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IndustryArena Forum > MetalWorking Machines > Benchtop Machines > Harbor Freight Mill CNC Conversion?
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  1. #1
    Join Date
    Jun 2013
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    4

    Harbor Freight Mill CNC Conversion?

    Hi Folks,

    I'm looking to gather some opinions about a CNC conversion I'm considering - I've recently ordered and received a rather nice Harbor Freight mill (Milling / Drilling Machine - 1-1/2 HP). It's actually quite a dandy for the price and I'm seriously considering what it would take to get some stepper based control on it. For the sake of a starting point, here's what I've compiled thus far and where my thoughts are headed - any feedback or direction would be greatly appreciated (I'll be sure to post the results of the build when completed if I do elect to go this path).

    Thus far, based on quite a lot of reading, a self-imposed budget constraint and pouring through some technical information I've located the following stepper motor package. (Free SHIP to US 3Axis Nema34 Stepper Motor 880oz in Dual Shaft Control CNC Mill | eBay). 256 microsteps should yield 51,200 steps per revolution, which is quite decent resolution (although I have read the feedrate can suffer, this is acceptable for my application).

    My Concerns and areas I could use some guidance:

    1. Mechanically, does anybody have any experience with this mill or any like it? Should I expect to have significant tolerance issues with screw backlash or general play in the table without upgrading to ballscrews? The lead screws are 10 TPI.

    2. My experience with CNC controls is with purely with Fanuc - servos as we all know are more capable, more accurate (semi-closed or closed loop typically) and just generally better in every way than steppers. That said, the Utopian world in which I throw Fanuc Beta motors on my mill is unfortunately not this one, my concern is the fact that the stepper system will be open loop. Two parts to this one - is there any reason I shouldn't be concerned with this and if I should be, does anybody have any thoughts as to how I could upgrade stepper motors to semi-closed feedback? (I intend to be controlling the mill with Mach3).

    Anyway, I really appreciate the feedback folks - thanks for your time!

  2. #2
    Join Date
    Jan 2005
    Posts
    1943
    I had a harbor freight round column mill that I converted. Mine wasn't as big as yours, but you can get an idea by looking at my thread http://www.cnczone.com/forums/bencht...ill_build.html The biggest problem with a round column mill is that you essentially only have the quill travel for cnc which limits you in terms of tool changes and such. You have to be creative with setup and tool usage. With that said, mine worked well and I got some good use out of it. I have since switched over to a g0704 dovetail column machine.

  3. #3
    Join Date
    Jan 2005
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    1943
    Had to go ahead and post so I could look at your link to the steppers. Those steppers are 15 mH inductance which means they really want about 120volts. They are not a good match for the 60 v power supplies or the drivers in that kit.

    As for the lead screws, you will lose LOTS of speed and torque just turning the screws compared to ball screws. On top of that running around under motor power is going to wear the nuts fast and you will be constantly readjusting the backlash. Now f you are talking about using the stock screws just to get cnc for making better parts then that is another thing. A temporary cnc setup using the stock screws can be done and would be a real benefit for making conversion parts for the ball screws. I would not count on running the stock screws for long though.

  4. #4
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    Jun 2013
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    Quote Originally Posted by 109jb View Post
    Had to go ahead and post so I could look at your link to the steppers. Those steppers are 15 mH inductance which means they really want about 120volts. They are not a good match for the 60 v power supplies or the drivers in that kit.

    As for the lead screws, you will lose LOTS of speed and torque just turning the screws compared to ball screws. On top of that running around under motor power is going to wear the nuts fast and you will be constantly readjusting the backlash. Now f you are talking about using the stock screws just to get cnc for making better parts then that is another thing. A temporary cnc setup using the stock screws can be done and would be a real benefit for making conversion parts for the ball screws. I would not count on running the stock screws for long though.
    Excellent point on the steppers - that's exactly the type of feedback I was hoping to get, thank you 109jb. I had a brief thought about running two of the power supplies in series (alas current considerations), but I'd just prefer a kit with adequate hardware if I'm going to take the dive. I don't suppose you might have a recommended stepper kit that may suit this application?

