knee mill/router hybrid??


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    Default knee mill/router hybrid??

    Hey guys, ok a bit over a year ago I started a small business manufacturing some small wood and plastic objects, I build bottom feeder squonk vape devises. when I first started I knew almost nothing about CNC but I did a lot of research and came to the conclusion that a small cnc router should fit the bill nicely and I winded up buying a Romaxx 2X3 router table. I really wanted to buy a cam master stinger at the time because from all of my research this looked like the best machine in the range I was searching but it was just not in the budget, but seeing as this Romaxx was ~ a $5K machine and it looked pretty beefy compared to most of the other wimpy machines I looked at I figured it should at least be decent, right? yeah, ok :P

    since then I have learned quite a bit more about cnc and about routers and machines in general and I quickly learned that the machine I bought was really not as great as I originally thought. it uses V wheels that ride on steel tracks and even though it at first appeared to be a robust machine with welded steel frame, all ball screw and not some cheap Chinese extruded aluminum type deal, it still is just very flimsy where it really counts. I can actually grab the router when the Z is extended down and literally move the cutter tip back and forth over .25" with just flex alone, and this much flex has just made it impossible to really hold any kind of tight tolerances because no matter how well I try to dial it in, cutting forces can just cause so much deflection. I now learned the hard way that V wheels just suck :P

    being stuck with this machine I did the best I can to make due and I have been getting by with it until now but I am now at the point where I just really want something better. from the beginning I have always looked at both routers and milling machines and of course if I had the money I might have just went for my at the time dream machine which would have been a tormach 1100 at the time, but that was just no where near my budget and still isn't, and I also was really Leary about restricting myself to such a small work envelope compared to even a small router. in the end I came to the conclusion that the only way I'm going to really get the right machine for myself is to just build it myself. I have fabrication and welding experience and I would want to build an extremely rigid and beefy fixed gantry welded steel machine was the conclusion I came to.

    I went out and ordered up some steel and started to design my ultimate machine, I have been slowly collecting linear rails and I just recently bought a 2.2Kw ATC spindle, I planned to weld up a beefy frame and gantry and have it heat treated after and then either have the linear surfaces milled flat or pour an epoxy bed for the rails, I also planned to design the gantry in such a way to be adjustable to fine tune the geometry and get a perfectly square machine that was really 90* on all axis. I do still believe this would be a great plan and know I can build something that would be ideal, but I also now realize that a build like I have in mind is going to take a lot of time and I will def not have this machine done any time soon. lately my Romaxx has been getting worse and worse and I'm at the point now where to continue to use it for a few more months even, I will need to at the very least at least replace all the V wheels just to buy myself some more time so I started AGAIN to look at alternatives...

    I am new to CNC but not to machining, and I actually have a small manual floor model knee mill in my garage. it is a decent machine, made in Taiwan cast iron from the 80's but it the equivalent of the grizzly G0730 or G0678 machines, just from a different importer and a different name tag slapped on it. I have in the past considered converting this machine to CNC but the spindle is very slow, like 2k rpm slow and the bearing growl whenever I even get close to that kind of RPM, I just run it very slow, not at all ideal for the mainly plastics now that I work with. well in my deliberations and options searching and trying to think of a solution to my situation a light bulb finally went off over my head, what if I used the base and the frame and XY table all from the knee mill, but replace the head with my new 2.2Kw ATC spindle and mount that on a pair of linear rails for the Z axis...

    the more I think about this idea the more I am liking it, I think this machine can really give me a great end result, the X and Y are very solid and are nice and co-planar to each other already and very rigid compared to anything in the router world really, and now with the 15-30K rpm ATC spindle, it would be awesome for the work I do. I can build the Z axis with only ~6-8" travel and make it extremely rigid not having to worry so much about weight since it is fixed to a relatively heavy cast iron mill column, and having a knee that can manually be raised and lowered means I can use whatever vises or fixtures or even 4th axis I could ever want even with only 6-8" of Z stroke because I would have another 17"+ that I could raise or lower the table too, something no router table could ever do. this might actually be the best of both worlds for me

    this is the machine I have more or less without all the variable speed head electronics:




    one pretty cool thing is the head is a turret style and can actually spin around 360*, so my plan is to actually spin the whole head around and replace the rear motor mount with the new Z mount and ATC spindle, this will actually allow me to keep both the low speed head for steel, and then rotate it 180* and switch to the high speed spindle when I need to. since the stock spindle is belt driven I can easily just mount the existing spindle drive motor above the existing spindle like a bridgeport style head, giving me 2 completely different heads 180* from each other, on a turret that only needs to loosen 3 bolts to spin it around. I can even build in positive stops to quickly relocate it to the same co-ordanats with each head swap so no re-dialing in and re-tramming making swapping from the slow spindle to high speed as painless as practical



    knee mill/router hybrid??-g0678_det2-eb55892120fb36c10c10e203237bc096-jpg

    now I know this is not the end all/ be all when it comes to CNC knee mills and it is not a bridgeport or other really heavy duty CNC mill, but remember I am comparing it to a router table here and my main use is CNC'ing plastics mainly, although with this turret head I can actually wind up still being able to do even steel when I eventually remount the motor and add a servo to drive the quill on the stock head side of the turret. the 8" X 18" work envelope is a bit smaller than I would have ideally liked but the items I build are only 2" X 3.5" in size so I can def manage with eve this smaller work envelope given the trade off of much more rigidity than I would get with most router table designs.

    so, thoughts? I am going to have a few questions about the actual conversion and I would have liked to start a build thread once I get this underway but this is such an odd ball hybrid that there really is no ideal place for me to put this thread, sure as heck couldn't put it in the router build forum, so here made the most sense to me I think.

