I Need A CNC Threading for Dummies Book

[JimDawson]

I don't believe our systems are fast enough to tweak the feed rates thousands of times per spindle rotation and have the mechanical parts respond. Mach3 apparently only makes the feed rate calculation once between passes. In my 10-32 example, it used the same Z-axis feed rate for 24 turns.

Some interval between adjusting Z-axis feed once in 24 turns and "several times per turn" seems like a better thing. Your encoder may be giving your 4000 PPR, but you're very likely using very little of that.

Mach3 is probably the worst case in that it grabs a sample prior to the start of the cut and as far as I know does not make any adjustments while cutting. Not sure if Mach4 has the capability of adjusting things on the fly.

My system is a bit different in that it tries to adjust the RPM every 10ms or so, but I know the system can't react that fast. The primary difference is that I'm using electronic gearing so the Z and A axes are tightly coupled with a position update every 62us, so absolutely constant RPM is not a critical factor, Z just tightly tracks A at the set ratio.

 

[MontanaAardvark]

My system is a bit different in that it tries to adjust the RPM every 10ms or so, but I know the system can't react that fast. The primary difference is that I'm using electronic gearing so the Z and A axes are tightly coupled with a position update every 62us, so absolutely constant RPM is not a critical factor, Z just tightly tracks A at the set ratio.

That's the way it should be done, and the whole reason I'm going down this road of trying to get my Sherline to make all of my threaded parts is to avoid hard gears. The gear change system on a manual Sherline is inconvenient and the gear arrangements for some threads (finer than about 28 TPI) go beyond inconvenient to dangerous on my SC4 lathe. The gears don't fit inside the cover, so that you're standing at the controls to stop the motor with your hand a few inches from a moving gear.

This is Florida, so I don't wear long sleeves in the shop (not regularly) that would be a bad combination.

 

[mmcmdl]

It really looks like the part might be springing away from the tool on the start of the thread.

That's what it looks like to me also , and if it is , you could program a tapered thread to compensate for the spring . I don't believe multiple spring cuts would make much difference .

Or just run a die up on it for finishing .

 

[MontanaAardvark]

That's what it looks like to me also , and if it is , you could program a tapered thread to compensate for the spring . I don't believe multiple spring cuts would make much difference .

Or just run a die up on it for finishing .

I've run a nut up the last couple of threaded pieces and it works well as a "sorta die". It doesn't have slots for chip relief and doesn't produce visible chips but after running it back and forth up the threaded rods, they get smoother.

This makes me wonder if perhaps the headstock isn't exactly parallel with the Z-axis. How Sherline does that is they used about a 4" long square key to line it up and then single screw to lock everything in place on a tapered shaft. I should try to check that with my sensitive DTI.


For illustration, this is an accessory riser that raises the headstock to allow bigger work pieces, but shows the square alignment key and the tapered pin with the large screw that holds the headstock in place. If that was sitting slightly out of a perfect position, I could see the headstock axis not being exactly parallel to the Z-axis.

 

[MontanaAardvark]

For posterity - and those who might be reading and not commenting.

I tried more spring passes and it's definitely better. Much better.



Now it's just the first couple of threads and not the first quarter inch. I think I can see the brass deflect at the end as its cutting, and then the spring passes taking more at the right end. I did 8 because I had no idea what number is needed. I might retry this with a LOT of them. Like 20 or something. But - why is the end moving?

I wanted to try a lathe center or some way of supporting the end to keep the blank from moving. I don't have a follower rest and don't know if that's the answer either. My live center is long enough, but wide enough that the cutter crashes into it, and my dead center seems narrow enough but is too short. I get this:



I need a longer dead center. Maybe I need to make something - I've never seen any mention of this before.


Bob

 

[JimDawson]

But - why is the end moving?

Looking much better!

Material springs from the cutting forces, that is just a fact of life. But you can minimize that with really sharp tools with the proper geometry. The tool that you are using is not very sharp, and has no rake on the cutting edges (just flat on top). You need some positive rake to have a more knife like edge presented to the work. What you have there now is more like a scraper rather than a cutter. Works fine for roughing cuts with a lot of cutting pressure.

I'm not recommending that you purchase an insert type tool, but here is a pretty good illustration of what it needs to look like.
https://www.sandvik.coromant.com/en-us/products/threading-inserts-grades/pages/default.aspx

You might take a look at these also, they seem to be good tools. I use them on both my CNC and manual lathe. But I do tailor the inserts for the specific material and job. The inserts that come with the set are pretty good general use inserts.
https://www.amazon.com/AccusizeTool...6203334&sr=8-19&keywords=accusize+tool+holder

Hanging your tool out a bit farther from the holder might get you clearance for the tailstock. Also, most of the right hand side of that tool is doing nothing except getting in the way, grinding as much clearance as you need on that side wouldn't hurt a thing.

 

[MontanaAardvark]

No problems on the tool recommendations - thanks. I don't know enough about them. I have some replaceable insert tools with triangular flat carbide cutters, but no experience with threading inserts like you link to. What I have looks like this, only not the name brand:
https://www.amazon.com/Accusize-Ind...-2380-5022/dp/B00I4AW722/ref=lp_257517011_1_3

I'm using this brazed carbide cutter because it's the best thing I've got, but shopping for replacements is high on my priority list. Over the years, I've had better results with brazed carbide cutters than the replaceable insert carbides, but never used one like that AccuSize kit.

At some point, I want to try internal threading, which I've never done with single point cutting.

Unfortunately, hanging the tool out farther isn't the axis I need more room on. The tailstock is hitting the right edge of the cross slide so it can't get any closer. If the tool were about 0.2 or .25 to the right that might get me there, because the slide would be farther to the left. The danger there is crashing the cross slide into the chuck. I don't believe I have a tool holder that would get me there.

 

[JimDawson]

You could always grind up a HSS tool bit for threading, I keep a few around for those non-standard jobs that require odd shapes for clearance. The cutting area on the tool doesn't have to be any deeper than the thread depth plus a little clearance.

 

[Glenn Goodlett]

I finally won a three day fight with my Shopmaster CNC trying to cut threads. The ultimate take away was that my machine, for whatever reason, will not reliably cut threads using a G76 cycle. If I use a wizard or write the G code manually that describes each pass on a separate line, the threads are cut perfectly. Go figure.

 

[MontanaAardvark]

I finally won a three day fight with my Shopmaster CNC trying to cut threads. The ultimate take away was that my machine, for whatever reason, will not reliably cut threads using a G76 cycle. If I use a wizard or write the G code manually that describes each pass on a separate line, the threads are cut perfectly. Go figure.

Good to know. That's a hard won insider secret, so thanks for sharing!