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Tooling selection and process refinement - small slots in titanium

cgrim3

Cast Iron
Joined
Dec 4, 2020
Location
Baltimore
Hi all,

I have a question for you all. I have a job where there are numerous small slots (1mm, 1.5mm width) in the face of a rectangular titanium part. The depth of the slots is about 1mm. Some of the slots have contoured radiuses so slotting saw/woodruff cutters may not always be the best option. I have quite a few parts to machine and am looking to buy the tooling/machines to do the job right. Fixturing is vacuum fixture.

I want to optimize the process to get cycle time down. Conventional methods would be to use HSM and climb mill and rough the slots in axial steps of .005" - .010" with the tiny endmills and leave material radially and axially for a finishing operation. However, I am trying to save time and money and want to rough/finish the slots in 1 pass on the mill with minimal burrs.

I am having difficulties with the small 1mm, 1.5mm endmills holding up and will have to dial in the feeds and speeds and endmill selection. Chips sometimes wrap up around the endmills and cause them to break and the shear amount of material being removed with the small endmill in one pass is causing the endmill to break unfortunately. But I am trying to come up with a solution. Also, I am using synergy 735 coolant.

I am leaning more towards the 3 flute and 4 flute variable helix coolant thru endmills since they are a little more stout than the 2 flute endmills. I am buying bull endmills and they last longer than the square endmills but want even more performance. I'm curious - would a cubic boron nitride (CBN) or whiskered ceramic endmill hold up better for this application than a tungsten carbide endmill? They are certainly more expensive but am unsure if they would be right for the job. Any thoughts?

What else would you suggest? Some micro machining tips and suggestions for good endmill brands for mico machining would be much appreciated. I was leaning towards Micron, Guhring, and Emuge endmills but was wondering what others here thought? Possibly a custom ground CBN or whiskered ceramic rougher/finisher combo endmill?

I considered ram EDM for the part but the logistics of it and the cost of the electrodes and the replacement factor of the electrode doesn't make economic sense for our application.

Additionally, the part sometimes distorts after machining. One solution is to rough the entire part, stress relieve it in the heat treat oven, and re-fixture and re-machine to finish the part, but this would take too much time. Any thoughts of how to reduce warpage in the part? Vacuum does help suck the part down during machining, but warpage can be a problem after removing the part from the fixture.

Any thoughts?

Thanks,

Chris
 
Last edited:
Hi Chris:
First, I concur with your conclusions about trying to sinker EDM these...titanium is slow to burn and uses up electrodes with enthusiasm.
It's doable, and you might do OK if you get an applications engineer to tweak the process for you but it's not going to be simple.
If you want to try, reach out to Brian Pfluger from Makino...he knows his stuff and can advise you.
He recently posted his contact information in the EDM subforum on this site.

I've had good success plunge roughing slots, and that is because the endmill is much stronger if you load it axially than if you load it radially.
It's definitely worth experimenting with.
You may have to poke in a start hole with a carbide drill so you don't overload the endmill on the first plunge.
You also have to keep the slot clean...whatever you have to do coolant wise will help you immensely.
If you have to rig something special to point the coolant stream right into the slot, it's worth it.

Second use stubby bullnose endmills wherever possible and use 4 flute cutters over 2 flute cutters to take advantage of the stouter core of a 4 flute cutter.

Third, wherever you have room, use a trochoidal roughing strategy and go full depth.
I aim for a radial DOC of about 10% and a chipload of about 0.0005" for a 1 mm cutter.

Sadly if you overspeed a cutter in titanium it'll burn up in a heartbeat, so you have to play with it to find the top end SFM for your cutters...I'm guessing it's going to be under 400, which for a 1 mm cutter is still ripping along pretty respectably.

With respect to stress relief, can you toss your blanks into the oven before you even begin on them?
It might not be quite as good as relieving after roughing, but it might still be better than nothing at all.

Other than that, there's probably no magic...small cutters tend to break before they wear out so I doubt an exotic ceramic will help you and I have no idea if PCD will help either.

Cheers

Marcus
www.implant-mechanix.com
www.vancouverwireedm.com
 
there's probably no magic
There is at least some magic. If he is trying to optimize for titanium, cryogenic machining would be the way to go. Also, you can measure the harmonics of your machine while it is in the cut as well as the thickness of your chips and have custom cutters made that cancel out the harmonics allowing cutting at 3x higher speed or more while maintain stability without chatter. Look into the work of Altintas et al for the exact method. It is done often sometimes in aerospace.
 