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    Attached Thumbnails Attached Thumbnails knee mill/router hybrid??-g0678_det2-eb55892120fb36c10c10e203237bc096-jpg  
    Last edited by turbocad6; 01-08-2018 at 05:02 PM.


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    Default Re: knee mill/router hybrid??

    just for comparison, this is the machine I am currently working with with a porter cable router mounted on it, not rigid at all,l but to a newbie it looked good to me at the time I guess

    knee mill/router hybrid??-wd3dpic1-540x481-jpg

    and this is a rough mock-up of what I was planning on building as my ideal machine before this knee mill hybrid idea entered my head. it would have had added gussets and supports welded in too and cross beams below the deck and I think it would have made a nice machine actually. would have had a 20"X20" work envelope and the 2 dual 6" vises would have been semi-permanently mounted, could clamp a flat plate above them for flat work if ever needed. I haven't completely ruled out eventually building this but the more I think about it the more I think the knee mill with router head may actually be all I will need so this now goes to the back burner/maybe someday area of my head. worse case the new knee mill conversion can help me build this new machine eventually too if I ever decide that it is still necissary for me to build it

    knee mill/router hybrid??-img_1631-2-jpg

    knee mill/router hybrid??-img_1636-2-jpg

    and here is the spindle I plan on mounting on the knee mill. I also have a pair of extreme overkill linear rails that I will mount to a 1" thick steel plate for the Z, to keep the Z axis as rigid as the rest of the machine. pictured is just one of the rails but I have a pair of these, they are HRW35's and they are massive, just the thought of going from the flexy wet noodle of a machine I am using now to this has got me pretty excited and it really shouldn't be too hard or take to long to build either I think

    knee mill/router hybrid??-img_1695-jpg

    Attached Thumbnails Attached Thumbnails knee mill/router hybrid??-img_1631-2-jpg   knee mill/router hybrid??-wd3dpic1-540x481-jpg   knee mill/router hybrid??-img_1636-2-jpg   knee mill/router hybrid??-img_1695-jpg  



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    Default Re: knee mill/router hybrid??

    in the future if I ever get ambitious I could also convert the XY table of this knee mill to liner rails instead of the cast iron ways it's on now too, but for now I think the ways will be ok, they are in great shape showing no real wear and I will add an oiler to it when I convert it to ball screws too. I did consider doing this from the beginning and extending the new router head a few extra inches from the column to increase the work envelope but at this point that would just complicate things and make this also into a much longer project too, and kill off much of the benefits of already having everything already perfectly aligned and square as it already is right now, buy utilizing the existing XY ways and table setup as-is this conversion should be relatively quick and simple for now.

    I am a little concerned about stick-slip action from the ways on very small moves but there are so many other cast iron way machines out there like the tormachs and most other medium sized mills that do just fine with cast iron ways still, so for now I think they will do just fine. I do really prefer linear ways of course but that could be done in the future, as well as maybe adding a higher horsepower much better columbo spindle, the room for growth with this machine will still be there later if I want to take it even further but even as it is right now I think it will be a nice and versatile machine by just adding ball screws to the X&Y and adding that high speed ATC spindle with the heavy liner rails for the Z, should be much more stout than any average router table at least, thats for sure



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    Gold Member Jim Dawson's Avatar
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    Default Re: knee mill/router hybrid??

    Sounds like a great idea. Unbolt the mill head, and bolt on the high speed spindle / Z axis in its place.

    Buy a ball screw / CNC kit for that mill, and you're good to go. A pretty easy conversion

    Don't worry about the slip/stick, it will work fine. There are thousands of machines that work that way.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: knee mill/router hybrid??

    instead of unbolting the mill head I'm going to leave it right there and rotate the turret 180 degrees, rotate the existing spindle head to the rear of the machine where the motor is mounted now and then remove the existing motor and make my new spindle mount where the existing motor mount is, then I can actually have a turret head that can switch between the milling head and high speed spindle by just rotating the whole top turret head 180*. after I will remount the existing motor to belt drive the existing mill head above the existing spindle like a bridgeport, it will be pretty easy and it will then have the best of both worlds, milling head for steel and high speed spindle for aluminum and plastic work

    only thing is, no one makes an off the shelf conversion for ball screw for this machine so I will have to do that myself, only thing to deal with there is the end mounts and ball mounts and grinding a little clearance for the new ball screw, pretty straight foward like most any other conversion really I think



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    Gold Member Jim Dawson's Avatar
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    Default Re: knee mill/router hybrid??

