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Thread: (Another) LMS SX2 Mini Mill CNC Conversion

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    Default (Another) LMS SX2 Mini Mill CNC Conversion

    Hi, hope there's room for yet another SX2 CNC conversion thread. I bought the LMS Hi Torque 3900 mini mill over a year ago and am very happy with the machine. I always wanted/planned to convert it to CNC, but with no machining skills (at least not metal, lots of woodworking) it took a long time to learn the basics and get enough tooling in place to do anything serious. After reading and learning from the many build threads here, I also need to thank Hoss, DJBird, and many others on this forum--without their excellent work I wouldn't have known where to start. Anyway, this is going to be a sort of "catch up" thread as I am pretty far along on the build. I didn't do very well on documenting while I was doing the build, so I've been doing some catch-up, taking a lot of pictures and will cover what I've done to date, and then to completion of the project. The picture is of my mill, when it was brand new and clean, mounted on it's dedicated bench/flood table.

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    Attached Thumbnails Attached Thumbnails (Another) LMS SX2 Mini Mill CNC Conversion-img_4861s-jpg  


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    Default Bench buildup

    The first thing I did after getting the mill (and doing some playing around of course!) was to build a solid bench. Probably jumped the gun but I decided to build a flood table from the start. I almost didn't allow enough room side-to-side to allow for the X-axis stepper, but it looks like it will work out. The finished product will have a angle aluminum uprights and 24" plexiglas sides. The bench is pretty sturdy, with a 1.5" plywood top (two layers of 3/4" plywood) on 2x4 and 4x4 frame. The outside was eventually skinned with 1/2" plywood. The pedestal for the machine is placed offset to the left to allow for the x-stepper motor. Once the pedestal was in place (made of several layers of 3/4" ply), I located and drilled the drain holes and added the stringers and masonite panels to make the sloped surfaces. The top was then covered with overlapping flashing aluminum to provide a sturdy surface. The aluminum was secured with contact cement, and clear silicone calk along the seams. I have yet to run a drop of coolant even though the pump and lines have been run, but hopefully soon! The attached show some of the table construction details and the nearly finished product. I later added the remaining sheet and some aluminum angle around the border to form the bottom for the eventual side walls.

    Attached Thumbnails Attached Thumbnails (Another) LMS SX2 Mini Mill CNC Conversion-img_4784s-jpg   (Another) LMS SX2 Mini Mill CNC Conversion-img_4787s-jpg   (Another) LMS SX2 Mini Mill CNC Conversion-img_4789s-jpg   (Another) LMS SX2 Mini Mill CNC Conversion-img_4788s-jpg  

    (Another) LMS SX2 Mini Mill CNC Conversion-img_4822s-jpg   (Another) LMS SX2 Mini Mill CNC Conversion-img_4856s-jpg  
    Last edited by mduckett; 03-21-2013 at 07:02 PM.


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    Default Development plans

    Ok, with the machine mounted on a suitable work bench, the next thing was the CNC development plan. A primary goal was to do as much of the mill work on this machine for the conversion as possible. Keeping that in mind, here is the rough CNC conversion design plan:

    1) Use the proven X,Y ball screw conversion demonstrated by Hoss’s X2 and many others subsequently. Since the saddle needed to be modified to fit the x-axis ball nut, this would require a spare part to be purchased in order to do that machining on this mill.

    2) For the Z-Axis, I went back and forth many times trading off the CNC fusion side-mount vs. a more centrally located lift point design for the Z-axis ball screw. I’ve seen a couple of centered designs; one uses a small diameter ACME or ball screw with the nut mounted inside the carriage approximately where the pinion drive shaft normally passes through. The other popular centered design is the StirlingSteele and Hoss rotating driven ball nut that secures the screw to the top of the carriage and uses a combined drive pulley, ball nut and bearing to move the carriage. The centered design finally won out as I really prefer the look and mechanical balance it seems to afford. This isn’t a knock on the cncfusion kits, PLENTY of those out there working fine, just my personal choice. From the two center mount choices, I decided to attempt the embedded ball nut approach, figuring that if I failed I could always fall back to the other proven design. Either of these designs is complicated by the far back positioning of the spindle motor near the column. In order to modify and make parts for the carriage, I would also need a spare carriage part.

