Jacobs Taper and chuck runout question

Rog,

First, congratulations on your rebuild of the Orbit! It doesn't get a lot better than "0" runout on the spindle.

I've attached a spreadsheet with data I got from Machinery's Handbook 26th Edition, and checked against Jacob's Website's Technical Bulletin. They do not match perfectly, and the Jacob's document has at least one signficant error in it (Taper No. 1's Length in inches is shown as 0.56525 instead of 0.65625). Other minor errors exist in both documents; the things you discover when you make a spreadsheet!

I don't know of a "2-1/2" Jacobs Taper, but a "2 Short" exists, and is shown in the table below.

Basically, here's the problem. Your data is shown in red & white at the bottom of the table. It doesn't really match any Jacobs Taper very closely. It certainly doesn't match either the 2 or the 2 Short. The closest match is, as you pointed out, a No. 6 taper.

As you can see from the table, a No. 33 Chuck will "stick" (sort of) to the spindle measurements you made. It will, however, really only hold at one circumferential ring on the taper, and this is clearly why your chuck has got the wobbles. Good job troubleshooting, by the way.

As I see it, here are your choices:

1. I'm wrong: a "2-1/2" Jacobs Taper exists, and someone else can tell you about it. Oops, sorry!
2. I'm right, and one of your 3 measurements is wrong. Make all 3 measurements again to be sure.
3. Your measurements are fairly accurate, and there's a No. 6 Jacobs Taper in there right now. As noted above, the No. 33 Chuck will kind of hang on. If this scenario is true, then probably the easiest thing to do is buy a new chuck with a No. 6 Jacobs Taper.

The nameplate does specifically state that the Spindle is a "2-1/2 JT". If "2-1/2" means the same as "2 Short", then someone may have changed out the spindle (or a mandrel fitted to a seperate spindle) to a No. 6 taper. "JT" does mean "Jacobs Taper", so that part's right anyway.

Here's Orbit Machine Tools' Website. Interestingly, they refer to their drills as "Special Purpose Machines". They have a lot of nice products, and their customer list is pretty impressive.

Good luck!
Dave

Here's a handy taper chart in text form from Logan:

http://www.loganact.com/tips/tapers.htm

Thanks Dave!

I REALLY appreciate your information. I'm beginning to suspect that someone previously changed out the spindle but I'll definitely recheck the measurements before doing anything else (even though I measured it about 10 times when I got the discrepancies before). I passed up a cheap JT6 reamer last week and now I'm kicking myself because I may have been able to tweak the chuck a bit with that since its so close.

Thanks for the great charts ... very helpful. I'm guessing that this DP is a 1970s model from what little info I can find so maybe I'll ask Orbit if they invented a JT 2 1/2 back then and forgot to tell anybody.

Enjoy your day!
Rog

Good morning everyone, newbe here with a problem, I know this is an old thread, but I was searching for some info on an Orbit drill press and ran across this. I have the same drill press that is missing the chuck and the label says JT 2 1/2. Just wondering if you ever found out more info from Jacobs or anywhere else? Thanx for any input !!!!

Sir,

I never got a response from Orbit on the JT 2 1/2 spindle label. I concluded that the spindle on my drill had been changed out from the original and I turned the spindle to fit the chuck. I suggest that you measure the spindle and check the result against the thumbnail chart provided by Dave or the chart in the link provided by USMCPOP to see what you have and go from there.

Good luck

Thanx for the info, I will take some measurements and see what I come up with

More on the Mystery Chuck

I too have an old DP w/ a chuck that alleges to be a "2 1/2." The chuck will accept a maximum of a half-inch drill bit. I've done some web investigation and found the following:

1. There is a wikipedia article on machine tapers that lists a Jacobs Taper of 2 1/2.
2. On the JacobsChuck.com website, I found a chart for a medium duty chuck (model 3326A). The mount for this chuck is listed as "0.625" rather than 1JT or 33JT or any of the more standard types. I note that 0.625 is the same number as the small dimension from the "2 1/2" Jacobs taper I found in the wikipedia article.
3. These are distinct from the 2JT chucks -- neither of which has a 1/2" capacity.

I have attached a few photos. Does anyone think the model 3326A is the long-lost Jacob 2 1/2 chuck?

Thanks,

Michael

Michael,

You present a convincing case! I think you must be correct. Your measurements, and the records you've found of the No. 2-1/2, are very close to the measurements of a No. 6 Jacobs Taper. I'll bet there's an interesting story somewhere about why the 2-1/2 was even created.

Dave

The 2 1/2

Dave,

Thanks for the reply. The JT 6 is indeed close! That makes me wonder why the 2 1/2 existed. Here is one, hare-brained idea I came up with:

The plate on the DP indicates it was manufactured in Taiwan. Also, the side of the chuck opposite from the photo I attached earlier has the words "Best Fujijirushi" stamped across an outline of a mountain. The words are worn, but under good light, legible (see attached).

Perhaps "Fuji" + the mountain logo imply the chuck was made by a tool works in Japan.

I've heard that 30-40 years ago, the market was flooded with inexpensive, imported drill presses. Maybe they produced and/or contracted out their own spindles (my press has no removable arbor) and chucks and didn't use any of the standard Jacobs tapers.

Then, later, a market grew for replacement chucks. Seeing an opportunity, Jacobs made a better-quality replacement chuck for these, and the company designated that part the 3326A. Like I said: it's only an idea.

At some point, I suppose I'm just going to have to phone the Jacobs company and embarrass myself with ignorant questions until I reach a dead-end or have my curiosity satisfied. It wouldn't be bad to have a better chuck on that old drill press either.

Michael

Atta Boy!!!

Michael,

Atta boy for proving that a Jacobs 2 1/2 exists. I wish I had known this a few years ago when I was trying to figure out why the drill press I had then had such weird measurements. Now I know that the spindle was right but it had the wrong chuck on it.

I'll be interested to see what you learn if you pursue the rest of the story.

Rog