by Jim Schroeder
Material(s): Steel and Acetal Plastic
Excerpt: This article will show you an easy method of reducing backlash to a relatively small amount (.001 to .002″) and at the same time correct the lead screw error that is inherent in the G0602. Much of this is equally applicable to other lathes, mills and devices that use the lead screws to position things.
Old time machinists will tell you that you need to learn how to manage backlash, it is part of the business of using machine tools. Today we have new technologies that allow the hobbyist to achieve accuracy that not many years ago would have been impossible. Most of these new technologies are not available on the equipment that we purchase. As hobbyists, we usually end up with the best bare-bones machine we can afford. Our machines are usually Grizzly, Harbor Freight, Enco or Homier; not Clausing, Hardinge or Bridgeport.
Many hobbyists spend much of their shop time improving the tools they have or making new ones, which is why the hobby is somewhat self-perpetuating. One of the most common problems we face and one of the more difficult to solve is backlash. It is a constant and ongoing nuisance. Fitting ball screws and other high-tech solutions is usually not an option as cost, size and other factors usually constrain the path we take.
This article will show you a relatively easy method of reducing backlash to a relative small amount and at the same time correct the lead screw error that is inherent in the G0602. Much of this is equally applicable to other lathes, mills and devices that use the lead screws to position things. On the G0602 the cross slide nut is cast iron and the compound nut is bronze. On my lathe both have backlash in the order of 4-8 thousands despite routine adjustments, and they wear quickly.
Several recent posts have focused on making the cross slide area more robust. You may want to review those posts and incorporate them as part of an overall plan for improvement. From my personal experience, if you incorporate all of these changes into your lathe you will be able to manage heavy cuts without effort. Here are two posts that may be of interest:
This is a long procedure, but do not be intimidated. Each step is very straight forward and if you take your time you will be successful. The most difficult portion is threading a long slender rod with a non-standard left-hand thread; look at it as a chance to learn something new. There are no modifications to the lathe and your equipment is operational until we fit the new cross slide lead screw.
I came across a posting on another site by Evan Williams that was so appealing that it falls into the “ah-hah moment” category. Evan, who is obviously a very experienced machinist with knowledge in a wide variety of subjects, put forth a very simple method of making lead screw nuts from acetal plastic. Acetal (Delrin is part of the acetal family) is widely available and inexpensive. It has mechanical properties that make it ideal for a lead screw nut. Evan’s original post can be found at:
This article will deal with the specifics of using Evan’s method to fabricate a nut for the cross slide on the G0602. The resulting nut is mechanically compatible with the original nut; no changes to the lathe are needed. A mill is needed to fabricate the shell that the nut fits into.
The Cross Slide: Start by Making a New Lead Screw
The original cross slide lead screw was made using a 1.5mm thread pitch. The calibrated dial indicates a total of 0.060” of movement per revolution. However, the 1.5mm thread will actually result in a total movement of 0.0591”. This difference can become significant over relative short distance; thus the need for a new screw.
Photo 1 shows a new cross slide screw and a threading chase, we start by making the threading chase since this will give you a chance to test your threading setup without risking a larger and more complex piece of work. Using a five inch piece of ½” round steel stock chuck it up and turn the first three inches to a diameter of 0.400”. This diameter can be plus or minus say 0.005” but what ever the final value is it must be the same as the lead screw which we will make next.
Once you have a turned a three inch portion to size set up your lathe to thread a 16.6667 TPI left hand thread. You will not find this thread size on your chart but thanks to RWey50, who created the software for the threading chart and interactive tool; we can achieve this thread by using the following setup:
- The top gear (“a” gear) is 52 tooth
- Next the normal 120 tooth gear used for metric threading is replaced with the 104 tooth gear. The 104 tooth gear will drive a 46 tooth gear (“b” gear).
- You will need to be able to reverse the lead screw direction (this is a left handed thread), if you have built and installed the “Simple reverse idler”, you can use the 120 tooth gear for that purpose.
