Screw cutting up to a shoulder

[John]

How do you cut a thread close up to a shoulder?

With a lathe, the width of the tool stops you cutting a thread right up to a shoulder.

Using a die, all the ones I have, have a tapered entry both sides so that when the body of the die meets the shoulder the cut thread is 2-3 mm back.

Can be done, as many (but not all) bolts have the thread cut right up to the head of the bolt.

Maybe there are types of dies that will cut right up to a shoulder?

John

 

[Swede]

John, most of the bolts you can buy at a hardware store have rolled threads, and their automated machinery allows this happy state. For us, it is a different story.

First, I would try my best to engineer or otherwise avoid the NEED to use such a fastener. If that doesn't work, you have a couple of options. First, I would get the screw threaded as much as possible using traditional means. I know you mentioned the taper in your dies, but most that I've seen have significant taper on only one side... the back side has much less. This you reverse and run it onto the screw backwards, which avoids the starting taper. This should give you decent threads almost all the way; just the last turn or two will be incomplete.

It is possible on a lathe to kill the power, and using a hand crank or similar, thread as closely to a shoulder as the machine can do, perhaps with a sneaky thread cutting bit especially ground for the job. But this would be tedious, to say the least.

The last option I can think of is to fabricate the fastener from two components. In other words, you have a section of threaded rod, and this in turn is screwed into what will ultimately become the head of the fastener. You can then either weld, silver solder, or simply apply enough torque on the "head" to firmly secure it to the threaded section.

Anyone else?

 

[Davyboy]

-I've used all the techniques decribed by Swede, last time was using a 8-32 set screw inserted on the end of a shaft. Drill & tap the shaft, then loktite the set screw in place.
-On larger fractional or industrial size applications, I like to cut a relief groove at the base of the shoulder, to remove the incomplete threads. Model sized app's may not have enough room. Also, the groove can be a weak point for breaking the end off.
-Grind the threading tool on the left side, so the point can get close to the shoulder.
-I've used a threading die as Swede says, but had the back ground off with surface grinder to remove the tapered threads. If you have two dies, use the ground off die as a third step after the front and back of a tapered die.

Good luck, let us know how you make out. Dave

 

[Mcgyver]

first thing to try is design change, can you recess the threads in the mating part etc. next is grind a threading tool with the point as far over to the left as will still allow the required depth of cut. you might be surprised with the lathe running slowly how accurately you can end a thread by winding out the cross feed and simultaneous disengaging the feed...you can cut a thread in the lathe where the looks like it was cut with a die, ie the V just ends without a minor dia undercut. with correctly a ground tool as mentioned, concentration and a bit of practice you can thread this way pretty darn close to a shoulder - but pay attention or you'll have a crash!

It if needs to be even tighter than that, put the lathe motor in reverse, put the compound at 29.5 to the left, turn the tool upside down and thread from shoulder to tailstock. turn a small relief groove to the minor dia (use a rounded end tool to avoid stress risers) start each pass with everything engaged before you hit the power. This will get a thread to a shoulder that will screw in flat to a threaded piece with no major dia counter bore.

 

[John]

Thanks guys.

As usual lots of usefull advice. I have found some dies among some assorted old bits and pieces that only have a taper on one side so will go with this method. All the newer dies I have all have tapers both sides.

I like the idea of cutting threads in reverse, away from the lathe chuck so there is no possibility of a crash. Will need to make sure the carriage is snugged down as there will be some tendency to lift it off the ways.

In the past I have turned my lathe by hand when cutting fiddly threads in plastic. Works well.

Thanks
John

 

[DickDastardly40]

I almost without exception machine a recess .010" under the thread minor diameter when screwcutting and either disengage the leadscrew or stop the machine (depending on if cutting metric or imperial threads) when the tool enters it. The width of the recess varies acourding to the size of the tool. To my mind it looks neater and if on a bolt will be used with a washer in most cases, if not screws in to full depth.

Al