Servo Driven 4th Axis for CNC Mill

[awemawson]

The Heidenhain TNC355 (which is very similar to the TNC150/151/155 ones) outputs an analog signal in the range +10v to -10v representing the motor speed and direction. This goes to the Bosch servo driver card that beefs it up to the 140 volt max and 26 amp absolute max peak that the motor will take. Into the Bosch servo card goes the tachometer output that is integral to the motor (7 volts per 1000 rpm).

At this point the TNC355 has no idea if the motor has moved at all so it takes in signals from a rotary quadrature encoder (also made by Heidenhain so conforming to the same spec) fixed to the servo motor (in my case through a belt drive as I need to change the ratio) giving it 36,000 counts round the 360 degrees of table rotation. Also on the table is a 'reference point' switch. When the controller is first switched on, all axis, including the rotary one, have to transverse across their reference points. From then on the TNC355 keeps count of where the table is.

The servo card has some more sophistication in that it has a form of 'three term control' (proportional, differential and integral) that when set up right allows it to respond to, for instance, a step function increase in speed without either overshooting or approaching too slowly.

All this is ahead at the moment until the mechanicals are sorted 

Andrew

[mgnbuk]

You will have a very hot motor if you manage to stuff 26 amps into it !

IIRC the peak current value is the maximum the motor can tolerate before de-magnetisation occurs - continuous stall current is 4.4 amps for an MT30 H4-44, which I think is the one you are using ?

Regards,

Nigel B.

[awemawson]

Yes that's the one Nigel. I am amazed that at 5 volts on a psu that limits at 1 amp it is turning the table, and while it's doing that (remember the encoder is removed) I cannot stall it by grasping the rear end of the shaft with my fingers - the current just goes up from 250 mA to about half an amp. Incredible torque.

Presumably the Bosch servo driver limits the current to a safe level ?

Andrew

[awemawson]

Pulleys and belt for the encoder reduction drive came in the post this morning, no time to fit as we have guests for lunch

Andrew

[mgnbuk]

There should be some method of adjusting the current limit on the drive. I am not familiar with the Bosch drives - I've tweaked them & swapped one or two, but can't recall the setup - usually I was just fine tuning the standstill offset or the speed to get the trailing error correct. It maybe a potentiometer, where you set a voltage on a test pin to a particular value for a particular current limit, but some drives require a resistor changing to set the limit. Most of the Bosch drives on Bridgeports would have been set up for the axis motors, which were 3.5 or 4.5 Nm, so the current limit will probably set much higher.

If you Google the Bosch drive part number, you may be able to find a copy of the installation / set-up manual.

HTH

Nigel B.

[awemawson]

Thanks Nigel. There is a pot on the servo card to set the limiting current when stalled and a full 10 volt command input is applied. I have had the manual on file for a few years as this project has been long in gestation   

Andrew

[awemawson]

So this morning I've finish off the CAD drawing for the reduction belt drive to the encoder. Now I just need to work out how to hold the stock when machining. (Stock is 200 mm off that long plank of 100 mm x 20 mm aluminium) As I want to cnc mill it at one setting I obviously cannot just grip it in the vice, and there are no convenient lands to put bolt in from underneither that won't finally be cut away. 

Any suggestions welcome 

Incidentally you can see on the rendering that in an ideal world this would be cut from 110 mm stock, but as I've said before, you work with what you have 

Andrew

[John Stevenson]

Any suggestions welcome 


Andrew

Buy a laser, find an old plywood packing case and start off with this.



A bit of PVA and here's one I made earlier 

[Pete W.]

SNIP
 As I want to cnc mill it at one setting I obviously cannot just grip it in the vice, and there are no convenient lands to put bolt in from underneither that won't finally be cut away. 

Any suggestions welcome 

SNIP

Andrew

Hi there, Andrew,

Surely, it's OK to design in features that facilitate manufacture, even if they don't have a function once the component is in use?

How about a boss between the two holes with one or two tapped holes in their underside?  Is there room between the pulleys?

Drill and tap the holes as stage #1, then screw the blank to a holder-plate.  (Studiously avoiding the terms 'jig' and/or 'fixture'!)          
Turn the sandwich over and clamp down via the holder-plate. 
Then, stage #2, machine the part according to your drawing (but leave some meat round the tapped holes). 

Agreed, this would require two sessions of machining, but ....... 