The setup should look like this:
A few comments on threading before you start. This is a left hand thread so the cutting tool you use must have the right hand side (as you look at it from the operators view) as the cutting edge with the appropriate top rake running back from that edge. I use a seven degree top rake and seven degrees of clearance. Your compound slide should be set to minus 29.5 degrees so that the direction of feed is towards the cutting edge of the tool, this can be seen in photo 4. The total depth of the thread as measured on the compound slide dial is 0.042”. Prior to threading use the parting tool to turn a “landing” area at the base of the thread where it meets remaining portion of the shaft, this should be turned 0.042” deep and 0.125” wide, this will provide a starting point for the threading tool.
Once you are satisfied with the thread I use a 3M abrasive pad over the threads to remove the burrs. This is done while it is running at speed in the chuck. Take appropriate measures to protect your hands during this process. Next, set your compound slide to 7.5 degrees and cut a small taper on the end of the chase. The depth of cut should just remove any trace of threads of the first one or two threads. Next we need to mill a step into the chase to give it a cutting edge, see photo 3. The bottom of the step should be at the mid point (0.200” down from the top when mounted in the mill vise), be 0.060 deep and run for a distance of 1.000”. Mill a step on two sides. Notice which side the step is milled into, since this is a left hand thread we want a cutting edge to be revealed.
Once the chase is complete we move on to the new shaft. Diagram 1 will provide the dimensions.
This shaft is non-hardened so most steel types will do; I have used 1018 and 4140 both with near equal results, the latter cuts a little cleaner leaving a nicer thread. With a piece of 0.500” round rod 11.775” long chuck it up in the lathe and drill a center hole in one end. Position the rod so that enough material is exposed to turn and cut the 7.250” threaded portion. You may need to extend the tailstock shaft several inches to gain enough clearance for the cross slide tool holder. This is a long and unsupported piece of material so we need to take a shallow depth of cut using very sharp HSS tools. Do not try and use carbide or inserts for this process.
The first step is to take a light skim cut over the entire 7.250” length. When this is done use your micrometer to measure the ends (we are looking for any sign of a taper). If the tail stock end is larger or smaller than the chuck end, more than 0.001-0.002”, then you need to adjust the tailstock. This is most easily accomplished by the use of a dial indicator to keep track of exactly how much you have moved, we are usually talking a few thousands. If you had to adjust the tailstock then take another skim cut and measure the result. One word of caution, do not apply too much pressure to the tailstock center, as the material heats up from cutting it may bow out in the middle.
When you have the tailstock properly centered check the middle measurement to see if it is larger than the ends, if it is within a few thousands then the final finish cut should bring it back to the desired dimension. I usually take 0.010” using a razor sharp HSS tool bit, which means four or five passes to get close to our 0.400 finished dimension. When you are close to the finished size make the last one or two or cuts of 0.001-0.003”. When you are finished you should have a uniform dimension of .400 from end to end. The exact diameter is not important as long as the chase and the shaft are the same size and uniform.
Use the same threading setup that you used to make the chase. Don’t forget to cut a landing area at the base of the shaft. The depth of the thread is the same 0.042” as measured on the compound slide dial. When the threading is complete and de-burred, chamfer the end of the shaft. We now need to reverse the shaft and turn the other end to desired dimensions. To hold the shaft in the chuck I used a piece of ½” aluminum tubing that was slit down one side. This held the shaft in the proper location and protected the threads. Drill and center this end similar to the other end. Preserve an 1/8” wide collar as shown in the diagram. Turn the remaining portion of the shaft to a diameter of 0.392”. You must hold this diameter closely since the bearing races will fit over this portion. When that is complete you can relieve the center section by ten thousands or so and then turn the end portion to a diameter of 0.306” for a distance of 0.393”. After this is complete turn a landing area at the base of this part of the shaft, 0.040” deep, see diagram 2. Next thread this end of the shaft for 8 x 1.25 mm and finish by adding a chamfer to the end of the shaft.
Once this is complete we return to the mill to cut a key slot in the shaft to hold the key used to couple the knob to the shaft. The original key is about 0.114” (4.5 mm) wide and 0.305” long. I used a 0.125” slot drill to mill a 0.125” x 0.305” slot that was 0.084 deep. This is not a perfect fit but if you are careful it will work satisfactorily. The alternative is to visit the local hobby store and obtain some square 1/8” brass square stock and fabricate one from that. The following diagram provides detail dimensions for the above.
