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Milling, cutting in both directions !

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Nige11/07/2018 20:45:49
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361 forum posts
64 photos

The other day I had two pieces of 12mm square MS in the mill. I was levelling of the ends using a brand new 12mm dia 3 flute cutter. I was running it at about 800rpm and taking a 0.2mm DOC. I was finding that having taken the cut, when winding the work back the cutter was taking a second cut and it was quite a cut as I could see the flakes of swarf coming of. I was quite surprised and having a job understanding what is going on. Any explanations please ?

richardandtracy11/07/2018 20:59:51
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938 forum posts
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Does the table show any twist on its dovetails when you go from one direction to the other? I assume it was on the side of the cutter that you got the cut. If the gib strips allow the table to slide, there will be some potential for this to happen.

This can be measured with a dial gauge. On the cross slide of my lathe (a Warco Combi lathe/mill) the amount this happens when the gibs are set to be smooth for lathe work but is being used for milling, is 0.07mm over the width of my 80mm milling vice.

Regards

Richard.

Andrew Johnston11/07/2018 21:02:03
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4037 forum posts
497 photos

Lack of rigidity somewhere in the setup from the cutter all the way to the mill, or the work is moving slightly after the first cut is taken. Cold drawn materials are particularly prone to distort when asymmetric cuts are taken.

Andrew

Joseph Noci 111/07/2018 21:09:10
327 forum posts
631 photos

What sort of mill do you have? If the table-top type mill/drill, it could be deflection in the vertical column, the spindle quill, the table/column mount, etc all adding to the amount of deflection of the cutter away from the work piece.

If the cutter is held in, for example a taper fit ER collet chuck, or the like, is the chuck taper a good fit in the spindle taper? If not, the cutter/chuck can be deflected away from the cut when side milling

Any play in the vertical column dovetails, etc...Side deflection would be more evident when milling on the side of material, or course, but any vertical play ( spindle bearings?) would have an effect when end milling as well.

And if the cutter is held in a collet chuck, ensure the collet is pulled up tight - if loose, the cutter could be pulled into or pushed away from the work.

My pennies worth..

Joe

Nige11/07/2018 21:10:35
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361 forum posts
64 photos

Richard: Yes I was using the side of the cutter. Ill check for any play in the gib strips etc.

Andrew: Not sure if the material is cold drawn or not but it was bought as free cutting bright mild steel.

Nige11/07/2018 21:16:11
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361 forum posts
64 photos

Joe: It is a new Sieg SX2.7 mill with an R8 ER type collett chuck. I ensure the collett is done up tight but not checked that the chuck is tight in the quill but would not expect the draw bar to loosen; can that be a problem, not come across it yet. Not expecting any problems with bearing or quill wear as the mill is brand new. I will give it a good looking at with a dial gauge tomorrow.

XD 35111/07/2018 21:44:27
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982 forum posts
38 photos

Are you locking the axis you are not using to traverse the part being milled ? ( if you are using the y axis for depth of cut and x axis to feed the work along you should be locking the y axis between cuts and also the headstock / quill lock as well .   are you climb milling or conventional milling ? Also check the gib ajustments ..

Andrew Johnston11/07/2018 22:16:25
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4037 forum posts
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Posted by Nige on 11/07/2018 21:10:35:

Andrew: Not sure if the material is cold drawn or not but it was bought as free cutting bright mild steel.

If it's bright then it will be cold drawn. The alternative is hot rolled, which has a black mill scale on the surface. Cold drawn will have sharp well defined corners, whereas hot rolled will have rounded corners.

Andrew

John Reese11/07/2018 23:06:06
542 forum posts

I experience the same thing even with a full sized Bridgeport mill. The cutting forces cause deflection. A dull tool will deflect more than a sharpone. The collet chuck sticking out of the spindle will increase the deflection. Use an R8 collet directly in the spindle. If you have a swivel base under your vise it will allow more deflection. Take it off and store it until you actually need it. I am not saying hat one shouldn't use the collet chuck or the swivel base. Just understand using them will cause greater deflection and plan for it.

A small, light machine will show more deflection than a large heavy one under the same cutting conditions. Most hobbyists are restricted by available space to smaller machines.

I generally start with a climb cut, then reverse the feed (without additional infeed) and take a pass in a conventional cut. That acts s a spring pass. It generally leaves some fuzz on the cut surface. I make another pass climb milling to take off the fuzz.

XD 35112/07/2018 05:21:28
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982 forum posts
38 photos

Looks like my little enemy - the random winky thing is back !

not done it yet12/07/2018 06:57:00
2042 forum posts
11 photos

‘Spring cuts’ are commonly used when cutting on the lathe to tidy up items. That may be spring in the workpiece holding, the cutter or the machine. Same applies to milling machines - particularly the less rigid ones. So I am in total agreement with John, above. Slack in Gibs is another option, so it could be a combination of several factors.

Cutting in the same direction at the same setting may well sort out if there is deflection with the original cut or slide way slack.

Andrew Johnston12/07/2018 07:45:11
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4037 forum posts
497 photos

When cleaning up the end of stock in the mill I usually conventionally mill for the heavy cuts and climb mill for the finish cuts. I generally increase the feedrate for finish cuts to take account of chip thinning. An exception is plastics, where conventional milling gives a better finish than climb milling.

Andrew

Mick B112/07/2018 10:13:56
671 forum posts
42 photos

Looks like a spring cut due to job or cutter deflection on the first pass.

I get this in spades milling on my Myford double-swivel vertical slide in the Warco WM250V lathe.

Solid carbide mills 6 - 10mm. diameter run at over 1000 rpm with a gentle feed are best for avoiding this.

Lock all the slides except the one being fed.

Climb milling back across the cut with no additional infeed usually cleans it up, unless there's been snatch due to slack gibs, when you might get shallow furrows. Those can take a few thou more cut to get out, so you need to be careful to counteract any slack when approaching finish size on a tight tolerance.

It's only a matter of scale. I can remember meeting the same issues back when I was running Bridgeports and a big Varnamo vertical mill at work in the 70s. Whatever you've got you tend to run to its limits.

JasonB12/07/2018 11:06:27
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Moderator
13224 forum posts
1203 photos

As the others have said it's a bit of spring / flex - there is slight deflection of the tool away from th ework as it makes the initial cut and as there is less force on the return cut it is not deflected away so will take a small amount of metal off. I get similar on the X3 and just did a similar cut to what you describe on the SX2.7 and got a small amount off on the return cut.

Just a case of getting used to the machine and working with it, so in future you may want to use a shallower final cut and or take a couple of passes back and forth at the same settings.

Nige12/07/2018 12:17:21
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361 forum posts
64 photos

Thank you everybody😊 Looks like I don't have a problem though I will take a look at any play there might be just for interests sake. Also need to get in the habit of locking off the axis I'm not using.

Andrew thanks for the drawn/rolled explanation. 😀

Thanks Jason that is exactly what I was getting 😀

Ian S C13/07/2018 12:52:08
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6962 forum posts
224 photos

Steel is all rolled, BMS/ bright mild steel is cold rolled, and while more accurate as bought, it has built in stresses that can lead to distortion after being machined. Hot rolled steel has less in built stress, has a layer of oxide/mill scale all over, the corners of square bar, and strip are rounded, and strip can sometimes what is described as dog boned, looking at the end the centre of the bar may be thinner than the edge, not usually a problem, and if you don't want it, get you strip cut fron a piece of hot rolled plate. HRS is much cheaper.

Ian S C

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