[QuinnSjoblom]

Wasn't really sure where to put this, it can be moved if needed.

I'm pretty happy with my home built machine the way it is, I can mill parts from 7075 with excellent accuracy(.01mm) but there's a couple limitations I've been trying to think of a way to get around. I've already posted about trying to get consistent tool length when changing tools in my er20. Few options there but either expensive, not that accurate, or not that fast. The other limitation I have is rpm. 6k minimum is a pretty big limiter. Also the er20 itself. I would really like to be able to use larger drills, boring heads, and face mills.

One solution I can think of is buying a drawbar type spindle cartridge and mount it next to my 2.2kw spindle on my z plate with a few belt drive options. Haven't done a ton of research yet, but tormach has a few options for around 500 bucks and they weigh about 20 pounds. No problem for my gantry to lug that around. Really wouldn't be hard to put it all together. I could make a few 1 inch thick plate mounts that hold both the spindle and cartridge side by side (2.2kw spindle upside down). I already have some experience milling htd pinions and pulleys so could come up with a pretty robust belt drive setup with some gearing options.

This could potentially address all the limitations I currently have. Faster tool changes, consistent tool offsets, all the rpm range I could ever want, bigger tool holding for facemills, boring heads, etc.

Has anyone tried putting a setup like this together? Any reason why it would be a bad idea? I don't plan on doing it soon, just something I'm looking into as a possibility.

[Goemon]

Wasn't really sure where to put this, it can be moved if needed.

I'm pretty happy with my home built machine the way it is, I can mill parts from 7075 with excellent accuracy(.01mm) but there's a couple limitations I've been trying to think of a way to get around. I've already posted about trying to get consistent tool length when changing tools in my er20. Few options there but either expensive, not that accurate, or not that fast. The other limitation I have is rpm. 6k minimum is a pretty big limiter. Also the er20 itself. I would really like to be able to use larger drills, boring heads, and face mills.

One solution I can think of is buying a drawbar type spindle cartridge and mount it next to my 2.2kw spindle on my z plate with a few belt drive options. Haven't done a ton of research yet, but tormach has a few options for around 500 bucks and they weigh about 20 pounds. No problem for my gantry to lug that around. Really wouldn't be hard to put it all together. I could make a few 1 inch thick plate mounts that hold both the spindle and cartridge side by side (2.2kw spindle upside down). I already have some experience milling htd pinions and pulleys so could come up with a pretty robust belt drive setup with some gearing options.

This could potentially address all the limitations I currently have. Faster tool changes, consistent tool offsets, all the rpm range I could ever want, bigger tool holding for facemills, boring heads, etc.

Has anyone tried putting a setup like this together? Any reason why it would be a bad idea? I don't plan on doing it soon, just something I'm looking into as a possibility.

I started out with similar thoughts but I learned a few things on the way...

First, there are adapters that allow you to fit larger end mills but they won't do much good. Your spindle is not powerful to make use of anything larger than 1/2" when cutting aluminum (and probably not stiff enough either). For woodwork, you can use router bits which mostly all have 1/2" or 1/4" shanks.

You can easily add a low speed / high torque belt drive milling head next to your high speed spindle but it has to be matched to the stiffness of your machine. A 2.2kw 24,000 rpm spindle motor is not particularly powerful (.7 ft lb of torque). A 2.2kw 1800rpm milling head motor is 3x times the size and weight. They put out 8.8ft lb of torque at the base speed and more with a gear reduction - more than enough to cause injury or bend a flimsy T-slot build. There is a reason why milling machines mount spindles on 2000lb solid cast iron columns.

I am putting an R8 spindle with a 2hp 1800rpm motor next to my 7.5hp high speed spindle but my frame is a lot stronger than most. My gantry is fixed and it's 8" thick (with no voids in the middle), I have 25mm rails on all axis and 25mm ball screws. It's a very robust build. I wouldn't want to try with an 80/20 t-slot gantry....

lMS is the cheapest place to buy an r8 cartridge. They also sell complete 500w R8 Milling heads from their "high torque mini mill" for $650.

