Trepanning Revisited

Topics include, Machine Tools & Tooling, Precision Measuring, Materials and their Properties, Electrical discussions related to machine tools, setups, fixtures and jigs and other general discussion related to amateur machining.

Moderators: Harold_V, websterz, GlennW

User avatar
rmac
Posts: 283
Joined: Tue Jun 26, 2012 12:48 am
Location: Phoenix, Arizona

Trepanning Revisited

Post by rmac » Tue May 11, 2021 4:56 pm

rmac wrote: I'm not planning to try this [trepanning exercise] again, since it seems iffy even under the best of conditions.
Harold_V wrote: A shame, as staying the course would be a great learning experience. We gain strength by failures like this.
rmac wrote: You're right. A better plan is to go ahead and make these engine parts using the mill and the rotary table, and then to continue with the trepanning and tool grinding exercise later.
The above is from a couple of months ago when I had some trouble with trepanning when I was working on one of the Moriya engine parts. So now it's later.

I ground a proper tool this morning from a HSS blank rather than the cut nail I was using before. It's got just a little front relief, and no top rake. Although it chattered pretty bad when I first tried it out, I was able to get good results by slowing the lathe way, way down. The chips didn't curl up into little watch spring spirals like they do sometimes with a parting tool, but otherwise it seemed a lot like a parting operation.

Question: Given that my cutter seemed to do the job, is there any reason to think it would work better--whatever that might mean--with a little top rake?

-- Russell Mac

trepanning_revisited.jpg

User avatar
BadDog
Posts: 5066
Joined: Wed May 17, 2006 8:21 pm
Location: Phoenix, AZ

Re: Trepanning Revisited

Post by BadDog » Tue May 11, 2021 6:20 pm

Harold is obviously the best source for an answer, but from my perspective as a fellow traveler on this path, top rake can be very useful for a few things, that may or not be important for you.

First, combined with a ramp feature to form a chip breaker may help with chip control. With a bit of dishing, this can include folding the chip a bit to reduce rubbing on deeper cuts, much the same as a prating tool, but in a more complex application.

Second, and more generally important, it can reduce pressure, and thus reduce chatter in common trepanning tooling setups. But nothing beats a rigid setup end to end (including absolute minimal bit stick-out), and proper feed/speed used to manage that chatter tendency.
Russ
Master Floor Sweeper

User avatar
Bill Shields
Posts: 6616
Joined: Fri Dec 21, 2007 4:57 am
Location: Somewhere in the World
Contact:

Re: Trepanning Revisited

Post by Bill Shields » Tue May 11, 2021 9:33 pm

PH Horn and others make very good tools for this method of metal removal.

The translation from German is called axial grooving if you care to look in their catalogue.

As previously mentioned..rigidity is tantamount
Too many things going on to bother listing them.

User avatar
rmac
Posts: 283
Joined: Tue Jun 26, 2012 12:48 am
Location: Phoenix, Arizona

Re: Trepanning Revisited

Post by rmac » Tue May 11, 2021 10:58 pm

Thanks for the hints.

I added some top rake to the HSS tool and while it didn't seem to make a huge difference, it did seem to cut a little smoother.

So just to see if going too fast was what got me before, I ground another Neanderthal tool from a cut nail and tried it with the lathe running at the same very low speed that I was using with the HSS tool. It seemed to be doing okay, but dug in and broke off for some reason.

So then I reinstalled the HSS tool just to play with it a little more, and IT dug in and broke off, too. I'm not sure what happened in either case. Maybe I was just feeding too fast, although a fairly aggressive feed seemed to reduce the chatter. Would the top rake make the tool more likely to dig in? It kinda seems like it to me, but (obviously) I don't know.

Looking back at the thread where I tried all this originally, I see that Harold recommended trying 2024 aluminum. I'm still using what I think is 6061, so a change of material is another thing to mess with.

No photos of the broken tools. :)

-- Russell Mac

Harold_V
Posts: 18849
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Trepanning Revisited

Post by Harold_V » Wed May 12, 2021 3:42 am

rmac wrote:
Tue May 11, 2021 10:58 pm
Would the top rake make the tool more likely to dig in? It kinda seems like it to me, but (obviously) I don't know.
It tends to encourage "hogging" (self feeding), but you can control its ability to do so by limiting front relief. It's a good way to balance cutting characteristics, and it strengthens the tip of the tool. You are best served in this situation, assuming you single point the cut, to have a chip breaker, which forms the desired positive rake. As feed rate is limited, the chip breaker shouldn't be long, and shouldn't create an abrupt wall that might cause chips to stack against the breaker. That's pretty much a guarantee of another broken tool.

Breakage in cuts like this is quite common, and often caused by the sides of the tool making contact with the resulting cut, or by chips wedging between the tool and the cut on the sides. You must be diligent in limiting tool contact (it should narrow slightly as it leads to the shank), plus you must ensure that the chips are removed completely. The widest portion of the tool (not including the shank) should be the cutting edge. Keep the tool as short as you can, and snug up any free moving slides. Rigidity is quite important in this instance.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

User avatar
GlennW
Posts: 6969
Joined: Sun Apr 15, 2007 9:23 am
Location: Florida

Re: Trepanning Revisited

Post by GlennW » Wed May 12, 2021 7:02 am

Here is a ValCut trepanning tool showing serious positive rake with minimal front relief to give you an idea.
DSC03316.JPG
These things really work.

https://www.youtube.com/watch?v=gviQG3k ... nel=raybmc

I have yet to figure out how to post a YouTube video...
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!

