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Auto Tool Changer for an ER32 Mill Spindle

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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  :dremel:


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. :scratch:

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

 :beer:

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  :bugeye:

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