Modified flank Inside threading away from the chuck question

[benmychree]

It does seem that most amateurs are mesmerized by the half degree thing, as far as needing to measure it, there is no need to do so; finding a compound rest with half degree graduations would be quite rare, I would think, and if there was one, it would be difficult to read, particularly on a small lathe, but it is easy for the eye to interpolate a central setting between two adjacent degree lines. To add to confusion some lathes start the numbering fromthe cross slide centerline, and some from the long axis of the lathe centers.

 

[3strucking]

you can use a 30 degree angle block to check your cross slide angle with. Place the flat end against your chuck body and turn your compound to meet the angle. Then you can put a mark on your cross slide if it doesnt have one.

 

[WobblyHand]

I was so intent on doing this setup, I transposed a gear pair. Ouch. I can flip the part around and do the other side. Can't salvage this side. I will have to indicate to center on the bore, rebore and thread.

However, got it all set up and it was cutting the thread pretty nicely. Seemed like less load on the lathe. Too bad I cut the wrong pitch. I put the 65T on the bottom and 50T on the top. It should have been the 50T on the bottom and the 65T on top. So I got 0.89mm pitch instead of 1.5mm. I always face the gear teeth number outwards, so I can read off the numbers. For some reason today, I faced the stack the opposite way.

The way I set everything up was poor. I couldn't get a thread pitch tool in, because my dial indicator and the bar holder was in the way. The 1" indicator was fully compressed, because I couldn't fit indicator on the chuck side. And because the thread was metric on an imperial lead screw, I had to leave the half nuts closed. I'd say I painted my way into a corner. Live and learn. So I get to do it again Hopefully in a better way!

I did find a formula that works for mini-lathes. Kind of the reverse of the standard gear calculator. It tells you what you actually cut, with the gears that you rightly or wrongly chose.

TPI = L / ((A/B) x (C/D)) where L is the TPI of the lead screw, A is the number of teeth on the top gear, B is # of teeth on the inner intermediate gear (on the bottom of the shaft), C is # teeth on the outer intermediate gear, and D is # of teeth on the gear on the lead screw.

 

[T Bredehoft]

Just to raise dust, what difference does it make which side the cut is taken from?

I see that by having the compound in the normal position, you advance the tool away from the chuck when cutting behind the work. Is this a problem? If you have the compound swung toward the chuck, and advance the tool behind the work you advance the tool toward the chuck and therefore toward the shoulder we are so anxious to stay away from.

 

[epanzella]

I set my compound to 77.365 degrees and then feed with the cross slide.

 

[MrWhoopee]

Like John, I was taught by a retired Navy instructor, precision flat-top haircut and all. Harry taught us that the 29.5º setting was simply to allow the trailing edge of the tool to take a very slight cleanup cut on each pass. If set at 30º, because of the imprecise nature of the graduations on the compound, there is the possibility of the trailing face having slight steps from each successive pass. There is nothing magic about the number, just an RCH under 30º. The same thing can be accomplished by feeding straight in with the cross-slide for the final pass.

 

[WobblyHand]

Just to raise dust, what difference does it make which side the cut is taken from?

I see that by having the compound in the normal position, you advance the tool away from the chuck when cutting behind the work. Is this a problem? If you have the compound swung toward the chuck, and advance the tool behind the work you advance the tool toward the chuck and therefore toward the shoulder we are so anxious to stay away from.

With the compound the way I have it, the tool is advanced in the direction of the cut. With the lathe in reverse, and the cutter on the backside of the bore, the compound advances towards the tailstock, which is away from the shoulder.

 

[MrWhoopee]

Just to raise dust, what difference does it make which side the cut is taken from?

If the tool is cutting on the trailing edge, the cutting forces are in the same direction as the feed. This is similar to climb milling. Any lash in the drive train will allow the cutting forces to push the tool and carriage forward. It will then pause until the lash is taken up, then repeat.

 

[aliva]

A few years ago I made a bullnose for my tail stock, which required some inside threading for a end cap to hold the bearings in place. Not knowing anything about inside threading, i said to myself this is no different than external treads, right. I set my compound to the standard 29.5 degrees used an external threading insert and cut away, towards the chuck Everything worked out fine. I don't know if I was lucky or all the hype about internal threading is bunk. I've used the bullnose many times and never had an issue with threads.