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Using backgear locked to remove chucks.

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  • Using backgear locked to remove chucks.

    I know I'll probably catch heck for this, but I like sharing interesting facts. Just thought I'd share some research I've done. Since I've never believed the adage that you shouldn't lock the backgear to remove a chuck, and do it regularly, I thought I would do some calculations on that. I used Boston Gear's suggested engineering equations on my Sheldon EXL lathe. Using data for the lowest carbon untreated steel, the maximum allowable torque is as follows: For the spindle gear nearest the chuck, max torque=1306 lbf-ft. For the large half-shaft gear, max torque=1270 lbf-ft. The lowest max torque was on the small gear on the half-shaft, and I use a gear face width equal to the meshing gear since it provides a concentrated load. It's max torque=392 lbf-ft. The caveat is that the torque applied to this small gear will be reduced using the ratio with the meshing gear, which results in 1176 lbf-ft at the spindle. Now if you use a hammer on the chuck, you could get that much torque. But if you do like I do, and use a 2 ft lever, you're unlikely to get there. I've always figured if there were broken teeth it was due to a hammer on the chuck or a crash.

  • #2
    I always use the back gear to removed the chuck. But I make sure not to pound on the chuck and always take up any slack in the gears before using my hand to tap the chuck loose.

    Where the problem lies is if a guy uses a hammer and doesn't take the slack out of the gears before hammering on the chuck. The shock is what breaks teeth.
    Andy

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    • #3
      While you're technically correct, I much prefer holding the gear on the end of the spindle. I have a block of aluminum that's bored and split with some bolts that allow it to clamp onto the gear on the outboard end of my 10K spindle.

      I'll use the back gears when I know the chuck is going to be reasonably loose, but if it requires more than just a little pressure, I'll stop. Even if the chances of damage are slim, back gears are expensive and a PITA to replace.

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      • #4
        I certainly wouldn't use them to remove a stuck chuck. Since I change/remove the chuck quite often, it's just convenient. Actually I'm surprised the manufacturers didn't bother to add a locking feature for that purpose. Angle grinders have spindle locks.

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        • #5
          Not to mention that there are back gears and then there are back gears. The back gears on the Sheldon are quite robust, those on smaller lathes not so much. Broken gear teeth are not at all uncommon on 9" South Bend, Atlas and similar lathes, less so on larger machines.

          On my Sheldon, I do use the back gear and a 10" crescent wrench on a chuck jaw.
          Jim H.

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          • #6
            I use the back gear all the time to lock the chuck then use the chuck key and give yank. Done! A 1/8 spin on the chuck to tighten has worked for me and I've never unscrewed one while cutting on the backside. I don't jam them on as it were. It's important to keep the registration areas clean so there's no grit or chips to act as a lock. A wipe with light oil helps too.

            You're using the strength of the teeth on two gears not just one. Anyway, there's probably more stress on the keyways than on the teeth. I'll keep doing it that way until I'm too weak or old to move chucks around or break something. I'm already "officially old" now--Just received my Medicare Card.

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            • #7
              Some people think they have to hammer them tight for some reason.

              An old SB I have, the chuck rusted/hammered/dunno and the teeth broke out in 3-4 places. I just applied gentle pressure with a 4 foot bar and off it came. I had hung a bucket of rocks from it the night before. Apparently the previous shop hammered it on and tried to hammer it off. I noticed a carbide tool in the toolpost - sideways. Really. They gave up and put the lathe in a dumpster for me.

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              • #8
                Problem is, the OP's calcs were down on the basis of the gears being made from a low carbon steel, but a lot of backgears are/were made from cast iron which is a different kettle of fish. Yes when I had such a lathes with threaded spindles, I did lock the backgear, but restricted myself to putting a 10" adjustable wrench on a chuck jaw, and hitting it firmly with my fist. That worked 99% of the time, even when I'd been doing some heavy intermittent cutting. I was probably lucky, I never succeeded in breaking a gear tooth.
                'It may not always be the best policy to do what is best technically, but those responsible for policy can never form a right judgement without knowledge of what is right technically' - 'Dutch' Kindelberger

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                • #9
                  A sure way to break teeth on a cast iron gear. Do not ask me how I know.

                  Jim

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                  • #10
                    I remember when searching for a lathe on E-bay that many, many of the descriptions said that the back gears or the bull gear were broken. This seems to be one of the most common problems with used lathes. There has got to be a reason and I doubt that it was from turning.

                    I have to agree with the mention of using a hammer. A hammer blow, even if the backlash in the gears has been taken up first, will impart a heavy instantaneous force to the gear teeth. If you have a stuck chuck, you should avoid the use of a hammer at all costs.

                    I have a long crank handle that installs on the rear of the spindle for threading. I would try this if I had a stuck chuck.
                    Paul A.

                    Make it fit.
                    You can't win and there is a penalty for trying!

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                    • #11
                      When setting a file handle do you whack the file into the handle with a hammer or do you bang the handle on the bench and let inertia set the file?

                      Same for releasing a chuck. Make a timber mask to fit in the jaws and spin the chuck smartly backwards until the timber hits the bed. Chuck breaks loose from the spindle's inertia - no damage to chuck, bed or gears. Change the timber when it gets too beaten up.
                      Peter - novice home machinist, modern motorcycle enthusiast.

                      Denford Viceroy 280 Synchro (11 x 24)
                      Herbert 0V adapted to R8 by 'Sir John'.
                      Monarch 10EE 1942

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                      • #12
                        Originally posted by Richard P Wilson View Post
                        Problem is, the OP's calcs were down on the basis of the gears being made from a low carbon steel, but a lot of backgears are/were made from cast iron which is a different kettle of fish. Yes when I had such a lathes with threaded spindles, I did lock the backgear, but restricted myself to putting a 10" adjustable wrench on a chuck jaw, and hitting it firmly with my fist. That worked 99% of the time, even when I'd been doing some heavy intermittent cutting. I was probably lucky, I never succeeded in breaking a gear tooth.
                        When I got my 9" SB I removed the chuck exactly the same way, adjustable wrench on a jaw, and whack it with the palm of my hand. I figured "How hard can I possible hit it with just the palm of my hand?" and I found out...hard enough to break a couple of teeth off one of the back gears. I've since made a tool that clamps on the outboard end of the spindle to hold it solid.

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                        • #13
                          I used to give the chuck key a welt with a hide hammer, the end next to the chuck that is, and always felt guilty. I made up a strap wrench of 1" belting with a handle a foot long, lock the back gear and feel a lot better about it. The Smart & Brown model A has a 1 3/4" Whitworth thread and I wind chucks up to within about 1/8 turn and then attempt to spin them the rest to nip them up (without locking the spindle). This works for me as I never use reverse over 30 rpm.

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                          • #14
                            My old 11" South Bend had several teeth missing on the backgear, including 2 that were side-by-side, rendering it useless. I always wondered what kind of hack did such damage...

                            Then one fine day I tried removing the chuck (using backgears as lock) with about a 2' bar. Gears fully engaged, backlash taken up, firm smack with hand. It had 1 less tooth when I sold it So no, I don't recommend using backgear for this purpose.


                            Ed

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                            • #15
                              My $0.02, most back gear teeth are broken by buffoons engaging under power, i.e., motor/spindle turning. The rest taking off a threaded chuck,

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