[awemawson]

OK John, I KNOW I made a masking tape mock up, but a plywood mount is going too far 

Peter, Yes I'm going to have to make some form of jig to hold it and do it in two sessions. I did try tentatively using a longer  (*) piece of stock clamped with four hold downs on a sacrificial piece of contiboard. Needs the clearance for the 22 mm profiling cutter to pass round the ends. I put double sided tape between the contiboard and the aluminium to try and reduce vibrations on the finishing cuts, and reduced the through bores by 0.5 mm to leave a solid base to hold things in place. However it let go 

I'd already reduced the feed to 50% of programmed rate and had my hand on the E Stop button so it wasn't too dramatic, but that length of aluminium won't now be the finished item 

However as I'd drilled the holes to mount on the Servo motor and the holes to mount the encoder at least I was able to use it as proof that I'd got my hole locations correct 

I'm being dragged to a nieces wedding in Bath this weekend so progress will be delayed

(*) I've not yet found how to define 'keep out' areas in Featurecam so I need a fair space between the work and the clamps.

Andrew

[jb3cx]

Hi Andrew I may be able to help you on the plug for the 4th axis ,pm sent ,
Peter

[Pete W.]

SNIP

I'm being dragged to a nieces wedding in Bath this weekend so progress will be delayed

SNIP

Andrew

Hi there, Andrew,

        
I've seen adverts for clockwork feeders for domestic cats and dogs - do they make them of a size to suit your livestock?          

Joking aside, I hope the sun shone on the bride.                   

[awemawson]

Well Pete, happily a good neighbour has been dealing with the stock while we were away, but my brother in law told me this weekend he had modified a timed cat feeder to supply his dog between visits from the dog walker while he was away for this wedding !

Now we are returned so perhaps I can get on with things, but I have 600 hedging plants arriving either Monday or Tuesday to plant a new 100 metre hedge across the field where we moved that mountain of earth last year, so that's going to hold things up   

Andrew

[Pete.]

What are you going to do hire a petrol post-hole drill?

[awemawson]

Should just be 'insert spade, wiggle it, drop in bare root whip, fit rabbit wrap, insert cane, tred in . . next '

Andrew

[awemawson]

Progress 

I decided to mill the inner form of the belt drive housing, then make some hold down disk and mill the outer profile as a second operation on a spoil board.

So:

a/ Slice another slab of stock

b/ A few passes with the roughing cutter and finishing cutter

[awemawson]

Then make some heavy duty hold down disks and re-tram onto the correct location

[awemawson]

And go at it again with the roughing and finishing cutters

[awemawson]

It'll do, but there's a bit of hand blending needed where the stock wasn't quite wide enough. Next thing is to get it mounted on the servo motor and fix the encoder so that I can decide how much dishing the cover will need to accommodate the thickness of the pulleys.

Bit of a delay tomorrow as all those hedging plants arrived today and need planting -it's a BIG heap

Andrew

[Pete.]

Nice

[awemawson]

Thanks Pete.

Not much done today as I was planting hedges - a 100 metres of 'em 

However I've bored and reamed the pulleys and test mounted the drive enclosure on the back of the SEM servo motor, and fitted the encoder thus proving that the allowance for belt tension is ok. But is also shows me I need to bore a hole to let me fix the encoder pulley's grub screw. I'll tap it and seal it with a larger grub screw when the settings are finalised.

Andrew

[Pete W.]

Hi there, Andrew,

I'm confused!                   

You seem to have mounted the larger pulley at the smaller end of the casing.  Is that because it's just a trial assembly?  Or have I missed something? 

[awemawson]

Pete,

Yes you ARE confused! 

The housing has it's two circular ends the size that they are to accommodate the existing mounting holes for the SEM servo motor (larger end) and the Heidenhain encoder (smaller end). However the encoder needs to be slowed down to give 100 counts per motor rev, and not the 125 that one revolution of the encoder gives. Hence the encoder has a 50 tooth pulley, and the servo motor has a 40 tooth one.

Thus one 'table revolution' gives x90 (worm wheel ratio) x4 (gear train ratio) x100 (encoder pulses per motor rev)

so 90 x 4 x 100 = 36,000 which is the count the TNC355 controller needs to give the required accuracy of 100 turns counts per degree

Andrew

Andrew

[philf]

Thus one 'table revolution' gives x90 (worm wheel ratio) x4 (gear train ratio) x100 (encoder pulses per motor rev)

so 90 x 4 x 100 = 36,000 which is the count the TNC355 controller needs to give the required accuracy of 100 turns per degree

Andrew,

Shouldn't "100 turns per degree" be "100 pulses (or counts) per degree"?

Phil.

[awemawson]

Phil yes it was a typo,

The TNC355 requires either 18,000 or 36,000 encoder 'counts' per 360 degree 4th axis revolution (set by parameter) - I've settled on 36,000 as it makes the gearing easier. So there are 100 encoder counts per degree.

To turn the table by one degree takes one revolution of the servo motor.

  I keep re-working those figures but I >think< that's right 

Andrew