Design Discussion: High Performance CNC Spindle

[macardoso]

The overall goal is to come up with a spindle design that is smoother, quieter, and faster than the stock design, that can be made for reasonable cost using machines available to hobby users. I don't love outsourcing the work to professionals because then I have a nice end product without the pride of making it.

I totally agree with you on that one. Also because of the cost. I just don't know how I could go to a grind shop and have them do it for less than the value of the entire machine. I see the prices we pay at work and it is crazy stuff. I think you could probably start with some pre-hard material and do all the turning on a hobby lathe (not that 7x12 stuff though). Bet a 12x36 or 13x40 would handle it.

My goals for the spindle are (at the moment):
1. A smooth 10k RPM
2. Rigid tap capable
3. ATC ready
4. 2-3 HP

Sounds reasonable! Are you considering any kind of low gearing? Do you do any work with steel?

For the taper, I am going with BT30 for a number of reasons.
1. Tooling is readily available and inexpensive (TTS is restricted to Tormach and a few knockoffs of questionable quality)
2. Tooling is more likely to be sharable with future machines
3. Substantially more variety available for tools
4. Better retention with a pull stud (compared to TTS using R8 collet and 3/4" shank)
5. Much easier to implement an ATC
6. Fits well for the size of the machine

I've long considered this, but the cost of the tooling is driving me away. I've invested a few hundred in Tormach TTS and have a lot of holders to show for it (28 of them actually). They're almost always loaded up and ready to go. At $100 each from Maritool, I don't see how I could afford BT30. Are there cheaper suppliers that are decent?

I would like to be able to fit the spindle on the machine with minimal permanent modification to it. The biggest restriction there is going to be the bore that the outside of the housing needs to fit into (2.36" on my machine). That restricts the size of the upper bearing slightly as well as the spindle wall thickness. The alternative is either making a new head for the machine (Either with welded steel or aluminum screwed together), which is a possibility, but one I would prefer to avoid if there isn't a significant gain in performance. However, the way the head separates from the Z slide, a new one would not be out of the question.

This is a really important point. If we had the room, the easiest option would be to buy a finished Tormach spindle cartridge rated for 10K.

The top bearing would be free to float axially to allow for thermal expansion while the bottom pair will be fixed.

That's a really solid idea. Building your own design certainly would give you the flexibility to place the bearings in the most ideal locations.

 

[macardoso]

My home shop/garage is pretty far away from the house ... not terribly concerned with noise levels.

Hopefully one day for me. I'd even settle for a ground level shop right now. Basement makes it difficult sometimes.

Pending approval from management, I have access to a fully equipped machine shop for personal projects (manual and CNC). This includes ovens for heat treatment that are large enough to fit anything we can dream up. Precision grinding would be my limitation, absolutely none of that equipment in our shop.

I know we want to keep this in the realm of a hobbyists ability to produce, but I do not mind prototyping.

That's a really generous offer! Hopefully one of us will be in a place to need that soon! I need a new mill before I can really get a dog in this fight. Everyone cross their fingers and do a rain dance for me that I can get my hands on a RF45 sized mill that I've been stalking.

 

[bakrch]

I've been leaning toward a PM940 for my next build, but RF45 size is plenty big enough for me if it means I can participate in this effort.

 

[macardoso]

I've been leaning toward a PM940 for my next build, but RF45 size is plenty big enough for me if it means I can participate in this effort.

I was hoping to keep this generic. I have a G0704 (hoping for a BF46 Vario), shooter has a PM-25, I'm blanking on @spumco 's machine. I think they're probably all more similar than different. I think the spindle is often the most difficult part of a CNC conversion and it's fun to toss around ideas.

 

[bakrch]

I was hoping to keep this generic. I have a G0704 (hoping for a BF46 Vario), shooter has a PM-25, I'm blanking on @spumco 's machine. I think they're probably all more similar than different. I think the spindle is often the most difficult part of a CNC conversion and it's fun to toss around ideas.

Definitely keep it generic. Much of what I am throwing out there is just to put all of the cards on the table so-to-speak. This way we can all plan our moves, or at least I can.

I'm fairly flexible right now and have no real timeline (my PM-25 is plenty sufficient to get me by for now).

I've read somewhere (not sure how true it is), that the G0704 and PM25 have the same head, but when you get to the rest of the castings everything is more or less different. If we all have that to start with, we should definitely exploit that.

 

[macardoso]

I think in size and features, the G0704 and PM-25 are pretty similar. In construction, the PM 25 has some real advantages, especially that the column mounts on top of the base (G0704 bolts to the back - very hard to adjust). The spindles seem identical? I bought a CAD model of the PM-25 to help me design and all the parts of the G0704 spindle seem to fit exactly to the CAD model. General fit and finish of the PM-25 is better too. My G0704 has kinda poor fitting gibs and ways. I'm sure Shooter can elaborate here better than I can.

I started with the G0704 because it was all the rage 8 years ago in the wild west days of Hoss Machine. I really didn't know much about anything (had just started college) and I don't think Precision Matthews existed. I certainly wish I had bought a bigger machine, but it was the right call at the time and I've had a blast with it!

 

[shooter123456]

I totally agree with you on that one. Also because of the cost. I just don't know how I could go to a grind shop and have them do it for less than the value of the entire machine. I see the prices we pay at work and it is crazy stuff. I think you could probably start with some pre-hard material and do all the turning on a hobby lathe (not that 7x12 stuff though). Bet a 12x36 or 13x40 would handle it.

