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Motor fully size for Craftsman/Atlas 12 inch lathe... I want to make one

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  • Motor fully size for Craftsman/Atlas 12 inch lathe... I want to make one

    I am working on a Craftsman 12inch lathe (obviously more than 50 years old). I have a Sears Craftsman owners manual I pulled off the "Vintage Machinery" website. I can't find any dimensions for the motor pulley (not that I really expected to find any). I have the pulley that was on the motor on the lathe but I am concerned that it is the wrong size since the motor is only 1/3hp and I'm guessing that this is not the original motor. I advised my friend to buy a 1hp motor (lesson Farm Duty). The bore on this pulley is only 1/2 inch and all the 1hp motors come with 5/8inch or bigger shafts (as far as I can tell). He has ordered the motor but now the pulley won't fit and as mentioned above, I am concerned that this pulley is not the original pulley.

    If somebody here has information on the pulley size or a website I can look it up on, he and I would be grateful. In advance, thank you!!

    Jeff Conti

  • #2
    You can determine motor pulley size by working backward from a known spindle speed, simple math.


    • #3
      Also, it is a simple matter to bore out the hole in the pulley and broach a new keyway.
      Kansas City area


      • #4
        Is this your lathe?

        Somewhere on the internet is a list of the spindle speeds for your particular lathe. Working backwards from one spindle speed through its pulley ratios and assuming the motor RPM was 1750 you can reverse engineer the motor pulley diameter using ratio and proportion.

        Example: assume for a moment the top spindle RPM is 750 (a fairly common top RPM for a lathe of this size and nature), the spindle cone pulley diameter is 3.5" and the counter shaft pulley diameter is 6.5"

        Countershaft RPM = Spindle RPM x Spindle pulley dia / Countershaft dia;

        Countershaft RPM = 750 RPM x 3.5" / 6.5" = 404 RPM.

        Calculating motor pulley size is a little different because we have two RPM's and one diameter. Assume the countershaft driven pulley is 13" dia.

        Motor pulley diameter = 404 RPM x 13" / 1750 RPM = 3.00"

        This is just an example to show a way to figure motor pulley diameter from known data. You'll have to measure and calculate from manufacturer's data to suit your particular machine.

        I see the spindle of your lathe is mounted on taper roller bearings so there is no real restriction on spindle RPM (within reason.) You just have to pick a top spindle RPM that suits your requirements.

        A 12" lathe optimum motor HP depends on a variety of factors but making a few assumptions based on past experience I suggest you can use any motor between 3/4 HP to 3 HP. 1/3 HP may run the machine but it severely restricts stock removal rate. A 1/3 HP motor is barely adequate for a small drill press.

        Before you commit to that farm duty motor consider that farm duty motors are intended for chicken house blowers and grain augers. These applications require mechanical power but expensive to execute features like smooth power are not provided. There are farm duty motor and there are farm duty motors. Flop a full sheet of 3/8 plywood across a pair of saw horses and set the motor on it. The plywood acts as a sounding board. Connect the motor to power and let it run idle. Can you hear it hum or feel a buzz in the plywood? If so, you do not want that motor for powering an engine lathe.

        I assume you wish to run the machine from single phase. Here's something to think about: Single phase capacitor start induction motors are inherently rough running. The motor you select may vibrate enough to set tools on your bench buzzing or record wavy patterns on the machined surface. If you're confronted with irregular finishes and you've eliminated tool, material, work support, etc as sources you're left with machine vibration from the motor. This vibration does not originate from motor balance but irregular motor torque within an individual revolution - a unique phenomena common to single phase induction motors and the way the powerful magnetic fields snatch at the armature as it drifts slightly in RPM from cycle to cycle.

        You may wish to consider a DC or three phase motor; if operated from clean power, both run silky smooth unaffected with this irregular motor torque. Both types require an electronic drive to make them work. If you elect to use a DC or three phase motor there are plenty of past posts and good advise here to lighten the burden of how-to. If cost is an issue there are sure to be good scrounging resources near you..

        For example, check with your dad's old friend. Maybe he'd have something, motor-wise
        Last edited by Forrest Addy; 02-21-2016, 08:06 PM.


        • #5
          IMHO a 1/3 or 1/2 hp motor is quite suitable for that lathe. A 1750 rpm motor and the pulleys you have should give a reasonable range of speeds. You should not expect to do heavy cutting that would require more power. Atlas is a light duty lathe, but within its limitations you can do good work with it.


          • #6
            There is such a thing as under-doing while thinking you are being careful and conservative or perhaps economical.

            "Light duty" should not imply weenie performance. I've run many small lathes over the years and the more satisfactory of them were equipped with adequate motors. Those powered by motors salvaged from washing machines and fan motors from junked furnaces were poor performing and, in top RPM, barely had power to drill middling sized holes.

