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  • Replacing the back gear mechanisms on a lathe

    Hi folks,

    I have an old Denford Viceroy lathe which I bought for cheap and have been restoring and ultimately plan to convert to CNC.

    The headstock contains a layshaft which connects, via a backgear mechanism, to the spindle. The motor is underneath in the stand and connects to this layshaft via a linkbelt.

    Unfortunately all of the gears (4 in total) in this backgear mechanism have one or more missing teeth.

    I'm replacing the motor with a 3hp 3 phase unit and a VFD, so at the moment I'm planning on ditching the backgears completely and replacing the mechanism with a pair of taperlock timing pulleys connecting the layshaft to the spindle at a 1:1 ratio. The spindle is too close to the top of the headstock to take the main drive pulley, so I can't ditch the layshaft. I would install a mechanism (an arm with a pulley on a bearing) to set the belt tension.

    Does anybody see a problem with doing this? I don't think I'll need the backgear any more, the motor has a stepped pulley anyway and I can get it down to 600rpm at full motor speed on the lowest ratio, then I can slow that down further with the VFD, which should provide good power still since the motor is so big.

    My other reasoning is that the belt drive will be smoother and quieter than the gears anyway. Only trouble is the belt snapping would require spindle removal in order to replace, but I just thought I'd use a really wide belt to make it strong.

    I'm confident about the above plan but being relatively new to this I don't want to bugger it up so thought I'd ask for advice first.

    Cheers,
    Rich

  • #2
    Should work fine. If you want to be sure you will need to find the torque capacity of all of the components. I know this info is available on the web, but my search skills aren't up to finding it.

    One other thought - Why not use a poly-V belt(s) to drive the spindle? One or two of those can transmit a lot of torque, but less likely to break in the event of a crash. Could be tensioned in the same way.

    Comment


    • #3
      Originally posted by Abaker View Post
      Should work fine. If you want to be sure you will need to find the torque capacity of all of the components. I know this info is available on the web, but my search skills aren't up to finding it.

      One other thought - Why not use a poly-V belt(s) to drive the spindle? One or two of those can transmit a lot of torque, but less likely to break in the event of a crash. Could be tensioned in the same way.
      The spindle and the layshaft are relatively close, and the spindle is close to the top of the headstock casting, meaning the pulleys can be a max of about 60mm diameter (possibly even smaller).

      In my experience V belts don't tend to like tight bends due to the deep profile, whereas a timing belt is flatter in profile and thus will take that radius with ease.

      Also, to a certain extent, I don't want an unbreakable belt - I would be concerned about a serious stall bending the spindle. Given that the motor will be 3hp (it really only needs a max of 1.5hp) I don't like the idea of a V belt which digs in harder and harder.

      Cheers,
      Rich

      Comment


      • #4
        I agree, that is way too small for a V belt. I was suggesting a Poly-V belt, which is a different animal. Over here we call them serpentine belts. It's a flat belt with a number of small Vs on the inside. Being flat they are much more flexible than traditional V belts. I don't know how unbreakable they are, but they use friction instead of tooth engagement like a timing belt. If you adjust the tension a bit in the light side then the belt will slip (screech like a banshee too) instead of break.

        An internet search on "poly V belt" turned up this site among others.
        http://www.goodyearep.com/ProductsDetail.aspx?id=3880

        Besides I think spindles are a bit more robust than you do. Some riggers will lift lathes by their spindles.

        Comment


        • #5
          Remember that with your 3hp motor at full power with 600 rpm, at 60rpm output on the vfd you will only have about 0.3hp output. However, it's not really practical to expect your VFD to perform well at that output frequency. I find 12hz-15hz is pretty much the bottom end for any real torque. At 15hz your bottom speed will be 180rpm. Pretty high...

          I also recommend Polyvee belts. Look for J-section, maybe 4-5 ribs. Don't try to use loose belts as a safety fuse! Your vfd correctly programmed will stop any overload in a heart beat!
          Last edited by lakeside53; 07-30-2012, 01:20 PM.

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          • #6
            I agree on the torque factor. I have a 3hp Baldor on my Logan, and while it's not VFD, but DC, I frequently have to switch to the backgear more for torque reasons rather than speed. I have plenty of power at 3hp, but there's just no substitute for a backgear.
            Wayne

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            • #7
              This is why you see huge motors on relatively small cnc lathes. A friend of mine has a 20hp motor (factory) on his 16x40, mostly for low speed torque. if you fit such large motors, definitely limit the power in the vfd!

              Comment


              • #8
                Originally posted by lakeside53 View Post
                Don't try to use loose belts as a safety fuse! Your vfd correctly programmed will stop any overload in a heart beat!
                I never thought of that. I do kind of use the belt drive as a safety fuse, but it's on a 1938 Sheldon that was designed to run with flat belts at a max of 600 RPM. I'll take it to 900 if I'm feeling like talking a walk on the wild side.

                That's why I like this place, I learn so much new all the time, I don't even mind getting schooled.

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                • #9
                  I just betcha you can find stock gears with the same pitch and close enough tooth counts to replace the existings gears. You will have to modify them perhaps boring out the replacement gears and turning down the existing gears so one be can slipped over the other and be retained by pinning, anerobic bushing locker, or drilling and tapping at the joint to insert a screw. The ratio may not be exact but it will be close.