You can't use your high speed router spindle to drive a 2000 rpm R8 (or BT30) cartridge. You'll need a low speed motor with pulleys and a belt.

Installing a second spindle and adding an extra 60lb + to your Z axis is not without it's challenges. It's not worth it unless you have a specific need imo.

[QuinnSjoblom]

I started out with similar thoughts but I learned a few things on the way...

First, there are adapters that allow you to fit larger end mills but they won't do much good. Your spindle is not powerful to make use of anything larger than 1/2" when cutting aluminum (and probably not stiff enough either). For woodwork, you can use router bits which mostly all have 1/2" or 1/4" shanks.

You can easily add a low speed / high torque belt drive milling head next to your high speed spindle but it has to be matched to the stiffness of your machine. A 2.2kw 24,000 rpm spindle motor is not particularly powerful (.7 ft lb of torque). A 2.2kw 1800rpm milling head motor is 3x times the size and weight. They put out 8.8ft lb of torque at the base speed and more with a gear reduction - more than enough to cause injury or bend a flimsy T-slot build. There is a reason why milling machines mount spindles on 2000lb solid cast iron columns.

I am putting an R8 spindle with a 2hp 1800rpm motor next to my 7.5hp high speed spindle but my frame is a lot stronger than most. My gantry is fixed and it's 8" thick (with no voids in the middle), I have 25mm rails on all axis and 25mm ball screws. It's a very robust build. I wouldn't want to try with an 80/20 t-slot gantry....

lMS is the cheapest place to buy an r8 cartridge. They also sell complete 500w R8 Milling heads from their "high torque mini mill" for $650.

You can't use your high speed router spindle to drive a 2000 rpm R8 (or BT30) cartridge. You'll need a low speed motor with pulleys and a belt.

Installing a second spindle and adding an extra 60lb + to your Z axis is not without it's challenges. It's not worth it unless you have a specific need imo.

I agree with a flimsy frame it's a bad idea. While my build does have some t slot in it, it gets it's rigidity from large solid 1/2 inch plate that triangulates everything together with large gussets. Doesn't rely on the extrusions for rigitity. It's a very rigid machine. All 20mm ballscrews and 20mm linear rails, dual stepper, dual ballscrew x axis.

As for the lack of torque from the 2.2kw spindle, isn't that the point of gearing down? If i start with .7ft lb and gear it down 10 to 1, does that not give me 7 ft lb at 2400rpm?

I would be a bit hesitant to add 60 pounds to my z axis, but 20 to 30 pounds, no doubt i think my machine could handle it without a sweat. The tormach cartridge I saw was 24 pound shipping weight, not sure what the cartridge itself weighs. I'll check out the ones you listed.

I'm not looking to swing a 3 inch facemill through steel or bore huge holes. Never plan on cutting anything harder than aluminum. If I could turn drills up to half inch, run an inch and a half facemill at shallow depth and bore holes up to an inch, I would be happy.

I agree it's not optimal and I've also been thinking about doing a second more heavy duty build, but I'm pretty confident in my machine design even though it's only about 300 pounds. I think if I came up with a light enough assembly (50 pounds max on the z plate) it could work within my expectations.

Another crazy idea I had was utilizing an rc brushless motor to drive a spindle. These things are mind blowing. My 700 size helicopter has a brushless motor on it smaller than my fist, weighing about a pound, and can put out 5kw continuous, with peaks of 10kw. Runs at 50v dc and can handle up to 200a. Normally powered by high performance lithium polymer batteries but would use a large dc power supply in this case. Hard to believe a 1lb motor can outperform a 30 pound watermelon but that's just where the technology is. Then the weight basically comes down to Just the spindle itself. Just another idea though. I would have to figure out how to control the electronic speed controller with mach3.

[Eldon_Joh]

gearing down a high speed spindle or motor to drive a lower gear ratio is certainly an interesting option to reduce the weight of the moving mass, but it adds another failure mechanism, the belt or the gears. also you have two sets of bearings to cool off.


anyhow i'm not saying don't do it, but i would add at least a 5 pound flywheel to a 200-2000 rpm spindle intended to turn .5 to 2 inch diameter cutters in steel.

