What spindle for CNC mill ?

[compact8]

I am looking to get a small CNC hobby mill which has three options for the spindle. The two I am interested in are

1) 1.1 kW servo motor 3000 RPM
2) 2.2 kW AC motor 12000 RPM

Option 1 can do rigid taping up to M8 so I believe it can go to very low RPM and still offer sufficient power for doing decent cutting. I will see it as something similar to my manual mill ( 0.22 kW AC motor, spindle speed controlled only by gear ratio )

Option 2 is something new to me. why such a high RPM ? If I run the spindle at the max speed of 12000 RPM, I believe the DOC will need be small, may be 0.05 mm on aluminum using climb cut and do many passes ? is flood coolant a must ? how will it be different from doing the same on my manul mill at 2000 RPM, taking several deep conventional cut followed by a shallow climb cut for finishing and no coolant ?

 

[RJSakowski]

I am looking to get a small CNC hobby mill which has three options for the spindle. The two I am interested in are

1) 1.1 kW servo motor 3000 RPM
2) 2.2 kW AC motor 12000 RPM

Option 1 can do rigid taping up to M8 so I believe it can go to very low RPM and still offer sufficient power for doing decent cutting. I will see it as something similar to my manual mill ( 0.22 kW AC motor, spindle speed controlled only by gear ratio )

Option 2 is something new to me. why such a high RPM ? If I run the spindle at the max speed of 12000 RPM, I believe the DOC will need be small, may be 0.05 mm on aluminum using climb cut and do many passes ? is flood coolant a must ? how will it be different from doing the same on my manul mill at 2000 RPM, taking several deep conventional cut followed by a shallow climb cut for finishing and no coolant ?

My Tormach PCNC 770 has a maximum spindle speed of 10,200 rpm. One of the reasons that I chose it vs. the 1100 was the higher spindle speed. I use the higher speed when machining printed circuit boards and working with wood. It does have a two speed pulley and the maximum speed in the low speed setting is 3250 rpm. The biggest disadvantage of the higher speed is the lack of low end torque. The 770 has a 750w. 3 phase ac motor driven by a VFD and even on the low speed setting, torque is pitiful at 170 rpm.

I have the flood coolant option but haven't used it for a number of years as the coolant tends to go bad and I don't use it often enough to justify the additional cleanup after use. It is nice though, for a better finish. Especially when machining plastic. If I used the mill on a daily basis, I would probably use the flood coolant.

 

[B2]

Hi @compact8

A 2.2KW motor does not sound like "small CNC hobby mill". I have an older version of the PM940M CNC VS machine that weights in at about 1500+ lbs with stand and it has a 2HP (1.5Kw) 3 phase motor.

2) Folks who do engraving, where the cutter is only a few thousands of an inch in diameter, usually use high speed spindles and run at 10000 or higher. The small tool diameters result in very fragile tools so you have to have very good control of motion, especially in the z-axis, or the tool breaks. This is usually done on a small mill. I have a big mill with a maximum spindle speed under 4000 RPM, but which has too much backlash and dynamic head nod to have the control required to move small cutters up and down without jamming into the material. Likewise, backlash and errors in the x-y directions cause breakage. The higher the RPM then the cutter can be moved a little faster. Perhaps a better tool for this these days is a laser system.

1) I assume you understand the z-axis vs spindle phase control for rigid taping. To do this right you need some kind servo system for the spindle and these usually have limited power at some RPM. I wish I had rigid taping ability. So I would be interested to know what 1.1kW servo motor/technology you are looking at and if it works out for you.

 

[Eddyde]

It depends on what kind of work you do, what kind of materials you most often use?

 

[B2]

I use the higher speed when machining printed circuit boards and working with wood.

I used to try to do PCB via milling blank pcb board surfaces. I will not do that again, as having PCBs made yields a much better product is just plain inexpensive any more. See the links below. I have also use my machine to do some wood working, but I think a CNC router would be much better. But you use what you have at the time.

I recently designed a circuit board using KiCAD (User group Free software ) which works quite well and has many device libraries with it, or downloadable as add-on libraries such as from Digikey. There is even an auto router available that actually works pretty well. This KiCAD software will generate a complete design package which you can upload to a PCB manufacturer. I recently did this as a test to replace my electronics board in my

VFD conversion using solid state electronic components.

After I designed this I used https://jlcpcb.com/ to make the board. You upload the design and get 5 two sided boards back in about a week and the total cost is only about $25-30. It is all automated. The boards I got back looked great and worked despite a small routing error I made at one of the interface connectors! I ordered my boards without components installed, but they will stuff the boards for you for little more. If you choose to use SMD components they will also populate the boards inexpensively. @ptrotter recently posted his version of my VFD conversion for his PM1340GT which he was also merging with a Cough42 ELS design. He used the jlcpcb.com boards with the installed smd on the boards.

