Making cross feed screws or lead screws, tips and tricks

[4GSR]

I started a thread on my B & S 612 Surface Grinder covering making a new cross feed screw.

Here's a few hints / tricks on machining new screws that I've used over the years.

1) Buy a stocked length of your favorite screw material if you can. That way you can skip most of this mentioned below.

2) Use a on size material for your screw, rather it's cold rolled steel, turned, ground, & polished stress proof, drill rod. It just makes life much easier doing so. Avoid using hot rolled materials, even if its all you have. For my new screw, I used a piece of 7/8" OD 1018 CRS I bought from a local source. I wanted drill rod, 3/4" was the biggest they had without ordering a piece.

3) Use a lathe equipped with a follow rest. You pretty much must have one to get it done correctly. I used my old faithful 9" SBL to cut the new screw.

4) use a threading tool ground for a pitch or two smaller than what you are cutting. Example, the thread I cut for the cross feed screw I made was a 10-pitch Acme thread. I used a 14-pitch tool I had on hand, 12-pitch would have worked too, if I had one.

More to come....

 

[4GSR]

I’m not going to go into detail of chucking up your material and getting it running true. You should already know this. But I will say this, get it running what call “dead true” within .002” or better run out, with the ends running .000”.

For Acme threads set your compound to 14 deg. off of perpendicular of centerline of the part. 60 deg. threads set it for 29-1/2 deg.

Set the follow rest jaws so they are snugged against the OD of the screw blank. Do this at the headstock end of the screw blank. Be sure to oil the lead screw and carriage, well.

With threading tool set and follow rest set, hand crank the carriage from end to end to make sure you have traveling clearance and you don’t crash into the chuck or tailstock while threading.

Take a .005” depth trial cut to check pitch/lead of thread being cut.

If all looks good, apply your favorite cutting fluid/oil or both and start taking cuts, about .005” on the compound dial per pass until you get to about 2/3’s the depth needed. Start reducing the amount per cut to around .002”. This will vary according to how small of a lathe you have or how big a lathe you have. I’m basing this on my 9” SBL. My Sheldon/ Sebastian lathe, I can almost cut this thread in a couple of passes!

At 2/3’s depth, take time to de-burr the top of the thread with a flat file to remove the burr. Next re-adjust the follow rest jaws. Apply more oil to the lead screw.

Let me make a comment here about threading tools. I’m spoiled to using threading insert type tooling, but on a small lathe and even on larger lathes, its important how to use the insert and make it cut. I’m not an expert by any means, so if someone wants to chime in and give expert advice, go for it! The inserts selected must have little or no hone to the edges at all. Yea, I know, you got to break the edge some, but for this kind of threading, keep it as minimum as you can, at least for your finishing passes.

Again, keep the part flooded with your favorite cutting fluid. I prefer Tap Magic for this kind of threading. Lathes with coolant systems, you’re on your own on this one!

The last third of cutting depth is done in small increments of .001-.002”. After you have established your depth, back off the compound and start shaving the sides a .001-.002” at a time on both thread flanks until you get the thread to the correct PD or measurement over wires dimension. Keep the screw flooded with your cutting fluid.

Once you get the thread to the desired dimensions, take a triangle file and break the edges of you newly cut thread. Next, clean out the thread to remove oil, shavings, etc. Take a piece of emery and polish the thread and make it look pretty. And you got it!

Again, if anyone wants to add to this or make a comment, please do so.

 

[savarin]

what is the purpose of point 4 in the first post please.

 

[Andre]

What is the need to use a smaller threading tool for the thread pitch? Is that so there is more space between the crest of the threads accommodating for nut wear?

Also noticed on your lathe the brilliant flaking marks, is it almost new lathe or has it been rescraped and/or reflaked?

 

[4GSR]

The reason for using a insert or threading tool one pitch smaller. It allows you to sneak up on cutting the thread and getting it exact the first time without scrapping the leas screw and having to do it again.

Once you get the depth correct, start shaving metal off the flanks of the thread. Doing this will give you a fine finish thread that will almost look like it is ground. But its also important to use a good cutting fluid like Tapfree or Tapmagic along with a good cutting oil on the finishing passes. by using the three wire method of checking the PD of the thread, you can get it within a few thousandths of being correct.

You don't want to be taking material off of three faces at the same time using a full width threading tool trying to cut a lead screw. You wind up with torn thread flanks that you cannot fix without going undersize on your thread.

I use this method for cutting most any Acme threads on larger and smaller manual lathes. Especially if your trying to cut a 4 pitch full Acme thread. I'll rough out the thread with a 6-pitch insert get it to depth, change the insert to 4-pitch and finish the thread. That way, you maximum life out of your 4-pitch insert on many parts.

 

[4GSR]

.........Also noticed on your lathe the brilliant flaking marks, is it almost new lathe or has it been rescraped and/or reflaked?

My dad did that about 35 years ago. I went through and reconditioned this lathe about 15 years ago and finished the flaking job dad started. Dad's flaking is much nicer than mine will ever be! Sadly, he no longer around...

My grandpa gave this 9" SBL lathe new to my dad for Christmas in 1948. It has well been taken care of by me for most of my life, and hope my son will do the same.

EDIT: I still have the original sale receipt and the payment plan they used to buy the lathe from SBL!