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marbles
27-04-2020, 11:00 PM
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

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marbles
27-04-2020, 11:22 PM
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.

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Voicecoil
27-04-2020, 11:38 PM
Wow - you're not joking about it being "outsized" :-o What were you carving out of that may I ask?

marbles
27-04-2020, 11:57 PM
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.

m_c
28-04-2020, 03:57 PM
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)


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..

marbles
28-04-2020, 05:26 PM
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.
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BobTSkutter
28-04-2020, 06:19 PM
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

marbles
28-04-2020, 06:41 PM
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.

m_c
29-04-2020, 04:31 PM
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

Boyan Silyavski
29-04-2020, 09:15 PM
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.

marbles
29-04-2020, 09:25 PM
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


I was in the workshop today trying to learn how to program simultaneously 4th axis and had a bit of wood on the 4th. When I put some pressure on the edge of the wood I can see a small amount of movement at the motor. Iíve got a digital driver on that 4th and wonder if it might also Ďhave a go atí self correcting, not as good as a closed loop thought iím sure.

From the movement I could see it looks like the wee pulley on the motor needs some mechanical advantage help so I think your idea of the rotary table might be better. I see people using 30:1, 100:1 gearing, what kind of gearing is on a rotary table. At this point I wish I knew more about mechanical engineering. ..

All things considered was pretty surprised my attempts without using specific kinematic actually worked to program some simultaneously 4th axis action just using a basic mach3 post processor?!

m_c
29-04-2020, 09:26 PM
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.

I'm guessing you mean the InTurn thread on MachSupport?
IIRC you could swap the belt to get a double reduction for more torque.

You can get bicycle disk brakes from 120 to 220mm, however I wouldn't use one for a brake as they're too flexy. I think the InTurn used a motorbike/scooter disk, as some of those use pretty small disks, but the disks are also pretty substantial and won't flex like a bicycle disk. He also built his own caliper setup. All you need is a lever, and a suitable taper.

Boyan Silyavski
29-04-2020, 09:42 PM
Just checked my MTB and yes, if you push them from side, discs could bend but not easily, they are steel. But if mounted properly there will not be a problem. If a disk holds 100kg guy racing downhill with 60km/h, it will hold the machining too.

marbles
29-04-2020, 10:08 PM
Thanks for the ideas and support, much appreciated.

I’m frequently asked if I can do two types of statue milling. 1: where a client wants a copy of a sculpture, If I can do these as a continuous 360 helical mill I will do but often that strategy can’t get all the detail so that why i’mcurrently learning 2: index milling to get better 4 sided definition and detail. Or to put it another way have more control over the access to difficult areas. So yes I as I don’t need speed I just need holding torque I could do with a reduction to help the motor pulley side but also a brake so yes the bicycle or moped hydraulic brake might be fine.

Thinking extra laterally here for a moment as a short term solution what I could do is mill a few 300mm wooden disks with degree marking added or included as holes for pegs and just use separate files to orientate the stock to the angle I want and lock it off with a g-clamp. I know I know not very high tech but if all I have to do is remember to remove and reclamp the g-clamp to hold it steady i’d go for that.

marbles
30-04-2020, 06:30 PM
I just remembered I bought a few of these a few years ago. Very high quality wormscrew mechanisms. Might be worth a tryout
Rino Ondrives Worm Gear Reducer 30:1 Vexta Nema23

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Voicecoil
01-05-2020, 01:32 PM
They look like a top bit of kit - accurate to about 0.1 degree if I've read the Ondrives data correctly.

m_c
01-05-2020, 06:09 PM
I just remembered I bought a few of these a few years ago. Very high quality wormscrew mechanisms. Might be worth a tryout
Rino Ondrives Worm Gear Reducer 30:1 Vexta Nema23


They're certainly worth trying.


They look like a top bit of kit - accurate to about 0.1 degree if I've read the Ondrives data correctly.

Looking at the OnDrive website for Nema mount version, it looks like you can get either an 8 arc minute, and a 30 arc minute version, which is 0.1333, or 0.5 degrees.
It all depends what kind of accuracy is needed. 0.5 deg works out about 1.3mm of movement at a 300mm diameter. Off course the 0.5deg figure is the worst case scenario.

marbles
12-05-2020, 09:02 AM
Just out of interest i've knocked up this manual trunnion positioner to allow me to 5 faces foamboard up to 750x400x200mm. Ideally yes this would all be automated one day with the OnDrive Rino units I have but in these COVid times i'm doing a temp improvisation with what i've got in the workshop to test the proof of concept. Hopefully one day this will be a fully automated unit with amazing holding torque and a brake but for now just got to test it works. Watch this space for an update.

I should point out the top plate is attached to the foamboard with jumbo helical insulation screws and that plate is mounted to a 360 deg turntable, which should allow for quick changeover of stock. I have afterall got 144, 5-sided parts to mill!!

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