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Face "pocketing" on a lathe?

KenSta

Plastic
Joined
Dec 19, 2023
I've looked for a long time and never found a good answer. I'm hoping that I'm just not describing the problem in the right terms. Most of the small parts I make have a circular pocket in the top or bottom (or both). pockets range in diameter from 25mm to 32mm and depths from 0.5 to 2mm. The floors need to be smooth and flat. A radius in the wall/floor edge is not a problem at all.

Historically, I've just turned blanks on the lathe and milled the pockets. More recently, I bought a new lathe with live tooling and Y-axis which allows me to mill the pockets, but it seems like there must be a way to turn them.

I've tried using a boring bar for V inserts with the tip pointed forward "SVMBR" (drags on the walls), standard boring bars (rubs on the surface), small solid carbide bars (poor finish on the floor), etc. I've seen face-grooving tools used to make wide grooves, but never tried one and the demos never show them machining to the center. I've seen some single-insert drills that look like they might work, but I don't know how flat of a floor they would make.

Parts are Aluminum, 17-4PH in the H1000 condition, and plastic. Pics attached are an example and drawing.
 

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Perhaps for roughing, but there are a lot of different hole/pocket diameters. I tried using a static endmill, but at if I'm adding a roughing tool, I might as well mill the pocket on the lathe. I'm hoping there's a type of boring bar or face-groove tool that can both plunge into the stock and turn from OD to a little past center to clean up the floor.
 
I'm hoping there's a type of boring bar or face-groove tool that can both plunge into the stock and turn from OD to a little past center to clean up the floor.
This is a pretty small recess ... I've seen little face-groovers but I think it'd be better to rough the recess first then take a finish pass. Even a regular boring tool that'd cut in both directions would work then, and you could get up to a 3/8" or little bigger body on it, which would help.
 
If you finish in the correct direction that should not be a problem:
- Drill as big a starting hole as will fit with with something left for turning.
- Rough turn towards the chuch with CCMT boring bar of suitable size (if you drill leaves too much for finish turning).
- Finish bottom of pocket and radius with the same insert, from the center and out.
- Finish the wall turning from Z0 towards the chuck. Blend the radius. Depending on the material it will probably be ok to just turn the radius a second time.
 
Quickest and easiest way is a ball socket bar and a face offset toolpath. Will have easier chip control than a face groover and will leave a better finish.

edit: very basic example below. If you did it with adaptive stepover it would work even better.

202421014_d50_0_0~tl05_00.png


Screenshot 2023-12-20 083601.jpg
 
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You can buy bars with that geometry down to like 4mm solid carbide, maybe less.
Yeah, for finishing they work good but they suck for roughing. Just plunge in there with a center-cutting end mill or an insert drill then use a little boring-bar type tool to take a nice finish pass. They don't plunge for shit, they don't remove metal for shit, it's not a good method as an exclusive tool.
 
Your part looks great!

I would use a 1/2 or so carbide flar drill then a 3/8 tcmt or ccmt bar to finish. The flat drills work great. Kennametal and Nachi make them maybe Garr and Harvey too. The drill will last forever and they leave a great finish. I'm not sure how big they go but they start getting pretty expensive in bigger sizes.. Get one with a double margin they work much better.

Another option with live tools is to spiral down with a 1/2 center cutting endmill to get a bigger hole. Only one finish pass required then. I've also used indexable endmills for this sort of thing.

I've never found a workable way to bore without some sort of starter hole. Roughing with a boring bar will work but it doesn't work well as you approach center.
 
Quickest and easiest way is a ball socket bar and a face offset toolpath. Will have easier chip control than a face groover and will leave a better finish.

edit: very basic example below. If you did it with adaptive stepover it would work even better.

202421014_d50_0_0~tl05_00.png


View attachment 420295
This is the kind of answer I was hoping for. I have this kind of tool, but when I try this approach. it still rubs on the inside wall. It's already a 0.25" IC (VCGT221) insert so going smaller is possible. Solid carbide was supposed to be the answer, but turning from OD to ID to completely clear the floor didn't work well. I suspect it was because the solid carbide wasn't designed to cut on the outside and inside of the tip. It cut fine while moving down the wall in Z, but crummy while moving across the surface to center.
 
