Tool Holder Falls from the Spindle!

[AndrewZ]

More problems with an old Dyna Mechtronics EM-3116 mill. The mill can be described as a Bridgeport knee-mill clone with CNC including an air powered drawbar. Seemingly runs R8 collet and the drawbar has threads at the bottom.

The tool holder (or collet) falls from the spindle during operation. It does not fall all the way to the table or ground, but remains in the spindle loosely. What is the most likely mode of failure? Belleville washers, ball bearings or the drawbar?

As next step, the spindle should be removed to have a closer look. I have not done so before so it is not clear what kind of mechanism is in place (the documentation is useless). Is the spindle removed by moving it all the way up to the point of error and then hammering it down from above where the top of the draw bar is (with or without the drawbar in place?)?


Previously, the tool refused to tighten up properly, the drawbar was pulled out and oiled, and the air pressure cabling was changed which fixed the issue. The collet alignment screw was also changed.

 

[Mtndew]

Normally with air drawbars you need a longer drawbar than standard. Is yours a longer one?

 

[GENERALDISARRAY]

I am not a fan of power drawbars.
Dont take the quill out. There is nothing in there to see.

"Previously, the tool refused to tighten up properly, the drawbar was pulled out and oiled, and the air pressure cabling was changed which fixed the issue. The collet alignment screw was also changed."

Take the drawbar out and run it by hand into the threads of all you collets. Does it run free through all of them?
With the drawbar still out. Put all of your collets into the spindle. Do all of them go in all the way without binding?

 

[AndrewZ]

I am not a fan of power drawbars.
Dont take the quill out. There is nothing in there to see.

"Previously, the tool refused to tighten up properly, the drawbar was pulled out and oiled, and the air pressure cabling was changed which fixed the issue. The collet alignment screw was also changed."

Take the drawbar out and run it by hand into the threads of all you collets. Does it run free through all of them?
With the drawbar still out. Put all of your collets into the spindle. Do all of them go in all the way without binding?

The threads go through the specific collet that dropped out without obstacles. That said, there are still threads remaining on the draw bar after screwing the collet what seems to be all the way in, by hand. The drawbar is quite oily.

The collet also went all the way to the spindle. At first when lowered somewhat it binded enough to probably hold its weight, but after moving it in and out couple times any binding seems gone.

 

[AndrewZ]

Normally with air drawbars you need a longer drawbar than standard. Is yours a longer one?

The drawbar came with the machine and has worked before for years.

 

[cngbrick]

Can you post a picture of the drawbar? Some drawbars are in two pieces with a dowel pin holding the two together. It's possible the dowel pin sheared and all that is holding the pieces together is friction from the pressfit. Machining forces may slowly be pulling the drawbar apart when cutting material.

Edit: never bring a hammer anywhere close to a spindle!

 

[Booze Daily]

The drawbar might not be fully disengaged after tightening, so if you turn the spindle on before making a Z move it could be loosening the collet.

 

[AndrewZ]

Can you post a picture of the drawbar? Some drawbars are in two pieces with a dowel pin holding the two together. It's possible the dowel pin sheared and all that is holding the pieces together is friction from the pressfit. Machining forces may slowly be pulling the drawbar apart when cutting material.

Edit: never bring a hammer anywhere close to a spindle!

Yes. The circular object at the bottom moves downwards (on the picture) to the shaft, and is on the most upward position (on the picture) it at least easily moves to. The picture is upside down relative to how it is put together in the machine, with threads on top.

 

[AndrewZ]

The drawbar might not be fully disengaged after tightening, so if you turn the spindle on before making a Z move it could be loosening the collet.

The holder came out after Z adjustment downwards. Spindle was turned on only afterwards.

 

[cngbrick]

Yes. The circular object at the bottom moves downwards (on the picture) to the shaft, and is on the most upward position (on the picture) it at least easily moves to. The picture is upside down relative to how it is put together in the machine, with threads on top.

It's hard to tell from the picture if the shaft is pulling out but I do see either a roll pin or dowel pin there. Can you remove the pin? If the pin is sheared the shaft hole will be misaligned and you won't be able to tap it out.

Another thing that can happen with threaded draw bars is that the threads can elongate with time and effectively change the pitch to the point where it won't screw in all the way. Typically this happens with wrench tightenend drawbars and a heavy handed machinist. Can you run a nut the whole length of the threads?

 

[AndrewZ]

It's hard to tell from the picture if the shaft is pulling out but I do see either a roll pin or dowel pin there. Can you remove the pin? If the pin is sheared the shaft hole will be misaligned and you won't be able to tap it out.

Another thing that can happen with threaded draw bars is that the threads can elongate with time and effectively change the pitch to the point where it won't screw in all the way. Typically this happens with wrench tightenend drawbars and a heavy handed machinist. Can you run a nut the whole length of the threads?

Here is a better picture of the shaft. It does rotate a few degrees freely (including the very end). I will try to remove the shaft.

Unfortunately, there are no correct size nuts! The diameters is not in use here, I recall it did not fit any size exactly, but was quite close to 7/16" IIRC. Sign of elongation? I wonder where a new bar could be obtained from. But first verification the bar is at fault is needed.

