Proper cutting tool form

[Privateer]

I'm making a part to affix to my lathe's spindle, and I'll be cutting an internal thread of 1.5"-8. In the past when I've cut these threads its been on a part where the thread was completely through the part. This time I'll need to put in a relief cut so I can move the cross slide when the tool clears the thread. I've not made a tool to accomplish this particular task before, and I'm a little fuzzy on the most appropriate shape. I've got a 5/8" boring bar with the 1/4" slots for HSS tool bits, and I am able to grind my tools, with varying degrees of success so far.

The other aspect is, should I be just doing a plunge cut into the ID or should I be trying to make short passes. All blind work, of course, so I'm a little leary about jumping right in.

I'm making a collet holder that will attach directly to the spindle so I can put the work through the collets and subsequently through the spindle. The tool I have now doesn't allow me to do more than hold the work.





I've got a piece of crs that I'm using to make this piece, and I've got the ID bored to the min diameter of the thread now. I just need to get the relief cut made before I proceed any further with it.


Opinions from persons more experienced than I would be appreciated. Thank you.

Terry

 

[rgray]

If I were doing it (might not be correct) I would thread in reverse way from the bottom.
I would set up a magnetic dial indicator on the way and zero on the depth that I wanted(tip on carridge).
I would set the compound at 29 1/2 deg pointing away from the part.
I would initialy set my compound and cross slide so the cross slide handle is in the down position.
All avancing for the cut is done with compound after that.
Choose thread pitch of course...Engage half nuts and make light first pass.
Leave half nuts engaged....back off 1 turn on cross slide...reverse back to starting point with the jog/inching button (goes towards headstock). go past zero approx .010 then turn forward by hand to zero...turn cross slide in one turn...add the amount you want to cut next to compound (usualy .005)...run reverse again.
Sounds complicated and time consuming but realy is not bad...My lathe's inching button will usualy get me within .020-.050....then I turn past by hand and then in the cut direction(reverse) till zero(can realy see the backlash in the gear train this way and that is the reason to go past and then turn in rotation to get to zero).


That's how I do it...If your lathe has a screw on chuck some will not like this procedure(mine has cam locks).
If you don't have a jog/inching button...just running in forward is the same(not quite as quick so might not get within .050 and then requires more had turning).

Right wrong??? it's just the way I do it.
I like that the tool bit is visable when cutting also.

 

[Privateer]

rgray,

My lathe is an Atlas 10" circa 1932ish, and I don't have a means to secure the chuck to the spindle, so running in reverse isn't really an option. That and I don't have the motor wired to be able to do so. But otherwise that would have been a great way to accomplish it, without having to cut the relief in the work.

Terry

- - - Updated - - -

Shawn,

I see what you mean, and my boring bar has those nice little graduation marks every 1/4", very handy. One other detail I failed to mention, is the thread is only about an inch in length, so I'll be able to see some of the results on the opposite side of the work as I make the cut. Off to the grinder.

Terry

 

[stevecmo]

Terry,

First off, what collets are those? They don't look familiar to me.

I've done what you are about to do only making an ER-40 collet chuck. Same threads - 1 1/2"-8. You can see a post I made here: http://www.hobby-machinist.com/showthread.php/7920-ER-40-Collet-Chuck(s)

You've done it right so far. You just need to grind a HSS bit with a grooving profile. The relief groove doesn't need to be very wide assuming you're using back gear on your lathe when threading. It's just sort of a landing area to disengage the half nuts.

If you are concerned about a crash, you might consider making a had crank for the spindle so you have more control over the carriage travel. That's a pretty easy mod and is useful for fine threading.

Take it slow and you'll be fine. Ask if you need more info.

Hope that helps.

Steve

 

[Privateer]

Steve,

They are SYIC brand, its an asian brand that Grizzly peddles. the shank is MT3 and the collets are EOC25, similar but not quite the same as ER25s. And finding more than the small set I have is a task.

I took a look at your link and that's precisely what I want to do. I've not managed to satisfactorily identified the threads on the collet holder yet, but I suspect they're metric too. I have several thread guages but none of them line up, and atm it appears to me to be 12.5 TPI.

When doing these internal threads I have been running with the back gears engaged, and I find I have plenty of time to manage the cut. Again, the difference with this one is that it will be blind as opposed to letting the thread complete through the entirety of the work.

I've made the tool, but I'm getting quite a bit of chatter on it, the saddle is locked and the gibs are as secure as I'm able. I've touched it up twice on the grinder and have managed to reduce it a bit, but just not quite getting it. ...

Just completed the blind relief cut.. it complained a LOT. Trying to find that sweet spot for rate/depth of cut is a pain when you can't see it.

Terry

- - - Updated - - -

Jono,

In the first picture you'll notice my spindle plug. That's the first thing I made before I began, so I wouldn't have to dismount the work to check the fit. Its actually longer than it needs to be, though I did intend to bevel the end on the grinder a bit too, just haven't gotten there yet.

Terry

 

[Privateer]

Steve,

I like the idea of making my own nut and disregarding what the thread was on the collet holder, BUT, the nut that came with it has this nice bearing tucked inside and I'd rather use it. So I'm stuck with determining the type of thread.

Terry

 

[Privateer]

Welp, my threading failed. I didn't quite have the QC toolpost tight enough and the holder rotated a few degrees, and it did it so gradually I didn't catch until about halfway through the total depth. Fortunately, it was only about 2 inches of 2" crs that I wasted in this first attempt on this part. And all the parts I need to complete the first stage are made and ready to be used again.

 

[Privateer]

I finally identified the thread on the collet holder, 48M x 2. And looking at how to cut metric threads on an Atlas lathe, it will be just a touch more troublesome than regular thread cutting.

Terry

 

[stevecmo]

Terry,

I did a Google search for EOC25 collets and didn't really find any useful info. I also searched Grizzly's site and didn't find anything. One thing I did see in the Google search was that they are primarily used in woodworking. Is that correct? Did you get this collet set with the lathe or is it something you purchased separately?

I'm wondering if you wouldn't be better off going the ER collet route. They are relatively inexpensive, have a wide tolerance on diameter grip, and achieve your goal of stock thru the spindle.

If you go that route, you can make the whole shebang (collet holder and nut) from hex stock and not have to bother with metric threading. Then you only need to buy the collets. Usually a half dozen sizes will do.

Steve

 

[ridgeway]

I scewed up 5 times trying to thread a 1.06 x 16 hole for a barrel indexing tool. I know where you are coming from! My final attempt I was sucessful threading from inside out and that almost failed...the carbide threading bit decided to break and crashed 3 threads. Luckily, the threads were deep enough and I counterbored the crashed threads away. I salvaged it and works great.