# Setting accurate depth of cut on a mill



## picclock (Aug 25, 2011)

(mod please relocate if this is not the right section)

My mill came with a Z axis DRO fitted to the mill head and spindle, which should make cut depths very easy, but I have a few issues with it. If I'm milling to a depth I usually touch the end of the mill to the part, zero the DRO and adjust the spindle to give the desired depth of cut. Except if I do this, the cut is always 4-8 thou (0.1-0.2mm) deeper than it should be. My initial thought was longitudinal play in the spindle bearings but there isn't any that I can physically detect, certainly none of that magnitude. I have the feeling that it may be the mill pulling the spindle/head down as it grabs the metal. 

Is this a common thing ? (excuse my ignorance - my first mill)

Are there any good fixes apart from allow 0.15mm on the depth setting. Is it likely using a thin paper (fag paper) stuck to the surface to set the DRO depth when running would improve things ? 

Any other info or ideas on this would be much appreciated.

Best Regards

picclock


----------



## Tin Falcon (Aug 25, 2011)

I almost always sneak up on the dimension. (Dial in a bit less than calculated cut. )
touching off on paper is a good practice. Also always shoot for the high side or safe side of tolerance easier to take a little more than put some back. Sounds like you are on the right track. 
Tin


----------



## picclock (Aug 25, 2011)

I think I would feel better about it if I could pin down the cause. If anyone who uses CNC is reading this perhaps they could comment on whether this happens with CNC mills, or is there a fiddle factor for this ? I would not expect it to happen if you mill a face then increase the depth of cut because depth would be truly relative, but if set from a reference face I think this may occur.

Its annoying at the moment for me because its pushing me to the edge of my tolerances, and for some time its been a problem that I must get round to sorting 

Best Regards

picclock


----------



## steamer (Aug 25, 2011)

The cutting force is in that direction. Tending to pull the cutter out of the spindle.

Are all your gibs adjusted correctly?....I'm wondering if your lifting the table.

Dave


----------



## steamer (Aug 25, 2011)

Additionally, compare your DRO output to a set of gage blocks or blocks of known thickness.  Make sure you don't have a scaling error.

Dave


----------



## Blogwitch (Aug 25, 2011)

Picclock,

This could just be a reason to find the limits of your machines. Most machines have some sort of play in them, and if you can find out how much, you can automatically allow for it.

I haven't got CNC, but I do have four axis DRO's on both my lathe and mill. One of them is on the mill quill and is a scale type, and is only used for drilling to depth, if perfect size is required, I use the glass scale on my normal z table axis.

Six of the glass scales are spot on when it comes to cutting to size, but having tried every which way, I can't get the cross slide on the lathe spot on, it always cuts 0.0002" undersize, so I always put an extra 0.0001" cut on to what is required, and that gives me perfect size.
Very rarely now do I need to be that accurate, but it is always in the back of my mind to put that extra bit on if needs be.

If yours is a round column RF type mill, then you will have trouble getting size anyway, unless you come up with some arrangement to hold the quill in it's upper position, as that can flop up and down by a fairly large amount and give the sort of readings that you are getting.


John


----------



## picclock (Aug 25, 2011)

@John
The mill is a WM14 column mill, on the whole its exceeded my expectations. The gibs are always snugged up and this morning I was using a 1/8th endmill to cut very shallow slots in Aluminium, 6061. Although I set the slot depth to 1.5mm (1.6 being correct) it very nearly made 1.7mm I don't think that low forces like that would be able to cause the table to lift. I set it to 1.5 instead of 1.45 as I figured a smaller mill would have less effect - sigh - wrong again. The column, X and Y axis were locked as I was working with a 6" rotary table. Once set to the correct depth the spindle was locked also. 

@steamer - that's the annoying part, the DRO is fine. If I mill at a depth X then at depth Y The difference between X and Y is a lot < 1 thou. However if I measure the depth of cut X from the surface it's always about 0.1 to 0.2mm lower. 

Its almost as though when the spindle is spinning it's getting forced lower - can't think why that should be but may be able to prove it with the fag papers (must get some).

Best Regards

picclock


----------



## Jasonb (Aug 25, 2011)

I wonder if when you are touching down onto the work surface that is pushing the spindle up by your error, then as you start cutting the tool can drop back to its lower position therefore giveing a deeper cut.

Hold a bit of faced bar in your collet chuck ( you are using one and nor a drill chuck?) and put a DTI onto the end of the bar and try moving the chuck up & down by hand this will show if there is up/down movement in the spindle.

