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  • Best speed range for a small lathe

    Hi folks,

    I've recently bought a small (3.5" x 12") bench lathe, a Faircut Junior, quite rare by all accounts, which I have had to fettle the drive system on as the original motor is missing and I have no clue what size pulley or what speed it would have been.

    I've bought a 1400rpm motor and have a selection of pulleys that can be made to fit, my issue is that as a lathe newbie I'm not sure what speed range is needed and as usual a google search is of no help at all!

    Working it out with a 100mm motor pulley driving the 260mm flywheel on the countershaft I can get 861.54, 538.46, 336.54, as my main speeds, and 149.57, 93.48, 58.43 as my backgeared speeds.

    Will this be of any use or do I need a higher top speed?

    Any help much appreciated, Dan

  • #2
    Hi,

    Sounds like a nice lathe. Post some photos please, I would like to see it!

    What speeds to have depends on what material you want to work with and the diameter of the material. The speeds you list so far, would be fine for steels. But far less than needed for aluminum or plastics. So you need to figure what you want to turn first. Then you can figure out what spindle speeds you might need.

    Dalee
    If you think you understand what is going on, you haven't been paying attention.

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    • #3
      Doesn't the Junior have plain spindle bearings? That might be what determines how fast you can run it.
      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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      • #4
        Is it this lathe?

        http://www.lathes.co.uk/faircut/

        If so, nice looking machine.

        The text mentions: "Although some contemporary British lathes of the 1950s still used a flat-belt final drive all Faircut Junior lathes seen by the writer have had a (cast-iron) V-belt drive headstock pulley with cones of 4", 3-1/4" and 2-1/2" diameters - and one can only assume that this was a standard feature from the start to finish of production."

        Also if you scroll down to the bottom there's a horrible pic that mentions the diameters of the flat belt pulleys used. Text above it reads: "An early 3.5-inch Faircut with 3-step flat-belt drive and pulley diameters of 2.25", 3" and 4"", and you might be able to make out the pulley sizes for the motor and intermediate shaft from the pic. Looks to me like "2-1/4" on motor. 8-3/8 on intermediate shaft". And from the other pics it looks like plain bearings, so not going to be running terribly fast.

        Last edited by adatesman; 10-19-2014, 12:37 PM.

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        • #5
          Ideally you want a speed range that will give you 80 surface feet per minute on your max swing and 360 surface feet per minute at 1/12 the machine's max swing. if your lathe is a little guy with a 3 1/2" swing. The ideal max range works out to 73 RPM and 4500 RPM respectively. That's ideally.

          The spindle RPM selection you have (roughly) 862/537/337 ... 150/94/58 has a nice geometric ratio in the diirect and back gear ranges but a big gap between. The geometric ratio is the ratio between adjacent speeds. In yours that ratio is 1.60 to 1. Between high and low ranges it's 2.24 which is awkward but not insurmountable. You have to adapt your machining operations to suit your limited range of speeds.

          What kind of spindle bearings? Ball or plain? If ball you can run the spindle RPM to most any practical range you wish. With plain you may have a max speed you can run at without overheating.

          What's your budget like? If you can afford a couple hundred - dollars? No location info but the 1400 RPM motor is a tip. Blighty?. If you can afford a hundred Euros or 70 pounds you can furnish the lathe with a house current VFD and a scrounged 3 phase motor. This will give you continuously variable speed to fill those awkward gaps. Lower budget but still satisfactory you can use a DC motor from a treadmill and a DC controller. The object is to get spindle RPMs between what you have.

          If you have ball or roller spindle bearings and need to keep the budget, I suggest two motor pulleys arranged for easily exchange, one low range and one high. The high range is selected to give a top RPM of 2000 (or 3000 RPM would be better for small work) and low speed to give you what you have now. Two pulleys, two belts - it could work if you don't mind fiddling as you shift from smaller work to larger and back.

          If you have plain spindle bearings, your options are limited. They may not be suited for high RPM. That will be an on-the-spot judgement call.
          Last edited by Forrest Addy; 10-19-2014, 01:31 PM.

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          • #6
            Hi all,

            Cheers for the replies, the pic posted is of the larger Senior lathe, though very similar, mines the Junior which is a cantilevered gap bed that most of that article talks about on lathes.co.uk.

            Afaik it has plain bearings so I'd imagine I'm limited top end, which is slightly annoying as I would like the ability to turn plastics and maybe aluminium as you never know when you might need a bush or what-have-you.

