Drawbar material

[pinstripe]

My horizontal/vertical mill has different drawbar threads for each spindle. Both are 40 taper, but the vertical is a metric thread (M16) and the horizontal is imperial (probably 5/8-11). I'd like to make a metric drawbar for the horizontal spindle.

My steel supplier carries mild steel, 1045 (medium tensile steel), 4140, and EN36A (chromium, nickel case hardening steel). Should I just go with mild steel, or would one of the others be better?

[Mcgyver]

it wouldn't hurt to use something stronger, but imo, mild steel is fine. its the taper that does the holding.....I just don't see a reason to ever tighten so much that the threads will rip out. Worst issue I'ver ever had is with a shear pin hold the hex section on the end, but never with the drawbar itself

[RichR]

... Worst issue I'ver ever had is with a shear pin hold the hex section on the end, but never with the drawbar itself

I've broken the roll pin twice. That's why I now choke up on the wrench when tightening the drawbar.

[BCRider]

I'm thinking that mild is fine from a strength standpoint. But I wonder at the long term durability of mild steel threads used as frequently as a drawbar is used.

[Paul Alciatore]

I totally agree. I have had the head's shear pin break. This will usually happen far sooner than the threads failing. Just use mild steel.

And if there is a failure in the threads, I would rather have it in the drawbar instead of in the collet. The drawbar is less expensive and easier to replace in the shop.

it wouldn't hurt to use something stronger, but imo, mild steel is fine. its the taper that does the holding.....I just don't see a reason to ever tighten so much that the threads will rip out. Worst issue I'ver ever had is with a shear pin hold the hex section on the end, but never with the drawbar itself

[Toolguy]

I would go with the 4140, probably the good middle of the road choice.

[Norman Bain]

I have used mild steel without any issues. For HSM use my finding is the "changes" are not frequent enough to wear one out.

For the "nut" end I have taken to using BMW wheel bolts ... the tapered kind. They have a quite long hex portion and a sort of "flat washer" portion. That is, once you have trimmed most of the taper there is a sizeable flat surface.

Hold the nut in the lathe by the hex, trim off the thread and most of the tapered portion to make a flat base. Drill thru in preparation for your bar.

With the nut prepared (but not yet attached to the bar) thread the bar. Assemble the bar into an arbour in the mill; not all the way in. Position the nut on the bar to determine how far on the bar it is to go and mark that position with a sharpie. Remove bar from mill and either weld the bar at the extreme end where it protrudes from the top of the nut; or pin and locktite it.

Norman