Multi Start Threading on Manual Lathe Question

[mitchapalooza88]

Hi all looking for some advice from the group. I’ve tried a couple times without success to cut a multistart thread and wanna reach out. I may be missing something in my setup or my math could be wrong.

I’m looking to cut a .75”dia 3 start 60° thread with a 4 tpi lead. So that should be a 12 tpi thread. I have the compound set to 90° and I use 0 - .083 -.166 for each thread cut set from the compound (each thread start progressing by the pitch). I am making my cuts with the cross slide to a double depth of .108.

When I do a scratch pass for the three it seems to line up perfect but after the first start is cut it looks off. And if I keep going it ends up getting skewed.

My only guess is that maybe the aggressive tpi is actually pulling my compound slide? As I don’t have a way to lock it.

 

[john.k]

Cut one pass on 4 tpi lead ,and have a look at that.

 

[wood2steel]

With any amount of backlash in the compound, you tool bit is literally floating throughout the cut with the compound set at 90°. Can't imagine you would ever get a uniform result. Better to go back to 29.5° on compound, turn between centers, and you can index the rod with lathe dog against the jaws on your 3 jaw chuck for you 3 starts

 

[eKretz]

Although it's possible that it could be moving your compound, that's easy to check, and to stop. Stick a dial indicator on it while you're threading and see if it's moving around. If so, you can lock it down pretty well by tightening the gib. Loosen it back up to move, then retighten. You could also follow the suggestions from others, set your compound to 29½° or straight in and use a different indexing method if you have one available.

A suggestion I'd also make is to make a mark on the part and a chuck jaw and make sure they don't move relative to each other. The work may be slipping in the chuck. Also ensure that your workpiece is positively located in the Z axis with either an end-stop or a turned shoulder butted into a jaw face so that it can't be pushed back into the chuck.

Assuming your threading tool is correctly ground, looking at the profile of your thread near the root it does appear as though something is moving in a way that it shouldn't be.

 

[LexD]

Your terminology is very confusing, there is no such thing as 4tpi lead, from what you have written I assume that you mean 1/4" lead which is the distance that a nut would move along the shaft in one revolution.
The threading tool also requires special attention to allow the leading cutting edge to cut feely without rubbing on the flanks due to the high lead angle, this could part of your problem.

 

[Cole2534]

Your terminology is very confusing, there is no such thing as 4tpi lead, from what you have written I assume that you mean 1/4" lead which is the distance that a nut would move along the shaft in one revolution.
The threading tool also requires special attention to allow the leading cutting edge to cut feely without rubbing on the flanks due to the high lead angle, this could part of your problem.

Agreed, I get something like a 7deg helix angle and that requires more relief than one might normally grind.

 

[LexD]

Could you post an end view of the cutting tool that you used?

 

[macsonboy]

do

Your terminology is very confusing, there is no such thing as 4tpi lead, from what you have written I assume that you mean 1/4" lead which is the distance that a nut would move along the shaft in one revolution.
The threading tool also requires special attention to allow the leading cutting edge to cut feely without rubbing on the flanks due to the high lead angle, this could part of your

 

[macsonboy]

does he mean 4 tpi on the lathe leadscrew??

 

[wood2steel]

Mitch,
Forgive my brief criticism here, but looking at the minor diameter relief groove appearance, I suspect there are several machine condition/toolbit grind geometry issues flagging that could also exaggerate the problem with your thread cutting. That simple groove cut should be crisp and smooth. Loose spindles, crossfeeds,etc; improper toolbit height, poor grind geometry are likely the culprit that caused its appearance and only compound the problems when threading, especially steep angle multi-starts.
eKretz has been dead on regarding multi-start threading on recent posts, but I believe you have a few other setup issues to iron out before going any further on that threading project .

 

[gbent]

Mitch, it looks to me like you need to step back to tooling and material 101. The finish on the threading neck and shoulder is horrible. That tearing against a solid surface will destroy something as fragile as a thread flank. It looks like you are using gummy mild steel like A36, along with dull tooling. Get your material and tooling right so you have crisp, clean cut surfaces. Then look to the other posts about how to set up your multi-start thread.

 

[michiganbuck]

You might start with a photo of your tool bit or insert.
When going straight in with the cross feed the heel (follow side) of the bit/insert can push the saddle forward.
But it seems you have more problems than just that.

Can the machine and tool bit/insert turn an OK OD turning operation?

 

[eKretz]

Lots of possibilities. I tend to lean away from mentioning basics like tool grinding only because they are rote to me, don't even think of them. That is a good suggestion.

