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John Kasunich
jmkasunich@fastmail.fm
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Mon, 08 Sep 2008

Spindle Project - Part 2 - Deciding on a Taper

The "Spindle Project" is my attempt to make something useful (and perhaps somewhat profitable) out of some surplus machine tool spindles that I bought in late 2007. This posting is about choosing a spindle taper and toolholder for the spindles. (See this posting for background on the spindles themselves.)

My existing milling machine is a Shoptask 3-in-1 that I have converted to CNC using EMC2 as the control software. It has a #3 Morse Taper on the mill spindle. I use " Tormach Tooling System" (aka TTS) toolholders on the machine, with a 3/4" MT3 collet in the spindle. The TTS system is a nice arrangement for small to medium mills - it is quite rigid for its size, and very repeatable in the Z direction. The toolholders have a flange which engages the spindle nose as the collet draws them in. Below is a drawing of a typical TTS holder, and a photo of my selection of TTS tooling. (Click drawings for a scalable PDF file, click photos for higher resolution.)

Since I already have some TTS tooling and plan to get more, it is naturally a leading candidate for these spindles.

Before I focus entirely on TTS tooling, I wanted to examine any other possiblities. The first thing to come to mind was of course R8. You can get R8 tooling anywhere, at very competitive prices. The main problem with R8 on these spindles is the length. As the drawing below shows, the back of the collet goes way up into the spindle.

The back of the collet would extend so far into the spindle that the step where the drawbar springs sit would be removed. That means I couldn't use the springs - I'd have to use a regular drawbar.

I would also have to figure out how to enlarge a 0.236" (6mm) hole that is over 2-1/2" inside the spindle, and make a precision 0.950" bore 4" into the spindle. The spindles are hardened (at least case-hardened), so the machining wouldn't be fun. I have a carbide boring bar with cermet inserts that can cut hardened steel, but it is far too large to get into the 6mm hole. I have no idea if an ordinary drill would touch it, and I don't want to think about what a carbide drill might cost.

And finally, the 1.25" diameter at the gage line leaves very little of the "step" on the front of the spindle. I would have to remove the step and machine the front of the spindle flat. That would move the taper another 0.160" deeper into the spindle, making all the other problems even worse. So, R8 is out.

NMTP-30 taper tooling is roughly the same diameter as R8, but considerably shorter. Tooling is less common and more expensive, but not outrageously so. So I dug out Machinery's Handbook and drew up a 30 taper toolholder. The result is below.

Although I would be able to use the drawbar springs, and wouldn't have to do a precision bore 4" into the spindle, the 30 taper has all of the other problems that make R8 impossible. And it costs more. So NMTB 30 is out.

The Tormach Tooling System toolholders only extend 1-3/8" into the spindle - that seems perfect since I really don't want to try machining deep inside the spindle. The standard way of holding TTS tooling is by either an R8 collet or a #3 Morse Taper collet. Tormach sells both collets and they aren't very expensive. I already know that R8 is too long, so I measured up the MT3 collet from my Shoptask, and added it to the drawing.

The MT3 is better than R8 in several ways. It is short enough that I could use the drawbar springs. It doesn't remove so much metal from the nose, so I can keep the 0.160 step on the front of the spindle. I would need to make a 0.160" or slightly thicker "washer" that would mount to the spindle nose, and be turned or ground square for the toolholders to seat against. That won't be too hard - there are already four M3 threaded holes in the spindle nose that would work perfectly. The washer would also cover up some other holes in the spindle face to make it a bit smoother and neater.

Unfortunately, the MT3 collet is still too long - I would have to clear out the 6mm area of the spindle. It wouldn't be a critical dimension, since the collet only bears against the taper for the outer 1-1/2" or so, but it would still be difficult to do.

Also, if you look closely at the drawing, you can see that the Morse Taper surface intersects the original taper near the end of the spindle. The existing taper is mirror smooth and very hard. If the spindle is case hardened, my new taper will transition from very hard to less hard material. That will make it hard to get a good fit and finish.

The conclusion is that I could probably use the stock MT3 collet, but it will require some very tricky machining.

Since the main problem with the MT3 collet is its length, I started considering a shorter collet. TTS shanks are short enough that it should be possible to remove 3/4" or so from the back of the collet and still have enough flex for it to close properly. However, simply cutting down the existing collet won't work - the threads don't go deep enough. I started desgning a collet that I could make from a hardened and ground MT3 to MT2 reducing sleeve.

However, tonight I realized that if I'm making a custom collet, I might as well go all the way. A Morse taper is about 0.6 inches per foot. The existing HSK taper in the front of the spindle is 1.2 inches per foot. If I make a collet to fit that taper, it will probably be easier to release from the spindle. In addition, I won't need to do as much critical machining. I still need a nicely fitted straight area in the back of the collet, but I won't need to deal with the taper, and I won't have to worry about the transition between hard case and softer core. The drawings below show the collet, and how it fits into the spindle.

I have a small electric oven that gets hot enough to harden drill rod, and is big enough to hold this collet. I'm leaning towards this approach, in part because making a precision collet would be an interesting challenge.

I haven't decided for sure whether to do a collet as shown above, a shorter MT3 collet, or the full length MT3 collet. I just ordered a MT3/MT2 reducing sleeve, as well as a chunk of 1" drill rod. We'll see what works out best.

I haven't just been drawing and thinking. Over the last weeks I've built some tooling needed to machine the spindles, and have done some experimental turning and boring. Those results will be the subject of my next posting.

(posted: 08 Sep 2008 01:09) (permalink)