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  1. #1
    Hi,

    I'm starting to mill some larger blocks of hardwood on my 4th axis using rotary indexing machining and I can see from the play in the wood when roughing at the extents of the block that the issue will only accelerate as I mill out to the edge of the stock. I've not had this problem with continuous 4th axis machining as i've been parallel milling down the centre line of the axis where there is least radial load on the stock.

    I've not really looked into this it only occurred to me tonight that maybe what I need is to be able to lock off the 4th axis between indexed rotations. Looking at my 4th i'll have to attach a brake rotor to the back of my 72t pulley.
    How have people solved this problem before?

    Anybody got a neat solution or a link?

    ps since the 4th was built i've changed the design a bit but its a standard belt and pulley set up with a nema 34 and an er32 shaft

    Click image for larger version. 

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  2. #2
    Just out of interest this is the kind of outsized 1200x600mm block of foam I attached to the 4th for milling out a sculpture in the past. Strictly parallel milling right down the centre of the 4th axis.

    Click image for larger version. 

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  3. #3
    Wow - you're not joking about it being "outsized" :-o What were you carving out of that may I ask?
    Last edited by Voicecoil; 01-05-2020 at 12:27 PM.

  4. #4
    Quote Originally Posted by Voicecoil View Post
    Wow - you're joking about it being "outsized" :-o What were you carving out of that may I ask?
    I was asked to make copies of two sandstone statues from a building in Glasgow from photogrammetry captured models.

  5. #5
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 18 Hours Ago Forum Superstar, has done so much to help others, they deserve a medal. Has been a member for 9-10 years. Has a total post count of 2,755. Received thanks 334 times, giving thanks to others 8 times.
    Quote Originally Posted by marbles View Post
    How have people solved this problem before?
    More torque or more braking :)

    Only real solutions are, a bigger motor, more gearing (could go to double belt reduction, or screw and pinion), or add a brake.
    1st is likely to be the most expensive.
    2nd you lose speed.
    3rd you need to consider programming it (typically done through custom M-codes)

    Quote Originally Posted by marbles View Post
    I was asked to make copies of two sandstone statues from a building in Glasgow from photogrammetry captured models.
    Have you posted about this before, or have I read about that somewhere?
    I seem to remember reading about models being made from photos in Glasgow..
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

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  7. #6
    Thanks for the feedback. Hmm I might have posted before about the big 3D foam milling, probably not this particular project though, see pic.

    I’ll start to look into the custom brake On/off coding and see if its something I can think incorporate without too much hair loss :). Yeah bigger motor could do, similarly the gearing as not concerned with speed just needs to work within the motors torque sweet spot.

    I was actually thinking not for this machine but another about making a big manual 4th/5th axis trunnion for a post lockdown job coming in. Something in which I can manually orientate large foam blocks, to orientate the part to mill a 5 sided objects using indexing milling strategies. I can just program each face separately, orientate the block, lock it off and run the file. As these will be in foam a small amount of rotational deviation will be ok and saves me tonnes of work and money trying to make a electromechanical version at short notice. As long as the tool tip is set accurately to axis centre/s should be ok.
    Click image for larger version. 

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  8. #7
    Would a closed loop stepper motor be any use? I would have thought the "closed loop" would prevent un-requested movement and behave a bit like a brake.
    regards
    bob

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  10. #8
    Quote Originally Posted by BobTSkutter View Post
    Would a closed loop stepper motor be any use? I would have thought the "closed loop" would prevent un-requested movement and behave a bit like a brake.
    regards
    bob
    I suspect the answer is probably yes but the gear thats on it isnt too bad, its not closed loop though.

    Current 4th set up
    3DM860, 24~80VAC 30~100VDC 7.5A 20KHz PWM
    Nema 34 , 8.5Nm, 5Amp
    80V + 600W switching power supply.
    Last edited by marbles; 28-04-2020 at 05:57 PM.

  11. #9
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 18 Hours Ago Forum Superstar, has done so much to help others, they deserve a medal. Has been a member for 9-10 years. Has a total post count of 2,755. Received thanks 334 times, giving thanks to others 8 times.
    Quote Originally Posted by BobTSkutter View Post
    Would a closed loop stepper motor be any use? I would have thought the "closed loop" would prevent un-requested movement and behave a bit like a brake.
    regards
    bob
    Yes and no, would be the answer to that.
    The big benefit of using a closed loop stepper in this application, would be they do tend to self correct to a certain amount so would move the stock back into position, and they would let you run things far closer the motors limit, however any movement could still scrap the part.
    It's a case of balancing up your needs, and what compromises you can make.

    I have just thought of one option if you don't need much speed, how about mounting a stepper onto a cheap rotary table?
    They should be adjustable for backlash, and they'll give you lots of gearing in a compact format
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

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  13. #10
    On the best diy heavy duty mill/lathe 4rth axis as far as i remember relation was 1:7 and there was possible via second belt change to raise that. And i remember talking to the guy and he was using 120?mm mountain bike hydraulic brakes and discs. Check ebay the standard size of brakes nowadays, could be around 200mm. Hydraulic brakes is the way to go i believe, fix the discs on the shaft somewhere.
    project 1 , 2, Dust Shoe ...

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