The Stovebolt Site Forums The Shop Area General Truck Talk Jim G’s SLURP Project – Posting 35: Making Red’s “billet” Pitman arm!

This posting is one in a series. If you have not yet read the earlier postings in the series, you’ll need to, as this posting won’t make much sense without having done so! Just do a search using the word “SLURP”.

In my last posting, I described how we determined what characteristics and dimensions we need for Red’s custom steering system Pitman arm.

In this posting, we’ll see how a skilled machinist, who also understands the forces at work in a roadgoing vehicle, custom made that Pitman arm.

Mitch Goss, as I have mentioned in an earlier posting is a highly skilled guy with medical issues that preclude his working at a regular job anymore. He took on this project for me after he and I got to know each other a bit during the process of my buying his beloved 1952 Packard from him. Mitch’s nephew, Chris, will actually be taking over the small custom machining business that Mitch had run out of a small but well equipped shop built specially for the purpose behind his regular garage. Mitch made sure that Chris was involved in the setup and prototyping run for this Pitman arm, as Mitch and I both see a strong potential market for Chris in building this type of custom hotrod part.

As you will see, the Pitman arm came out really, really well. That was no surprise to me, since Mitch’s background is in the precision aerospace machining industry (where he made parts for the space shuttle), AND he is also himself a dedicated streetrodder. So, he knows and understands what is needed on a vehicle, what materials to use, and how to do the machining in a manner that maximizes the quality.

Mitch began by analyzing the measurements he had carefully obtained from the Flaming River steering box, Red’s stock drag link, and my own dimensioned sketch. Mitch pointed out to me that there were several key features on the Pitman arm I needed:

- A properly tapered hole to correctly engage the tapered shaft on the drag link tie rod end, so that the connection would be tight, with all movement occurring via the tie rod end, not via slip between the Pitman arm and the tie rod shaft.


- A very precisely sized tapered and splined hole that engaged the output shaft on the Flaming River steering box. This hole has a very subtle taper - only a degree or two – to ensure tightening action as it is forced into position by a hexnut, 4 “master” splines that ensure that the Pitman arm is properly “clocked” relative to the steering box shaft (so that the steering box is at center of travel when the vehicle’s wheels are aligned for straight forward movement), and 32 “regular” splines whose dimensions must be as perfect a fit as possible on the matching steering box shaft splines, in order to avoid any play whatsoever that would lead to rapid wear of both the shaft and Pitman splines.


- The center-to-center distance between the two holes on the Pitman arm must of course be the 8.5 inches that I had calculated would be necessary to ensure the correct “laden angle” of the drag link with the truck at its equilibrium load height.

Mitch cut a suitable “blank” out of a solid piece of a type of steel he selected for the application based on its proeprties. He then set up his milling machine to produce the two holes, each in a separate operation.

The splined hole was particularly interesting of course, due to all the required features. Here are some photos of the process that Mitch let me observe:

Here, Mitch has set up the angle of the milling machine to make the subtle taper angle in the splined hole:

http://i252.photobucket.com/albums/hh16/JimGnitecki/Pitmanarm004taperangleonspline.jpg

Here Mitch is cutting an individual spline (one of 32 “regular” splines), to a non-final depth. Mitch had to cut each of the splines in an iterative pattern that took off less and less material as the final finished depth was approached:

http://i252.photobucket.com/albums/hh16/JimGnitecki/Pitmanarm002cuttingonespline.jpg

Here is a photo of the splined hole at one point during this iterative process:

http://i252.photobucket.com/albums/hh16/JimGnitecki/Pitmanarm003partialdepthsplines.jpg

The final finish cuts were made in very small increments so that Mitch could custom fit the splined hole to the steering box shaft for a tight, precise fit.

The next step was to use a cutoff saw to chamfer the corners of the Pitman arm:

http://i252.photobucket.com/albums/hh16/JimGnitecki/Pitmanarm005roughcutcorners.jpg

Then, Mitch used a grinding wheel to radius the chamfers by hand:

http://i252.photobucket.com/albums/hh16/JimGnitecki/Pitmanarm006smoothoutradii.jpg

You can see how much he was enjoying himself on this artistic step where he hand chamfered the edges of the Pitman arm!


http://i252.photobucket.com/albums/hh16/JimGnitecki/Pitmanarm007chamferedges.jpg

The next step was using a surface grinder to shave the arm level and remove any surface defects and oxidation:

http://i252.photobucket.com/albums/hh16/JimGnitecki/Pitmanarm008grind-polish.jpg


Here is the finished Pitman arm ready for paint (a shame to paint such a beautiful “billet part”):

http://i252.photobucket.com/albums/hh16/JimGnitecki/Pitmanarm009readyforpaint.jpg

I don’t have a trial fitting photo of the Pitman arm in position on Red, but Randy and I HAVE already trial fitted it, and it fits perfectly, hangs at the correct angle, and suspends the drag link at just the right working height for the new leafsprings! (next posting!!)

Those of you needing any custom parts for your street rod or restoration project might consider contacting Mitch to get it done for you. He showed a great deal of skill and patience with me on my project, and I am very satisfied with the end result. Contact me to get his contact information!

Jim G

Nice... I'm a machine tool student... fascinating stuff there.

Thanks for all of the pictures.

-W

I'm always impressed by machining because I can't do it.Are you sure you really want to paint it? Why not just clear coat it.

You might be right. Clear coating would be less durable than the POR15 I plan to use, but would certainly look better!

I'll think about that option.

Jim G

If you want a clear finish you could have it clear powder coated, that works well.

Grigg

jim, i love the pics of the pitman arm.
i am a green horn machinst, and was wondering if you know or could ask your buddy mitch how he put the vertical splines into the holes when a mill turns horizontaly ?
i have put splines on the o.d. of a shaft before, rotary table mounted horizontaly, standing up on the bed, but cant for the life of me see how you can put splines with the set up you show.
it must be a tool i have never seen.
like i said, i am a green horn. LOL. but i love to learn. i even showed my buddy at work who is teaching me and he couldnt explain.
he said it looked like it was broached. is it a broaching tool in the mill ? i doubt it, since you really need some force when using a broach.
anyway, thanks for your help.

leonard

I to am a machinist...
If I were to do that job it would be nice to have a slotter, basically a vertical shaper with a built in rotary table.

From what I can see in the pictures I would say he used the quill of the milling machine as a slotter with a stationary tool to cut the splines, advancing the table slightly for depth of cut, never even turning the mill on.

It is nice work!

Grigg

Here is what he did:

- The mill is NOT turned on

- He used a broaching tool that he handmade

- The tool is stroked down by HAND for each spline, and all 32 splines are done to a shallow depth first, then broached a little deeper on the second pass, a little deeper on the third pass, and then custom fitted to the steering box shaft via .002" cuts until the tapered spline goes onto the shaft JUST far enough.

- The rotary table is roated eactly 10 degrees each time (32 regular splines plus 4 master splines = 36 splines in 360 degrees, or 10 degrees per spline

- The hand broaching easy when done just a small depth at a time, and with a tool bit that is much harder than the steel.

Jim G

wow. very cool. thanks for the tip.
now i understand.
pretty cool. you guys are very talented.

leonard