Threads On Mill Drawbar

[JR49]

So, I'm thinking about making a new (better) drawbar for my new PM 932 mill. There is a long threaded section on all the R-8 collets, as well as on the original drawbar. Do they really need that much thread engagement? That's a lot of spinning in and out for each collet change, and I recall Mr. Pete saying that when threading any particular size thread, any more thread than what is on a nut of that same thread size is unnecessary. Of coarse, he wasn't referring to collets and drawbars. Opinions Please ? Thanks, JR49 PS. yes, I know, do a power drawbar mod. Maybe someday.

 

[Andre]

More thread engagement means more surface area under load, and less wear on the threads. Although you may only need a few threads before you hit yield strength of the drawbar they can't hurt.

 

[janvanruth]

i made the threads way longer than necessary
also the top end of the drawbar is longer so the hex is an inch higher than normal
if the threads are worn i cut the worn piece off and turn the bottom of the top end to length

 

[projectnut]

Mr. Pete is right when the bolt or fastener is being held in place with a nut. However the maximum holding strength of a bolt is achieved when a threaded portion equal to 1.5 times the diameter is captured. There are many applications where a longer thread is required. That's why the length of coupling nuts are always at least 3 times the diameter of the rods they are joining.

 

[Bob Korves]

If you have problems with wear on the threads of your drawbar, make another one out of some real steel. 1.5 X diameter is plenty of engagement.

 

[derf]

Typically the threads on drawbars are rolled threads for more strength. I just repaired one with a grade 8 bolt.

 

[P T Schram]

However the maximum holding strength of a bolt is achieved when a threaded portion equal to 1.5 times the diameter is captured.

When I was building the rear suspension for my off-road truck, I did a lot of research into thread engagement and the strength of the resulting bolted joint.

I was AMAZED to find that three threads is sufficient to achieve the maximum clamping force of the bolted joint. Even more amazed to find that this rule applied regardless of the size of the fasteners under analysis.

After I had done the math (using Marks' as a reference) I kept finding truth to the three threads rule.

 

[mmcmdl]

i made the threads way longer than necessary
also the top end of the drawbar is longer so the hex is an inch higher than normal
if the threads are worn i cut the worn piece off and turn the bottom of the top end to length

Same principle . Machine your drawbar thread an extra inch or so . Add that length to the rod end . Slip on a 1" spacer on top . When your threads wear , part the worn threads off and reduce your top spacer accordingly . Use a good grade of steel and follow the 1.5x diameter rule of thumb .