Yes, cost is another obvious factor. Material cost should be reasonable but I imagine that professional work would be at least $1000. Material selection is something I have not yet figured out. I am also not sure about hardening or grinding afterward. I think a toolpost grinder could do the trick, though I know a lot of people won't grind on their lathes. I have not decided how I feel about it yet.

Sounds reasonable! Are you considering any kind of low gearing? Do you do any work with steel?

I am not sure. I was considering a 2 step pulley, but I don't do much work with steel. I have done maybe 3 small parts in steel on the machine in the 2 years I have had it. My motor is 6000 RPM, so maybe a high range bumping up to 10k, then a low range gearing it down to 3k.

I've long considered this, but the cost of the tooling is driving me away. I've invested a few hundred in Tormach TTS and have a lot of holders to show for it (28 of them actually). They're almost always loaded up and ready to go. At $100 each from Maritool, I don't see how I could afford BT30. Are there cheaper suppliers that are decent?

I have the same problem, but I keep thinking that if I ever get a larger machine (and I really hope to some day), I will be buying all of that tooling anyway. There are a lot of machines that I have my eye on for one day in the future that are BT30 spindles (something like a robodrill or Syil tapping machine) which will mean purchasing all of the tooling anyway. There are a ton of suppliers for BT30 tools that are less expensive than Maritool, but I don't know about quality for all of them. Like anything else, there is the option to have less expensive tools where they will be sufficient and purchase the better ones for the important stuff. For example, drill chucks and roughing tool holders can be on the lower end and finishing tools one the higher end. I know of at least these suppliers that should have decent stuff though:

Tormach - $50-60 for set screw or collet chucks
Shars - $50 - 60 for set screw holders (They also have a set of 4 that comes to $35 each)

You also need to factor in a little bit more for the pull studs, which will add maybe $10 to each tool. There are some import tools for as low as $15 each. Quality will be a question, but like the cheap TTS tools you found, at $15 each, it may be worthwhile to buy several, measure them, and use the best ones or return the ones that don't meet their specifications. It would be a hassle, but an option.

 

[bakrch]

I think in size and features, the G0704 and PM-25 are pretty similar. In construction, the PM 25 has some real advantages, especially that the column mounts on top of the base (G0704 bolts to the back - very hard to adjust). The spindles seem identical? I bought a CAD model of the PM-25 to help me design and all the parts of the G0704 spindle seem to fit exactly to the CAD model. General fit and finish of the PM-25 is better too. My G0704 has kinda poor fitting gibs and ways. I'm sure Shooter can elaborate here better than I can.

I started with the G0704 because it was all the rage 8 years ago in the wild west days of Hoss Machine. I really didn't know much about anything (had just started college) and I don't think Precision Matthews existed. I certainly wish I had bought a bigger machine, but it was the right call at the time and I've had a blast with it!

On paper the PM-25 certainly has advantages. I can confirm the dovetail and gib fitment is very good on my machine, seems to be the common opinion. The column however, even with the better design it was poorly executed on my particular machine. It takes .02 in shimming to get the X square, but only .004 for the Y/nod.

I'm not sure what is typical, but needing .02 for X is very unacceptable to me.

I'm getting off topic here, so I'll stop now ..haha.

 

[macardoso]

Thought I'd try to get a couple more posts on the topics in the first post as promised.

Closed Loop

Accurate control of speed and position (indexing & tapping) requires encoder feedback. The encoder can be mounted to the motor (either integral or external) which is good, belted to the spindle - also good, or directly attached to the spindle - best. The encoder should have an index channel.

One issue with the spindle encoder is the speed. Very few encoders are rated for speeds above 6-8000 rpm, and the receiving circuitry is usually not fast enough to track these signals. Special encoders must be purchased and the number of counts per rev (PPR) is typically much smaller than an axis motor to limit the output frequency.

Mounting the encoder to the spindle is also tricky as the pneumatics for tool release is positioned above the spindle tube. A hollow bore encoder might be considered here. Heidenhain for example makes a high speed hollow bore encoder specifically for machine tools. No price though

NEW Encoders for High Speed Spindles - HEIDENHAIN

HEIDENHAIN ERM 2400 encoders are ideal for applications that offer little space and yet require a relatively high accuracy of angular measurement.

www.heidenhain.us

The encoder needs to be relatively immune to vibration, contamination, and temperature leaning us towards a magnetic encoder over optical.

Differential outputs and a differential receiver would be a big plus.

 

[macardoso]

Common Taper

As discussed above, the spindle should have a common taper to allow for easy access to tooling. This selection is critical as it determines tooling cost, accuracy, rigidity, and tooling vendors.

• R8: Not ideal due to long tool change length and balance issues. Tooling is widely available and inexpensive.
• R8/TTS: Quick change Tormach Tooling offers moderate stability, limited but low cost tooling options, and a short tool change length. Most of us already own a lot of TTS tooling.
• BT/CAT/ISO 30: Greatly increased stability. Toolholders balanced to 10k rpm. Lots of tooling vendors. Slightly longer tool change height than TTS. Moderately expensive
• BT/CAT/ISO 40: Too large for a benchtop machine.