 

[AndrewZ]

How is the pin supposed to be removed? Not coming apart easily...

 

[cngbrick]

View attachment 381053


Here is a better picture of the shaft. It does rotate a few degrees freely (including the very end). I will try to remove the shaft.

Unfortunately, there are no correct size nuts! The diameters is not in use here, I recall it did not fit any size exactly, but was quite close to 7/16" IIRC. Sign of elongation? I wonder where a new bar could be obtained from. But first verification the bar is at fault is needed.

That looks like a Kurt drawbar, I believe they can make one to fit if you give them the dimensions. For an R8 collet the thread will be 7/16-20.

Can you take a picture with the drawbar on a light background, such as a sheet of paper and without touching up the picture? Cutting out the background is preventing us from clearly seeing the condition of the drawbar. You'll also need to move the thrust bushing away from the head of the drawbar so we can see if there are any telltale signs of the shaft pulling out.

 

[cngbrick]

How is the pin supposed to be removed? Not coming apart easily...

If it's sheared it won't come out unless you pull out the shaft first. If it's good, just use a punch. You may have to try both sides if they used a taper pin.

 

[AndrewZ]

If it's sheared it won't come out unless you pull out the shaft first. If it's good, just use a punch. You may have to try both sides if they used a taper pin.

The pin is only on one side. Hammer is too much punch?

The shaft does not seem to want to come out easily either.

 

[cngbrick]

The pin is only on one side. Hammer is too much punch?

The shaft does not seem to want to come out easily either.

If the pin is only on one side then it has to be drilled to remove it. Don't do this unless you've determined it's actually bad.

First thing is to check for elongation:
You'll need to take some accurate measurements of your spindle to determine if the drawbar shaft is elongated or pulling out. Put a collet in the spindle and hold it in place so you can measure the distance between the top of the spindle to the top of the collet. Put back the thrust bushing on the drawbar and place a mark on the drawbar shaft corresponding to the measurement you took from the spindle (measure the location of the mark from the bottom of the thrust bushing). Now try screwing your collets on the drawbar, if any don't reach the mark then your drawbar is the issue.

To check for a sheared pin:
Find a heavy wall tube or pipe that will fit over the shaft but will butt up against the head and be just long enough for to be able to slip a nut on the end of the drawbar. Mark where the shaft enters the drawbar head. Put a 7/16-20 nut on the end and apply a moderate torque to the nut. Remove the nut and check if the shaft is pulling out of the head. If it moved then your pin is sheared. Don't apply so much torque that you actually shear the pin though.

 

[AndrewZ]

That looks like a Kurt drawbar, I believe they can make one to fit if you give them the dimensions. For an R8 collet the thread will be 7/16-20.

Can you take a picture with the drawbar on a light background, such as a sheet of paper and without touching up the picture? Cutting out the background is preventing us from clearly seeing the condition of the drawbar. You'll also need to move the thrust bushing away from the head of the drawbar so we can see if there are any telltale signs of the shaft pulling out.

Here are more pictures.

 

[cngbrick]

They threaded it into the head, shear pin should be ok as there is not much force on it. Measure for elongation and see if that is the issue.

Did you check to see that the collet alignment pin/screw is actually engaging properly in the spindle?

 

[AndrewZ]

If the pin is only on one side then it has to be drilled to remove it. Don't do this unless you've determined it's actually bad.

First thing is to check for elongation:
You'll need to take some accurate measurements of your spindle to determine if the drawbar shaft is elongated or pulling out. Put a collet in the spindle and hold it in place so you can measure the distance between the top of the spindle to the top of the collet. Put back the thrust bushing on the drawbar and place a mark on the drawbar shaft corresponding to the measurement you took from the spindle (measure the location of the mark from the bottom of the thrust bushing). Now try screwing your collets on the drawbar, if any don't reach the mark then your drawbar is the issue.

To check for a sheared pin:
Find a heavy wall tube or pipe that will fit over the shaft but will butt up against the head and be just long enough for to be able to slip a nut on the end of the drawbar. Mark where the shaft enters the drawbar head. Put a 7/16-20 nut on the end and apply a moderate torque to the nut. Remove the nut and check if the shaft is pulling out of the head. If it moved then your pin is sheared. Don't apply so much torque that you actually shear the pin though.

Thank you, I will complete the steps. Have to get a correct size nut first, however.

One question, what do you mean by thrust bushing?

 

[AndrewZ]

They threaded it into the head, shear pin should be ok as there is not much force on it. Measure for elongation and see if that is the issue.

Did you check to see that the collet alignment pin/screw is actually engaging properly in the spindle?

The collet alignment screw was changed. However, there is a other screw on top of it, and it was impossible to screw in properly as it wouldn't engage the thread. It's still on top of the alignment screw, but not screwed in.

The screw itself was changed according to the documentation. First screwed in so much so that the collet won't go in, then turned back 1/4 turn until the track of the collet pulls through. The screw holds the track on the collet in place, but no other checks have been made.