J


----------



## ronkh (Aug 25, 2011)

picclock,

I have a SIEG Super X2 mill with a fitted DRO (vernier type) which, after a while, gave me a similar discrepancy to what you are getting. The screws (2off) worked loose so you may want to check that these screws holding the vernier shaft(?) to the spindle have not worked loose as mine did. 
This is presuming that yours is held on the same as mine?

Regards,

Ron.


----------



## Russel (Aug 25, 2011)

A method that I use with my little Sherline mill is to bring the mill bit down to the part, then back off until the play is taken out. The Z axis on my mill usually has about .003" to .005" play. When I bring the mill bit in contact with the part I wish to machine I typically end up going a little beyond just contacting the part and in order to make up for that I back off very gently until I reach the end of the play. I hope this makes sense.

Another way of looking at it is that when the mill bit come in contact with the part I can rotate the handwheel back and forth and feel the .003" to .005" of play. What I do is simply set the handwheel on the "up" end of the play with after bringing the mill bit down in contact with the part then zero the DRO. I kept getting inconsistant results until I figured this method out. It does take a bit of feel. Give it a try, it may work for you.


----------



## shred (Aug 25, 2011)

You aren't locking the quill touching the part, are you? The lock can sometimes drive the cutter deeper. Good way to trash carbide endmills.


----------



## lordedmond (Aug 26, 2011)

this may sound dumb and I am not trying to teach you to suck eggs

is the cutter pulling out?

is is held in a manner that will restrain it in the axial direction



Stuart


----------



## picclock (Aug 26, 2011)

@ jasonb
Checked the endplay on the spindle again this morning with reasonable force I managed an overall movement of 1.25 thou - I think that's OK certainly not the dominant 4-8 thou that I am getting. ER32 collet with 1/8th carbide endmill at this time. But it seems to occur on all sizes of endmill regardless of speed or material. Its a nuisance and I don't like things I can't fully get to the bottom of.

@ ronkh Screws checked and fine, its a very solid fixing. As I said before milling a surface and then setting a depth relative to that produces accurate results - so its not the DRO or its fixings.

@russel endplay is only 1.25 thou max - I expect in normal use the value should be lower as I used reasonable force to get it to that figure. The forces using a 1/8th mill on 6061 Aluminium with a cut depth of 1.6mm are minimal.

@ shred I use the handle for drilling to just touch the mill to the work. It gives good feel and is quite repeatable according to the DRO readout. 

@lordedmond Nope all cutters are good. Check DRO after milling on an unmilled piece and reading the same. 

Thanks for the suggestions

Best Regards

picclock


----------



## Lakc (Aug 26, 2011)

Although at first it seems counterintuitive, I wonder if the helix of your endmill is picking up the part/vise/table and drawing it towards the spindle?

How about making a test cut with an non-helical cutter, like a flycutter or insert cutter?


----------



## Omnimill (Aug 26, 2011)

I think it's just that end mills have a tendency to pull into the work and hobby machines aren't rigid enough to resist it.

Here's an idea that _may_ prove the point, try putting on a cut with a Fly Cutter and see what you get.

Vic.


----------



## mzetati (Aug 26, 2011)

picclock,

that wm14 (correct?) looks much like my BF20: there should be a ring nut for adjusting the preload of the spindle bearing,: make sure it's correctly set.
No other idea at the moment.
Marcello


----------



## Sshire (Aug 26, 2011)

How does one know when it is correctly set?
Is there a technique or measure for setting it?
Thanks
Stan


----------



## Journeyman (Aug 26, 2011)

picclock

Have you checked the mounting screws that hold the DRO head in place? There are four socket head grub screws at the bottom of deep holes on either side of the head. They are also used to adjust the squareness of the DRO head. You can see the screws if you remove the front plate from the head (you need to remove the plate to change the DRO battery). I have noticed when using the quill on my WM14 that you can get some vertical movement if the Z-axis is not clamped up.

Cheers
John


----------



## el gringo (Aug 26, 2011)

Are the depth setters I see @ Enco worth the bother?

Depth Setter Repeatability: .0002" Height: 1 Width: 2 Length: 3 Battery Type: AAA 
Repeatability (Decimal Inch): .0002"" 
Height (Inch): 1 
Width (Inch): 2 
Length (Inch): 3 
Battery Type: AAA 
Manufacturer Part Number: 54-575-750 
Ray M


----------



## shred (Aug 27, 2011)

picclock  said:
			
		

> @ shred I use the handle for drilling to just touch the mill to the work. It gives good feel and is quite repeatable according to the DRO readout.


Then what do you do after pulling the mill down to touch the work? Locking the quill then and there could be the problem. If your quill sits still when you let go, try backing down the knee a tiny bit before locking the quill (assuming you have a DRO on the knee or some other way to return it to whence it was)


----------

