Air micro die grinder review

Page 2

How they operate

The die grinder is basically a small single bladed turbine mounted on a shaft with ball races, driving a collet chuck, and housed in an alloy body slightly larger than a standard marker pen.

Two small wrenches are used to tighten the collet chuck - see below.

At the rear of the unit an air supply line comes up though the centre of another larger hose, which is the exhaust outlet.

The small hose feeds air into the body through a rear barrel type speed controller and drives the turbine.  It then vents out of the rear of the unit through the bigger diameter hose, away from the user.

So any oil in the air supply (lubricant) is vented well away from the job/work area.

Using the die grinder

These do everything that a regular Dremel can do - cut, grind, engrave, polish etc.

With the air supply set at 90 psi and a tool fitting in the collet, connect the air, and then rotate the aluminum collar at the rear of the unit.

This is a firm and very precise controller ( no slop or air leaks) and the unit spins up to speed the further you rotate it.  It has an off position.

holding a die grinder

Vibration is dependent upon the balance of the tool in the collet, and any runout in the collet chuck.  The larger the diameter and weight of the fitting, the more likely vibration will occur if everything is not symetrical. 

In fact, you can actually see the metal shank of a grindstone start to flex/whip at seriously high speeds if imbalance is present.   So be careful - these grinders can rev hard.

Air consumption is quoted as 2 CFM per minute at maximum speed, however as you don't have to spin it that fast for many applications, air consumption is fairly modest.

An average 7 CFM air compressor would hardly notice it was attached.

You simply dial in the speed you want, and then lightly touch the tool bit against the item you wish to machine.   The higher the speed, the smoother the cutting action.


The unit feels solid and looks to be well made.

There is no apparent looseness in the ball races and minimal run out in the collet chuck.

However, the standard 3mm collet has a degree of axial runout.

This is caused by the the basic design, and loose tolerances required to prevent it from jamming in the mount.

It has to be made this way given the design of the grinder.  If a collet was to jam in the drive tube, there is no easy way to remove it without possible damage.

The degree of runout varies depending upon how well the collet is centred in the chuck nose cone when tightened up.

Runout is generally minor and has little effect on hand held grinding, but it can be an issue if used in a fixed position, such as mounted on a lathe toolpost.  It will cause vibration and tool whipping at high RPM.

Axial runout becomes much more noticable as the unsupported length of the tool bit shank increases, so the tool bit unsupported length should be kept as short as possible.

Another factor is the diameter of the shank fitted.  There seem to be two sizes sold as suitable for 3mm collets.  These measure up at 2.97mm and 3.12mm respectively. 

The 2.97mm will pass completely through the length of the collet and have full support with little run out.

The 3.12mm will only go in about half the length of the collet.  The long shaft overhang causes considerable flex at normal and higher operating speeds.

A simple modification

I largely overcame this issue by drilling out the additional and un-used 3/32" collet to 1/8" for use with the larger diameter burrs.  

This allows the 3.12 mm shank to pass completely through the collet, as seen below.

collet modification

This gives much greater accuracy and control, and whipping is reduced.   You can see how much closer the burr is to the grinder body, compared to a picture on the previous page.

Done correctly, it will also grip the smaller diameter shanks.  I use this collet for everything.

My collets are made of brass and easily modified with a small lathe.

Drill out the collet in the non-compressed (open) position by gripping the non slotted section of the collet in the chuck.

The three collet jaws will spread when drilled, and need to be re-compressed to fit in the threaded end cap.

You can use a three jaw lathe or drill chuck to do this.  Clean off any external burrs with a file before use.

Other modifications

A small Allen grub screw near the barrel speed controller locks the assembly together. This can vibrate loose when used in a rigid mount - put some Loctite on it.

The rubber exhaust hose can slip off of the die grinder body - put a ring type circlip around it to engage with the groove in the alloy die grinder body.

Collar thread

The collet collar thread of the air grinder is 32 TPI.  This is not the same as a Dremel 40 TPI thread, so any screw type chuck or accessory intended for a Dremel will not fit this unit.

Replacement collets

Dremel collets can be used as a replacement, but the profile and angles are quite different, and they need reworking to fit reasonably well.

They also have a smaller diameter shank where they fit in the drive tube.

So they are far from ideal.

See below, pencil grinder collet on left and Dremel on the right.

dremel collet

Original Dremel collets are fairly light weight and made from aluminium.

You can buy brass Dremel style collets off Ebay very cheaply - less than 50 cents each.  I bought a few and modified them.

The end result is still not perfect, but useable if you centre them in the chuck correctly before tightening it.

Use a friction disc in the air grinder to re-shape them.

reshaping a collet

The original pencil air grinder collets supplied will be either brass or hardened steel.


Awesome - for it's size it has plenty of power.

Even if you do stall it, you can't damage the motor. 

I've used it for many applications including:

- as an engraver,

-  with a round emery tube to clean up centre bored holes,

- with carbide burrs to shape metal and remove weld,

- with a diamond burr to open out undersize spring washers,

- with a friction disc to cut through a hardened steel shaft/ key.

In each case the little unit did a great job, quickly and easily.  

I thought the small cutting disc would be useless against such hard metal, but at full speed it sliced through the 6 mm square key without effort.  The cutting disc was barely worn.

Here's a shot of it with a 1.5" (30 mm) friction disc cutting through a steel shaft.

With a decent set of burrs it will munch through the hardest of steel with ease.  

The bigger rotary burrs can quickly and easily slot or open up drill holes which would otherwise have to be laboriously filed by hand. 

Seen below engraving mild steel with a small round headed burr.


It can even take on HSS (high speed steel) and win, as demonstrated on the next page.  

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