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Old 09-27-2011, 08:14 PM   #1
dieselpilot
 
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Default High speed CNC spindle

So it's time to replace the spindle in my Ebay parts CNC mill. I work on mostly small parts with 1/4" and under cutters so I decided on a fairly high RPM design using a brushless motor from the RC world to drive it. I have an industrial type controller but rated at only 500W, so I may just use an RC speed controller to operate it initially. I began today with the hope of getting along quick, but so far have made only a small dent in it.

An almost perfectly good motor was stripped down to the parts I need.



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Old 09-27-2011, 10:15 PM   #2
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Default Re: High speed CNC spindle

If you haven't sawed through 2.25" 304SS round bar by hand you haven't lived! After a lot of sweat, I have a 6" long stick of said material in the chuck and will begin cutting some parts. As I was already thinking about surface finish, I did some tests the on OD of the bar. Steel inserts work OK, but for the finish pass I decided to try an aluminum insert. All I can say is the finish is much better than I expected and it take off as little as you want. This should work very well for finishing the bearing bores.

I'll post a sketch of the overall layout later.


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Old 09-27-2011, 10:21 PM   #3
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Default Re: High speed CNC spindle

Looking forward to it. I have thought of the same thing. I bought 3 - 15,000rpm motors and thats as far as i got.
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Old 09-28-2011, 12:33 AM   #4
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Default Re: High speed CNC spindle

I'll be the first to admit I know less than nothing about computers or CNC, But if you haven't already thought of it I'd design and build your spindle to allow the use of some of the smaller series of ER collets. Obviously balance and dead true running will be a requirement at those speeds. Are the bearings your using designed to take the side and end forces?

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Old 09-28-2011, 02:26 AM   #5
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Default Re: High speed CNC spindle

The design uses a pair of angular contact ball bearings in the bottom and a single angular contact bearing in the top. The tool holder is a Chinese ER16 straight shank holder. Motor is a Neu Motors 1912/1Y. The rest is going to be made. There are a few threads over at CNCzone that get into the design aspects. This is what it will look like.

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Old 09-28-2011, 07:52 AM   #6
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Default Re: High speed CNC spindle

That's the way a spindle should be made - with two close coupled bearings as close as possible to the load and set to take out longitudinal lash - the tail end bearing merely acts as an outrigger to support and allow expansion and contraction of the spindle.

A common design error (my Chinese lathe and minimill) place the bearings at opposite ends of the spindle - so if set to zero lash - then as the spindle heats up it loads the bearings rapidly wearing them and introducing lash at stabilised temperatures once again.

Some industrial NC machines (Gildermeister) use angular contact tandem pairs which require unbelievable precision for both to carry the load.

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Old 09-28-2011, 12:37 PM   #7
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Default Re: High speed CNC spindle

dieselpilot,

Some thoughts based on my own current experience of building a CNC HS spindle:
  • [liHow do you intend to hold the spindle - this is the 'right' moment to build in lugs, ears, or any other type of attachment - having a near perfect run out but a wobbly or not truely aligned spindle is not helpful]
  • your base design which puts the motor at the back - if you move the motor to the middle and move the top bearing to the very end, you will have a stiffer spindle assembly for side loads
  • if you use a RC speed controller prepare to do some electronics to make it compatible with Mach3 - the servo/RC controllers expect a 1-2ms pulse every 20ms where 1ms = no speed and 2ms = full speed, whereas Mach3 outputs a true PWM signal where 0.2ms = zero and 19.5ms = full speed. The two are not compatible and require some playing to get them to work together
  • Power supply - I am currently (for testing) using a small car battery to provide enough amps to drive my ~500W outrunner since the power supplies I own did not like the peak start current - even with lots of C to help

I am watching your progress with interest and may even get around to reporting my own progress here...
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Old 09-28-2011, 01:11 PM   #8
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Default Re: High speed CNC spindle

A few years ago I picked up a Myford OD grinder that was missing the workhead. I built a workhead spindle from scratch and it has worked well so far. I agree with KenI that there should be 2 angular contact bearings on the tool end next to each other to set end play and 1 bearing on the other end as support for the spindle. Some high strength spindles use more than 2 bearings on the tool end but that is probably over kill.
One of the toughest jobs is to machine the bores for the outer races true to each other. The tool end will be bored for the 2 angular contact bearings and the other end of the housing will get bored for the support bearing. These bores and their shoulders must be true with each other. Any misalignment will create heat when running and assembly problems. I think maybe you could rough bore the housing first then fit it to an arbor between centers to finish each ends bores to keep alignment. I am quite interested in this project as I would like to do the same so keep us posted. my 2c, Dave
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Old 09-28-2011, 02:07 PM   #9
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Default Re: High speed CNC spindle

Yes, I understand I have a bit of compromise by putting the motor on the end. I had a version drawn with the motor in the middle, but it complicated the components and assembly quite a bit. This makes it easier to build as the body is only ~90mm long. Chances are this would never see a 3/8" endmill except for light facing. Mounting on the machine will be the typical method for such spindles, a bored block with screws to clamp. The intention is to complete this in the next day or two.

Bearing suppliers tech specs describe numerous ways to assemble spindles. I agree though that is thermal expansion is not accounted for in material selection, a rigid system like Ken describes is bound to fail.

Back to the shop!
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Old 09-29-2011, 02:36 AM   #10
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Default Re: High speed CNC spindle

I had a battle the PC today. I didn't print the drawings yesterday and it wouldn't boot today. Nothing new the PS for this particular Dell is trash I have 3 bad ones now. I was swapping 2 everytime they went down for some reason after a while resting they would work again. After days or weeks, after shutting down it won't give a "Power OK" signal after POST. So, today I fixed it semi-permanently by rewiring an ATX PS to meet the proprietary Dell connector standard. The only thing I don't have is fan tach signal from the PS. It runs with an error on boot, so I'm not going to complain for now.

Anyhow, I only manage to get the cone for the spindle shaft made today. Photos tomorrow after I get it mounted on the shaft and finish turned. Turning that 304SS is slow going.


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