Keep crashing internal threads on Grizzly 4003 lathe

[ridgeway]

I have no idea what I'm doing wrong or if there is something wrong with my machine or all above. I have my set up just like the picture below. I am doing 18 TPI with a minor diameter of 1.005" to screw over a gun barrel tenon. I tried this 4 times, same thing. I make several passes that cut fine and boom, threads get crashed! I spend a lot of time turning, drilling and boring to prep for the threads. Not sure if there is lash somewhere that is upsetting the travel? I am engaging the half nut on the same number each time. Each pass, I take .005" on compound.

Any tips of suggestions to try? I'm at a loss.

I've can thread external threads no problem. Below is pictures of 18 TPI on 304 SS and 1/2-28 on cold rolled bar stock. They come out fine.

 

[Bill C.]

I have no idea what I'm doing wrong or if there is something wrong with my machine or all above. I have my set up just like the picture below. I am doing 18 TPI with a minor diameter of 1.005" to screw over a gun barrel tenon. I tried this 4 times, same thing. I make several passes that cut fine and boom, threads get crashed! I spend a lot of time turning, drilling and boring to prep for the threads. Not sure if there is lash somewhere that is upsetting the travel? I am engaging the half nut on the same number each time. Each pass, I take .005" on compound.

Any tips of suggestions to try? I'm at a loss.

I've can thread external threads no problem. Below is pictures of 18 TPI on 304 SS and 1/2-28 on cold rolled bar stock. They come out fine.View attachment 60492View attachment 60489View attachment 60490View attachment 60491

I would suggest making sure to remove all the backlash is out of the compound and cross feeds. Also I would use an indicator with a magnetic or fixed base as a part stop. I have used a zero to one inch travel indicator. I know it is frustrating but must be some little problem causing such a headache.

 

[Ray C]

Need to see the cutter and threading bar setup... Looks like you're bottoming out.

Ray

 

[markknx]

have you checkedfor play in your compound, and cross slide

 

[ridgeway]

I agree, it has to be something simple. As for backlash, I do have some in both. I am feeding toward me, so both compound and crossfeed should be tight against my stock.

I am using a dial to stop plus .010" more so i dont bottom. Who know, maybe it still is. If I try again, i may cut a relief at the end with a boring bar.

Ray...tooling is 1/2" Mesa threading bar with carbide insert.

 

[DBlue]

.....

 

[schor]

I agree, it has to be something simple. As for backlash, I do have some in both. I am feeding toward me, so both compound and crossfeed should be tight against my stock.

I am using a dial to stop plus .010" more so i dont bottom. Who know, maybe it still is. If I try again, i may cut a relief at the end with a boring bar.

Ray...tooling is 1/2" Mesa threading bar with carbide insert.

I also believe your compound is incorrect, you need to either place it like in the diagram or you would need to turn it to the back side of the lathe at 30. (or 29)

 

[schor]

More on what's happening.

I went out to the shop for a quick demo of how your crashing your threads. The position of your compound is incorrect and the forces of cutting the thread are pushing back against the compound causing the backlash. It's a bit hard to see the backlash in the video but just go try it yourself, push back directly towards the tailstock and you will see the same.

[video=youtube;StzhjogT47g]http://www.youtube.com/watch?v=StzhjogT47g&feature=youtu.be[/video]

 

[ridgeway]

I also believe your compound is incorrect, you need to either place it like in the diagram or you would need to turn it to the back side of the lathe at 30. (or 29)

I did have it set on the backside. I just threw the threading bit on my tool holder to take the picture. Sorry for the confusion.

 

[Ray C]

Here's a bunch of random thoughts because I don't see a smoking gun.

1) I can't count the number of times I've done inside threading and didn't bother to change the compound to 30[SUP]o [/SUP]the other way. It doesn't seem to make a difference because I do straight-in feed with the cross slide most of the time and when I do, I set the compound 90[SUP]o[/SUP] perpendicular. If you are going through all the steps of manipulating the compound by a few thou here and there then yes, you should have it the other way. I'm not advocating you do the way I do -just saying I seem to be successful at getting away with it.

2) It looks like you're cutting brass or bronze which tends to make "powdery" swarf that easily packs up under the cutting bit. A little bit of lube will go a long way and also, keep the bit clean (be safe now).

3) Try making a spring pass (no additional dial-in) after every second cut. Brass and Bronze are usually pretty cooperative and cuts clean but, it's worth a shot especially since there does not appear to be face or top relieve on that cutter. -But I can't see it well enough to tell really.

4) Are you sure that bit is dead-on perpendicular and sure it's real tight in the bar, and is the bit centered?

5) With inside threading, you need extra face relief because the curve of the piece is coming right at the base of the cutter. If there is little to no face relieve on that cutter bring it up above center by about 1/8" or so -or better yet, grind some relief on that cutter.

That's about all I can think of today but, I'm kinda whipped from a headache at the moment. If more comes to mind, I'll post.


Ray