Torque needed for MT3 drawbar?

[zoltan]

Pretty simple question: how much torque is needed for the MT3 drawbar on my mini mill? I hate doing things by feel.

 

[RonGinger]

I am not sure about the MT3, but I was just today reading an article on the Tormach site about R8 drawbars and they suggest 30 ft-lbs. That would seem pretty close for an MT3.

MT tapers are locking tapers, so its going to take force to get it loose- some drawbars have a shoulder so you can apply force with the wrench, some just expect you to whack it with a hammer. It always bothered me to be whacking on my spindle, but its the only way. That does help you learn to not make it to tight.

 

[TorontoBuilder]

Pretty simple question: how much torque is needed for the MT3 drawbar on my mini mill? I hate doing things by feel.

wait, you mean I'm supposed to be using a torque wrench on my drawbars? Oh dear. Well good thing I haven't used my mill much yet

 

[Swifty]

You got me wondering, so I tightened my R8 collet on a cutter, used the ring spanner and tightened it up like I always do. I then checked it with my torque wrench, it was 30 ft/lbs.

Paul.

 

[Blogwitch]

You should find that most general purpose spanners come in standard lengths.

There is a reason for this.

When an 'average' person tightens up a nut or bolt with an 'average' sort of force, you should be within the torque range for that said nut or bolt. All this was worked out many years before torque wrenches were used, and as mentioned above, is why many spanners come in standard lengths.

Or just measure up the length of the spanner and use a spring balance to work out how much force should be used.

If you are anything like myself, white knuckles is the usual setting.

John

 

[hanermo3]

The A is very little.
R8 and MT3 are completely different.

MT tapers need a lot less force, as they are locking tapers.
1-2 Nm on MT2 on a bridgeport is enough.

1-2 Nm (just push/slap it in) on an MT3 on my lathe (350 kg) is enough.

 

[Swifty]

Hanermo3, I understand what you are saying, but there are MT collets available that need to be tight enough to hold the cutters. If it's just a MT shank on the end of a collet holder, that's a different situation.

Paul.

 

[Forestgnome]

Hanermo3, I understand what you are saying, but there are MT collets available that need to be tight enough to hold the cutters. If it's just a MT shank on the end of a collet holder, that's a different situation.

Paul.

No, it's pretty much the same. It takes very little tension on the drawbar to hold a cutter in a Morse or B&S collet, unless you just like beating on the end of the drawbar.

 

[velocette]

Pretty simple question: how much torque is needed for the MT3 drawbar on my mini mill? I hate doing things by feel.

Hi zoltan
Engineering is all about hands on and using the hands to feel.
Firm but gentle hands will help get things done.

Eric

 

[zoltan]

I think the issue I'm having isn't the drawbar being too loose, but that the drawbar keeps loosening up on me while being used. I think I'll try Loctite on the drawbar to keep it in place.

 

[rodw]

I think the issue I'm having isn't the drawbar being too loose, but that the drawbar keeps loosening up on me while being used. I think I'll try Loctite on the drawbar to keep it in place.

If you do, get the one that is designed for adjustment screws that wicks in. 290 I think.

 

[digiex-chris]

if it's an endmill holder, just enough to keep it from coming loose. If it's a collet, just enough to hang on to the part or endmill. I find that a morse taper will stay put properly all by itself if you slap it in there, the draw bar is just to prevent it from loosening up from side forces. Definitely less for a morse taper than an R8 or 30 taper, way different angles. I would imagine loctite on a drawbar would be annoying when trying to remove a flush collet.

I often slap the endmill holder in, then tighten the draw bar by hand till it stops, then tighten it 1/4 or 1/3 what I'd feel on a similarly sized fastener. Collets are a bit of an experience thing. Just enough that it'll hang on to whatever you need to hang on to, and just loose enough that you can still get it out with a reasonable tap on the draw bar. If you have to really slug it hard to get it out, try a bit looser next time.

It's tough to find drawbar torque numbers published for morse tapers. Like many things that were spec'ed out when Morse tapers were new, it's a feel thing.

