# Turning 4140



## Johnwa (Jul 9, 2018)

The attached picture is of a replacement part for the braking system on a paraplegic’s racing trike. I made it from a cutoff of one of the 4140 rounds we got last year.  Since my order for carbide tooling is currently swimming across the Pacific I had to turn the OD with HSS.  It worked well once I realized I had to run the spindle at 100rpm.


----------



## PeterT (Jul 9, 2018)

Looks nice John. How thick is it?


----------



## Johnwa (Jul 9, 2018)

Peter
It’s about ⅛” thick. The racer doesn’t have any grip strength so she operates the brake by reversing the crank. This is part of the latching mechanism that engages the brake cable to the crank.


Sent from my iPad using Tapatalk


----------



## PeterT (Jul 9, 2018)

That's about what I figured. So how did you hold it onto lathe faceplate or whatever to do the uniform thickness turning?


----------



## Johnwa (Jul 9, 2018)

I held it in a 3 jaw. The only decent carbide tooling I have is a boring bar. I used that to face one side almost to the OD. I then ground off the remaining lip in 3 places so it wouldn’t  interfere with the jaws when I reversed it in the chuck.


Sent from my iPad using Tapatalk


----------



## PeterT (Jul 9, 2018)

I see. Reason I ask is I had to make some thin disks, not that big of diameter though. They were just too thin for the chuck jaws to grip properly. On aluminum stock & smaller diameters I had pretty good luck CA gluing the material on a sacrificial machined piece of scrap. Then use mild torch heat to unset the glue (off the lathe of course). But it still makes me nervous. On larger, heavier stock like yours no way I would trust adhesive 

My other idea, which I never got around to, was kind of a universal tooling plate that gets bolted to face plate from behind. It has an array of closely spaced threaded holes & I could use thin edge holding clamps around the perimeter. There are some fancy clamps that move inward with cam action, but I found that basically 0.1" thick steel washers hold pretty good especially if there are lots of them around perimeter to share the load. I wouldn't trust this for eccentric turning.

If there any holes on the part itself to serve double duty to bolt to faceplate with standoffs, that's another solid technique. But the bolt heads have to be flush for cross feeding (or at least temporarily if they can be drilled out later), so only works in certain applications.


----------



## DPittman (Jul 9, 2018)

John,
I got a few pieces of that metal in the same group buy and I remember seeing 4340 (I think) on some pieces.  Do you know for sure what stuff is 4140 and which might be the 4340?

By the way, nice work. I wouldn't have know to turn so slowly.

Don


----------



## Johnwa (Jul 9, 2018)

Don
I believe the 5” round with the 1” hole in the center is 4340. If you search the forums for 4340 there are a few posts about it.

As for turning speed I didn’t know either until I did a bit of research last night. I found a table that gave 100 sfpm for turning annealed 4140 with HSS. The 4” round is about 1 foot circumference therefore 100 rpm. 

John


Sent from my iPad using Tapatalk


----------



## DPittman (Jul 9, 2018)

Thanks for the info.  I always learn something on this forum with more ease than many other forums!

Don


----------

