Pocket milling

[ChipsAlloy!]

I was wondering how you guys achieve a precise depth when machining a pocket in a part . Of course I'm talking about a job done on a pm932/45 hence the post in this section.

I would venture saying that you use the crank to lower the head (cutter) into the material because I normally raise the knee when at work. But I might be wrong. Is there another technique for these smaller machines ? Like maybe there is a way of using the quill and control the descent ?

I do have the z axis on my dro so I can tell how much I lowered the head with the crank. I guess I am wondering if there is a better way than the one I figured.

 

[mattthemuppet2]

I'm trying to avoid doing work so I'll give it a shot, even though I'm no expert (ha!)

I'd plunge to 80% of depth with a center cutter using the quill, use a rougher (love 'em!) to get most of the pocket out, then a finisher to end and side mill the pocket to depth and width. Set the quill to depth, lock and go to town It's probably not the quickest way to do it, but it should help with chips too.

How deep is the pocket? Within the range of the quill travel?

 

[darkzero]

With my PM45 I use the quill "fine feed" handwheel to plunge with DRO for measurement. It's not a very rigid quill so I try to limit quill extension by lowering the head ahead of time to compensate. I have a DRO for head Z axis also but I find it to be pretty much useless.

 

[ChipsAlloy!]

Fine feed of the quill... Hmm I will have to check that out using the handwheel. I will try it this evening and see how it goes.

So Using the head crank may not be the way to go for that job. In that case Dark, you might be right the z axis would be pretty much useless.

But for now I have to go to work..

 

[ChipsAlloy!]

How deep is the pocket? Within the range of the quill travel?

Yes it is about .375" deep in aluminum so well within quill range.

I admit I was afraid of doing it with the quill because lowering it by hand I would not have much control ( can go too deep if the cutter bites into the piece or even big vibration) .

Now I cant wait to try with the fine feed and see how it goes.

 

[wrmiller]

Pocket milling on these little bench mills is the worst case scenario in my opinion. I too am used to/learned on a BP with a scale on the knee, so naturally I got used to raising the part into the cutter for operations like this. On my PM I have a scale on the head and normally just drop the head slightly below target depth and bring it back up so that I have a consistent load on the Z-axis screw (and subsequently locking the head down of course. However this sequence doesn't work for pocket cuts.

But in talking to someone else who has a larger machine/larger quill, they use the quill to plunge the cutter into the part when making pocket cuts. My only concern here is 'axis creep' whereby the cutter falls slightly lower during the cut due to slop in the quill mechanism as downward cutting pressure on the tool is reduced as material is removed. I guess you have to lock the quill to prevent that from happening, but then what happens when you unlock the quill? Do you have to stop the cutter before doing so?

Bill

 

[zmotorsports]

I will admit this was one of my concerns when I had originally ordered the PM932PDF as I too am used to locking the quill and raising the knee precisely where I need it for roughing and then finishing cuts. I figured I would just have to figure something out once I got the machine as my budget and room really couldn't accommodate a full-size BP/BP clone.

It is interesting to read how you guys are compensating for this and dealing with the head vs. knee type of movement.

I just wanted to subscribe to make sure I didn't miss out on useful information.

 

[JimDawson]

What I did to compensate for the quill feed backlash is to counter balance the quill it's self. I am using a cable that is connected to the quill stop that runs back to an air cylinder that provides a constant 50lbs of up force on the quill. http://www.hobby-machinist.com/show...CNC-Conversion&p=199564&viewfull=1#post199564 This takes all of the backlash out of the system in both CNC and manual mode.

Using this system it seems to hold about 0.0003 when in CNC mode, at least that is what the DRO says.

 

[Falcon67]

With my PM45 I use the quill "fine feed" handwheel to plunge with DRO for measurement. It's not a very rigid quill so I try to limit quill extension by lowering the head ahead of time to compensate. I have a DRO for head Z axis also but I find it to be pretty much useless.

Same here. Have a 6" digital scale on the quill, and use the fine feed. The scale is required on my Griz because the fine feed falls behind about .002 every inch or so of downfeed. You'd lose track real quick using the micrometer dial. There's almost no backlash in the fine feed and for maybe .3 in either up or down the dial is dead on. Go an inch, you are off .002. Another inch, now your .004 off. Etc.

 

[wrmiller]

Same here. Have a 6" digital scale on the quill, and use the fine feed. The scale is required on my Griz because the fine feed falls behind about .002 every inch or so of downfeed. You'd lose track real quick using the micrometer dial. There's almost no backlash in the fine feed and for maybe .3 in either up or down the dial is dead on. Go an inch, you are off .002. Another inch, now your .004 off. Etc.

That seems kinda weird Chris? Metric screw or rack and Imperial dial?

This morning Paul at CO told me that they put the Z-axis on the quill on their manual machines. That would require me to do this like you guys, i.e. move the head close then use the quill for precision movements. I will have to chuck up some test pieces and quantify exactly how accurate this method will be. I hope the 12Z comes with a digital on the quill, so I don't have to depend upon a dial. If not, I'll put a cheap one on until I can afford a DRO. I've bought too many machines and tools this year and my 'fun money' is about gone.

Bill