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dvk
07-30-2004, 07:44 PM
I'm making a morse taper spindle with the end threaded to screw on adaptors to hold end mills, I originally thought about going 3/8 16, but now thinking about using the largest tap I have to make the adaptors which would be 1/2 13 or 1/2 20 I'm using 3/4" leaded steel for the taper spindle. What would give the best holding power course threads 13 per in. or fine threrads 20 per 1".
Thankyou in advance. I'm going to start machining this spindle to 1/2" diameter tonight so If I could have some help on this question soon would be appreciated.
Thankyou.

coles-webb
07-30-2004, 08:29 PM
My mill-drill has a MT3 and the threads for the drawbar are 3/8-16. Going to 1/2 inch will give you more than enough strength and less chance of bending the drawbar if the MT3 gets stuck as they sometimes will. I bent a few of them, now I have 2 spares and just cut off the bent part and weld on a bolt. As for Fine or Coarse threads it is my understanding that fine threads are stronger in the fact the minor diameter is slightly larger making it a larger diameter so therefore stronger. I would go with coarse myself as it will unscrew faster and there is more meat in the threads if you have to pound out the MT3. As mentioned earlier, my mill-drill collet system uses a 3/8-16 drawbar so going to 1/2 inch in anything is more than strong enough.

Mike

Forrest Addy
07-30-2004, 08:45 PM
There's a standard for Morse taper shanks retained with a threaded draw bar and standards should be used wherever possible for interchangeability with applicable off-the-shelf tooling.

My Enco catalog page 302 shows the following for Morse taper endmill adaptors draw bolt threads (and collets):

#2 = 3/6-16
#3 = 3/8-16
#4 = 1/2-13
#5 = 5/8-11

About the largest taper you can safely seat in a 3/4" dia spindle is #1 Morse with a 0.474 big end. #2 has a .700 dia big end leaving but a .050" wall. Unless you move up a stock size or two you'll be restricted in the size of tooling and cutters you can use.

As for discusssion of the applications of coarse Vs fine thread series, here's a link:

http://www.accuratescrew.com/TechTips/?cb=1

I have some reservations about the use of leaded steel for a spindle. This stuff is very machinable but in low carbon versions they are not hardened nor are they hardenable except by case hardening quenching and tempering with its inevitable distorion.

The taper will receive considerable wear for frequent tool changing. Usually spindles in this application are hardened to Rc 50-55.

I suggest you find a piece of pre-heattreated steel like 4130 or 4140 Ht to Rc 30-35. This stuff is available off the shelf. While not as hard as a factory made spindle this it will have much superior wear propertiese compared to low carbon steel. This material is still very machineable with ordinary tooling but it is hard and you have to slow down a bit to preserve the tool life.

I suggest you use a heat treated alloy steel drawbolt as well equivalent to Grade 8. sometimes you have to sock them pretty hard to unseat the taper.

[This message has been edited by Forrest Addy (edited 07-30-2004).]