User avatar
rmac
Posts: 283
Joined: Tue Jun 26, 2012 12:48 am
Location: Phoenix, Arizona

Re: Trepanning Revisited

Post by rmac » Wed May 12, 2021 9:09 am

Bill Shields wrote:
Tue May 11, 2021 9:33 pm
PH Horn and others make very good tools for this method of metal removal.
Thank you. Their miniature boring bars and such look really interesting, too.
Harold_V wrote:
Wed May 12, 2021 3:42 am
assuming you single point the cut,
What do you mean here by "single point"? I've only heard that term used when cutting threads.
GlennW wrote:
Wed May 12, 2021 7:02 am
Here is a ValCut trepanning tool showing serious positive rake with minimal front relief to give you an idea.
Wow. I had nowhere near that much rake.

From what you folks are saying, I think both broken tools were due to chips piling up in the cut. Before I added the top rake to the HSS tool, the chips (as you can see in the picture) were relatively straight, and they kind of just scooted over the top of the tool and out of the cut. But with the top rake, the chips were more curly and tended not to clear themselves as well. Also, after adding the top rake I may have gotten lazy and failed to hone the tool as well as I could have.

If I was in a huge hurry to make some part, I think at this point I would go back to what worked (no top rake) and press on. But I'm not! So I will try to make another tool that incorporates these many hints. Thanks for all of them.

-- Russell Mac

User avatar
BadDog
Posts: 5066
Joined: Wed May 17, 2006 8:21 pm
Location: Phoenix, AZ

Re: Trepanning Revisited

Post by BadDog » Wed May 12, 2021 12:38 pm

I probably should have mentioned front relief to prevent self feed, but didn't specifically consider it, probably because as a general rule I always minimize clearance with purpose ground tools both to limit feed and to support the tool (in this case, cutting edge).

As a result, with complex geometries, I may encounter rubbing more often than someone providing a more generous clearance, so I always (try to remember to) review/check the tool for such when working with a new grind, or applying it in a different operation.
Russ
Master Floor Sweeper

User avatar
rmac
Posts: 283
Joined: Tue Jun 26, 2012 12:48 am
Location: Phoenix, Arizona

Re: Trepanning Revisited

Post by rmac » Wed May 12, 2021 1:51 pm

BadDog wrote:
Wed May 12, 2021 12:38 pm
I probably should have mentioned front relief to prevent self feed, but didn't specifically consider it, probably because as a general rule I always minimize clearance with purpose ground tools both to limit feed and to support the tool (in this case, cutting edge).
Actually, Harold mentioned minimizing front relief in a post a couple of months ago when we were all first talking about this. I remembered that, and tried to provide "just a little" front relief. But I didn't/don't know exactly what "just a little" means yet. Hopefully that's part of what I'll figure out from playing around with all this now.

-- Russell Mac

Harold_V
Posts: 18849
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Trepanning Revisited

Post by Harold_V » Wed May 12, 2021 2:45 pm

rmac wrote:
Wed May 12, 2021 9:09 am
Harold_V wrote:
Wed May 12, 2021 3:42 am
assuming you single point the cut,
What do you mean here by "single point"? I've only heard that term used when cutting threads.
As opposed to a tool with multiple teeth. Like a hole saw.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

Russ Hanscom
Posts: 1837
Joined: Wed Mar 15, 2006 11:10 pm
Location: Farmington, NM

Re: Trepanning Revisited

Post by Russ Hanscom » Wed May 12, 2021 4:24 pm

How are you holding the bit? Is it mounted directly in a tool holder with minimum stick out?

User avatar
rmac
Posts: 283
Joined: Tue Jun 26, 2012 12:48 am
Location: Phoenix, Arizona

Re: Trepanning Revisited

Post by rmac » Wed May 12, 2021 6:19 pm

Harold_V wrote:
Wed May 12, 2021 2:45 pm
As opposed to a tool with multiple teeth. Like a hole saw.
Aha. Thanks.
Russ Hanscom wrote:
Wed May 12, 2021 4:24 pm
How are you holding the bit? Is it mounted directly in a tool holder ...
Yes.
... with minimum stick out?
Yes, but in this case, "minimum" is still quite a bit. The original goal was to make an annular groove with an OD of about 3/4", about 3/16" wide, and 5/8" deep. So the tiny little bit had to stick out at least 5/8". I was trying to meet those requirements yesterday with the first tool that I made, but today I'm not trying to go so deep just to avoid a bunch of grinding.

The most recent iteration was a tool that looks much like the one Glenn showed (again made out of a cut nail because I didn't want to sacrifice any more real tool bits to these experiments). It had problems at first because it was rubbing on the sides as Harold warned against earlier today. Once I fixed that, it worked as well or better than any of the previous attempts. The chips didn't really break, but they did exit the cut reliably and smoothly, much like I saw with the first tool I tried that had zero top rake.

I still think the original part was best made using a rotary table and an 1/8" end mill, but the experiments today were time well spent regardless.

-- Russell Mac

Post Reply