Knee mill draw bar failure

[cathead]

Today while tightening up the draw bar on my Enco mill, I felt something give way. The threads on the end of the draw bar were worn
and had slipped. I found a suitable bolt to modify to replace the worn threads.
In the front of this photo is the end of the draw bar as it came out of the mill. The head of the donor bolt was machined away and tapered
so I could put it back together.
This is how I positioned the parts to weld together with the acetylene torch.
Now the draw bar is welded in two spots but no big deal as long as the welding is good. I did experience a little binding so had to
reheat the weld area and give it a little tweak. It's back to it's old self now so I'm back in business, well play actually.

 

[projectnut]

It probably stripped because it's been overtightened many times since it was new. I've seen guys tighten the drawbar as tight as they could get it with a 2' wrench. Bridgeport recommends tightening it to around 40 ft. lbs. The recommended torque for a 7/16-14 grade 5 or grade 8 bolt is 37 ft. lbs.

 

[benmychree]

It probably stripped because it's been overtightened many times since it was new. I've seen guys tighten the drawbar as tight as they could get it with a 2' wrench. Bridgeport recommends tightening it to around 40 ft. lbs. The recommended torque for a 7/16-14 grade 5 or grade 8 bolt is 37 ft. lbs.

Yes, overtightening is likely the cause; excessive tightening does not make a setup more rigid, it just wears things out.

 

[cathead]

This is the tool I tighten the draw bar with. It's 8 inches long. Just for fun I checked to
see how tight it was and the reading on the torque wrench was 23 foot pounds.

It's the wrench from a 6 inch mill vise that I have used for well over twenty years.

 

[MrWhoopee]

We used to repair drawbars regularly. Even without major over-tightening they would still wear out in production use. Make a new bar from 1/2" ETD150, insert into the hex and cross pin. Makes replacement easy.

 

[C-Bag]

It probably stripped because it's been overtightened many times since it was new. I've seen guys tighten the drawbar as tight as they could get it with a 2' wrench. Bridgeport recommends tightening it to around 40 ft. lbs. The recommended torque for a 7/16-14 grade 5 or grade 8 bolt is 37 ft. lbs.

So makes me wonder about all those guys with those pneumatic drawbar tightners. Those butterfly guns go way more than 40ftlbs depending on the air pressure.

 

[aliva]

I've been using a power draw bar with the butterfly wrench. So far no issues but I've never checked the threads in over 6 years. It's on my to do list now

 

[ttabbal]

I do keep mine with the air turned down. It has its own regulator. I think it usually runs about 40psi. I also don't just let it hammer on the drawbar. I tried it then used a wrench and the force was similar. I didn't break out a torque wrench, but it was pretty close to what I did by hand with a normal size box wrench.

The 3/8" butterfly impacts I used to see all over the place were rated to about 50 ft-lb. I don't think I believe the 90+ ft-lb ratings I've seen around the internet today. Break-away maximum, maybe, but these things are pretty weak. I don't see them tightening enough to damage the threads unless you hold it on for a long time or run high pressure to it.

 

[seagiant]

Hi,
Any time, I have to weld a shaft that needs to be as straight as possible...

I use a piece of new angle iron to hold both pieces and tack and weld a little at a time to keep down the warp.

Has worked pretty good and I have welded up to 2" dia. stock!

 

[OOT]

I have bought new drawbars to solve this problem and nevertheless have to fiddle with them occasionally .....the crew tightens them silly as reported above, or they must loosen them quite a bit and then tap the collet out.

The threads are not tolerant of these antics.

I use a die, and a then with the draw rod in the lathe: a 60 degree thread file followed by a cratex stick - to get the drawbars back to being in shape.