Hi Chris:
First, I concur with your conclusions about trying to sinker EDM these...titanium is slow to burn and uses up electrodes with enthusiasm.
It's doable, and you might do OK if you get an applications engineer to tweak the process for you but it's not going to be simple.
If you want to try, reach out to Brian Pfluger from Makino...he knows his stuff and can advise you.
He recently posted his contact information in the EDM subforum on this site.

I've had good success plunge roughing slots, and that is because the endmill is much stronger if you load it axially than if you load it radially.
It's definitely worth experimenting with.
You may have to poke in a start hole with a carbide drill so you don't overload the endmill on the first plunge.
You also have to keep the slot clean...whatever you have to do coolant wise will help you immensely.
If you have to rig something special to point the coolant stream right into the slot, it's worth it.

Second use stubby bullnose endmills wherever possible and use 4 flute cutters over 2 flute cutters to take advantage of the stouter core of a 4 flute cutter.

Third, wherever you have room, use a trochoidal roughing strategy and go full depth.
I aim for a radial DOC of about 10% and a chipload of about 0.0005" for a 1 mm cutter.

Sadly if you overspeed a cutter in titanium it'll burn up in a heartbeat, so you have to play with it to find the top end SFM for your cutters...I'm guessing it's going to be under 400, which for a 1 mm cutter is still ripping along pretty respectably.

With respect to stress relief, can you toss your blanks into the oven before you even begin on them?
It might not be quite as good as relieving after roughing, but it might still be better than nothing at all.

Other than that, there's probably no magic...small cutters tend to break before they wear out so I doubt an exotic ceramic will help you and I have no idea if PCD will help either.

Cheers

Marcus
www.implant-mechanix.com
www.vancouverwireedm.com
Hi Marcus,

Thanks for your reply. Being that the cutters are so small, is trichoidal milling for these tiny slots an option? The slots are 1mm diameter and if we want to start trichoidal milling, we will have to start using smaller endmills than 1mm in order to trichoidal mill the slots.

Thanks,

Chris
 
For endmills in that size, you want either Mitsubishi or Mikron. Get your feeds and speeds and coolant flow correct, they'll last for hours.
 
You could also try ramping the slots instead of trac paths. I do this on skinny deep slots, just because chip evacuation is better.
 
Hi again Chris:
You are correct, trochoidal milling is not a reasonable option for those 1 mm or even the 1.5 mm slots.
The reason is that there is a non -linear relationship between cutter diameter and cutter strength.
So you will lose ground if you try to go in with say a 1/2 mm cutter.

However if you also have say a pocket with a 0.5 mm corner rad, I have done better clearing it out using a trochoidal path and a bigger cutter (Adaptive Roughing in HSMWorks) and then going in again and adaptive roughing the remnant again with a 1 mm cutter than just trying to ramp down the periphery of the pocket after the big cutter has done its work.

But no...I'm in agreement with you that trying to rough a 1 mm wide slot with a 0.5mm wide endmill just to gain space for the trochoidal movement is a fools errand.

Cheers

Marcus
www.implant-mechanix.com
www.vancouverwireedm.com
 
Would you guys use 2 flute Mikron endmills or 3 or 4 flute Mikron endmills? I feel like the 3 and 4 flute endmills will have a larger core and could handle the cut better but you would get better chip evacuation from the 2 flute endmill
 
3-flute for slotting. You don't want the cutter teeth engaged in both sides of the slot simultaneously if you can avoid it.
 
What rpm do you have available? 200 SFM with a .03" end mill is about 25,500 rpm. Get your runout as close to zero as possible because you will be running feed per tooth in the tenths.
 
I just did this a few days ago. Ti6Al4V-ELI, slot 1/32" wide x 6mm long.

1. Do not use dynamic / trochoidal motion on a slot of that scale on a machine driven with ball screws; you'll ruin the machine in short order via false brinelling, as the repetitive motion is smaller than the bearing spacing in the ball screws and thrust bearings, and will displace the lubrication without replacing it. A linear motor machine will not have this problem, except perhaps on the ways.

2. Perforate the slot with a tightly spaced row of drilled holes before slotting; this will reduce the work the endmill has to do, help with chip evacuation, and prolong cutter life.

3. After perforating, I did full slotting with a Redline 1/32" 4-flute, RE10302. 16658RPM, 80mm/min, .397mm depth passes, to full reach of the cutter. After dialing in this approach I didn't lose a cutter for the rest of the run.

Got my feeds and speeds from HSMAdvisor.

Here's what Harvey has to say about micro slotting:
 








 
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