    3) Identify the basic mechanical components such as ball screws and nuts,
    belts, pulleys, motors, and shaft couplers and sources for each.

    4) Develop some CAD 3D models to assist in designing the mechanical parts and selecting specifics of the mechanical parts such as bearings, etc. for the X,Y and especially the Z axes. Develop shop prints (well, not really, but good enough for the garage shop) to make the parts.

    5) Identify the necessary tooling. After I bought the mill, I had a pretty strict budget for tools so I carefully selected a number of key components to stay within budget but still give good functionality. The essentials were: 3” vise, R8 collet set, end mill set, 115 piece drill bits, boring head and bars, 6” caliper, 1” micrometer, 1” dial indicator, .030” test indicator and holder, and drill chuck as the LMS mill doesn’t come with one. Other tooling needed would be purchased along the way to spread out the costs.

    6) Electronics -- Identify motors and controllers, breakout board(s) and a computer to host Mach3. After searching and comparing prices, I narrowed the suppliers for these components down to either Keling or Probotix. Both have good offerings, but in the end I like the bipolar drivers and hybrid motors from Keling a little bit better. I still plan to buy a relay board to control flood coolant pump from Probotix, they have some nice options. I have an older but capable laptop with a docking station with a parallel port so that will be the Mach3 host.

    7) Additional Considerations—Most people with the tilting head mills have added some sort of column support to add some rigidity to the system. My plan added a length of 4” steel C-channel to the back of the column to boost its cross section, and also to add some triangular gusset braces between the column and rear base castings. Another area for consideration is the completion of the enclosure, the access openings, placement of the electronics controller, and the wiring scheme, including limit switches.

    So that's the plan, much of this has been completed so I'll start posting the design and build details next.



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    Default Z-Axis – Spindle Carriage Modification

    I decided to tackle the hardest part of the build first, namely the Z-axis. To get started with the effort I purchased the replacement part for the spindle carriage, i.e. the part that has the dovetails and is bolted to the spindle box. Fortunately LMS stocked this part for $65+shipping. Wished it were less, but I couldn’t proceed without it so ordered it and had it in a few days. The first order of business was to clean up the inner casting outlines at the back side opening (dovetail side) so that an insert would have straight edges to fit into. Also worked in from the wider opening (front) side to clean up the shoulder features and basically flatten and straighten up things a bit. Because the casting inner walls aren't parallel to the outside, no attempt was made to do anything with the sides, just the bottom areas where an insert block would be made to fit down against those surfaces. Once the part had at least some flat and parallel areas to work with, careful measurements of the features were made a 3D CAD model was developed. The column was also modeled and by placing the two models together the available spacing and design of the ball nut carrier were much easier to visualize. A 3D model for the ball nut was located on the RBS website, and that allowed orienting and positioning the ball nut for the best fit in the available space. Since there is a 16mm hole in one side for the rack shaft, that was convenient to use as the method to hold the insert. The hole on the opposite side was to be enlarged to match, and a pair of 16mm shoulder bolts would serve as the main fasteners for the ball nut holder. The smaller hole located forward and down would be re-drilled and copied on the other side to provide a second attachment point using some ¼”-20 button head screws.
    Using the 3D CAD features, the amount of space available for a ball screw could be accurately estimated, and a model of a shaft opening was developed into the part. The attached rendering gives an idea of what the model provides. I didn't do a complete model, only as much as was needed to determine if it was going to work, and what would need to be done to make it work. Playing with the model, it was clear that while there is almost enough room for a 5/8" ball screw to pass behind the spindle motor, the bearing block to support it would need to be located at least the height of the motor (6 - 8") above the top of the column in order to allow the motor to not interfere with the bearing at the top of its travel. That wasn't an attractive solution, so the unavoidable solution was to move the motor forward. This turned to be simpler than expected, as the modification only required about 1/2" of movement to get the needed clearance.

    The attached pictures are of the last model updates including the Z bearing block, which will be covered in detail later. If you look closely at the wireframe you can just see the ball return tube of the ball nut model.