At this point we should have a nicely finished new shaft and a threading chase. The next step is to make an acetal nut. To form the threads we will use heat. Once the acetal material reaches the flow point it will give us surprisingly good quality threads.
Making an Acetal Nut:
Start by making the acetal nut and then the housing. Using a 1.000” piece of acetal part off a piece 1.600” long. In the following photo I used a piece 1.5” acetal and turned down a section to 1.000”. Bore the inside using a “T” drill bit (0.358”); this will allow enough material to fill in the threads when heated.
Next, take the blank and saw into two pieces, they do not have to be exactly the same, if you saw them slightly off center it should not be a problem.
You are now ready to make the “sandwich”. Place a liberal amount of oil on the shaft in the location where you place acetal. I used a piece of 2×4 and drilled it to one inch then ran it through the saw to end up with two supporting pieces to hold everything together. The saw kerf removed enough material to insure the two acetal pieces would come together when heated. The saw kerf was 3/32”.
The wood will also keep the vise from acting as a large heat sink.
The sandwich is together and ready to heat. It is very important that the heat only be applied to the shaft not the acetal directly. The shaft needs to reach the temperature where the acetal thermally flows to the contour of the threads. I used two sources of heat, on the far end that is a 100W soldering iron and in the foreground is a 1200W heat gun. The soldering iron was probably overkill but speeds up the process. Heating both ends of the shaft with the heat gun alternately would have worked as well, it just would have taken longer. It will take 15-20 minutes for the shaft to reach a high enough temperature using just a heat gun so be patient and slowly heat the shaft and take up on the vise from time to time. As things warm up the vise will close in and towards the end you will begin to see acetal oozing from the ends. When you have reached the point where the vise has closed the two halves together and thermal forming has been completed then let the entire assembly cool. Wait until it is comfortable to hold before proceeding to the next step.
When the nut and shaft have cooled then we need to remove the nut from the shaft. Photo 11 will show what the nut looks like when it is properly formed, notice the excess material that has been squeezed out at the ends. The out edges of the two halves are not fused together but the inner portions are fused. Do not worry about them separating, even if you want to pry one open to see how the threads are you will have much difficulty in doing so.
Once the nut is removed we need to chase the threads, as the nut is removed it becomes obvious that the existing threads are much too tight. Using the chase that we previously made work the chase in a few threads at a time, backing it out to remove the material from the flutes. Once you have finished chasing the threads chuck up the cross slide lead screw (holding it by the recessed center section) and work the nut up and down the shaft until it moves with a moderate amount of friction.
Before we proceed with finishing the nut we need to construct the nut housing. It is important that we have moderate press fit into this housing and to achieve that we need to know the exact inside diameter prior to finishing the nut.
The Acetal Nut Housing:
The nut housing is made from a piece steel 1.000” x 1.025” x 1.500”. The following diagram shows the housing and the retainer plate.
Square up and surface the nut material as needed. The height is shown as 1.025”, on my lathe this was maximum that would fit into the trough without interference. When the material is squared up the next step is to drill and bore the center hole. Start by drilling a ½” hole all the way through the block, this hole is centered side to side and the center must be 0.665” down from the top. Next open the hole up to 0.625”, I used an end mill for this operation (photo 14), you could also bore the hole. Note that the 0.625” hole stops 1.400” into the cavity; this leaves a retaining shoulder to capture one end of the nut.
It takes three 6 x 32 screws to hold the retaining plate to the nut housing. I used a bolt hole feature on my DRO to locate these holes. Assuming that you do not have that capability the following diagram will provide the “X” and “Y” coordinates for those holes. Start by positioning your mill over the center of the 0.625 hole, from that location follow the offsets beginning with hole #1 then holes 2 and 3. Using this pattern drill and tap for 6 x32 for 0.500” in three places. Use a spotting drill or machine screw drill bits to prevent “wandering”, tolerances are tight in this area.
When you have finished boring drill and tap a 6 x 1.25 mm hole in the top centered side to side and end to end. Drill the hole all the way through the top and into the bore cavity. After tapping clean out any burrs. To finish the housing mill a step in each side, this provides clearance for the chip guard that rides under the cross slide housing.
