Locking the spindle on a mini mill

[redvan22]

Hi,
I'm interested in trying some broaching on my mini mill after seeing a recent video. The operator stressed that locking the quill (on his Bridgeport) was critical to prevent rotation. Mini mills move the entire head assembly up/down. How can this be done on a mini mill that doesn't have a locking mechanism?

I'm thinking Ill need some sort of device that would mount to the head assembly. I figure it would need a sleeve, an arm or a bracket, that would clamp around the spindle that could be tightened. However, before I go down this path, figured I'd better reach out for advice. I don't think it should be a permanent mount to avoid possible accidental locking while the machine is running.

Thoughts?

Mike.

 

[WobblyHand]

Hi,
I'm interested in trying some broaching on my mini mill after seeing a recent video. The operator stressed that locking the quill (on his Bridgeport) was critical to prevent rotation. Mini mills move the entire head assembly up/down. How can this be done on a mini mill that doesn't have a locking mechanism?

I'm thinking Ill need some sort of device that would mount to the head assembly. I figure it would need a sleeve, an arm or a bracket, that would clamp around the spindle that could be tightened. However, before I go down this path, figured I'd better reach out for advice. I don't think it should be a permanent mount to avoid possible accidental locking while the machine is running.

Thoughts?

Mike.

Gears or pulleys in the mill head? I have pulleys in my PM25. For that, the best I can think of is some sort of belt clamp. I'd like something that works. I don't like the idea of indicating using the spindle and having the possibility of rotation. But like you, don't want something that might lock during operation.

For gears? A pin, more like a tooth, that engages in one of the gears?

 

[benmychree]

I owned a Induma 1-S vertical mill that someone attempted a broaching operation on and managed to strip the rack teeth off the quill, fortunately, a customer was doing a trip to Italy on business and picked up a new quill and I was able to repair the machine. I can imagine that these Asian machines are considerably more frail than the Induma, which was heavier and more robust than a Bridgeport mill.

 

[homebrewed]

Some mini mills have a spindle lock that is used to keep the spindle from turning when tightening/loosening the drawbar. Others (like mine) have a hole on the side of the spindle where you insert a tommy bar to do the same thing. Either way, you might be able to use the built-in spindle lock feature (or at least the hole in the spindle....).

If that doesn't work out, you may be looking at some sort of clamping arrangement around the spindle. Wood is easy to shape and wouldn't mar the surface of the spindle. The tricky part will be the portion that clamps onto the head casting, since there's stuff on all 3 available sides.

Another thought occurs to me. You could use the drawbar to install a clamp. The part that attaches to the drawbar could just be a length of steel with a hole in it for the drawbar. The other end would be held in place with a woodworking clamp, but your clamping arrangement can't apply up or down pressure to the drawbar because you will be applying additional load to the spindle bearings. A simple plate with a hole in it would clamp the spindle but it most certainly would load up the bearings in the process.

For the above, I'm thinking that a T-shaped piece might do the trick. The top of the T would drop over the top edge of the head casting and that would naturally prevent rotation. To prevent ANY motion, maybe a screw on one end to take out the play.

Using the drawbar in this way might require a slightly longer one, since you also will use it to install your broach.

Any of these schemes would quickly go sideways if the motor is turned on.

 

[WobblyHand]

Having manually broached on a lathe, it stinks. Have very little mechanical advantage. Can't imagine doing it on a mill for the same reasons. Even with a 3T arbor press, it can require a bit of muscle at times. The small mills are not designed to take that amount of force. Unless you are taking really tiny slices, this is not a great idea.

 

[redvan22]

A friend said the same thing as you WobblyHand- "not a good idea. If it was designed to do that there would be a design feature to allow it."

So, although I hate to be defeated, I have to agree that this sort of operation is not advisable on my mini mill.

Thank you everyone for your time today.

 

[redvan22]

Some mini mills have a spindle lock that is used to keep the spindle from turning when tightening/loosening the drawbar. Others (like mine) have a hole on the side of the spindle where you insert a tommy bar to do the same thing. Either way, you might be able to use the built-in spindle lock feature (or at least the hole in the spindle....).

If that doesn't work out, you may be looking at some sort of clamping arrangement around the spindle. Wood is easy to shape and wouldn't mar the surface of the spindle. The tricky part will be the portion that clamps onto the head casting, since there's stuff on all 3 available sides.

Another thought occurs to me. You could use the drawbar to install a clamp. The part that attaches to the drawbar could just be a length of steel with a hole in it for the drawbar. The other end would be held in place with a woodworking clamp, but your clamping arrangement can't apply up or down pressure to the drawbar because you will be applying additional load to the spindle bearings. A simple plate with a hole in it would clamp the spindle but it most certainly would load up the bearings in the process.

For the above, I'm thinking that a T-shaped piece might do the trick. The top of the T would drop over the top edge of the head casting and that would naturally prevent rotation. To prevent ANY motion, maybe a screw on one end to take out the play.

Using the drawbar in this way might require a slightly longer one, since you also will use it to install your broach.

Any of these schemes would quickly go sideways if the motor is turned on.

I have the mini mill that has the hole in the spindle with which I use a C-spanner hook wrench to hold the spindle while tightening the draw bar.

Additionally, the "go sideways" issue is my biggest concern even though I have a panic safety switch that needs to opened first and then the on button depressed. (Getting old, would be just like me to forget to remove the gizmo and try to start the machine. )

 

[mmcmdl]

I'm thinking Ill need some sort of device that would mount to the head assembly. I figure it would need a sleeve, an arm or a bracket, that would clamp around the spindle that could be tightened

I've made many fixtures as above for manual shaping , high speed air and electric grinders for my Bridgeports . I use the original BP shaper attachment cutters and it works OK . Nothing heavy duty , notching sharp corners out etc . Not sure a mini mill would be up to the task , but maybe worth a try , just make sure the cutters are ground correctly .

 

[mmcmdl]

Cutters must be ground concave and clearance on all sides ( well at least 3 ) . A ground flat locates the cutters edges . If I could find a shaper head for a BP at a somewhat reasonable price I'd probably buy it .

 

[Bi11Hudson]

Right up front, broaching from the mill is a bad idea. Not only for the pressure involved, but alignment is appoximate at best. Most any "mini-mill" should have some sort of locking device, for changing the collet or tool holder if nothing else. On mine, there is a hole in the spindle that recieves a pin. Some ~8mm, it serves no other purpose, just as a restraining pin. But is a "tool" none the less.

I use an arbor press (cheap, HF) to do broaching. I haven't broken a broach, YET, but that much pressure has to be applied even for soft materials such as brass or aluminium. Rotary broaching on a lathe must be done slowly as well, it is not a speedy process. I wouldn't subject my mini-mill to that kind of abuse. For abuse it would be. . .

.