[QuinnSjoblom]

gearing down a high speed spindle or motor to drive a lower gear ratio is certainly an interesting option to reduce the weight of the moving mass, but it adds another failure mechanism, the belt or the gears. also you have two sets of bearings to cool off.


anyhow i'm not saying don't do it, but i would add at least a 5 pound flywheel to a 200-2000 rpm spindle intended to turn .5 to 2 inch diameter cutters in steel.

Belt failure is definitely a possibility, but most milling heads are already belt driven so I don't really see an increased risk.

I won't be milling any steel, only 7075 aluminum, also I have no need to turn an endmill larger than 3/8 or so. I want to be able to turn an inch and a half fly cutter or face mill at very shallow passes and run a 1 inch max diameter boring head, also very light clean up passes.

[Goemon]

I don't think it will work with a belt driven by a 24,000 rpm motor. There is a reason why Haas 24000rpm spindles use a pin drive instead of a belt. There is also a reason why no commercial machines gear down a 24000 rpm motor instead of gearing up a low speed motor. If you try it, let us know how it goes.

I haven't seen your machine obviously and i'm no expert but what you described doesn't sound stiff in the context of milling machines. I lost a lot of faith in aluminum T-slots when I tried to use them for a gantry frame to lift my CNC gantry onto the table with an electric hoist. It bent 2" x 4" and 3" x 3" t-slot profiles like they were butter and ripped through 5/8" aluminum bars like they were paper....

[QuinnSjoblom]

I don't think it will work with a belt driven by a 24,000 rpm motor. There is a reason why Haas 24000rpm spindles use a pin drive instead of a belt. There is also a reason why no commercial machines gear down a 24000 rpm motor instead of gearing up a low speed motor. If you try it, let us know how it goes.

I haven't seen your machine obviously and i'm no expert but what you described doesn't sound stiff in the context of milling machines. I lost a lot of faith in aluminum T-slots when I tried to use them for a gantry frame to lift my CNC gantry onto the table with an electric hoist. It bent 2" x 4" and 3" x 3" t-slot profiles like they were butter and ripped through 5/8" aluminum bars like they were paper....

Here's my machine. Like I mentioned, it doesn't rely much on extrusion for rigidity. The z plate that the spindle is mounted to would be redesigned wider and beefed up as well as the plate behind it that z rails are mounted to. I guess I don't see how rigidity is going to become an issue by adding another 25 pounds to the gantry. Currently I can reef on the spindle as hard as I can and only get a couple thou of deflection when indicating from the table. When milling aluminum with a pretty healthy stepover, I'm holding very good tolerances, definitely under a thou, usually within .01mm. It definitely wouldn't come close to the capability of a common milling machine but with my expectations do you really think rigidity will be an issue? Only aluminum milling, Light passes with inch and a half max fly cutter or face mill, light cleaning with up to 1 inch boring head, and maybe drilling up to half inch.

[Goemon]

Here's my machine. Like I mentioned, it doesn't rely much on extrusion for rigidity. The z plate that the spindle is mounted to would be redesigned wider and beefed up as well as the plate behind it that z rails are mounted to. I guess I don't see how rigidity is going to become an issue by adding another 25 pounds to the gantry. Currently I can reef on the spindle as hard as I can and only get a couple thou of deflection when indicating from the table. When milling aluminum with a pretty healthy stepover, I'm holding very good tolerances, definitely under a thou, usually within .01mm. It definitely wouldn't come close to the capability of a common milling machine but with my expectations do you really think rigidity will be an issue? Only aluminum milling, Light passes with inch and a half max fly cutter or face mill, light cleaning with up to 1 inch boring head, and maybe drilling up to half inch.

The issue with the light aluminum frame is not the weight of the new spindle. It's the torque and cutting forces from the low speed / high torque motor and cutting with larger diameter tools. Your machine is closer in weight to your typical mini mill and they use 350w or 500w motors and recommend end mills no larger than 5/8" (even though R8 toolholders can fit larger). Plus, mini mills are made of stronger material and use a stiffer design than your machine.

A high torque motor can easily bend 1/2" aluminum plate and extrusions. You could rip the Z axis right off or bend your gantry. It can be a safety issue as well as a cut quality problem.

