Issue: Head Deflection

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syh

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Hi,

I've got a HF minimill with a Hoss design 3 axis CNC conversion and little machine shop belt drive conversion.

I'm having a major issue. The spindle head (the whole part that moves up and down) is deflecting SUBSTANTIALLY when i'm trying to make a cut.

The X and Y axes (fun fact, the plural of axis is axes) are tuned in well (as evidenced by the fact that tightening the crap out of the Z axis yield no backlash on the X or Y).

The amount that I need to tighten the Z so it doesn't deflect is WELL beyond how loose it needs to be to move freely.

Also, my belt is slipping on the belt drive. I'm not sure if this is contributing.

Help! Please!
 
Oh. . . i tried searching, and couldn't come up with a fix, or rather first steps to diagnose.
 
syh said:
Hi,

I've got a HF minimill with a Hoss design 3 axis CNC conversion and little machine shop belt drive conversion.

I'm having a major issue. The spindle head (the whole part that moves up and down) is deflecting SUBSTANTIALLY when i'm trying to make a cut.

The X and Y axes (fun fact, the plural of axis is axes) are tuned in well (as evidenced by the fact that tightening the crap out of the Z axis yield no backlash on the X or Y).

The amount that I need to tighten the Z so it doesn't deflect is WELL beyond how loose it needs to be to move freely.

Also, my belt is slipping on the belt drive. I'm not sure if this is contributing.

Help! Please!
Click to expand...

When you say deflecting, are you referring to z axis movement (end play) or to deflecting in the x and/or y direction?
 
RJSakowski said:
When you say deflecting, are you referring to z axis movement (end play) or to deflecting in the x and/or y direction?
Click to expand...
It's deflecting along the x/y plane. So, if I'm roughing the side of a piece, the head isn't cutting as deep into the side of it as it should.

I don't think I'm taking too deep of a cut.
 
How deep a cut are you making? My smallish bench mill can be seen to bend when i attemp to make a massive cut :)

Stuart
 
OK, I think I got it. There's movement between the column and the base. I think tightening the crap outta those three bolts might yield me some better results. I'll try that and get back to y'all a little later.
 
More details are needed. What size end mill? What size cut? What material?
It sounds like you are taking too much of a cut with too big an mill.

Movement between the column and the base is a bit hard to believe unless the surfaces are not flat. Tightening the crap out of things just leads to broken things. They should be tight, but not hill billy tight. More likely you are just experiencing flex. The solution is take a lighter cut.
 
Deflection in the x and/or y direction can be caused by three reasons.
1. the z axis gibs can be loose. This will cause the head to pivot lightly from side to side and/ or to lift up slightly on the dovetail deflecting in the x and/or y directions. If the gib has a slight high spot in the center, you can tighten it to remove free play but it can still provide a pivot point to allow rocking
2. If you have a movable quill, it may be loose or worn permitting the quill to rock slightly in its bearings.
3. All metal will bend slightly under stress. Small mills are notorious for this because of the relatively small amount of mass in the frame of the mill. Flexing of the spindle, arbor, and chuck would be included in the category. Deflection under applied force is normal. That's why serious machines weigh tons. Heavy metal deflect less. Every cutting tool uses force to make a cut

Tightening your column mount may not improve your results. I would set up a test indicator to see I had a deflection when I pushed on the head and if it went away when I stopped pushing. I would then push in the opposite direction. Do this in both the x and y directions. If the head returns to the original position, the deflection you are observing is most likely just spring in the machine, probably the column itself. If the machine moves to a new position and stays when you apply and release force, then you need to look for play in your ways. Keep in mind that there will always be some play since you have to be able to move the head and table. This is particularly true for a CNC machine since it is constantly required to change direction unlike a manual mill where you can lock the 3 axes.

Bob
 
I was taking a .015" cut with a 3/8 end mill over .02 deep material. Those screws were so loose i could hand loosen them. The gibs were my first step, but I discovered that i could tighten them until I couldn't move the Z (even before it was CNC'd, by hand) and the problem persisted.

I could see the deflection looking at where the screws were with the naked eye, and it was substantial.

Thanks for the help!
 
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