While the mill is set up make the retaining plate from 0.125 steel to the dimensions shown in the diagram 3. I started with a small piece of 0.125 plate and after drilling the holes I used a slitting saw to trim it to size, see photo 15.
Finishing the Acetal Nut:
Trim the excess acetal, start by chucking up the nut and using a parting tool remove excess from the ends. Then screw the nut onto the shaft until it reaches the bearing shoulder, using a piece of tubing as a spacer and another piece to protect the threads mount the assembly so that the outside can be turned to size. See photo 16.
Make sure that the shaft is running true, if not then you may want to reverse the shaft so the tail stock center is running in the threaded end of the shaft and the tubing spacer on the right side press against the center. When you have the nut running on a true shaft then we are ready to turn it to the finish size which 0.003-0.005” plus the diameter of the hole we bored in the housing. You may find that the instrument you used to make the hole is several thousands more or less than the target 0.625”. If the diameter is 0.625” then add another 0.004” for a desired OD of 0.629”. Turn the nut to this size, and then remove it from the shaft place it in the chuck and square up one end and then cut a small chamfer on that end.
Now press the nut into the housing with the chamfered end down towards the retainer collar, once pressed into place there should be a tenth of an inch or so of excess material. Remove this excess in the mill, set the cutter about 0.005” above the face of the housing and trim the excess, we want a small amount to protrude and be compressed by the retainer plate. Next install the retainer plate and test fit the shaft; it may be too tight as a result of the press fitting of the nut. If so then use the chase to open up the threads until you have a firm but not tight fit. As shown below I ran the nut up and down the shaft several time until I had a good fit and modest amount of drag.
Assembly and Test Time:
Disassemble the cross slide removing the lead screw, nut, and bearing assembly. While you have the assembly apart and before reassembly clean and lubricate the ways and the shaft bearings. If your gibs need adjustment do that now, the table should move freely without binding and without play. Clean and grease the bearings then reassemble the bearings on the new shaft, add some preload to the bearings and secure the holding nut with the locking screw.
Reassemble leaving the nut retaining screw and the bearing assembly screw loose, crank the table towards you (in the operator position) until it is within 1 ½” of the knob flange. Now tighten the nut retaining screw and the bearing assembly screws and test the movement, if it is too tight loosen one or more screws and shift the bearing assembly in one direction or another and try again. When satisfied crank the table to normal operating position and measure the backlash, it should measure in the low single digits. With a very loose table and a very tight screw thread you can achieve zero but then a loose table and tight screw bring on other problems. What we want is a good balance between backlash, table tightness and ease of motion. By experimenting with the various adjustments you should easily be able reach a point where you have much improvement in all of these areas. My research on this subject left me with the impression that the acetal nut life should be many times that of the original cast iron nut.
Late update on making a new key.
On my lathe my backlash is 0.0015 with about 5 tenths from the shaft key fit. The loose fit resulting from an under sized key was resolved by machining a new key that fits snugly in the 0.125” slot and whose top is lightly filed down to fit the 0.118” slot in the knob. Here are a few pictures of the process.
After milling the brass piece to a width of 0.125 and a length of 0.305 I then filed the corners so that it fit the shaft slot.
I found that adding a few drops oil to the threaded shaft also helps make for a smoother feel. As time accumulates on the new nut I am sure it will continue wear in and become increasingly smoother and probably pick up a little more backlash.
Not Everything Works the First Time:
Here are some examples of the work that did not pan out. The white nut is one of several that I made from PTFE, the material proved to be too soft. Prior to RWey50 showing me how to obtain a thread pitch of 16.6667 TPI I made several screws using 20 TPI which would result in 0.050” per revolution of the dial. This would require a new dial and it would also result in a much slower transit rate. I looked at making a 10TPI which would solve the transit rate issue but the depth of threads would leave very little shaft remaining. Looking at the physical constraints, primarily the trough size, a shaft larger than 0.400” would be hard to accommodate using acetal for a nut.
I hope you find this project enjoyable and useful. If you have any questions or if you decide to tackle this project yourself, please share your experience via the forum.
Jim About the Author
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