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  1. #1
    Join Date
    Oct 2011
    Posts
    8

    Compact, low cost ISO25 belt drive spindle build

    Greetings all

    Sharing an idea I have been working on for a new spindle to replace the stock spindle on my Taig CNC milling machine. This is the first spindle I have designed so feel free to provide suggestions on how it could be improved. Still in the design stage so easier to change things now. Please note that my use case may differ from the next persons so don't be offended if I don't implement your suggestions.

    Some background
    - I have had my Taig mill for years and used it on and off, slowly doing upgrades as I get the urge. This project may or may not come to life, depends on if something more urgent comes up.
    - My mill is basically one of these hardware wise, it's not the fancy ballscrew version https://taigtools.com/product/micro-mill-2019cnc/
    - exploded view of Mill https://taigtools.com/wp-content/upl...Mill-Parts.jpg
    - I understand this is a mini mill / desktop mill. I'm not planning to strap a 3hp motor on it and expect it to hog metal all day long.
    - As mentioned I have had my Mill for many years now, the current models may be different but I'm not sure.

    Shortfalls of stock design (as I see it) / Why I'm looking to replace the stock one
    - Stock there is an aluminium dovetail plate that bolts to the Z axis carriage, the head stock then clamps to that via a clamping plate on the side. This is not the most rigid setup and doing adaptive tool paths previously I have had the head stock slip on the dovetail and get pulled down into the part.This has been mostly solved with a lower stop block to create a rigid stop but it's not ideal.
    - I'd like to move the cutter center line further forward to give more reach. Would be easy enough to do with a spacer plate but prefer to do a full upgrade
    - The ER16 spindle nose works fine (ER20 would have been better) but I want tools holders that I can change quickly instead of having to touch off after every tool change.

    Design criteria
    - Cheap, must be cheap. I'm not throwing a heap of money to get a super expensive spindle
    - Quick change tool holders, happy to run the draw bar manually, have a spare impact gun that I can use to speed up changes, but no more touching off all the time.
    - more rigid mounting, I never move the spindle up and down on the dovetail so why sacrifice rigidity for a feature I don't use.
    - Easy to make while still being nice and accurate, this means minimum setups for the critical dimensions. Not worried about secondary setups for non critical stuff, but don't want to be milling a pocket in one end then trying to get it lined up to pocket from the other side
    - compact and reasonable weight. The Taig is not huge, don't want to hang a 15kg headstock on the Z axis
    - readily available and budget bearings, I don't really run the headstock above 5k rpm, no point in paying a heap for fancy bearings to get high RPM ranges that I'll never use.

    Why ISO25
    Okay I'm sure this will be asked so may as well answer it straight up
    - Size, I considered BT30 or other 30 taper, but the fat end of the taper is 31.75mm. ISO25 is 25.2mm. ISO20 is 22.1mm. With restriction on outer housing size, ISO25 seemed to give best balance of the 3 tapers when looking at what bearings I could use.
    - Stick out, for the budget tool holders off AliExpress the 30 Taper ER20 tooling has a stick out of 70mm or 100mm where as the most common for ISO20 / ISO25 are 35mm.
    - Cost, I could find budget tool holders for about the same price before shipping for all 3 tapers. Price difference for shipping on ISO20 & ISO25 was negligible but 30 taper was significantly more. If I was to buy 8 ISO25 ER20 they work out at roughly 18usd each after shipping and tax where the 30 taper is nearly 30usd each.
    I know some things like face mill holders, ER32 collet chucks, ect will be easier to find in 30 taper, but the mill isn't big enough to want to use larger then ER20 or a facemill. I also don't currently have any 25 or 30 taper tooling so whatever I buy will just be for this mill and will stay with this mill.

    This is where I'm at so far with the design, the main section is a 100mm3 block of Aluminium, The spindle's largest diameter is 49mm to make stock selection easy. I'll probably end up going with an adapter between the new headstock and the Z carriage to make finding stock easier and mounting easier as the holes won't line up.
    I'm currently trying to find details on preloading for 5207 double row angular contact ball bearings. One would assume they are factory pre-loaded but I can't find anything about them. My local bearing guys normally supply NSK and they have
    "Double-row angular contact ball bearings are essentially a back-to-back mounting of two single-row angular contact ball bearings, but their inner and outer rings are each integrated into one. These bearings can sustain axial loads in both directions and offer good
    capacity for sustaining moment loads. They are often used as fixed-end bearings and contain cages made of pressed steel.".
    The original Taig spindle just has a couple of 6203's with some light pre-loading and has worked fine so huge preloading isn't required. If anyone is a bearing expert feel free to chime in.

    Attachment 476548

    Thanks for reading.

  2. #2
    Join Date
    Nov 2020
    Posts
    361

    Re: Compact, low cost ISO25 belt drive spindle build

    are you going to be 'screwing in' the toolholder each time? Or is that just visualization? No disc springs then?

  3. #3
    Join Date
    Oct 2011
    Posts
    8

    Re: Compact, low cost ISO25 belt drive spindle build

    Correct I will be screwing in the tool holder each time. Just removing the pull stud and using the thread there instead.
    With the taper being self releasing and cordless impact drill drivers being pretty cheap I will leave one near the mill.
    It won't be as quick or as fancy, but it will be quick enough and is cheap and easy to implement.

  4. #4
    Join Date
    Oct 2011
    Posts
    8

    Re: Compact, low cost ISO25 belt drive spindle build

    Spindle has been redesigned again, went for an even more basic lower cost setup with easier to get materials.
    The original plan was using 100mm square bar but found that can't get it locally, well at least not easily.
    It looks like 65mm square bar will be the easiest to get so have redesigned around that.

    I also followed up with NSK in regards to preloading on 5207 double row angular contact ball bearings and they came back to advise that they are not preloaded and have radial internal clearance. Their technical drawing has
    Internal clearance
    Radial 0.007~0.022
    Axial -----------
    Not entirely sure if this would be a deal breaker, I know This Old Tony used a double row angular contact ball bearing up front and a floating deep groove bearing up the back in his CNC Lathe router add-on. He didn't appear entirely happy with the outcome but wrote most of it off as being lack of rigidity in the rather instead of preload in the bearings.

    Regardless Taig uses 2 x 6203's in the stock spindle, I haven't really had an issue with the bearings in my use and although I will be upgrading the factory motor, it will still be under 1/2hp so I think a similar application will be fine.
    In the redesign I'm using 2 x 6006's which although being from the light duty range they have a higher load rating then the 6203's as they are larger bearings.

    Attachment 476956

  5. #5
    Join Date
    May 2018
    Posts
    1

    Re: Compact, low cost ISO25 belt drive spindle build

    Interesting! I went ahead and bought a Chinese BT30 spindle for my build, though it is definitely overkill for the size. I may be getting access to a CNC lathe in the near future so I'll be keenly following this thread. Have you considered powering the draw bar?

  6. #6
    Join Date
    Feb 2004
    Posts
    33

    Re: Compact, low cost ISO25 belt drive spindle build

    Nice design (both of them)! One of your design goals was to make the precision machining within one setup. The first design accomplished that. On the second design, how do you plan on making the two bearing bores concentric?

    I agree with your bearing selection on the second design. Its plenty good for the Taig.

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