    Attached Thumbnails Attached Thumbnails (Another) LMS SX2 Mini Mill CNC Conversion-z-axis-assembly-1-jpg   (Another) LMS SX2 Mini Mill CNC Conversion-z-axis-assembly-2-jpg  


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    Default Z-Axis Carriage Machining

    These pictures show some of the final work being done on the spindle carriage to cut the ball screw pathway through the carriage and the baall nut insert at the same time using a 3/4" ball end mill. The carriage was aligned to the x-axis of the mill using a dial indicator along the dovetail. The aluminum insert/ball nut carrier can be seen fitted into the middle. I'll show more details for the ball nut carrier in the next post.

    Attached Thumbnails Attached Thumbnails (Another) LMS SX2 Mini Mill CNC Conversion-img_5640s-jpg   (Another) LMS SX2 Mini Mill CNC Conversion-img_5642s-jpg   (Another) LMS SX2 Mini Mill CNC Conversion-img_5643s-jpg  


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    Default Z-Axis Ball Nut Carrier

    These pictures show the completed ball nut carrier insert and the ball nut fitted into the carrier. To make that carrier was a lot of work. The first part involved fitting a chunk of aluminum into the carrier housing. I began by marking out the areas and depths that would need to be removed to clear the shoulders in the bottom of the carriage. These were just approximated and then many, many test fits and removing of material until it fit down into the opening and flush with the back surface. Once that was complete, the shoulder bolt holes were drilled and tapped to 10mm x 1.0mm. The insert was installed into the carriage with the shoulder bolts (giving an appropriate Frankenstein look), to get ready for the ball end mill cuts to make the ball screw shaft as shown in the previous post. When this was completed, the cut out for the ball nut was made with the same setup. Here a 1” 5-flute end mill was used to make a 1” channel centered on the ball screw channel, and 1” deep, with length equal to the length of the ball nut. Then the insert was removed and rotated upright so that the 1” cutter could cut down from the top side into the channel to square up the end of the channel. When completed, this left the inner corners of the ball nut channel with fillets in the bottom corners that needed to be removed. I used a smaller end mill to remove more of the material, and in the end just scraped and chiseled out the remaining bits of material. A sharp 5/16” HSS lathe tool bit shaped roughly like a chisel was used to do some of the work. Finally, since the ball return tube faces downward, an end mill slightly wider than the ball tube was used to make a channel for that feature, and also made some plunges to accommodate the screws and the metal clamp. It just took a lot of patience but finally got it done. By this time I was wishing for a CNC machine to make this part! If I did it again, I would simply drill 1/8” holes centered in the corners for the ball nut to eliminate the need to make sharp corners.

    Attached Thumbnails Attached Thumbnails (Another) LMS SX2 Mini Mill CNC Conversion-img_6170s-jpg   (Another) LMS SX2 Mini Mill CNC Conversion-img_6171s-jpg   (Another) LMS SX2 Mini Mill CNC Conversion-img_6176s-jpg   (Another) LMS SX2 Mini Mill CNC Conversion-img_6181s-jpg  

    (Another) LMS SX2 Mini Mill CNC Conversion-img_6179s-jpg  


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    Default New Spindle Motor Plate

    As previously discussed, the spindle motor needs to move forward by about ½” to accommodate the ball screw and to clear the Z-axis bearing block. It turns out that there is enough room in the drive belt cover cutouts to accommodate the movement without any other changes, so I decided to make a new motor mount plate that would continue to use the rear two holes and would essentially stretch forward by ½” , thereby requiring only two new holes to be tapped closer to the drawbar opening. There is a large round cutout in the middle of the steel motor plate, otherwise it is a pretty simple part. I had access to a 6” rotary table, so that provided a solution for making the big hole. The steel thickness is a little less that ¼”, so it was easy to buy some ¼” x 4” steel for the raw material. After cutting a piece to length and facing both sides down to the nominal thickness, the layout consisted of using Dykem and then tracing the old motor mount at one end, including the large hole, then moving it to the other end and tracing the motor mount slots, and ready to machine. The mounting slots were cut first, then the plate was put on the rotary table. I used a conical center finder to center the mill in Y first by picking up the scribed arc and dividing the distances and cranking the to the center in Y and locked the axis. The X-axis was cranked to pick up on the scribed circle, then moved back by the diameter of the cutter (3/8”). There is a small amount of clearance for the motor so it wasn’t super critical. I did at one point early on move back by 7/16” and used that sized end mill to plunge completely through the part. Then the table was moved back and the 3/8” end mill was used to finish cutting out the hole. The larger hole allowed for a place where the movement could stop without generating chatter (allowed for clearing off the chips) and where the end mill could be lowered into the hole before starting the next cut. With a stiffer machine it might make no difference, but it seemed like a smoother operation on this machine using the relief hole.
    I installed this plate and used the mill with the shorter spindle drive belt with no problems. The shorter belt is a HTD5-330-15 that I got from Timing Belt Pulleys Taper Bushings Timing Belts Belt & Chain Tensioners Tension Testers Diametral Pitch Gears Gearheads. Later, when I disassembled the machine to do the final CNC installation, my machinist friend (actually his son) used a ¾” end mill to make the cutouts in the end of the motor plate and the pulley cover to accommodate the ball screw pass-through. I need to acknowledge my machinist friend RT, he has always been patient and quick to help out with expert advice and some occasional machining to support this project—Thanks RT! (and son!).

