Auto Tool Changer for an ER32 Mill Spindle

[philf]

Apologies first for posting this here as well as on a CNC forum. (It's been on almost a week and enough people have looked at it but not one reply or suggestion.)

I'm considering building an auto tool changer for my home built CNC mill (based on an Alexander engraving machine). I designed the spindle (perhaps short-sightedly) to take ER32 collets directly in the spindle. I did however put a 12mm hole right through the spindle with a drawbar in mind.

The taper on the ER32 is very similar to an R8 taper which is commonly used with the Tormach TTS toolholders. The TTS toolholders have a plain 3/4" spigot and an annular flange which is pulled up against the end of the spindle by the action of the collet pulling the toolholder into the spindle. This gives a repeatable tool height.

I am thinking of making a special collet with an 8 degree taper (as ER32) but with a plain section and 12mm thread for a drawbar which will be pulled up with Belleville washers.



I would make the collet out of a decent tool steel but don't contemplate hardening it as I don't have the facilities.

The spindle has a 3 step poly-v pulley arrangement and with a nominally 0.55KW 3-phase motor and VFD can run anywhere between 200 and 10,000 rpm.

Can anyone think of any good reason why this wouldn't work?

I don't particularly like the idea of a pneumatic actuator for depressing the drawbar as my compressor is deafeningly loud and it frightens the life out of me every time it starts. I've thought about linear actuators either acting directly on the top of the drawbar or through a lever system to amplify the force. I haven't tried to do any calculations but I'm thinking I will need a force of around 5,000N (roughly 1,200 lbf) to compress the Belleville washers. I'm not too worried about speed of operation as long as it only takes a few seconds rather than minutes to open the collet.

Another drawbar solution I've seen used is to use an electric impact wrench to tighten the drawbar but these are noisy and still need something pneumatic or motor driven to lower the impact wrench onto the drawbar hex.

Any other ideas?

Cheers.

Phil.

[RobWilson]

Hi Phil

I cant see any reason why your plan will not work , its the exact same quick change system  I am going to adopt  on my CNC mill (when done)  . I am not a fan of the rattle gun draw bar either .


I would and am going for an air cylinder as if the cylinder volume is not to large and the compressor tank is modest you should get quite a few  tool changes between compressor cycles .

Air will be faster 


Rob

[awemawson]

My Beaver Partsmaster has an air over oil pressure intensifier to pull up the drawbar. My (years ago) Moog Hydropoint 1000 had a simple claw that expanded within the tool holder gripping a grove cut into the hole in the top. Expansion was simply a tapered plug on the end of the drawbar screwed in expanding the petals of the gripper, and driven iirc by a separate electric motor (may have been an air motor), the gripper being sufficiently springy to contract to release when the motor reversed. Seemed to work ok. It was a Bristol Eriksson design iirc.

 

[philf]

Rob,

Maybe I dismissed the pneumatics too quickly. I might look at putting the compressor in a box with some soundproofing.

Andrew,

Interesting - so your Partsmaster doesn't have springs to pull the toolholder into the spindle. The air over oil device must therefore be pulling on the drawbar even whilst the spindle is rotating. Is it failsafe?

I've seen an air over hydraulic unit at Harrogate a few years ago. Expensive to buy - how difficult to make I wonder.

Already 2 responses in a couple of minutes - Thanks - It's what makes this the most friendly and helpful forum I use.

 

Phil.

[awemawson]

Phil,

I've never dismantled it - it's possible the air over oil is used to release rather than engage thinking about it. It's all buried in masses of covers so not easy to see, and I don't have any drawings for it.

In fact giving it due consideration it must be like that, as otherwise if left unused for a few weeks (which it often is) the holder would potentially drop out of the spindle 

[Fergus OMore]

I'm sorry but if this is based on the 1A machine, I simply believe that it will not have sufficient rigidity.
I had a converted  one years ago and it literally flopped about.  You may be luckier than me.


Meantime

My best wishes


Norman

[philf]

I'm sorry but if this is based on the 1A machine, I simply believe that it will not have sufficient rigidity.
I had a converted  one years ago and it literally flopped about.  You may be luckier than me.

Meantime

My best wishes

Norman

Hi Norman,

I think mine was a 2a which is not as rigid a setup as the 1a due to its swan neck support for the spindle to give clearance for big workpieces. (I have plans to bridge the swan neck to increase rigidity. This would be removable in case I need the space.)

So far, I've not seen any problems with rigidity but then I'm not using 25mm cutters with a huge depth of cut. With a 6mm cutter it works very well in mild steel giving a superb finish. It's a hobby machine after all.

It works far better than a Warco Major mill drill I have used

I do have a 1a as well - I would have used that but the table is much smaller (OK I could have swapped it with the 2a table) and the Z axis tightened as you lowered the table. I thought the dovetail had tapered with wear but it turned out that it was just a 'bruise' on the point of the dovetail which was easily filed and stoned off. By the time I discovered this I'd already finished the 2a conversion.

Cheers.

Phil.

[Fergus OMore]

Thanks, Phil. Glad you are aware of the situation.

I look forward to following your progress



Cheers


Norman

[PK]

Apart from being larger...and better thought out.... Your design is identical to mine.
See the bottom half of: http://www.caswa.com/cncathome/spindles.html

Tips based on my experiences (ie YMMV):
Oversize the pneumatic cylinder
Add some feature that stops the collet spinning in the spindle, unscrewing and destroying itself :-(

Re compressor starting and unwanted bowel movements; for a long time I had one of those compressors and a second one I built from an old fridge compressor connected tot he same air circuit. The little one was set to kick in 10-20PSI before the big one and was whisper quiet.  The idea is that, if you are using small amounts of air, only the little one cycles. If you start using a lot of air, the big one kicks in.

Hope you find this useful.

[philf]

Apart from being larger...and better thought out.... Your design is identical to mine.
See the bottom half of: http://www.caswa.com/cncathome/spindles.html

Tips based on my experiences (ie YMMV):
Oversize the pneumatic cylinder
Add some feature that stops the collet spinning in the spindle, unscrewing and destroying itself :-(

Re compressor starting and unwanted bowel movements; for a long time I had one of those compressors and a second one I built from an old fridge compressor connected tot he same air circuit. The little one was set to kick in 10-20PSI before the big one and was whisper quiet.  The idea is that, if you are using small amounts of air, only the little one cycles. If you start using a lot of air, the big one kicks in.

Hope you find this useful.

Hi Paul,

Thanks - your system looks very good and that's exactly what I hope to achieve.

I was already thinking that I'd turn the outside of the collet and the internal drawbar thread in the lathe and then finish the bore in the CNC mill. I was going to do this for the ER32 taper but it didn't need it. I just need to find a decent bit of material for the collet now.

By coincidence I was only looking at 'whisper quiet' compressors on eBay last night but my thought was to replace my noisy one. Your idea of using a smaller, quieter compressor to keep the tank topped up is appealing. My mate got a Bambi at his local council tip just as someone was about to throw it into the skip!

I've just ordered a plug in timer for the compressor. You press a button and it powers the socket for 30 minutes then turns off so there's no chance of the compressor kicking in in the middle of the night.

I do have a small diaphragm compressor which is quiet but it can only manage about 30 psi. I use this on the CNC for blowing chips out of the way and cooling the tool.

Cheers.

Phil.

[RobWilson]

Hi Phil

Just a thought if you go the air cylinder route and have space restrictions  you could go for the tandem cylinder approach ,compact and stackable  ,  4 tandem 100mm cylinders  = 1  single  200mm cylinder


  https://www.festo.com/cat/en-gb_gb/products_010600


Just a thought 



Rob