 

[Charles Lamont]

I think the issue I'm having isn't the drawbar being too loose, but that the drawbar keeps loosening up on me while being used. I think I'll try Loctite on the drawbar to keep it in place.

Loctite what exactly? This should not be necessary. If the drawbar is coming loose there is something wrong.

 

[zoltan]

Loctite what exactly? This should not be necessary. If the drawbar is coming loose there is something wrong.

Loctite on the drawbar threads. It doesn't surprise me that it's loosening considering it's not very tight, is spinning at 1,000RPM, and the head can vibrate.

 

[BaronJ]

Loctite on the drawbar threads. It doesn't surprise me that it's loosening considering it's not very tight, is spinning at 1,000RPM, and the head can vibrate.

Hi,
If its any help I have a "T" key that has a four inch tommy bar, two inches on either side for tightening the drawbar on my mill, which is MT3.

 

[Charles Lamont]

Loctite on the drawbar threads. It doesn't surprise me that it's loosening considering it's not very tight, is spinning at 1,000RPM, and the head can vibrate.

As I don't know your level of knowledge I am asking probably dumb questions to try to make sure we are all singing from the same sheet to get to the bottom of this for you.

Which threads? The drawbar should have threads at both ends, at the bottom to screw into the back of the taper you want to hold, and at the top to allow the retaining nut to be screwed down onto the top of the spindle to tighten the assembly. Is your configuration in some way different?

Is your technique right? I was taught to screw the nut well up the threads so you can screw the bar down until it bottoms in the taper, back off two turns, then screw the nut down and tighten it moderately.

The threads at both ends need to be used every time you change tooling, so I don't understand why you would want to use Loctite. The drawbar should be tight, but not very tight, and not come loose. The length of the drawbar should provide enough elasticity that vibration is not a problem. Do your tapers fit properly?

 

[velocette]

Hi Zoltan
What is the condition of the Morse tapers both in the spindle and the surface of the external taper as any burrs or swarf will not allow them to seat properly and may be the cause of the vibrations.
Remember thumb on the wrench head that is on the lock bolt and use small digits to nip up the bolt is normally enough to secure it.
Is the collar that is in the top of the spindle in place and lightly oiled.
Check your spindle speed and feed rate for the size of the cutter and the hardness of the metal that is being milled as this is often a cause of vibration.

Eric

 

[zoltan]

As I don't know your level of knowledge I am asking probably dumb questions to try to make sure we are all singing from the same sheet to get to the bottom of this for you.

Which threads? The drawbar should have threads at both ends, at the bottom to screw into the back of the taper you want to hold, and at the top to allow the retaining nut to be screwed down onto the top of the spindle to tighten the assembly. Is your configuration in some way different?

I just have a X2 mini mill and its drawbar is only threaded on the end which engages the tool, like this:

I'm going to put some blue Loctite on those thread to keep it from loosening from the tool. I could tighten the drawbar more to keep it from loosening, but at that point it becomes very difficult to release the tool from the taper.

 

[zoltan]

Hi Zoltan
What is the condition of the Morse tapers both in the spindle and the surface of the external taper as any burrs or swarf will not allow them to seat properly and may be the cause of the vibrations.
Remember thumb on the wrench head that is on the lock bolt and use small digits to nip up the bolt is normally enough to secure it.
Is the collar that is in the top of the spindle in place and lightly oiled.
Check your spindle speed and feed rate for the size of the cutter and the hardness of the metal that is being milled as this is often a cause of vibration.

Eric

Tapers are in good shape. In fact, I just replaced the spindle.

 

[digiex-chris]

the drawbar isn't bent or anything? the flat at the top that bears against the top of the spindle is square to the drawbar shaft? The top of the spindle is smooth and flat? The drawbar threads are smooth and in good shape? The collet or endmill holder or whatever is in the taper has threads that are smooth and in good shape?

I'm curious if a split style lock washer under the top of the drawbar would be a good idea or not. Would at least make it less trouble than untreading a loctite'd thread I think.