Uneven surface finish on steel

[sooncheng]

Hi,

I was facing a part. But, the surface finish of the part is uneven, only one small part has mirror-like finish and feels 'flat', the other has tool marks. Can anyone explain what could've possibly gone wrong? I'm pretty sure that the part is clamped down hard enough. Hasn't check runout and the inserts have different heights, I used the lowest insert to setup for my z-axis.

material: mild steel
tool: 50mm shell mill with 4 AMPT1604 inserts at 90 degree
speed: 382rpm
feed: 76mm/min
DOC: 0.15mm
stepover: 75% of tool diameter
coolant: blast

Photo attached

Thanks.

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[john-100]

Have you successfully used the same cutter before ?

what grade of steel are you using ? some grades don't give you a good finish

in the UK EN1A Leaded, also known as 230M07 Pb is a free cutting steel that makes to easier get a good finish it

unless you have to weld the part !

John

[burbingus]

It looks like the mill travels ~8 thousandths of an inch per revolution. If that corresponds to the average scalloping period, you may have something going on with a cutter. Here are some things to check:

-What is the condition of the inserts? How far offset are their heights? What is their material/coating? Are they all properly installed?
-Is your head tramming good?
-Is your tool being held fast and free from vibration? I think the sound of the cutter could give you some insight in to what may be going on here.

Good luck!

Micah

[sooncheng]

Have you successfully used the same cutter before ?

what grade of steel are you using ? some grades don't give you a good finish

in the UK EN1A Leaded, also known as 230M07 Pb is a free cutting steel that makes to easier get a good finish it

unless you have to weld the part !

John

I'm not sure what material I'm using, my vendor does not know it as well, unfortunately. I don't have to weld the part. The cutter marks leaving on the steel are very noticeable, I don't need a mirror finish but at least I can't feel them. I believe most grades of steel can at least achieve this?

[sooncheng]

It looks like the mill travels ~8 thousandths of an inch per revolution. If that corresponds to the average scalloping period, you may have something going on with a cutter. Here are some things to check:

-What is the condition of the inserts? How far offset are their heights? What is their material/coating? Are they all properly installed?
-Is your head tramming good?
-Is your tool being held fast and free from vibration? I think the sound of the cutter could give you some insight into what may be going on here.

Good luck!

Micah

Can you explain more about the scalloping period? I'm fairly new to machining.

- The inserts are new, the material is 'H6501' claimed by the Chinese vendor (I believe its carbide) with TiAlN coatings, inserts are installed properly. They differ at most 0.2mm for their heights.
- My machine is trammed well but I'm not sure about the tool. I'm using a BT40 FMB22 holder so I think I cannot calibrate the tool more.
- My tool is being held sturdy enough I believe and they sounded good for me. The strange part is the part where the finish looks good is where the cutter makes a squeaky sound.

Sorry if I don't know much. The thing I'm wondering is how can a same part ran with the same settings gives two different surface finishes?

[burbingus]

This is what I was thinking:

If you have one insert that is a lot lower than the others, it will be the main insert that affects the surface finish. All the inserts should be in the same condition and at the same height for a good face mill in order to get the best finish and keep a consistent chip load between the cutters.

It appears to me that one of the inserts is a good bit lower than the others and is responsible for the variable surface finish. This also makes sense that the corners have a better finish because they've been cut twice by the same cutter and from two different angles.

If the face mill appears to be in decent shape, I would recommend that you adjust your feeds and speeds until you get the finish you want. You will want to boost your spindle speed if possible and/or reduce your travel. I think 2-3x faster spindle or 2-3x slower feed or some combination of the two look like it will be about right to me.

Micah

[sooncheng]

This is what I was thinking:

If you have one insert that is a lot lower than the others, it will be the main insert that affects the surface finish. All the inserts should be in the same condition and at the same height for a good face mill in order to get the best finish and keep a consistent chip load between the cutters.

It appears to me that one of the inserts is a good bit lower than the others and is responsible for the variable surface finish. This also makes sense that the corners have a better finish because they've been cut twice by the same cutter and from two different angles.