            Also an over-sized motor mounted on the machine cannot damage it by merely existing. The machine draws only enough mechanical power from the motor to supply the sum of its loads and losses. Thus a 15 HP motor will supply only 1 HP worth of torque to a machine whose total requirements were 1 HP. That said, an over-sized motor may somehow through collision or carelessness overload a weak part and damage it, that is if the belts didn't slip.

            The motor with which you power your machine tools is best if sized in the "sweet spot;" that is, a motor that has adequate power and performance to run the machine throughout its work envelope allowing expeditious stock reduction and smooth quiet operation needed for fine finishes.

            I mentioned in my earlier post that in my experience a suitable motor for a 50 year old Craftsman 12" lathe would be in the 3/4 to 3 HP range. 3/4 HP will run it just fine.

            I've seen the effect of a 1/3 HP on a South Bend 10K belonging to my dad: the 1/3 HP motor was taxed to run the machine with all its bearings, belts, and index gearing to top speed in idle. There was little power remaining for chip-making unless the motor is run over full-load amps. Dad was a careful worker but still he smoked that 1/3 HP motor polishing a gun barrel in 1955. The next motor was a 1 HP that ran for the next 15 years when it came to me and 5 years thereafter until I sold the machine after buying my present lathe.

            "Just right" is just right and in most of life's situations "just right" covers a range on either end of which lies increasingly unsatisfactory results.
            Last edited by Forrest Addy; 02-21-2016, 09:10 PM.


            • #7
              It must be the 10" with the 1" block added with the open pulleys & not the later Craftsman Commercial with the cabinet & undermotor. I have both & the 1st is just a blocked up 10" Atlas refused to put there name on so it's the same as an Atlas 10", if so I can measure the pulley & actually have original Atlas motors in boxes. IMHO You may tear it up with much more motor. Let me know & I'll measure it.


              • #8
                It just so happens that I have a basket case sitting out back, so;
                Small 1.988
                Large 4.454
                Width 1.242
                Bore .625
                Part No 10-428
                This pulley is suitable for the old style Atlas 10 or the raised in the sand from 10 to 12 inch lathe that was made to Sears specifications, by Atlas, and was sold exclusively by Sears under the Craftsman name. The older style 12 was never badged or sold as an Atlas product.
                Check out Tony’s site,
                Good luck, Mike
                Last edited by mf205i; 02-21-2016, 10:55 PM.


                • #9
                  Sears recommended 1/2 HP minimum when I was buying my 12", so that's what I got. It's a ball bearing, factory color gold. Bought in 1956 +/- 1 yr. Motor still fine, billions of revolutions later, replaced belt end bearing once, years ago, cleaned centrifugal switch contacts once, replaced On/Off switch quite a few times!

                  The original countershaft pulley cracked quite awhile ago, fastened two pulleys together with screws after finding out I really needed that two-step choice. Found one subsequently that is a bit larger than original, I think, but speeds OK. imp
                  IF IT'S ELECTRICITY, IT'S ME.


                  • #10
                    Atlas still sells a new pulley for your lathe. The new pulleys have a 5/8" bore and a bushing to use with a 1/2" shaft motor.


                    • #11
                      My Craftsman is from '37. My lathe's original pulley is gone. The replacement is about 4.5" in diameter. I added VFD to my lathe. Because of this I can pretty much pick the speed and the pulley size doesn't matter much. The belt from this pulley connects to the smaller choice on the countershaft pulley. I recommend this route if you can do it.
                      Last edited by Tony Ennis; 02-22-2016, 08:43 AM.


                      • #12
                        Originally posted by Forrest Addy View Post
                        "Just right" is just right and in most of life's situations "just right" covers a range on either end of which lies increasingly unsatisfactory results.
                        Folks, that wisdom applies to most of life.


                        • #13
                          Consider the ConSew sewing machine motors we just discussed. You get variable speed, reverse , brake & It plugs in the wall but is a DC servo 3/4 HP motor with lots of power for about $100 new with the pulley.


                          • #14
                            Originally posted by flylo View Post
                            Consider the ConSew sewing machine motors we just discussed. You get variable speed, reverse , brake & It plugs in the wall but is a DC servo 3/4 HP motor with lots of power for about $100 new with the pulley.
                            Could someone point me to the Consew motor discussion? I looked but could not find it. I have a 12" Atlas that could really benefit from this upgrade. Thanks Steve


                            • #15
                              I've had good luck with ones like this eBay item number:161737355645 They are DC so they have good low end power, variable speed, reverse, an extra plug for a light & a brake if you need it. Let me know if I can help!