And yeah, I'm sure he meant 12 tpi pitch, ¼" lead.

 

[sneebot]

Get a 12 TPI tap. Grind a tool out of the tap with three teeth. Cut all three leads at one time.

 

[eKretz]

Get a 12 TPI tap. Grind a tool out of the tap with three teeth. Cut all three leads at one time.

Only if you alter the clearance though. Not an easy task (with the 3 cutters stacked right next to each other at 12 pitch) even with a tool and cutter grinder.

 

[mitchapalooza88]

Thanks for the replies, I am still learning the ropes so new perspectives are helpful from experienced machinists.
I apologize for messing up some of the terminology but I will get better as I learn.

Although it's possible that it could be moving your compound, that's easy to check, and to stop. Stick a dial indicator on it while you're threading and see if it's moving around. If so, you can lock it down pretty well by tightening the gib. Loosen it back up to move, then retighten. You could also follow the suggestions from others, set your compound to 29½° or straight in and use a different indexing method if you have one available.

A suggestion I'd also make is to make a mark on the part and a chuck jaw and make sure they don't move relative to each other. The work may be slipping in the chuck. Also ensure that your workpiece is positively located in the Z axis with either an end-stop or a turned shoulder butted into a jaw face so that it can't be pushed back into the chuck.

Assuming your threading tool is correctly ground, looking at the profile of your thread near the root it does appear as though something is moving in a way that it shouldn't be.

I did actually mark the chuck on my second attempt as this was also a concern I had (no slip for at least that run).

Using the dial indicator is also good tip, I'll give that a go.

I don't have a picture of my thread tool on me but it may be the culprit. I am using a Dorian AXA tool holder with a TNMC-32NV insert. It could be dull, I'll flip it to a new side. The TPI range for that is also 8-36 as I look it up, I was thinking it would be ok cause its like a 12tpi thread, but maybe the lead angle is too great cause the machine is cutting at 4tpi

 

[mitchapalooza88]

Mitch, it looks to me like you need to step back to tooling and material 101. The finish on the threading neck and shoulder is horrible. That tearing against a solid surface will destroy something as fragile as a thread flank. It looks like you are using gummy mild steel like A36, along with dull tooling. Get your material and tooling right so you have crisp, clean cut surfaces. Then look to the other posts about how to set up your multi-start thread.

Thank you, I will sign up for tooling and material 101. Appreciate the advice

 

[Cole2534]

Thanks for the replies, I am still learning the ropes so new perspectives are helpful from experienced machinists.
I apologize for messing up some of the terminology but I will get better as I learn.



I did actually mark the chuck on my second attempt as this was also a concern I had (no slip for at least that run).

Using the dial indicator is also good tip, I'll give that a go.

I don't have a picture of my thread tool on me but it may be the culprit. I am using a Dorian AXA tool holder with a TNMC-32NV insert. It could be dull, I'll flip it to a new side. The TPI range for that is also 8-36 as I look it up, I was thinking it would be ok cause its like a 12tpi thread, but maybe the lead angle is too great cause the machine is cutting at 4tpi
View attachment 385831

I think that has something to do with it. I could see where the lack of clearance and associated rubbing would push the tool rearward and that seem to tie well with your pics.

 

[Conrad Hoffman]

The tool needs to be ground with the right relief, or better yet, just tilted to the helix angle of the thread. Right now it's almost certainly rubbing. I posted about cutting a 4-start some days ago and as soon as the garage warms up I'm going to try and put theory into practice.

IMO, there are steels you just shouldn't use for this. If it has enough strength and wear resistance, try 12L14 or similar free-cutting material.

 

[michiganbuck]

Thanks for the replies, I am still learning the ropes so new perspectives are helpful from experienced machinists.
I apologize for messing up some of the terminology but I will get better as I learn.



I did actually mark the chuck on my second attempt as this was also a concern I had (no slip for at least that run).

Using the dial indicator is also good tip, I'll give that a go.

I don't have a picture of my thread tool on me but it may be the culprit. I am using a Dorian AXA tool holder with a TNMC-32NV insert. It could be dull, I'll flip it to a new side. The TPI range for that is also 8-36 as I look it up, I was thinking it would be ok cause its like a 12tpi thread, but maybe the lead angle is too great cause the machine is cutting at 4tpi
View attachment 385831

That insert should be Ok/good, and better coming in at 29 1/2 degrees. I prefer a back rake and did most of my thread turning woth a back rake and side cutting edge positive rake.
Fingernail shaving sharp is best for threading mild steel.