    Sounds like you have it figured out. It should work out fine.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: knee mill/router hybrid??

    ok so I just ordered 3 X 2520 NSK compact FA series ground C5 ballscrews for this thing, the space in here is tight for ballscrews with the thinner XY table and since it wasn't really designed for ball screws in the first place this should minimize the amount of extra clearance machining i will have to do to fit them. gonna run them with 3:1 belt reduction, should be able to hit over 600ipm rapids I think and still have very high resolution with the servo's I have I think. resolution will be .00006" per pulse if my math is correct. should be very low backlash with these screws too I hope, I didn't want to go with cheaper rolled ball screws and I really wanted the 20mm pitch so I can multiply the torque of my servos by 3X because they are a little small for this machine, so ground screws were my only real option for this kind of screw pitch, but with the gear reduction they should work real nice I hope now my biggest problem is once I take this thing apart I'm not going to have a mill to build the parts and machine what needs to be machined. I will at least build my Z axis before disassembling this thing so I can at least use this machine for that before taking it apart I guess.

    I hope I did my ball screw calculations right, I don't know for sure, I downloaded a calculator but don't really understand how to use it correctly so I hope I choose the right stuff here

    my servo's are 492oz peak torque, 100oz continuous and 6,400 pulses per revolution clear path servo's. the RPM is rated at 2,390 rpm but max rpm is 3170. I'm thinking a 3:1 belt drive should be about right but I'd def appreciate any input here if I screwed up or if this should work ok? I guess I could go 4:1 if I have to also, I'm not really sure but I'd like to have some good speed with this thing too, any input on this would be greatly appreciated. if I have to replace the servo's later I guess I could but I'm hoping it should at least be usable with the servos I have for now




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    Gold Member Jim Dawson's Avatar
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    Default Re: knee mill/router hybrid??

    Nice ball screws. Is there enough room under the table to stuff in a 25mm ball nut? That's the same size ball screws that are on the Shizuoka AN-S sitting in my shop which is a 4500 lb, 10x50 machine. Recommend you handle all of the thrust on the motor end of the table let the other end of the ball screw float in the support bearing/housing. Works well.

    Overall I think your design will work fine, good choice on the Clearpath motors. I did a retrofit for a customer using Clearpath motors and was very pleased with the result.

    600 IPM rapids on a small machine like that would be positively scary. You are probably going to have to tear the mill down twice. Once to get all the measurements, and then again to install the new parts. Think through every operation before you do it, time spent on planning is time well spent in a project like this.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: knee mill/router hybrid??

    thank you, that is reassuring. the compact FA series ball nuts are relatively slim, I'm pretty sure I can get them in there without a problem. the Y is not really much concern at all I have room to drop that screw down slightly even. it's the X that will be tight but the existing lead screw is 1" now so the ball nut itself is the only concern. I have already measured it and I'm pretty sure it will be fine, I think I will have to recess about 8mm deep semi circle into the saddle for clearance of the nut itself if I hang the flange off to the side of the saddle. shouldn't have to give up much travel at all doing that I think.

    yes I got these screws with one fixed side for the drive and floating bearing on the other end. the Y and Z probably won't even need a support on the floating end I think. luckily I can measure most things and make them before hand, the main thing I can not machine with this machine itself is the saddle and the table itself if it turns out I need to machine anything on the table underside or end, and of course the motor mount plate.

    one thing I'm a bit concerned about now is, my servo's are only .25" shaft and the timing belt choices with a bore that small are pretty limited, so far I have only been able to find an MXL pulley that will work but with a pretty narrow 3/8" belt. I would much rather use a gt2 or gt3 belt drive or something so I don't have any backlash in the belt drive and something a bit wider too but I can't find anything suitable with a small enough bore for the 1/4" shaft. the screw end is 20mm so no issue on that end but the drive pulley from the servo is looking to be a bit of an issue. not sure if trying to use a bushing or something is a possibility here.



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    Gold Member Jim Dawson's Avatar
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    Default Re: knee mill/router hybrid??

    Bushing the pulley bore is perfectly acceptable.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: knee mill/router hybrid??

    nice, that should open up my possibilities here, I imagine I would need a bushing with a bore for a longer setscrew to pass threw it to meet the motor shaft itself, right? is that how something like this is done?



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    Gold Member Jim Dawson's Avatar
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    Default Re: knee mill/router hybrid??

    Yes, you might need a bit longer set screw. The normal way of doing this is to press the bushing into the pulley bore and just drill through the the set screw hole(s) with a tap size drill then run a tap through. Then ream the bore to size, for 1/4 shaft I would ream 0.0005 oversize to get a tight slip fit. Measure the motor shaft to confirm the actual size.

    Even a bronze bushing would work fine, you can buy them in standard sizes.

    Jim Dawson
    Sandy, Oregon, USA


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knee mill/router hybrid??
knee mill/router hybrid??