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How does the back gear on a mill work?

Susquatch

Susquatch

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I've been noodling this for a while now. I figure it's time to swallow my pride and just ask the members.

Both my mills have a high and low speed range that is accomplished through a back gear. It is easy to see how the back gear is engaged through a fork on the small gear that connects the back gear belt pulley to the bull gear assembly.

But what disconnects/connects the other half of the system? Is there an over-running clutch of some kind between the top and bottom half of the spindle. All I see is a set of teeth between the top and bottom, but no way to connect or disconnect the teeth. Does turning the top of the spindle in reverse disconnect the top and bottom somehow?
 
When you move the speed change lever from high to low, instead of driving direct from the belt, you lower a large drive gear to mesh with a smaller “back gear” and your mill will drive in reverse unless you change the direction of the motor, hence the motor switch says “High and Low” instead of forward and reverse.

The lever stop you fixed raises and lowers the large drive gear to mesh with the back gear.
 
Here is a page from the Bridgeport manual from MIT. It kind of relates to your question.

B4DD5E11-A474-4B8A-AFBE-D72FB98E4FAB.jpeg


And here is the link to the full manual.

https://fab.cba.mit.edu/content/tools/bridgeport_mill/Bridgeport.pdf
 
Thanks for trying to help @RobinHood & @Brent H. Both of these answers relate to my question but don't address the core issue that I am having. I do understand how the back gear itself works and how it is engaged or disengaged.

What I am having difficulty with is how the top and bottom of the spindle disconnect in concert with the back gear. In my mind, you can not have two parallel power paths engaged at the same time. They can turn at the same time, but they cannot be connected at the same time. Otherwise, something will break.

My guts tell me that this disconnect happens at the horizontal set of teeth between top and bottom halves of the spindle. But there are no levers that act on that section of the power path. I have not disassembled that part of the mill and I have no experience to teach me. So I can't figure out how it works.

I suspect that there has to be an over running clutch or something similar in there. Since the motor and the top of the spindle are rotating in reverse and the bull gear is driving the bottom of the spindle in the forward direction at some lower speed, it ought to be easy to do. But when I look at the parts breakdown, I don't see it. Again, something in there probably lifts the top section teeth away from the bottom section. But how?
 
I think this picture from Precision Matthews does a good job of showing the difference in the path of power from the motor to the spindle between high and low range on a Bridgeport variable speed type head. There is also a neutral position possible when the dog teeth of #37 and 39 are not engaged with each other and also gears #83 and 40 are not in mesh.

TV head high-low range.webp
 
Ahhhhhhh, I see! That answers my question @John Conroy ! It also explains why @RobinHood & @Brent H thought they answered it.

All I can say is that my blindness was caused by owning two mills. A step pulley mill and a variable speed.

The step pulley engages the dog teeth with a lever on top of the mill, and moves the back gear with a fork on another lever.

I made the mistake of assuming that the back gear lever on the right side of the mills did the same thing on both mills - lifts a fork to move the back gear. In my own defense, it was an easy mistake to make since they are both in the same place. But in reality, they work differently.

On the VariSpeed, the lever does two things at once - it engages the gear and the dog teeth simultaneously by moving gear 40 and the dog teeth together.

On the step pulley, the lever doesn't move gear 40 or the dog teeth. It moves gear 83 and the dog teeth are moved by the top lever at the Drawbar.

Paradoxically, this also explains what I thought was a very poor parts diagram for the VariSpeed. In reality, I was only blinded by the fact that I thought the back gear engagement on both mills worked the same way. They clearly don't.

Thank you!
 
@John Conroy - I see you posted the step pulley mechanism while I was writing my response above. But they are both appropriate knowing that they were drafted and posted at roughly the same time.
 
@John Conroy - For what it's worth, your drawing also explains why the step pulley shift fork lever has no neutral position and the VariSpeed does. This is a question I asked quite a while ago but it never got answered till now.

Again, thank you.

And my apologies to @RobinHood and @Brent H for asking the question in such an ambiguous way. It's funny how the known prejudiced my perception of the unknown. If only they put the lever in a different place in the two mills! (at least that's my excuse).

Sure glad I never drilled a hole for the neutral position on my step pulley mill!!! Ya, go ahead and laugh! I was seriously thinking about it!
 
RobinHood said:
Thanks @John Conroy for the manual. That sure makes easy work of explaining how exactly the drive systems work.

No worries @Susquatch, I certainly learned something from your question.
Click to expand...

They are GREAT drawings. For the Hartford, it's exactly what I saw in my minds eye. For the Bridgeport, it's a cerebral transformation! My head is happy now!
 
