View Full Version : Milling Slots, Best Way?

Paul Alciatore
04-28-2004, 04:08 PM
I'm milling some 3/8" x 7/16" slots in mild steel for tool holders. The slots are 2.25" long and I'm using a mill-drill and a 3/8" end mill. Perhaps I'm just "chicken" but on the first cut, which is the full 3/8" wide, I'm taking cuts of 0.033" deep each pass and it's taking about 12 passes to get to the bottom. And then I have to make about four more passes to finish it to the full 7/16" width. This takes a long time. Oh, I'm running at about 500 RPM.

I tried a deeper cut at 0.050" but got more vibration so I backed off on the next pass. Afraid I might break the tool.

I know I could and likely should increase the RPM and I plan to try that on the next one but I am wondering if this is the best way to do this. Or am I just too cautious. Should I try deeper cuts? Or a faster feed rate?

Another thought I had was what about mounting a 4 or 6 inch cutter for a horizontal mill on an arbor and do this sideways. I assume they make R8 arbors for them. Can deeper cuts be made with such a set-up? Could it cut the full 3/8" depth at once? I need a better feel for the best way to do a cut like this on the equipment I have.

Paul A.

04-28-2004, 05:14 PM
Start out by increasing your spindle speed.
With a mill-drill a bit of experimentation seems to be required all too frequently. They will certainly let you know when you have exceeded their window of capability. I would be starting at about twice the spindle speed you are currently using. They seem to like more feed then a deep cut. I usually try and go a little faster on the speed, feed, and more DOC until it is obvious that the machine can’t handle it. Is your set-up as rigid as you can make it, and the head as low as possible? If you want to try a side cutter, watch out for the head to slip on the round column. I get better results with an end mill, and leave the side cutters for when I have no other alternatives. I assume you know to avoid heavy climb cuts http://bbs.homeshopmachinist.net//smile.gif

Let us know how it comes out.

[This message has been edited by Joel (edited 04-28-2004).]

Paul Alciatore
04-28-2004, 06:01 PM

Thanks. Yes it's pretty rigid, spindle is extended about an inch at the start of cutting. I like to leave a little room as when things get too close, more of those red lines tend to appear on my hands. My wife thinks I am trying to whittle my fingers off as it is.

I didn't think about the head rotating on the column. Oh well, one more thing to worry about when finding useful parts inside metal stock. What if I used the Y feed to make the cut?

Re: Climb Cuts
Yes, the dreaded climb cut. I am definitely avoiding that when milling the second side to the final dimension as I am only taking off 1/16 at that point. While making the initial, full width cut with the end mill, one side is climbing and the other isn't. I believe they cancel out each other. It doesn't seem to be a problem.

I'll try a higher speed tonight.

Paul A.

04-28-2004, 07:17 PM
Using the Y-axis feed will help, but it still wants to push. This is just my experience, you can always try it and see what works for you. Several side cutters, a slitting saw, and an arbor will be a useful addition to your tooling collection regardless. Besides, using the X feed means cranking by hand (for me), and I’m lazy.

As I am sure you have already done, double check that the column, quill, and unused axis, are all locked down securely. I replaced the column bolts with harder SAE ones, and added a hardened washer under the nut (everything well greased). The locking feels more secure as a result, and the factory wrench even fits better.

I climb cut all the time, but stick to light cuts. Directing an air nozzle behind the tool can help quite a bit when cutting a slot in a single pass.

I think a spindle speed of around 1k will work fine. Use a sharp 4 flute cutter.

04-28-2004, 07:20 PM
For cutting a slot a two flute cutter is much better.


[This message has been edited by Evan (edited 04-28-2004).]

04-28-2004, 07:32 PM
Evan - In steel? How do you figure?
Wouldn't going to a two-flute necessitate raising the rpm? I like my end mills too much to do that. I don't think the column mills will handle that. Flood cooling might help but who wants the mess in a home shop?

04-28-2004, 07:56 PM
It is how the mill interacts with the material based on the number of flutes. 2 flutes work better than four in steel or aluminum when cutting slots.



04-28-2004, 09:17 PM
Paul is cutting a 7/16" slot with a 3/8" cutter in steel, using a mill-drill. I stand by my suggestion.

04-28-2004, 09:55 PM
Have you considered using a 4 flute 3/8" roughing end mill to make the initial cuts then finish with a regular end mill. You can certainly take much more agressive cuts with the roughing end mill. They work great with my mill/drill.

04-28-2004, 11:12 PM
Joel and Evan are both right,only the way I do it is to plunge and rough out the slot with a 5/16,then use a sharp 3/8 two flute for the finish cut.Why?Two reasons,one it eliminates the "keyhole"condition that afflict deep slots,second much less chatter and pulling takes place as the cut is balanced.

As far as speed,when using coolant 1200 rpm is not uncommon for me,dry never more than 850.The thing that kills an endmill the fastest is cutting back through its own chips,this chips the edges of the endmill and dulls it quickly.Vaccum or blow out the chips while cutting.

J Tiers
04-28-2004, 11:39 PM
If he is using a 3/8 cutter and 500 rpm, that is only about 42 sfm, where closer to 100 is suggested for mild steel. It seems to me he could increase (double) his speed without a problem.

As far as how to really cut the slots? Horizontal mill anyone? (I know, I know, he has a mill-drill).

Paul Alciatore
04-29-2004, 10:04 AM
Well, I had much better results last night. I upped the spindle speed to 1200 and stayed with 0.033" depth per pass. And yes, I am using a 4 flute cutter because it's the only 3/8" that I have. I also upped the feed rate and if anything, the cuts went smoother. I managed to zip through 3 more in less time than the first one took.

The roughing cutter sounds like a good idea. I think I'll definitely get some but it's too late for this job as I plan to finish tonight.

Thanks for all the suggestions. As I've said before, this board - you guys are just great.

Paul A.

04-29-2004, 11:49 AM
sounds like you already got the job done,
maybe a suggestion for next time...

i dont know if the slots were blind- or through- but back when i had one of those
minimills, i found drilling out most of the material helped enormously, before 'cleaning up' with an endmill.


04-29-2004, 12:13 PM
roughing out the slot with a "roughing" end mill is the best solution...

When its time for you to buy some more endmills, give 3-flutes a try...They seem to give great finishes...

Situations will always be different for the "Home Shop" as compared to the "work shop"...Home shops usually have limited resources..and no one like to make a huge mess at home http://bbs.homeshopmachinist.net//smile.gif...Sometimes you have to do with what you have on hand...


04-29-2004, 01:21 PM
I see an issue with taking many light, eg. .030", cuts to mill a slot. That is all the wear on the end mill occurs at the end. So the mill is quickly worn out even though only a small section was ever used.

I think trying the smaller diameter roughing mill followed by a regular mill to finish is worth a shot.

Also, I read in Moltrecht that using a two flute mill can cause the slot to be formed with tilted side angles in a parrallelagram cross section. This is another reason to consider three and four flute mills.