Question: Mill Vertical Stop

[kquiggle]

My mill (LittleMachineShop Model 3900 HiTorque Mini Mill) has a stop on the vertical column (see picture below - it's the black plastic cylinder located in the middle of the picture, about 30% below the top). In the picture, the mill is raised almost as high as it will go with the stop in place.

This stop prevents the head from rising all the way to the top of the column, by a full three inches (no small amount on a mini mill). Out of curiosity, I removed the stop and raised the head (mill powered off, of course) - it looked to me like the rack and pinion were still fully engaged, and the gibs were fully on the dovetails.

I assume the stop is there as some sort of a safety measure, but why is it set so low? What would be the danger/harm of relocating the stop higher up? What am I missing here?

P.S. My mill is bolted to the work surface, so there is no risk of it tipping over. I point this out because I thought one reason for the stop may be to keep the center of gravity lower to reduce the risk of the mill tipping.

Also, one would not want to raise the head so high that the pinion went past the end of the rack, but so long as the top of the gibs do not go past the top of the dovetail, that does not seem to be an issue.

 

[Wizard69]

The first thing I thought was that the lathe operator making the stop was settings up for a run and you got the first part that came off the lathe that was too long. It would seem to me that the stop should hit the saddle. Right now it looks like it hits the housing well before the saddle but that could be an optical thing.

One quick way to determine what is going on would be to check the specs for your machine and see if you have proper maximum clearance. If you can't raise the Z to published specs then obviously the stop has issues. If you can get published clearances then something most have inspired them to put the stop there.

That inspiration might be the extra leverage exerted on the column and its bracket. Personally I'd go with the extra clearance and just make a point not to use excessive pressure.

My mill (LittleMachineShop Model 3900 HiTorque Mini Mill) has a stop on the vertical column (see picture below - it's the black plastic cylinder located in the middle of the picture, about 30% below the top). In the picture, the mill is raised almost as high as it will go with the stop in place. This stop prevents the head from rising all the way to the top of the column, by a full three inches (no small amount on a mini mill). Out of curiosity, I removed the stop and raised the head (mill powered off, of course) - it looked to me like the rack and pinion were still fully engaged, and the gibs were fully on the dovetails. I assume the stop is there as some sort of a safety measure, but why is it set so low? What would be the danger/harm of relocating the stop higher up? What am I missing here? P.S. My mill is bolted to the work surface, so there is no risk of it tipping over. I point this out because I thought one reason for the stop may be to keep the center of gravity lower to reduce the risk of the mill tipping. Also, one would not want to raise the head so high that the pinion went past the end of the rack, but so long as the top of the gibs do not go past the top of the dovetail, that does not seem to be an issue.

 

[kquiggle]

I bought the mill used and the stop was already in place. Looking at the exploded view in the manual for the mill, it appears to be a standard component. You are correct that the stop hits the housing well before the saddle, which I would also think was incorrect if this stop was not "factory original."

Checking the specs is a good idea: The specs say Z travel is 9.3" - I measure 8.5" from the top of the table to the bottom of the spindle (Is this the correct way to measure Z travel?). It looks like if the stop were adjusted to hit the top of the head instead of the upper housing, it might add another 1.75" of Z travel.

Anyone else out there with this model of mill who can comment?

 

[SilverSanJuan]

I have the same model. But, I've added the air-spring conversion kit. This kit removes that stop and adds one at the very top of the column. That gives you an extra few inches of travel as the stop just prevents the head from going above the dovetails on the column. It is a great conversion kit that is well worth the money spent. It supports the head much better than the metal coil spring that comes with the mill.

Todd

 

[velocette]

Hi
.The stop certainly does restrict the vertical travel. you can dispense with it and fit a new stop on the very top of the column a couple of holes drilled and tapped and a stop fabricated.
You can make the stop to allow the mill head to travel right to the top end of the rack and head to be a little past the end of the column.
An air spring conversion is a great improvement to this type of mill.
My own mill is fitted with an air spring with a yoke on the end to give a head travel of 285 mm
Took the liberty of attaching a couple of Photos with "Yoke in the top and bottom positions.

Eric

 

[chucketn]

I took the stop off all together when I put the air spring kit on my HF X2. Haven't missed it.

Chuck

 

[kquiggle]

Thanks to all - I think you have told me what I needed to know. I was concerned that there might be some sort of subtle safety issue I was missing, so I was reluctant to remove the stop.

Eric - I notice in your pictures that the rack on your mill extends all of the way to the top of the column, whereas on my mill it stops far short of that. Still, on my mill with the stop removed I can raise the head much further and still not over-run the rack.

With care (and a new stop) I think I can safely relocate the stopping point.

 

[chucketn]

On mine, I think the travel of the air shock prevents the head from going far enough to go off the rack. I often find myself raising the head untill it won't go any further.

Chuck