought "one time only use" tools that worked their way into production because of a particular ability. the point being it can be difficult to know where improvements are best applied. My standard now is find either a bottleneck, or something I hate doing to apply a fix to.
    The tormach system has one nice feature in that it can be applied to a new mill in the future.

  4. Those VFD's are expensive but are a sweet little item. They also extend the life of your motor. Around here you can't get three phase into residential garages so you are forced into buying an expensive phase inverter so that you can get 220 volts to work on three phase motors.

  5. I have a vfd on a series one Bridgeport and ended up with interference with my controller. I cured that by moving it to the other side of the head about 30" away and shielded by the iron. Just a heads up for positioning.