For the solid state lathe controls I used a slightly modified version of the controls that were designed by @B2. More information can be found at https://www.hobby-machinist.com/thr...nents-pm1440gt-vfd-3-phase.95058/#post-883089.

Clough42 ELS on my PM 1340GT

 

[compact8]

It depends on what kind of work you do, what kind of materials you most often use?

Aluminum, mild steel and occasionally 303 stainless and plastic ( ABS, POM, HDPE )

 

[compact8]

..... I would be interested to know what 1.1kW servo motor/technology you are looking at and if it works out for you.

I am looking at this mill which is intended for hobby use only. The mill in the web page has a standalone CNC controller and a big cabinet but they can also sell the mill with a Mach 3 controller board and other electronics housed inside a small box attached to the back of the column. I am not 100% sure what technology they use but should be something like this :

 

[B2]

Some random thoughts.

You are right, it is a pretty small and specialized machine. With only a few inches motion in the x and y axis you cannot work on very large pieces of material. Maybe this is ok for your applications?

My CNC machine came with a Chinese n-motion controller set up for Mach-3 (Newfangled Solutions / Artsoft). (Mach-3 is no longer supported via updates... but is still being sold... and has limited documentation about certain areas.... and has always had limitations. For example no where do they say exactly what variable addresses are allowed in the user G-code as opposed to being already used by the code itself. Mach4 is their new version, perhaps its documentation is more complete. Mach3 does work for most things I need to do, but ......??? It is widely used on low end CNC applications.) The n-motion controller has no readable documentation which apparently is not uncommon. It is not clear how many versions of the n-motion controller are floating around out there, but lots. It is not clear if the n-motion controller will work with Mach4. I am uncertain as to how Mach3 would work for rigid tapping as my machine has an induction motor not a servo ... so I have never really looked into this. There is no encoder in the spindle either. I would not start out with Mach3 if I were you. Because of the limited info on my n-motion controller I do not know if Mach4 would work with it. So if you are headed down this path, I would make sure the milling machine does EVERYTHING you want from it before buying.

You say "something like this" then link a video. So I assume this is not a video of the machine that you are looking to buy.

I think is kind of hard to find a good, inexpensive, CNC mill. Mine was only sold by PM for a short while and so there are not too many of them out there. PM does not sell a CNC mill of any kind any more. Mine came from mainland China, but I have no idea who actually made it or if it was made by the same folks who make the other PM-940Ms for PM. The mechanical quality of my machine is not as good as folks have reported on the PM940M non-CNC machines. I think all of the PM940M machines are in mainland China. I had very limited experience in milling when I purchased it so I did not really know the questions to ask. I got "mechanical" performance specs with the machine, but I do not believe they have anything to do with my machine nor do I think my machine ever was as good as the spec. sheet indicated. If the mechanics were better I might have tried to add encoder, servo motor, and working on getting rigid tapping. Other folks have purchased non-CNC 940Ms and have converted and claim that they have gotten accurate results. However, I have yet to find rigid tapping on these machines in the HM discussions. https://www.arizonacnckits.com/pm-940-cnc-kit.html makes kits for converting these and a few other machines to CNC and they seem to have an excellent reputation. You might want to look these over to learn how they work at it. For example, to minimize backlash you definitely want double ball nut lead screws and excelent bearings.

If I understand correctly, you have milling experience so should know what you are getting into. Also, the model you are looking at is a CNC machine: https://cncwmt.com/product/xk7113c/ But does not have rigid tapping and you have to have them add a mach3 controller etc. I would ask why? ! The CNC-M45 that they sell is written up as a drilling machine. Does it have tapping built it?

By the way, a lot of the folks on HM who have converted their machines to CNC use Centroid Acorn controller boards and their software, but it costs more than Mach etc. I suspect that once one person gets something to working others just follow. Newfangled solutions does not sell a controller board, but I suspect that there is plenty of chatter about various options on their user board.

Dave

 

[Huub Buis]

I build my CNC router for milling steel and aluminum and build a spindle running at 3000 RPM. That is the same RPM my manual milling machine runs and that is milling steel, aluminum and plastics for some years now. On the CNC router I also mill small double sided PCB boards (isolation routing) and they are pretty good, far better than the toner transfer method I used for +10 years.
Every choice is a compromise and you have to decide what is needed most.
Next year I will convert a new vertical milling machine to CNC and that machine will also have a 3000 RPM spindle. It will replace my manual mill and probably my CNC router.
On the CNC router, I mill the threads, that is easier than tapping. When the bored size is larger than 12 mm, you can use a lathe internal IR11 half profile threading tool to mill threads. Replacing the insert is far cheaper than replacing a thread mill or tap.