  5. #5
    Join Date
    Mar 2004
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    1804
    I fully concur with 109jb. I converted the same basic machine years ago and started out with the stock screws. Within about 2 months, the nut on the X axis was so worn that I could literally "walk" the nut down the screw without turning anything! And this is back when the machine they sold was a genuine Rong Fu!
    As to using 256 microstepping, from what I have read, anything beyond about 16 microsteps is counter productive. And according to Marris (Mr. Gecko) you should only plan on a resolution based on no better than I think it is 1/2 stepping. The microstepping is there to handle resonance problems.
    All in all, I have been very happy with my round column machine since its conversion back in about 2000,
    Art
    AKA Country Bubba (Older Than Dirt)

  6. #6
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    Jun 2013
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    Quote Originally Posted by Bubba View Post
    I fully concur with 109jb. I converted the same basic machine years ago and started out with the stock screws. Within about 2 months, the nut on the X axis was so worn that I could literally "walk" the nut down the screw without turning anything! And this is back when the machine they sold was a genuine Rong Fu!
    As to using 256 microstepping, from what I have read, anything beyond about 16 microsteps is counter productive. And according to Marris (Mr. Gecko) you should only plan on a resolution based on no better than I think it is 1/2 stepping. The microstepping is there to handle resonance problems.
    All in all, I have been very happy with my round column machine since its conversion back in about 2000,
    I was worried about this, did you have a difficult time installing ballscrews? To be completely honest, that's the most apprehensive part of the conversion for me and also the reason I'm taking my sweet time compiling information and being absolutely sure before I do anything. (It's a great manual mill right now and I'd be really upset if I ruined the table and turned it into an expensive drill press!)

    On a sidenote: Given that the leadscrews are intended to be run manually and relatively slowly (maybe 100 cranks per minute, or 10 inches on my table), does anybody see a reason the leadscrews and nuts wouldn't hold up to a 10IPM feedrate with steppers?

    Thanks again for your feedback, I really do appreciate it!

  7. #7
    Join Date
    Mar 2004
    Posts
    1804
    Anappa,
    I used a 5/8" ball screw and besides not knowing what I was doing, didn't really have a problem.I spent a lot of time cadding things up and trying to get it in my head before I made any parts. One of the main things I learned was that the bolt holes are NOT in the same location on all of these machines. In other words, measure your machine to make sure parts will fit!
    My old web page is still up at my previous ISP, so you might get an idea or two from it:
    My CNC Homepage

    10IPM will get real old real quick! I cut aluminum faster than that at times! My best advice is to put your steppers/servos on the machine and use it to make the parts necessary to convert to ball screws. As previously iterated, I wore mine out in about 2 months and I did not really run it fast and had installed a one shot oiler.
    Art
    AKA Country Bubba (Older Than Dirt)

  8. #8
    Join Date
    Jan 2005
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    1943
    When I. Converted mine I disassembled and measured carefully and planned out the ball screw conversion. I did not modify any stock parts. I made all new ones and if I wanted to could have converted the machine back to its stock manual configuration at any time.

  9. #9
    Join Date
    Feb 2011
    Posts
    3

    Converting HF RF31 to CNC

    For converting a RF31 to CNC are there any drawings, guides, parts lists, etc? I'm having problems understanding "where to start"?

  10. #10
    Join Date
    Jan 2005
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    1943
    Quote Originally Posted by melvinstanley View Post
    For converting a RF31 to CNC are there any drawings, guides, parts lists, etc? I'm having problems understanding "where to start"?
    For the X and Y axes, the conversion would be pretty much the same as any other benchtop mill. There are tons of threads to give ideas on that part of it. What you need to do is take the table and Y-axis slide off the mill and look at how the stock lead screws work and how they are mounted. Some guys just bore the stock lead nut and mount a ball nut directly to that. If you look at my thread that was linked in post #2 it will give an idea of how I did it on my Harbor Freight mill which has similar mounting for the lead screws and nuts when compared to the Rong Fu.

    For the Z-axis, you have a choice to make. Being a round column machine you can either move the quill using the stock fine feed or you can build a separate drive to move the quill. The easiest way is to use the stock fine feed, but you will need to reverse the quill return spring. The reason for this is because tools tend to pull into the work so you want the quill at the bottom of the slop in the gearing. The stock quill spring raises the quill which is opposite of what works best. Reversing the spring so that it pushes down on the quill works pretty darn well. The drive for the z is just hooking a motor up to the fine feed shaft. Again, my Harbor freight link shows how I did it and something similar would be doable on the Rong Fu.

  11. #11
    Join Date
    Feb 2011
    Posts
    3
    Thank you for the information. I now understand why the ball screw conversion needs some plotting and planning. One other question and then I'll try to be silent: The mounting and housing for the stepper motors, what do most people so about that?

    Thanks
    Mel Stanley

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