Author Topic: DIY 4th Axis with Brake  (Read 1071 times)

Offline mukanico

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DIY 4th Axis with Brake
« on: February 03, 2020, 09:44:59 AM »
Hello,

I would like to present the latest addition to my workshop. Itís a 150mm DIY 4th axis (based on a harmonic drive reduction) with an electromagnetic disk brake.

The basis of the project is a 100:1 reduction harmonic drive imported from Japan. For compactness, I went for one of their CSD units (CSD-25-100-2A-GR-SP) which has an overall length of just 17mm and an OD of 85mm. The rated torque for this model is 47Nm at 2000RPM, with a limit of 110Nm for repeated torque.

The input of the harmonic drive is driven by a 4.6Nm closed-loop stepper motor (Nema 34). The output side is attached to a 40mm OD shaft supported by a pair of P5 precision tapered roller bearings (32008 XA P5), with the assembly being held inside a 120mm cubed steel housing.

The interesting part of this project is, in my view, the electromagnetic brake. Most of the DIY 4th axis builds I found online that are based on harmonic drives donít include a brake. This results in some performance issues, as the harmonic drives are noticeably not very torsionally stiff devices. As an example, for drive I chose, a 10Nm torque at the output results in a torsional displacement of 76 arcsec. Thatís a linear displacement of 28 microns at a radius of 75mm. This means that we are limited to doing only very light machining or very small parts...

Since I plan to use this 4th axis mostly just for indexing, having a brake mostly solves this problem of stiffness.

The brake is composed of an aluminium disk attached to the main shaft through an axial flexure (I had to go through 2 versions of the flexure until I found the right design). By default, the disk is pressed by a steel plate against a stationary aluminium housing. This plate is preloaded by 8 stacks of belleville (spring) washers held on M5 socket head screws. The spring preload, which sets the braking torque, can be adjusted by tightening or loosening the M5 screws.

Having aluminium as the braking surfaces might seem an odd choice, but this combination of materials actually has a pretty high static coefficient of friction (around 1.0 depending on where you source the data). Since the purpose of this ďbrakeĒ is to lock the 2 surfaces together and not do any actual breaking (i.e. surfaces sliding together), I donít anticipate too many problems of wear or galling of the aluminium surfaces. But I might be wrong  :loco:, and if I am a new disk and housing can be easily made out of different materials.

When it comes to how the brakes are actuated, there are plenty of methods available to choose from (hydraulic, pneumatic, hydraulic over pneumatic, etc.). I went for an electromagnetic actuation as it was quite easy to integrate neatly into the 4th axis and straightforward to connect to the machining centre this will go into.

The actuator consists of a 700 turn, 0.65mm enamelled copper wire coil inserted in the front face of the steel bearing block. At rest, the gap between the steel plate that preloads the brake and the steel bearing block is 0.2mm. When the coil is energised with a current of 2.4A, the magnetic forces pull the steel plate away from the aluminium disk, hence releasing the brake. Once the steel plate is stuck to the steel bearing block, the current can be reduced to half (about 1.2A) as to not overheat and burn the coil. In practice, this is achieved by connecting a resistor in series with the coil through a delay relay.

The measured holding torque for the optimal preload was about 90Nm which seems reasonable for a 4th axis of this size that will be used mostly for machining aluminium parts. The measured backlash (which comes from both the hysteresis losses inside the harmonic drive and the discrete steps of the stepper motor) was about 18 arcsec under no load.

I hope this can be of use/inspiration to anyone thinking of building a DIY 4th axis.

I will include a few images of this project next. :wave:

Cheers
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Offline mukanico

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Re: DIY 4th Axis with Brake
« Reply #1 on: February 03, 2020, 09:49:42 AM »
The harmonic drive


Machining the steel bearing housing



Turning the main shaft


The main shaft and bearing preload lock nut


Machining the brake disk (version 2.0)


Brake disk inside the brake housing
« Last Edit: January 24, 2021, 05:19:34 PM by mukanico »
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Offline mukanico

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Re: DIY 4th Axis with Brake
« Reply #2 on: February 03, 2020, 09:52:22 AM »
Winding the brake coil


Brake coil and steel bearing housing



Layout of parts (with an earlier version of the brake disk)


Beginning of assembly



Preloading of the bearings with a custom made socket

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Offline mukanico

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Re: DIY 4th Axis with Brake
« Reply #3 on: February 03, 2020, 09:55:23 AM »
Assembly of the harmonic drive



