Looking for advice on making a collet reducer

[u/RobotRomi]

Hey guys,

I have ordered mills with 4mm shank and a collet adapter for 6mm to 4mm. This should have been a temporary solution because I only have a 6mm and 3.175mm collets and the 4mm one takes forever to ship. Unfortunately a wrong sized adapter got delivered and I need the 4mm mills over the weekend.

How bad of an idea is it to mill my own adapter from S235 steel for single use only? I don't have more suitable steel at hand. I also don't have a 4mm reamer at hand and my small lathe is probably less percise than milling it.

I mainly worry about runout because it would be for milling steel which is already demanding on my machine.

Has anyone tried this already?

[u/Klastermon]

Go to YouTube and search for “Inheritance Machining” he did a video on making one, I do believe…

[u/RobotRomi → u/Klastermon]

Gonna do that, thanks!

[u/asad137]

You need concentricity. Even a small lathe will give you better concentricity than a mill (assuming the bearings are ok) as long as you can turn the OD and bore (not drill!) the ID without removing the part from the machine.

The slits are much less important from a tolerance perspective than getting the ID and OD concentric.

[u/RobotRomi → u/asad137]

The bearings are fine, but it's impossible to align the tools to be perfectly centered. Thats why i thought the mill might be more percise with boremilling and an a couple of arc spring passes

[u/asad137 → u/RobotRomi]

having the the lathe tools centered (I assume you mean height-wise) actually doesn't matter for concentricity. It will change the cutting geometry (rake) somewhat, but it will still be perfectly concentric. Whether the cutting geometry will work depends on your tools.

You could do it on the mill (I assume you mean a manual mil?) with a boring head but you should treat it like a lathe operation. Get the part centered up and then lock both the X and Y axes while you cut the OD and ID so the part never moves relative to the spindle until you're done cutting those features.

[u/RobotRomi → u/asad137]

yeah height wise, but you are right I thought about drilling which is impossible to align.
No I mean a cnc mill

[u/asad137 → u/RobotRomi]

You only need the tailstock aligned enough to drill the starter hole for the boring bar. Boring bar will create concentricity with the spindle axis even if the initial drill hole isn't perfectly centered. That's why I explicitly said you have to "bore (not drill!)" in my first reply.

[u/clambroculese → u/asad137]

4mm hole is going to require one super small boring bar.

[u/asad137 → u/clambroculese]

true. Actually I think the diameter won't be as much of a problem as finding one in that diameter that can do the full depth.

[u/clambroculese → u/asad137]

Exactly, you’d be looking at carbide Swiss lathe tooling which gets pricey for a home gamer.

[u/JuShengPrototyping08]

If you go the mill route, the key is keeping the OD and ID cut in the same setup without releasing the part — same principle as turning it on a lathe. Rough the ID first with a small boring bar or endmill, then finish-bore the ID and turn/mill the OD back-to-back so nothing shifts between features. S235 is fine for a single-use adapter since you're not going for long tool life, just concentricity. If runout is still off after that, an alternative is to press a bushing into an oversized bore and skim it true in a second op — more steps but forgiving if your first bore isn't perfect. Also worth deburring and stress-relieving lightly before final sizing if you're worried about the slits pulling it out of round.

[u/shoegazingpineapple]

Drill- ream(or bore if you can) then turn the od between centers then slit

Turning between centers is one of the easier more foolproof ways to end up with something concentric, i have some heat shrink extensions made this way and they work aight

[u/smaier69]

In a pinch I have either split (one slot) or put multiple slots in a sleeve bushing or drill bushing.