                  As others have mentioned, you cannot simply turn down the motor speed and expect decent stock removal at low RPM unless your spindle motor is truely large. For constant HP, RPM is inversely proportional to torque. Thus mechanical advantage in the form of a multi-step mechanical transmission is necessary for machine tool spindle drives. If you employ a VFD and three phase motor you can vary the spindle speed over a wide range and take light cuts simply by dialing the speed. However, if you wish to remove significant stock you have to emply a huge motor whose RPM you can vary or train the multi-speed mechanical drive.

                  In the home shop where older machine tools are retrofitted with DC/SCR drives or AC/VFD, major RPM ntervals of the original mechanical transmission have to be retained.

                  I suggest you fix the backgear either along the lines I suggest, obtain replacement parts, or some other remedy like maybe an extra large V belt drive.

                  Comment


                  • #10
                    Originally posted by Forrest Addy View Post
                    .

                    As others have mentioned, you cannot simply turn down the motor speed and expect decent stock removal at low RPM unless your spindle motor is truely large.

                    As mentioned though the main pulley between the motor and layshaft allows 4 different ratios which gets me 600rpm at full motor speed in the lowest ratio, so 300rpm at half motor speed, and so forth.

                    Since I'm converting to CNC anyway and nearly always use carbide tooling I'm happy with this. 300rpm @ 1.5hp (roughly) should be fine.

                    What I'm more concerned about are potential negative effects of using belts instead of gears, eg longevity, uneven wear on spindle bearings, etc, none of which I know much about.

                    Cheers for the advice folks.
                    Rich

                    Comment


                    • #11
                      I suggest you repair or replace the damaged gears, you will get more satisfaction from your restoration and all options will be open to you.

                      It is interesting to see what www.lathes.co.uk has to say about your lathe:-

                      Viceroy machine tools were rarely advertised in other than the educational press and are still little known, even in their country of origin; consequently, the market under-values them and, if you want a neat, strong, very compact lathe with a generous capacity at a bargain price - a Denford Viceroy may be the machine to look for ..

                      Comment


                      • #12
                        I have a VFD on my lathe. Even so I was happy to have back gears when I was turning some cast iron. I could keep the motor RPMs up and gear down the spindle for some torquey goodness.

                        Comment


                        • #13
                          Originally posted by loply View Post
                          As mentioned though the main pulley between the motor and layshaft allows 4 different ratios which gets me 600rpm at full motor speed in the lowest ratio, so 300rpm at half motor speed, and so forth.

                          Since I'm converting to CNC anyway and nearly always use carbide tooling I'm happy with this. 300rpm @ 1.5hp (roughly) should be fine.
                          Might think differently with some tool steels or other nasty materials..... carbide at 3x HSS speeds might need lower rpm for larger pieces, .... you'd want some serious power, but at 1/4 motor speed, you'd have 1/4 motor power.

                          The other advantage I think was mentioned, which is that the belt is not as good for raw low speed torque, may slip etc..... gears on the OUTPUT side (to spindle) give a torque MULTIPLYING effect, instead of a power loss.

                          I have a small horizontal mill which has a 1/4 HP (!) motor. Your first thought is that might be OK for a small slitting saw, but weak otherwise.... But I have back gears and a good belt reduction..... That thing will chew a slab mill through steel without slowing down.... I am more afraid of breaking something than the motor stalling.

                          The gears can be redone by shrinking on blank hoops and re-cutting, or by putting on a modified stock gear..... the modified stock might have issues with web thickness vs the width of "landing" that it goes onto..... but a blank rim that you cut is makeable to any size/thickness you want.
                          1601

                          Keep eye on ball.
                          Hashim Khan

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                          • #14
                            Thanks for all the replies folks, I appreciate the comments about retaining the back gear.

                            Unfortunately I'm not confident about my ability to make new gears - they are quite big and relatively complex, and I'd need the lathe to make them on! Even acquiring stock for them may challenge me.

                            This may seem like an outlandish idea but I have a possible alternative I'm considering.

                            I've had an idea for a while of a selectable belt drive system whereby I would put two complete sets of pulleys (of different ratios) on the layshaft on bearings so they are free to rotate independent of the layshaft, then I would 'engage' one or another to the layshaft by means of a sliding dog gear which is keyed to the shaft. This is much like how the current system works.

                            The pulley pair which is not in use would harmlessly rotate at whatever speed was neccessary, but wouldn't be in sync with the layshaft as the dogs aren't engaged in it.

                            The reason I'm considering this is because:

                            1) I'm more comfortable that I can modify four cheap, easily acquired pulleys with the dog holes than trying to make four big gears

                            2) I like the idea of replacing the gears anyway as they are noisy and may introduce a 'vibe' to the surface finish as they look quite coarse

                            I hope this doesn't seem like a daft idea but would appreciate any comments on it.

                            Cheers,
                            Rich
                            Last edited by loply; 07-31-2012, 07:07 AM.

                            Comment


                            • #15
                              Rich, I am sorry you are not going to try to make/repair/replace the gears as I well recall the satisfaction I got when I managed to make some change gears for my Drummond lathe.

                              I think your scheme with belts and pulleys may, or may not, be a good idea and I suggest you make sure you have considered all details before starting modification to your lathe.

                              You might also consider making plastic gears. These are quite easy depending upon the plastic. I have made a few for light duty work. The technique is to fit a gear of the required gear family in your lathe spindle and mount your plastic blank on an axle fixed to your tool post. Then start the lathe and press the blank against the spinning gear, it will be very noisy (rumble) at first but as the teeth begin to form the noise will subside, then you increase the pressure some more and so on until the teeth are formed. A hot air gun playing on the edge of the plastic blank helps the process. Just a thought.

                              John

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