            I see what you mean about the big gap between effectively high and low ranges, I suppose the way round this may be to have a two step system on the motor to flywheel, giving 12 speeds overall.

            I'll post some pics when I get chance and have a space for the lathe to sit without ten other jobs etc crowding it!

            Dan

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            • #7
              Those speeds are faster than my old Myford ML7 runs with a 1750 rpm motor, and considerably faster than it would run with the 1420 rpm motor it would have had in Britain. 640,357,200,110,62, and 35 rpm are the speeds Myford states for a 1420 drive motor for that particular model.

              You don't need a higher top speed , though there will be times when it might be nice to have. You probably will wish for a lot slower speeds in both direct drive and backgear pretty often, if you do work of any size for that lathe.

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              • #8
                If I drop to the next size pulley I have lying about (67mm) I get these speeds 577.23, 360.77, 225.48 and on BG 100.21, 62.63, 39.15 which seem to be less of a jump from high to low range and nearer to what the ML7 is.

                Dan

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                • #9
                  The OP is in teh UK. Therefore his 3 1/2" lathe swings 7", since in the UK mostly the center height is quoted, vs the diameter. If so that affects the ratio s Forrest suggested.
                  1601

                  Keep eye on ball.
                  Hashim Khan

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                  • #10
                    My lathes lowest speed is 115 rpm, which is fine for parting off work, but too fast for threading. It has a speed of 550 rpm which I use for most 1" and less mild steel, a speed of 970 rpm which is fine for aluminum and light finishing cuts on mild steel, and a screaming 1620 rpm which is too fast for almost any of the turning I do but works good for polishing.
                    Brian Rupnow

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                    • #11
                      That's still a big jump in speed from back-gear to direct, but it's manageable.

                      For v-belt drive, the motor and flywheel sheaves are huge for the power that lathe can handle. Nothing wrong with that, but it suggests a possibility, perhaps for the future after you get running. As Forrest suggested, two pulleys, two belts. In your case, this could be a smaller pulley added to the motor shaft. Accepting a small amount of offset, both sheaves could drive to the flywheel.

                      For reference, my ML7 uses a pulley about 48mm diameter on the motor driving to a pulley of about 180mm on the countershaft. Slippage has never been a problem, nor belt wear.

                      If your use of the lathe is anything like mine, I think you would really appreciate the availability of a lower speed in direct drive, and occasionally, in back-gear.

                      The shaft and bushings of the ML7 countershaft are just about worn out after 48 years of use. When I dismantle the countershaft for repair, I will add a larger wooden rim to the face of the pulley. I'll try a V to flat drive, and only add a groove to the wooden rim if slippage is a problem. There are many occasions when I would much rather change belts rather than shift to highest back-gear.
                      Last edited by cameron; 10-19-2014, 04:23 PM.

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                      • #12
                        Yes should have said its a 3.5" centre height, something like 9" swing in the gap. There is scope to mount a twin pulley on the motor that would allow 12 speeds by switching the motor belt over but would involve faffing with bolts.

                        Might just do with one set and see how I go.

                        Dan

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                        • #13
                          One thing that many have not taken into account.

                          That design was pretty much a standard British design (dozens of different company's made almost identical lathes) going right back to the 1800's, when lathes where either overhead belt driven or treadle driven. 600 to 800 RPM's where about the most that they where designed for, the bearings would have a hard time with higher speeds.
                          The shortest distance between two points is a circle of infinite diameter.

                          Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

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                          • #14
                            Another thing perhaps not fully taken into account by some is this: while the lathe appears to be a very good example of it's type, that type is a small, lightly built lathe with a small diameter spindle. The limiting factor is usually chatter, and that has to be countered by lowering the speed. The option of plowing through with increased feed rate is just not there.

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                            • #15
                              Originally posted by J Tiers View Post
                              The OP is in teh UK. Therefore his 3 1/2" lathe swings 7", since in the UK mostly the center height is quoted, vs the diameter. If so that affects the ratio s Forrest suggested.
                              I was aware of that once but forgot in the rush to blab.

                              Regardless it is a lathe if a bit basic. Any limitations it may have may be overcome with patience, technique, and ingenuity. Sounds like you're on your way. Pick a pulley and make some chips.
                              Last edited by Forrest Addy; 10-19-2014, 06:14 PM.

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