# Newbie, back gear questions



## Theron751 (Sep 19, 2021)

Hello all,
I purchased an atlas mill model mf at an auction about a month ago. Serial 004048.
After 3 weeks of reading this forum and downloading specs for a drawbar, back gear wrench and manual I lubed it according to the chart and turned it on. Thankfully it just hums, can barely hear it running. Of course after 34 years of construction I say the same about jet engines!
Anyway I am happy with it but have a couple questions.

The back gear pin on my machine has a set screw holding it in. After making a wrench and fiddling with it for hours I figured that out, loosened the set screw and the pin popped right out.
So questions are1) is set screw normal or PO modification?
2) Is the purpose of back gear to increase spindle speed?
3) is there any way to reverse spindle direction or is that even usefull?

If these have been asked and answered maybe you could point me in the right direction?
Thank you.


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## wa5cab (Sep 19, 2021)

The set screw on the direct drive pin (back gear pin) is not factory.  My guess would be that a PO lost the detent pin and or spring.  Removing the direct drive pin without a rag over the hole for the detent and spring to capture the detent pin when you remove the direct drive pin has a tendency for the detent pin to try for LEO,

When engaged, the back gears decrease the spindle RPM by about 6:1.

The only way to reverse the spindle would be to reverse the motor.  You would have to work at it to dream up a reason for doing that.


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## Theron751 (Sep 19, 2021)

Thank you for that.


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## markba633csi (Sep 19, 2021)

Back gears on a machine tool do two things simultaneously: decrease speed and multiply torque
The transmission in your vehicle does the same.
Like a mechanical transformer, you gain in one parameter but lose in the other, by the same ratio
-Mark


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## wa5cab (Sep 20, 2021)

Note the correction in RED to my previous post on this subject.


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## WobblyHand (Sep 20, 2021)

wa5cab said:


> Note the correction in RED to my previous post on this subject.


fyi post #2 has no RED right now.


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## wa5cab (Sep 20, 2021)

I don't know how that happened but try it now.


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## JPMacG (Sep 20, 2021)

Just for clarification, LEO = low earth orbit.   LEOs decay over time (due to not being completely out of atmospheric drag), so in a few years your spring will likely return to earth.


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## Grinderman (Sep 20, 2021)

no message


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## WobblyHand (Sep 20, 2021)

wa5cab said:


> I don't know how that happened but try it now.


It's fine now.


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## Theron751 (Sep 20, 2021)

If it retuns from leo it will undoubtedly fall in the same place as various gun springs, knife springs and an infinite number of motor parts.
Large reward for the finder of that location!


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## Lastwagen (Sep 20, 2021)

For what it’s worth, I recently re-wired the SB lathe motor so I could run it in forward or reverse. I was cutting metric threads and used reverse to power return the apron after each pass. That’s how I kept the half nut engaged during the whole threading operation. The reverse capability was handy for that, but that’s about it. 


Sent from my iPhone using Tapatalk


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## Bi11Hudson (Sep 20, 2021)

Theron751 said:


> If it retuns from leo it will undoubtedly fall in the same place as various gun springs, knife springs and an infinite number of motor parts.
> Large reward for the finder of that location!


Bottom of the Marianas Trench. We know where but haven't the wherewithal to get there. A trolling magnet keeps getting lost on the long leader.


Lastwagen said:


> For what it’s worth, I recently re-wired the SB lathe motor so I could run it in forward or reverse. I was cutting metric threads and used reverse to power return the apron after each pass. That’s how I kept the half nut engaged during the whole threading operation. The reverse capability was handy for that, but that’s about it.


I have an MFC with the motor reversable. There are times work is mounted on an angle plate and milling cutters attacked through the MT2 spindle. As it works out, mandrel cutters in automatic feed run better one way and milling cutters run the other. I consider a reversible motor to be an absolute necessity.

My Atlas/Craftsman 12X36 is reversible, like the above. The biggest reason is threading, but there are other uses as well.

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## Theron751 (Sep 20, 2021)

Are there any other issues with the gears or any internals using a reversable motor?  I am just starting out and probably won't need that for awhile but can be collecting parts in the meantime.


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## Bi11Hudson (Sep 20, 2021)

The most obvious is the drawbar running backwards(loosening) Any other is unknown from my perspective. I haven't encountered any problem yet, but my machine time is fairly low. I will say, as an aside, that most of my run time is in back gear. Mostly because I don't know what I'm doing.

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## wa5cab (Sep 21, 2021)

The danger of running the spindle in reverse is that all of the Atlas machines have a threaded spindle.  So if the torque required to turn the spindle is high enough, the spindle chuck will unscrew itself and fall onto the bed.  So with a threaded spindle, you never perform any cutting operation in reverse.  However, if you properly seated the chuck against the register or shoulder at the back end of the spindle threads, it will take significant torque to break the threads loose in order to remove it.  So after 40 years of routinely engaging reverse whenever I needed to, I have never had a chuck start to un-screw itself.

The two operations that routinely require running the motor in reverse are OD grinding with a tool post grinder and cutting a metric thread.  In the latter operation, you MUST remember to back the cutter out clear of the work piece before starting the motor in reverse.


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## Bi11Hudson (Sep 22, 2021)

My MFC now has a proper drum switch. When I first set it up, I had a DPDT (double pole double throw) toggle switch wired into the start winding. A drum switch is essentially the same thing, but with a sort of interlocking control. The original switch was wired with a piece of 4 conductor "SO" cord out to the front of the machine.

Even with the drum switch, on a single phase motor, the *motor must come to zero speed before reversing*. I am familiar with that sort of control anyway, so the toggle switch was no big deal to me. The only adder was the third contact on the drum so I only have 1 switch.

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## wa5cab (Sep 24, 2021)

FWIW, all of the original mills came with a two position bat-handle toggle switch, not a drum switch.  I am not sure as to whether it was a SPST or a DPST.  The latter would be safer if you do not know where the Neutral and hot wires are or are using a 2-pin line cord plug.  However, there is really no excuse for either case.


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## Bi11Hudson (Sep 24, 2021)

The only situation where a DPDT switch would be in the LINE would be if the motor was 3 phase. And then only as a directional, not a line switch. In any single phase case, the DPDT switch would not be in the line cord at all. It reverses just the start winding, using a separate ON/OFF switch. In answer to the above, it would not be a SPST, single pole switch unless controlling a relay or starter. My use of the drum switch is my own modification to allow use the third contact set as an ON/OFF switch.

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## wa5cab (Sep 25, 2021)

Actually, although it wouldn't be as safe to use, an adequately sized SPST switch would work OK as a direct motor start switch.  It just wouldn't be quite as safe as a DPST, especially in an era when 2-pole line plugs were the rule rather than the exception.


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## Bi11Hudson (Sep 25, 2021)

It is a good practice to use a double pole switch as a line (ON/OFF) switch in most any condition. Especially if the machine has a dual voltage motor. It is quite possible, especially for home grown electrical installations, to have the polarity reversed on the 120V socket. Or even back at the fuse panel. Remember, black is hot and white is neutral, *theoretically* at ground on 120 volt systems in the States. For 240 volt installations, in any country, a double pole is a given.

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