Mini Mill Question

[Civilian]

Hey guys, I had a question for those of you with Mini Mill/Mill experience.

I have a Harbor Freight 44991 Mini Mill. A few months ago I learned that it had an interesting habit of dropping into my workpiece. The drop is about 1/8" but obviously big enough and bad enough to screw things up! I thought I could correct this by giving the mill some TLC. I spent hours taking this sucker apart, removing all the black sludge it came shipped with. I used white lithium grease on the moving parts, tightened the screws up all nice-like, and had her recalibrated to be 90 to the table (I dont have a single dial indicator here, but I do have a pro tram system I use).

It's been working fine until last night. I secured a small 1/4" pivot barrel to the table. I put a 3/8" carbide center cutting end mill into its 3/8" R8 collet, got it right above the work, locked the mill into the fine tuning mode (excuse me here, I have zero formal education and I get by on trial and error). cranked the mill up, locked the front/back column, and started turning the dial. Maybe moved it .005" came across the pivot, DROP.

It dropped the 3/8" center cutting end mill into the pivot, shattered the pivot.

I don't really understand this behavior so I'm reaching out to see if anyone else has seen this or have any thoughts.

Thanks!

-Robert

 

[Guster]

What specifically is dropping? The whole head, the quill or the collet?
Assuming you have:
* Tightened the collet using a drawbar.
* Thightened the quill in the head
* Tightened the head to the column
That should not leave anything to drop unless something is binding instead of tightening and the tool pressure or motor vibration is letting it 'unbind'. Still need to know specifically where it is dropping to give advice on where to look.

Cheers,

 

[Falcon67]

I can't figure what it may be doing, but you might find some more info in the mini-mill group at Yahoo. I had one for a short while, never had any issues with it. It was too small for what I do, so only had it about a year.

https://groups.yahoo.com/neo/groups/Mini-Mills/info

As above - unless the quill moves or the collet lets go of the end mill, there should be no movement of the mill when running.

 

[Civilian]

Collet is definitely tightened. Not sure about the quill. I'll have to google what that is! The head, which I assume is the large piece that runs up and down along the column, should be tightened. I had it off, then replaced after it was lubed and tightened.

I should have paid more attention to what dropped Im thinking it was the whole head, but now that you mention it, I cannot be certain.

 

[txvwnut]

Sounds like the balance spring for the quill is either broken or out of adjustment. Assuming it has one as I have not seen the HF mini mill to be for sure.

 

[Falcon67]

Run tests on scrap, that should show what's letting go. With the head locked to the column and the quill (part that lowers the bit - think drill press) locked and draw bar tight, nothing should move unless the collet can't hold the end mill. Could have an oversized bore in the collet.

 

[Civilian]

I'll get some pictures when I'm home. There's a nut on top that goes around...the...bolt? that spins while the mill is going. It was loose when i got it so I used the tools that came with the mill and tightened it. Maybe that's it.

 

[Falcon67]

Good parts source should you need it -
http://littlemachineshop.com/info/minimill_compare.php

 

[wolfsburged]

Known as the "head drop issue"
http://www.machinistblog.com/the-mini-mill-head-drop-problem/

There are air spring kits to rectify:
http://www.littlemachineshop.com/products/product_view.php?ProductID=2258&category=879658189

 

[Guster]

Also try and avoid using grease on any of the running surfaces or ways. It collects and traps debris which can become abrasive. Way oil is the preference though alternatives like chain bar oil are also used by many due to its viscosity.

Need pictures of the nut you are referring too. Sounds like you are talking about the quill pre-load nut which I hope it isn't.

On collet selection - collets have a limited range of clamping and the endmill should be a close fit to the collet before it is tightened. Normal endmills will want to pull themselves into the work. R8's also have an alignment pin+slot which keeps them from spinning in the tapper or while being drawn in by the drawbar. if the collet is not seated/aligned it will also not hold the endmill properly.

The quill is the spindle that the collet rides in that can also move up and down with finer control to the head's up and down adjustment. If it is not tightened it will also move in and out under tool pressure.

 

[Guster]

Known as the "head drop issue"
http://www.machinistblog.com/the-mini-mill-head-drop-problem/

There are air spring kits to rectify:
http://www.littlemachineshop.com/products/product_view.php?ProductID=2258&category=879658189

Ah... one of those. Should have checked the model that the OP listed. Doesn't have quill in the classic sense. Description of the issue makes sense too.

 

[Civilian]

Known as the "head drop issue"
http://www.machinistblog.com/the-mini-mill-head-drop-problem/

There are air spring kits to rectify:
http://www.littlemachineshop.com/products/product_view.php?ProductID=2258&category=879658189

WHOA! Thank you so much! This machinistblog entry is dead on what happens! I'll look into getting a few upgrades for the mill. LMS also has a pully conversion kit that apparently is a must-have as well!

You guys rock! Thank you so much!

-Robert

 

[Regnar]

Had the same mill. Air Springs, Belt Pulleys and a Butterfly Impact makes the mill much more enjoyable to use. I never did it but I remember someone Loctited on hex nuts to each axis and used a battery drill for power feed. Though that was pretty smart.

 

[matt_i]

As above, clean out any and all grease used in machine tool dovetailed slides and apply Vactra #2 (way oil)

Don't use carbide endmills in a manual mill. It does not have the rigidity and the cutter will break every time. You need a ballscrew CNC machine to keep one going long enough to wear it out.

 

[57RTO]

Try bringing your cutter down about a 1/4", then back up to your cutting depth. This allows you to remove all slack in the head.

 

[Civilian]

Try bringing your cutter down about a 1/4", then back up to your cutting depth. This allows you to remove all slack in the head.

Yup, I saw this as a recommendation as a workaround.

I ordered the kit today. When it gets here I'll install it and do some testing. Here's hoping!

 

[LutzTD]

Yup, I saw this as a recommendation as a workaround.

I ordered the kit today. When it gets here I'll install it and do some testing. Here's hoping!

looks like you have a workaround. But those mills will never be great. keep an eye on craigslist, youre in prime used machinery country. hardinge made a small mill and there are some other off brands made in the 70's and 80's that would be more rigid for you in the smaller footprint. I assume you dont have the space for a bridgeport, but if you do then dont worry about the 3ph, you can get a variable frequency drive now for <$200 that will work up to 3hp and run off of your dryer outlet.

 

[Civilian]

I dont have 220 in the house yet, so I'm working on 100. My workshop is a one car garage on a cement slab. I could always make room for a larger mill, but this one was what fit my needs and budget.

Other then dropping, it's been pretty great. I'm looking to get a nice mill vise next, so upgrading my mill will be down the line. I was just on Craigslist, so I know what you're talking about! So many radial arm saws, haha.