Harold Hall
Milling Cutter Chuck for Threaded Shank End Mills, Harold Hall
One of the earliest lessons a novice milling machine operator will likely learn is that the spiral action of an end mill will, when taking a cut, attempt to draw the cutter from the chuck holding it. The result is that it soon becomes apparent that the drill chuck usually supplied with the average mill drill is quite inadequate.
Having purchased my first mill drill for around £600, I soon learnt this was so but was then dismayed to learn that a suitable milling cutter chuck was going to set me back in excess of £100. This seemed quite disproportionate compared to the cost of the machine. Because of this, I was led to consider making one, which I did, with the resulting chuck, Photograph 1, being more than adequate for over 15 years of use.
I had though found that I would like to use endmills having a larger shank diameter than the 1/2 in./12mm that my chuck catered for, and I therefore made a larger version to suit 16mm diameter but following the same design as the first. This was to use endmills having threaded shanks which make it impossible for the cutter to be withdrawn in use. The larger chuck is seen in Photograph 2.
Briefly, the way that these chucks work is that the cutter is screwed into the collet until it contacts the base of the chuck’s body. If then, the cutter rotates a little in the collet due to the load on it, it will attempt to screw father into the collet but being in contact with the chuck's base it will draw the collet forward into its closing taper. This, as a result, increases the collets grip of the cutter. From this, it can be seen that it is impossible for the cutter to be withdrawn from the chuck as it will always remain in contact with the base of the chucks internal bore. Sk.1 should help to make the above understandable.
An alternative.
When I made my first chuck, I now know that the ER range of collets had been in being for around 10 years but was unaware of their existence. This because, at the time, it was obviously very much a collet for use in industry, and no doubt having an industrial price tag.
I now have though a set of these collets that I use exclusively for smaller diameter work, 20mm and below, on my lathe and find them very useful. For the viewer who is not conversant with these, each one covers a range of 1mm and are made in 1mm increments, typically, 7 to 6mm and 6 to 5mm. From this it can be seen that there are no gaps in the sizes that can be held and can therefore be used for metric and imperial diameters.
I do find though that they need a substantial tightening torque, especially if the collet has to be closed towards its smallest size, with the effect being particularly apparent with the smaller size collets. This is so much so that it borders on being impossible if closing to ones minimum diameter in this range. For this reason, I would not be happy using them for milling cutters, especially if say holding a 1/4” (6.35mm) cutter in a 7 to 6mm collet.
Whilst the ER system was first envisaged as just having metrically sized collets imperial sized collets are becoming available at the smaller sizes, obviously to overcome the situation. Even then, there is no absolute certainty that the cutter will not be withdrawn from its collet. For this reason I would recommend using a chuck designed for use with threaded shank cutters.
With either of my two cutter chucks, when closing, I frequently just hand tightened them without any major problem. However, I did find that if only taking a very light cut there was insufficient load on the cutter to cause it to rotate in the collet and tighten it fully. This resulted in the finish achieved suffering a little. I do now just very lightly tighten them using a C spanner and it works perfectly, not worrying that the cutter may be pulled from the collet or the need to use a substantial torque to fully secure the cutter.
Photograph 2 does show that I have made some solid non slotted adaptors for holding the smaller throw away cutters which do not have threaded shanks. However, as I have a few larger non threaded shank cutters I have made some larger adaptors also. These are also designed to eliminate the possibility of the cutter being withdrawn when in use. They work well as can be seen in Photograph 3 where a substantial cut is being taken without any problem.
Whilst economy milling cutter chucks can now be had, one that covers up to 5/8"/16mm and with collets for both metric and imperial sizes in the range 6mm to 16mm shanks would still be expensive compared to the cost of the milling machine. Making ones own would still be worth considering financially and with an interesting project as a bonus.
Details for making a set of these cutter chucks can be found in my book, “Lathework, a complete course” number 34 in the workshop practice series.
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All pictures can be clicked on to provide a larger view