If the face mill appears to be in decent shape, I would recommend that you adjust your feeds and speeds until you get the finish you want. You will want to boost your spindle speed if possible and/or reduce your travel. I think 2-3x faster spindle or 2-3x slower feed or some combination of the two look like it will be about right to me.

Micah

What do you mean by
they've been cut twice by the same cutter and from two different angles ?

Thanks for the info, I will try to work on my feed and speed when I get back to my workshop.

[krypt333]

i believe he is referring to the fact that when you changed direction of your cut, the lower insert in turn went over the same area twice.. one thing you could try that tends to help me when i am worried more about finish than time is to keep my cuts all the same direction. If your first cut is X-, then retract, move over make the second cut the same. Make all step overs the same as well, keeping the conditions between the workpiece and the cutter as close to the same as you can. I know on my old mill that is the only way i can achieve a decent finish. Otherwise i end up with steps, and tool lines.

[sooncheng]

i believe he is referring to the fact that when you changed direction of your cut, the lower insert in turn went over the same area twice.. one thing you could try that tends to help me when i am worried more about finish than time is to keep my cuts all the same direction. If your first cut is X-, then retract, move over make the second cut the same. Make all step overs the same as well, keeping the conditions between the workpiece and the cutter as close to the same as you can. I know on my old mill that is the only way i can achieve a decent finish. Otherwise i end up with steps, and tool lines.

Thanks, I get what he means now. Turns out that the corner appears to have better finished due to some vibration, my vise isn't large enough to hold the steel plate. Found this out because I clamped a larger plate and the effect was very noticeable, the surface did feel smooth but wasn't flat at all. Also, I noticed that the inserts themselves have various lengths causing the difference in height, the machine is well trammed as well as the tool holder I'm using. Guess I need to get a better cutter.

Cheng

[HarryC]

I couldn't see the picture that you tried to upload? What flute type are you using?
Also I've read that overtightening pull studs on holders can cause issues with the finish.

[hazzard1]

Looks like maybe blunt tips, maybe spindle speed too slow also head bearings play so cutter pressure pushing cutter up. have a check and get back thanks. most likely is blunt tips.
good luck

[sooncheng]

I couldn't see the picture that you tried to upload? What flute type are you using?
Also I've read that overtightening pull studs on holders can cause issues with the finish.

I'm using a 50mm shell mill and inserts like this one :
https://sea.banggood.com/4-Flute-BAP...r_warehouse=CN

[sooncheng]

Looks like maybe blunt tips, maybe spindle speed too slow also head bearings play so cutter pressure pushing cutter up. have a check and get back thanks. most likely is blunt tips.
good luck

I use new inserts, shouldn't have blunt tips. How low spindle speed contributes to poor surface finish?

[NobleManufacturing]

I agree with Krypt, you should be trying to run your finish pass all in the same direction. Also, did this part have 'scale' on it prior to this facing? If this is a hot roll material, and you only cut deep enough to remove the scale that will almost always lead to poor surface finish. The .2mm variation in your inserts height along with the inserts having different lengths. Are the corner radi on the inserts all the same?
90 deg cutters are more prone to giving you a poor surface finish then a true face mill with inserts at a 45 or 30 deg angle.
Have you checked to see if the plate is warping as well? Pulling up or down in the center? This will be more common in cold rolled materials than hot rolled. However, if you are removing scale off of a cheap mild steel it is common for some deflection as the scale holds alot of stress. If this is the case you can try skimming the scale off then unclamping and reclamping then taking a few very light skimming cuts (.15mm) to achieve you required surface finish.
Also, if time is not a factor and you would just like a very clean surface finish. You can always remove all but 1 insert in your cutter body (effecting making it an old school face mill) and running your passes like that. Keep you rpm's the same but reduce you cutting rate to about .1-.15mm per rev.

Noblemanufacturing.net

[sooncheng]

Thanks. What are the ways I can check my plate is warping? I tried using an indicator to measure the flatness of the steel and it shows the plate is flat with almost no difference (0.005mm) across every corner of the plate.

Cheng