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Mini Mill Spindle Dropping

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domperna52

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I recently got a Harbor Freight Mini Mill (branded Speedway, but it looks like a Sieg X2D) fairly cheaply at a resale shop that cleans out houses and estates. There were several items missing that I needed to purchase to get it back to operating condition. That done, I used it to make some t-nuts for the table as a small "get familiar with the machine" project. I noticed that the Z-axis has a lot of backlash due to the rack and pinion set up and fine adjustment arrangement, and that if you are not careful the spindle assembly could have an unwanted drop. This could gouge and possibly ruin a part being machined. I did some research on the problem and found several very helpful tips to help eliminate excessive backlash which I am currently incorporating (shimming the rack, remaking the handle key, etc). While I was working on these items I had a thought that part of the problem was that the spindle head wasn't completely counter-balanced by the torsion spring/arm arrangement, and that a little more spring tension might keep the head in place even when the lock is released. I disassembled the counterbalance spring assembly to see if there were any kind of adjustments similar to a drill press. There was no easy adjustment possible. The fixed end of the spring fits into a hole through the assembly into the main column. There was enough room to drill a new hole about 45 deg. counter clockwise from the original that would provide a little more wind up. So I moved the fixed end of the spring into the new position and reassembled everything. It takes a little more effort to move the spindle assembly down in fine adjustment and drill press mode, but when I set a head position and release the lock, the head stays put. I just thought I would pass this along since I didn't see it in the research I didn't know if anyone had tried this approach. Hopefully it might help someone out. Thanks.
 

cbellanca

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My knee mill has the same problem. I'll have to look into as you did. Thanks for the info.
 

hman

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Head positioning is a known problem with these mills. The monkey-motion used to position the head has a large number of links, each of which can contribute to backlash. And the spring arm changes tension with position, as you wind the spring while moving the head up and down. I have an HF 44991, and here are the things I've done to help alleviate the problem:
  1. Always tighten the gib lock, once the head is positioned
  2. I've added a DRO that reads head position directly ... so backlash is no longer an issue
  3. I've replaced the spring arm with an air spring kit from Little Machine Shop (https://littlemachineshop.com/products/product_view.php?ProductID=2258&category=) $49. You not only get a more even spring force, but also extended Z travel.
 

homebrewed

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I employ all 3 approaches described by hman. Works very well for me.

The castle nut scheme used to engage the fine feed is a source of backlash. On my machine it contributes the lion's share of the backlash. As a matter of curiosity I installed a pipe clamp over the castle nuts and tightened it down once the fine feed was engaged. It substantially reduced the backlash, but was a pain to use. That was before I installed DROs. After that, the pipe clamp went back into the plumbing drawer.
 

domperna52

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Thanks for the responses. A couple of things.
  1. I made a replacement key to remove some backlash between the lever arm and pinion shaft, which helped a bit more. I actually had to create a step key to make it work as the slot in the lever arm was slightly wider than the keyway in the pinion shaft. When machining the key I noticed a slight motion of the head when using the gib lock. I learned to compensate for it but it took me three tries to get the key dimensions right. It sounds like a DRO at least on the Z axis will make this machine much more user friendly. Any recommendations on DRO brand/supplier/etc.?
  2. Just ordered the air spring kit from Little Machine Shop.
Thanks again.
 

homebrewed

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The cheapest DRO setup & the one I use is 3 iGaging Acuremote units. They are capacitive type scales and can be noise sensitive (although I have heard that the latest version is better in that regard). You also just get position info on the axes, no stuff like calculating bolt circles, angles etc. But you can pre-calculate all that if you really need it.

The nice thing about the iGaging units is that they have remote readouts (hence their moniker), so you can put them in a convenient place rather than peering at a readout on the scale itself. That gave me the freedom to install the scales without having to drill/tap any new holes in my mill.
 

domperna52

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Thanks for all the help with this. I replaced the torsion spring arrangement with the gas spring set up I got from Little Machine Shop, and the head seems well balanced now. I also installed three axis iGaging Acuremote units. I have milled a couple of things since then and the machine is MUCH more useful and user friendly now. I feel like I actually have some control over the machining operations, and it is also much better using the digital readouts rather than the crank dials set up for 1/16" per revolution. Now I need to find a spindle speed readout.
 

homebrewed

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The SX2 style of mill has a motor controller that can output the spindle RPMs so it is possible to DIY your own tach. I found a web site that details how he rolled his own tach readout here. If electronics is not your cup of tea (or you have an older X2 mill that doesn't support that feature), you might consider a standalone tach -- that way, you can use it on more than one machine....
 

royesses

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Here are a couple of tachometer options for the mini mill.


I built a MachTach kit for mine but he seems to have closed down his business. It does direct rpm/sfm readout.

Roy
 
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