CNC mill conversion - part 23

Pneumatic drawbar

There are quite a few ways for a mill to hang on to a cutting tool. Let’s take a look at three of the more common with small machines.

Being that the mini-mill is an obvious budget machine, it is normally sold with a chuck pressed onto an arbor much the same way as a on a drill press. The arbor shape obviously must match the spindle, usually a modern R8 shape, or as in my case an older MT-3 style, both are equally functional.

Chuck pressed on an R8 arbor

Chuck pressed on an MT3 arbor

Mini-mill with chuck

This arrangement is good enough for drilling holes, but dangerous for milling, since the chuck is not designed to take any of the side loads it would encounter. This is usually where the first of many “necessary upgrades“ begins. 

Trying to replace a chuck system, one could go with in-spindle collets, or with ER collets. 

With the former, different size collets slide up into the spindle, and a drawbar (basically a large bolt) is inserted from the top, and screwed into the collet’s top end. Tightening the drawbar pulls the collet higher into the spindle and closes the collet around whatever tool one has put into the collet. When the tool needs changing, one has to loosen the drawbar, and tap on it with a mallet in order for the collet to release from the spindle enough to eject the tool. 

In-spindle R8 collets

After purchasing my mill I chose to go with ER collet. ER collets are easier to use since the collet holder is the only part permanently installed in the spindle, and the different collets are housed outside the spindle. This did however eat up some of my Z axis real estate versus the in-spindle collet, but that’s never been an issue.

R8 arbor with ER chuck and collets

ER collet system being used on my mill

Tool change is unproductive time, and this can last quite a while, depending on a number of factors, including the need to account for the distance from the part to the cutting end of the new tool. Damage to the part, tool, or even machine might ensue, if this is improperly done.

Modern CNC machines use tool holders that are dedicated to each individual tool. These are accurately measured ahead of time, accounted for in the controller software database, and can quickly be replaced in and out of the spindle, sometimes automatically by an Automatic Tool Changer. 

Automatic tool changer

A popular tooling solution for hobby mills is the Tormach TTS (Tormach Tooling System). The Tormach mill uses a 3/4” in-spindle collet  opened or closed by the action of a 3 stage pneumatic piston pressing down on the drawbar. A set of 6 Belleville washers returns tension to the collet when the piston pressure is released, and the tool holder is thus firmly gripped.

I decided to follow in the footsteps of others, and implement a popular upgrade design for the mini-mill (part of the CNC plans CD I bought online), with some of my own modifications. The upgrade consists in a pneumatic drawbar able capture and retain a TTS look-alike tool holder set. Everything is made from scratch of course.

Initially, I bought a MT-3 style ¾” collet, and 1 TTS tool holder from the Little Machine Shop. I wanted to make sure this idea was even possible with my older spindle type.

Drawbar, MT3 style collet, and tool holder.

Items mounted outside of the spindle

Collet (not visible) grasping the tool holder

Since that worked fine, I then tried to make a tool holder on the newly  computerized lathe, to prove I would be able to machine enough of them to accommodate the majority of my tools. These would have to be made out of steel, something the once manual lathe had not digested very well in the past.

Steel slug getting turned into a tool holder

First try at making a tool holder

Amazingly, it turned out that the CNC lathe does not mind being fed steel bars any more, as long as the cuts are not too deep, or the feed too fast.

With all the individual pieces of the puzzle coming together, all I needed was to design and build the darn thing, so here’s what I came up with, after modifying the drawings from the CD a bit.

Power drawbar design for my mill

Same thing viewed from the rear

Making the right side bracket

Right bracket completed

Left and right bracket

Top plate

Testing for initial fit

All linkages completed

The thickness of 6 Belleville washers forced me to have to increase the effective length of the drawbar. I did this on the lathe, by turning 1” of the bolt head into a ½" shaft.

Turning the drawbar bolt

Drawbar modified to hold 6 Bellville washers

Before and after

Drawbar and washers in place

After a lot of fiddling with it, it was finally done.

Finding a good place to drill the bolt holes

0.050" (1.3 mm) clearance from bolt head to steel cam

Rear view of the power drawbar assembly

Power drawbar in action