Any references you have from cutting at higher speeds with smaller tools aren't relavent. The cutting forces at high speed with smaller tools is a lot less. You are talking about using 8 times the torque.

The other issue with low speed / high torque spindles with larger tools is the vibration. Mills are made of heavy cast iron that absorbs the vibration. Aluminum is not good in this respect.

I'm not trying to be negative for the sake of it. I only know all this because I started out wanting to do something similar and after a lot of research and testing I found that a thin aluminum frame just wasn't stiff enough for a milling head.

I don't believe in just giving up though. IMO, if you need this capability, the correct answer is to build a new and suitably stiff gantry and frame to match the spindle motor.

As another option, they do make face mills with 1/2" shanks for aluminum on CNC routers (for use at high speeds). Might be a better option for a router...

[QuinnSjoblom]

The issue with the light aluminum frame is not the weight of the new spindle. It's the torque and cutting forces from the low speed / high torque motor and cutting with larger diameter tools. Your machine is closer in weight to your typical mini mill and they use 350w or 500w motors and recommend end mills no larger than 5/8" (even though R8 toolholders can fit larger). Plus, mini mills are made of stronger material and use a stiffer design than your machine.

A high torque motor can easily bend 1/2" aluminum plate and extrusions. You could rip the Z axis right off or bend your gantry. It can be a safety issue as well as a cut quality problem.

Any references you have from cutting at higher speeds with smaller tools aren't relavent. The cutting forces at high speed with smaller tools is a lot less. You are talking about using 8 times the torque.

The other issue with low speed / high torque spindles with larger tools is the vibration. Mills are made of heavy cast iron that absorbs the vibration. Aluminum is not good in this respect.

I'm not trying to be negative for the sake of it. I only know all this because I started out wanting to do something similar and after a lot of research and testing I found that a thin aluminum frame just wasn't stiff enough for a milling head.

I don't believe in just giving up though. IMO, if you need this capability, the correct answer is to build a new and suitably stiff gantry and frame to match the spindle motor.

As another option, they do make face mills with 1/2" shanks for aluminum on CNC routers (for use at high speeds). Might be a better option for a router...

Yes, in the end a new machine will definitely be best. The current one that I built needed to have a lot of versatility for what it could do and it's been great. I've designed and built a whole rc helicopter airframe with tolerances much tighter than I ever imagined i would get from a home built machine. I needed to be able to mill aluminum parts as well as large sheets of carbon fiber. In the long run I think the best approach is to keep my current machine for cutting carbon fiber and occasional larger parts, and build a smaller more rigid frame specifically for milling. So far i havent come across a part I need that doesn't fit in a 6 inch kurt so my current machine with a 20 by 26 work space is massively overkill for that.

Gonna be a while before I build a new machine though. I would want to use much higher quality ballscrews, linear rails, etc. So there's gonna be some money in it. I guess I was hoping that this could work within my limitations. I know you keep mentioning the torque will be too much for my current machine to handle, but are you talking about milling in general? Or the torque required to do the specific things I mentioned? Will it really create enough torque to cause problems by spinning an inch and a half fly cutter through aluminum at maybe .2mm depth of cut? Or bore a 1 inch hole just cleaning up .2mm or so? I definItely agree that most things a proper mill could do would rip my machine apart, but just so I'm clear, are you referring to the specific tasks of very light cuts I mentioned? If you think those light operations would be an issue, I'll take your word for it, just want to make sure we are on the same page for what I'm expecting it to handle

As for the 1/2 shank face mills you mentioned, I assumed 6k rpm is still too much. At inch and a half or 2 inch diameter, sfm is through the roof at 6k rpm. Are they designed to handle crazy high sfm?

I attached a pic in my last post, not sure if you saw it

[skrubol]

A 3:1 or 4:1 ratio might be possible, higher belt ratios aren't usually recommended in general. Another issue is most belt profiles aren't meant to spin at 24k RPM. (Haas 30k spindles are belt driven. No idea what the speed of the motor is though, much slower I assume.)
It does seem strange I've never seen a success/failure story of someone trying it..