John Conroy said:
No problem, both drawings came from the owners manual for the PM1054 mill I am getting from Precision Matthews. Here is a link to the manual on their website.

https://www.precisionmatthews.com/wp-content/uploads/2017/01/PM-935T-949T-1054T-v3-2021-2-1.pdf
Click to expand...

This is perfect. I didn't even know these mills existed! I can see why you ordered one. Which model are you getting? VariSpeed or StepPulley?

I wonder how many of the step pulley model parts are interchangeable with my Hartford?

Especially the timing belt and pulley.......
 
I ordered the PM1054TV with a 3 phase motor.

I'm pretty sure you can achieve a neutral on the step pully head. The Hi-Lo lever shows a neutral position. If the dog gears are not engaged and the countershaft gear is lowered to clear the bull gear you would have neutral.

TS model spindle neutral.webp
 
One of the things that attracted me to PM is the extra effort they take to provide usable owners manuals. The manual I got from Modern Tool for my lathe is just about useless but luckily the PM1440HD is pretty much the same machine and I was able to download the PM manual and use it for my Modern GH1440W lathe.
 
Yes, the manuals I downloaded are simply awesome!

I'm also happy to see that you ordered the step pulley model. Maybe after it comes, I could get you to measure the width and pitch of the timing belt. That can be done fairly easily without taking it apart. My feelings about my Hartford being an orphan would forever change if I can get those parts from PM.

What HP is it?

Is the motor VFD rated?

John Conroy said:
The Hi-Lo lever shows a neutral position.
Click to expand...

Yes, but only on the label. I don't see an actual detent for the actuator. My Hartford has no label at all nor a detent. But having said all that, if the PM works the way their drawing says it does, there really is no true neutral. The gear is either engaged or it isn't. There is no such thing as an in-between for engaged and disengaged. It's sort of like leaving a light switch half way between on and off. You can do it, but why would you ever want to?

Btw, I cruised their website a bit. They have a lot of stuff there and their prices for other things are VERY reasonable!
 
I actually ordered the TV model which stands for Taiwan made, variable speed vs the TS which is a step pulley model.
 
Yeah my Ferro mill (variable speed head) had no detent when the lever way in the neutral position either but the lever would just fall into the neutral position unless it was very purposefully put into 1 of the 2 detents. With the step pulley head the lower lever only moves the countershaft gear so if that gear is not in mesh with the bull gear you will have neutral when the upper lever disconnects the dog gears.

I thought you had solved your belt problem with your call to MSC. Was Bando not able to supply a new belt after all?

It is 3HP and the motor is VFD rated. I would have probably ordered a step pulley model in 3 phase for use with a VFD but they are not available for months so I went with the TV model. It's still taking months-LoL
 
John Conroy said:
Yeah my Ferro mill (variable speed head) had no detent when the lever way in the neutral position either but the lever would just fall into the neutral position unless it was very purposefully put into 1 of the 2 detents. With the step pulley head the lower lever only moves the countershaft gear so if that gear is not in mesh with the bull gear you will have neutral when the upper lever disconnects the dog gears.

I thought you had solved your belt problem with your call to MSC. Was Bando not able to supply a new belt after all?

It is 3HP and the motor is VFD rated. I would have probably ordered a step pulley model in 3 phase for use with a VFD but they are not available for months so I went with the TV model. It's still taking months-LoL
Click to expand...

My Bridgeport VariSpeed did the same thing until it didn't. Sometimes it just slipped into HI speed. Things break when they do that under load. So that's why I made the neutral block for it and adjusted it per the manual.

Yes, that's what I meant by what I said earlier about neutral on the step pulley mill. There is really only engaged and disengaged. Disengaged could just as easily be called neutral. A rose by any other name......

The bando belt has not arrived yet. I am hoping it will be correct, but I am not confident. Regardless, someday I will need another replacement and they may not be making custom belts when that day comes. That's what I meant by orphan equipment earlier. The other problem is the timing belt pulley. I couldn't find that anywhere so I repaired the one I have. My fingers are crossed on that one! The pulley is sintered metal and I repaired it with devcon plastic steel.......

All the above said, @Dabbler made a GREAT suggestion. Since I have a VFD and the motor I got for it is VFD rated, he suggested just removing the back gear! The VFD/Motor will easily match anything a back gear could do! Need 20rpm for some reason? Dial it in and go!

3HP eh! Wow! Should be a great mill when it arrives!

I may give them a call and see if I can get them to give me specs on the appropriate parts. Surely they will be more helpful than H&W was.
 
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