    Attached Thumbnails Attached Thumbnails (Another) LMS SX2 Mini Mill CNC Conversion-2012-05-12_14-12-43_873s-jpg   (Another) LMS SX2 Mini Mill CNC Conversion-img_6197s-jpg   (Another) LMS SX2 Mini Mill CNC Conversion-img_6194sss-jpg  
    Last edited by mduckett; 12-02-2012 at 01:50 AM. Reason: Added motor plate comparison picture


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    Great build! Great Pictures and I love the bench!



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    Quote Originally Posted by BAMCNC.COM View Post
    Great build! Great Pictures and I love the bench!
    Thanks! Lots more to come.



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    Default X, Y Axis Bearings and Motor Mounts

    By this time I wanted to get started on the X and Y axis ball screw details, but I couldn’t start working on those until the X,Y bearing details were figured out. I spent a lot of time trying to decide on the axis bearings, and had even bought some of the thrust type bearings thinking at one point that I had it settled. Then I saw where a lot of setups were using angular contact pairs. In the end, I got realistic, stopped over thinking it, and settled on using double row angular contact bearings. These have the additional advantage of being sealed units too. I bought a bearing from vxb.com to measure for use in the CAD design. The unit is a 5200-2RS (10mm x 30mm x 14.3mm) double row angular contact bearing. Although not rated as high for load carrying capacity as a pair of ACs, these seemed to be adequate in my estimation for this size machine and the forces it will generate on these axes. With that finally decided, the designs for the bearings were pretty easy. The Y bearing block assembly is basically a modified Hoss design with a spacer and the bearing block. The X bearing block assembly consists of a new table end piece with the bearing block attached. The bearing blocks consist of two pieces: a bearing holder and a lid. Both the holder and lid are designed with recesses for shaft oil seals, although since the bearings are sealed these will probably not be used. This is a typical design that you will see described at 5bears.com and other places. This design calls for the bearing recess to be a couple thousands shy of the true bearing width to allow the lid to apply some load to hold it in place. Shims can always be used if the machining doesn’t accomplish this so no worries there. The motor mounts were inspired by djbird’s design that uses two flat plates for the motor mount. This gives a nice appearance of a one piece design and also provides some motor heat sink as djbird mentions. The attached are some of the CAD drawings that show the general design and a rendering of the Y axis. I couldn’t find the X axis rendering but you get the idea. In the next section I’ll cover how these were made.

    Attached Thumbnails Attached Thumbnails (Another) LMS SX2 Mini Mill CNC Conversion-y-axis-bearing-assembly-jpg   (Another) LMS SX2 Mini Mill CNC Conversion-y-axis-bearing-assembly-3d-jpg   (Another) LMS SX2 Mini Mill CNC Conversion-x-axis-bearing-assembly-jpg  


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    Nice work! I'll be reading along..... superb so far!

    Q: How many tools does it take before a simple task becomes a project?
    A: Just one. I'm the Tool that turns a simple task in to a project.


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    Quote Originally Posted by TroyO View Post
    Nice work! I'll be reading along..... superb so far!
    Thanks TroyO!



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