This is the kind of answer I was hoping for. I have this kind of tool, but when I try this approach. it still rubs on the inside wall. It's already a 0.25" IC (VCGT221) insert so going smaller is possible. Solid carbide was supposed to be the answer, but turning from OD to ID to completely clear the floor didn't work well. I suspect it was because the solid carbide wasn't designed to cut on the outside and inside of the tip. It cut fine while moving down the wall in Z, but crummy while moving across the surface to center.
Sometimes I have had to finish by plunging in z to the bottom, retracting coming back in in z on center and turning out to the wall instead of in to center.
 
Your part looks great!

I would use a 1/2 or so carbide flar drill then a 3/8 tcmt or ccmt bar to finish. The flat drills work great. Kennametal and Nachi make them maybe Garr and Harvey too. The drill will last forever and they leave a great finish. I'm not sure how big they go but they start getting pretty expensive in bigger sizes.. Get one with a double margin they work much better.

Another option with live tools is to spiral down with a 1/2 center cutting endmill to get a bigger hole. Only one finish pass required then. I've also used indexable endmills for this sort of thing.

I've never found a workable way to bore without some sort of starter hole. Roughing with a boring bar will work but it doesn't work well as you approach center.
I like the idea of a flat-bottom drill. The problem with using anything like that for roughing is the variety of diameters I have to make these bores. Some sizes are made-to-order because volumes of any particular size are so low. It might be that the best approach to preserve flexibility and still finish it just going to be to keep milling out the pocket with the live FEM and finish as you suggest with the boring bar. This also would allow me to control the diameter with the finish tool and leave the offsets on the endmill alone.

Thanks for taking the time!

Ken
 
I've never used a round button insert before... But isn't this something they'd be good at?
That would be too much radius on the wall, but the idea is solid. That's what I was trying to accomplish using an insert boring bar. The nose radius acts like a little ball so it can cut the wall to the outside of the insert and also the floor which is basically removing material on the left side of the insert. The problem seems to be that the insert was just too tall to cut the wall without dragging. I have a Y-axis so maybe moving the tool off-center in Y would provide the clearance for the wall of the insert inside the pocket.
 
Solid carbide was supposed to be the answer, but turning from OD to ID to completely clear the floor didn't work well. I suspect it was because the solid carbide wasn't designed to cut on the outside and inside of the tip. It cut fine while moving down the wall in Z, but crummy while moving across the surface to center.
It can't, if you think about the geometry and what you are trying to do. You need one type of top rake going down the wall and another for moving across the face. You can't get both in one tool. About the best you can do is neutral rake which won't be perfect for either cut but won't be terrible for the other one.

I have seen little face groovers which might work well for finishing, since they are sort of dished inserts which gives you a bit of positive rake in both directions. They might work okay for finishing, taking just a few thou off each feature. I think you have to rough with something else, nothing is going to cut well in both directions in that small a size. But for readily available, the flat neutral rake carbide boring bars are probably easiest to get, then hand sharpen to give you clearance in both directions.

It'd be good to get someone in here who does a lot of small parts tho, cuz stuff gets different way down in tinyland where you are working.
 
Well I must say that I'm impressed with all the responses. Based on what I'm hearing, it looks like I'm probably not going to get away without a roughing plunge of some kind. That opens up the options for the finish tool. If nothing else, I think this puts to rest the thought that there was some "perfect" tool for this that I just hadn't found.
 
It can't, if you think about the geometry and what you are trying to do. You need one type of top rake going down the wall and another for moving across the face. You can't get both in one tool. About the best you can do is neutral rake which won't be perfect for either cut but won't be terrible for the other one.

I have seen little face groovers which might work well for finishing, since they are sort of dished inserts which gives you a bit of positive rake in both directions. They might work okay for finishing, taking just a few thou off each feature. I think you have to rough with something else, nothing is going to cut well in both directions in that small a size. But for readily available, the flat neutral rake carbide boring bars are probably easiest to get, then hand sharpen to give you clearance in both directions.

It'd be good to get someone in here who does a lot of small parts tho, cuz stuff gets different way down in tinyland where you are working.
Maybe two tools for finishing would do the trick. The little carbide bar for the wall and something a little different for the floor. I don't have the capability to grind tools so I'm stuck with what I can buy. Worst case scenario is just to mill these pockets so it's not like they won't get made.
 








 
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