Lathe spindle MT3 drawbar torque question

[dml66]

This is the my first time using an MT3-tapered chuck in my mini-lathe, it's an ER20 collet chuck.

I have a 12mm x 120mm drawbar with a washer set in the lathe spindle through-hole in order to secure the ER collet chuck, what I'm not sure about is how much torque I should apply to the drawbar? Without a torque wrench, I found I could easily tighten it enough to where I have to rap the loosened drawbar pretty stoutly to get the chuck to release from the lathe spindle.

Thanks for the help!

 

[OTmachine]

I had a Steinel milling machine with MT3 taper that I would tighten the draw bar, which I believe was 12mm, to about 15 ft/lb torque for most cutters. If I would be using a cutter that was slapping the material harder, ( like a flycutter machining factory heat treat 4140), I would apply more torque to ensure that it did not come loose. Max torque for a grade 8.8 M12 bolt is about 50 ft/lb depending on conditions, but that would be extremely hard to remove if tightened to that amount. HTH, Keith

 

[Makintrax73]

I use a MT3 to ER32 adapter as a collet chuck. My experience is it takes very little torque and it isn't going anywhere. Couldn't tell you how much since I can't get a torque wrench on it. I made a custom draw bar with a removable handle....but I just snug it and then turn it somewhere around 1/8 turn after it's snug.

 

[dml66]

Thanks for the tips. I'll be taking very light cuts on brass, I think a hand-snug with a 10mm hex wrench should be fine. I don't have any hex sockets on-hand, I'll pick some up on my next wallet-lightener so I can quantify the torque .

 

[dml66]

Figured I'd share my final mini-lathe MT3 drawbar setup in case it helps another member in the future.

For the drawbar itself, I went with 130mm length rather than 120mm which is what LMS sells; I found 120mm doesn't quite get enough threads into the chuck IMO, I became concerned that if I ever had to really pound on it to release the chuck, the threads might fail. I turned a piece of scrap UHMW plastic rod so it fits close inside the spindle through-hole, this ensures the drawbar is parallel to the spindle axis, for whatever that is worth.



I used a 30mm hole saw to open up the change gear cover hole enough so I can install and remove the drawbar assembly without removing the cover itself.



Thanks again for the help regarding drawbar torque!

 

[VicHobbyGuy]

It looks like you are all set- nice work! The only reason for the drawbar is to keep the end mill in the collet from pulling the MT collet holder out of the spindle. Once the MT taper is seated, most 7x machines don't have enough torque to make it slip- and if it does, it's probably preventing some sort of crash. I agree with @Makintrax73 that fairly gentle tightening is all that's necessary. I'm assuming you are using the collet setup for an end mill , or short workpieces. I bought an ER32 MT3 holder for my 7x14 and quickly realized that a faceplate ER32 would be better for most of the stuff I was doing, since I could slide a longer workpiece into the spindle bore. I really like ER collets - I use them with MT2 holders in the tailstock, to hold the center drill, etc..

 

[dml66]

It looks like you are all set- nice work! The only reason for the drawbar is to keep the end mill in the collet from pulling the MT collet holder out of the spindle. Once the MT taper is seated, most 7x machines don't have enough torque to make it slip- and if it does, it's probably preventing some sort of crash. I agree with @Makintrax73 that fairly gentle tightening is all that's necessary. I'm assuming you are using the collet setup for an end mill , or short workpieces. I bought an ER32 MT3 holder for my 7x14 and quickly realized that a faceplate ER32 would be better for most of the stuff I was doing, since I could slide a longer workpiece into the spindle bore. I really like ER collets - I use them with MT2 holders in the tailstock, to hold the center drill, etc..

I agree with you regarding the faceplate ER32, it's what I use most of the time, that 5/8ths plus pass-through is nice to have .

I wanted to use the MT3 mount for both an ER20 collet chuck, and for an ER40 collet chuck and yes, for short parts.

ER20 collets are better suited for the slightly tapered (<0.2°), ≈1/2" O.D. parts I turn (the tapered section is inside the collet), I find the ER32 collets a bit on the stiff side in this case. What I particularly like about the chuck is, when used with a Techniks Precision ER20 collet, it holds a 1/2" diameter gage pin to <0.0004" TIR. I'm sure that's just dumb luck, but I'll take it!

I use the ER40 chuck only when I need to hold short parts that exceed the maximum clamping range of the ER32 faceplate chuck; it's faster for me than mounting a 3 or 4 jaw on the faceplate. Speaking of which, if someone invented a clever and convenient way to hold the faceplate nuts and get them started on the studs, I'd be eternally grateful, my fingers are just large enough to make it a challenge .