Assembly of the brake assembly and face plate



Closed-loop stepper motor installed



Stepper motor housing (a 3D printed part in ABS with a fibreglass reinforced outer shell)


4th axis installed in the machining centre

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Offline awemawson

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Re: DIY 4th Axis with Brake
« Reply #4 on: February 03, 2020, 10:35:42 AM »
Very nice  :thumbup:
Andrew Mawson
East Sussex

Offline PekkaNF

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Re: DIY 4th Axis with Brake
« Reply #5 on: February 03, 2020, 11:07:41 AM »
Great work. Is that RoboDrill? How does that brake works? Is the rotating disc aluminium or which part is stationary, which rotary and which is actuated by the coil? Maybe I need to have another look.

Offline mukanico

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Re: DIY 4th Axis with Brake
« Reply #6 on: February 03, 2020, 12:17:28 PM »
Thanks. It's a Brother TC-215, or at least the mechanics of one (I have done a full retrofit in the past with a new controller, drives and servos...).

The aluminium disk is attached to the main shaft and rotates with it when the brake is disengaged (i.e. the coil is energised). The disk is sandwiched between a stationary aluminium housing and a steel pressure plate (that's attracted by the coil).

Here is a photo of the brake unit assembly where you can see the steel pressure plate and the 8 belleville spring stacks. The fingers in the middle of the assembly belong to the disk brake (this bolts on to the main shaft).


I hope this helps explaining it.

Cheers
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Offline RotarySMP

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Re: DIY 4th Axis with Brake
« Reply #7 on: February 03, 2020, 01:22:25 PM »
Nice design and workmanship.
Mark
Best regards, Meilleures salutations, Mit freundlichen GrŁŖen, Cu salutari
Mark
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Offline PekkaNF

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Re: DIY 4th Axis with Brake
« Reply #8 on: February 03, 2020, 01:50:43 PM »
OK. Thank you. I get it now. The brake disc has high radial stiffness to spindle and the bell housing is fixed to main housing. The pressure plate doubles as a solenoid "plunger" and it does not need be indexed accurately into housing.

The construction looks very good and sound design.

Offline philf

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Re: DIY 4th Axis with Brake
« Reply #9 on: February 03, 2020, 02:26:46 PM »
Wow!

That's superb.

Much more substantial than mine!

My 4th axis also uses a Harmonic Drive but mine's 200:1 and is driven by an ordinary stepper so the speed is very slow. (The Harmonic Drive came out of a semiconductor silicon dicing saw which had been scrapped so was free = otherwise I'd have gone for 100:1 or 50:1.) I also use taper rollers in my design.

I've really only used mine for engraving so the cutting forces are low. Mine has a threaded nose to take Myford accessories so I do worry about damaging the Harmonic Drive when I'm tightening or undoing a chuck or faceplate. I have included slots for a c-spanner to oppose those forces but it's difficult to not try to turn the spanner when you're fitting or removing something.



Phil.
Phil Fern
Location: Marple, Cheshire

Offline mukanico

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Re: DIY 4th Axis with Brake
« Reply #10 on: February 04, 2020, 04:21:19 AM »
Hi Phil,

Have you considered attaching some sort of a split collar clamp to the headstock? You could use it to lock shaft rotation while you tighten and loosen your accessories.

https://images.homedepot-static.com/productImages/dd9879e8-b05d-4639-847e-474be05d599f/svn/metallics-climax-composite-fasteners-1c-062-64_1000.jpg

Cheers
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Offline philf

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Re: DIY 4th Axis with Brake
« Reply #11 on: February 04, 2020, 05:10:52 AM »
Hi Phil,

Have you considered attaching some sort of a split collar clamp to the headstock? You could use it to lock shaft rotation while you tighten and loosen your accessories.

Cheers

Mukanico,

I have - only last night after my post!  :thumbup:

It will have to clamp over my slots but that shouldn't be a problem.

What speed are you getting out of your hybrid stepper?

Cheers.

Phil.

« Last Edit: February 04, 2020, 07:44:13 AM by philf »
Phil Fern
Location: Marple, Cheshire

Offline mukanico

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Re: DIY 4th Axis with Brake
« Reply #12 on: February 04, 2020, 05:37:58 AM »
"Great" minds think alike :thumbup:

For the moment I'm running the motor at 1500rpm when indexing (15rpm at the 4th axis output). But I haven't really tried to find its limit. For my needs, this speed is sufficient...

Cheers
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