Sunday, September 19, 2010

A Stubby Jack Lever

Sometimes when jacking a vehicle with a small floor jack, you have to get the jack quite far under the vehicle for it to reach the jacking point that you're after. Then you go to insert the lever and start raising the jack, and you realize that the standard lever is too long to give you anywhere near full piston strokes with the low clearance you have above the jack. What's needed in those situations is a stubby lever that'll give you enough leverage to get things going. Once you get the jack raised, and a bit of jacking begun, you can switch over to the standard length lever and carry on.

That nasty-looking piece of 3/4" diameter steel rod is what I've been using. Since I've been sprucing up some of my workshop's gear, I thought I'd prettify it a bit and install that 10-24 screw-eye at one end for hanging it up when it's not in use. There'll be an interesting point to be made here about cutting female screw threads with a tap. I'll get to that shortly. First, I'll chuck that rod in the lathe, square off and chamfer the ends and clean it up to make it fit for painting.

- - -

There. That's done. Here's the rod chucked in the lathe, centre-drilled and ready for boring with a tap-size drill:

Cutting threads in blind holes to any appreciable depth is fraught with peril. To install the full length of that screw-eye's threaded stud, I'd need a 5/8" depth of female thread in the rod. To attempt to cut that deep a thread with a tap, even in mild steel like this, would be foolhardy -- begging for a broken tap. (I'll leave it to the reader to guess how I've come to know that.) So, I'm going to cheat a bit here.

First, I'll shorten the screw-eye's stud so that I'll only need a 1/4" threaded depth. That should be doable.

And second, I'll drill the hole oversize of the normal 75%-thread drill size you get from the charts. All the thread is ever going to have to bear is the weight of that rod hanging from a hook. A 50% thread will more than suffice here, especially if I assemble it with blue Loctite. Also, it doesn't have to be a blind-tapped 1/4" deep hole; I can drill deeper than needed and thread it in one go with only a plug tap.

For a suitable drill size for a 50% thread, I only have to look up the pitch diameter of a 10-24 thread. A thread's pitch diameter is its diameter at a point half way between the crests and roots of the thread. For a 10-24 thread, the figure is 0.163". The nearest number size drill is No. 20 (0.161"). That will give me just a tiny bit more than a 50% thread, while considerably easing the load on the tap. I'll drill 5/8" deep to give clearance for the tapered front portion of the tap, and this should work out fine.

- - -

And here we are with a deep enough thread and an unbroken tap:

(You can see the shortened screw-eye stud in the shot as well. That material protecting the rod from marring is 0.020" thick scraps of aluminum exterior window casing material. It's excellent for the purpose.)

To have gone any deeper would have been pushing my luck big time. I should also mention that for both the boring and the threading operations, it's wise to back out the drill/tap frequently to clear chips.

Anyway, my point here is that while orthodoxy is all well and good, it's not a strait-jacket. When it comes to tap drill sizes, you're at complete liberty to deviate from the 75%-thread drill size when a situation calls for it. Consider what the thread is actually going to be asked to do. In this case the answer was "not much" -- there was no need for a 'strong' thread, only an adequate one. An adequate one was doable. A strong one might not have been.

- - -

I'll give that rod a nice gloss black paint job before installing the screw-eye for good.

- - -

And here's the lever out on the job site. (I should have painted it orange for this photograph.)

# # #


# # #

No comments:

Post a Comment