Issue tapping a hole

J

johnpaulh

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Hello,

I'm sure someone can help me here, I have a head scratcher...never had this issue occur before and I was unable to solve it myself.

I am trying to tap a 1/4-28 hole, .75 deep, it is a thru hole, so no bottoming out. The material is aluminum (6061) I bored a thru hole on the lathe with a #3 drill as a pilot. Selected my run of the mill, Irwin tap from the hardware store (new tap) double checking to be sure I had the 1/4-28 tap. Mounted it in my tap handle, put a few drops of cutting oil on it and with the piece still in the lathe, used a spring center to get the tap started. Seemed to be cutting fine, but about 2/3 of the way thru, it seemed to get easier.....much easier. Then it got harder again. When I backed the tap out, I had a nearly smooth hole about 2/3 of the way thru, with some nice threads in the last 3rd. What?

I thought maybe the pilot should have been a bit bigger, so tried again with a slightly larger pilot hole...same thing. I thought, "the aluminum maybe...it came from a hobby store...maybe, maybe.....

So I grabbed a piece of mild steel, bored with the #3 again...same dang thing!

I have never encountered this before...what am I missing or doing wrong?

Thanks in advance!!
John
 
It was harder to turn at first because you were reaming the hole instead of threading it. Could be too much tailstock pressure, or a chip not properly cut and pushing the edge into the opposite side of the hole. Have a close look at the tap. Just because it is new doesn't mean flawless. Could be a problem with it. Run a few test holes and dulling just a bit may be the trick.
 
Try tapping a hole free hand to see if the tap feeds properly. If you have a hard time starting it, the tap is likely bad. Make sure the taps you buy are two fluted, high speed steel, spiral point machine taps, not carbon steel hand taps, which may have three or four flutes. That will get you a ground tap vs. a cut tap and the performance will be much better. You can tell a ground tap from a cut tap by looking at the thread; a ground tap will be shiny and crisp looking, a tap with a cut thread will crude and rough looking and usually be coated.

Unfortunately just because a tool has a big name attached to it doesn't necessarily mean it's good quality. A lot of American companies are having their tools made overseas these days and the quality just isn't what it used to be. I get most of my taps from places like McMaster Carr and MSC where I know exactly what I'm getting and can be assured of good quality tools.

Tom
 
Erwin Taps

I dont know about the quality of Erwin taps. I did have a job I was doing a while back, and I had to tap 1/4 20 threads. I was using the Erwin taps. I get them over at the local Princess Auto for a low cost. Well I broke 3-taps in a row. I never got the chance to see how bad the threads looked, the taps were spapping off flush with the surface of the hole.

I ended up making a new piece, and took a run over to Fastenal, and bought one of there taps. Still using it without a problem. Its not like I was cranking the Erwin taps in with heavy pressure, I was taking it slow and easy, as I do with all small taps. Id just buy a different brand, your problem will most likely go away.

Paul
 
Thanks to all for the suggestions. I will definitely give that tap a careful inspection, and if it is messed up, adios......I have a Fastenal near work, so maybe will hop in there and pick one up after work.

John
 
Irwin stuff does not have a good reputation from the posts I have read.
 
I suspect there was a typo, Jim. The censor will catch only a few words that are manually set in the module. I suspect maybe a "u" instead of an "e" slipped in there. That's the only thing I can see as a possibility.
 
Thanks Jim, I did not bevel the hole....it was colder than heck in the garage and this was the last task, so I kind of rushed it...I know better.
I was using a cutting oil that works well for tapping aluminum, and was backing out, but not every 1/4 turn. I was worried about the length of the hole due to the diameter of the tap and will try doing a short tap, remove and clean, then go at it again.

On inspecting the offending tap last night I didn't see any deficiencies, but everyone is right, they are really crappy taps compared to my "good ones" (which I have been buying singly as needed from a "real" tool supplier....I live about 20 minutes away from Wholesale Tool and Production Tool Supply so am fortunate in that regard)

I'll have another go at it soon since the project is waiting for me....my son and I are building an electric upright bass, and the threaded ferrule is part of the bridge height adjusting system. We could have bought commercially made adjusters, but they are really thumbscrews with a pin on the side opposite the threads, which bear on the bottom of the bridge, the threads go directly into the wood on the upper portion, to me a recipe for stripped threads in the future....and since I tend to overthink/build things, I made a bearing cup for the pinion side (like a watch bearing) and wanted a threaded ferrule in the upper portion of the bridge for durability and ease of turning...those strings are under a LOT of pressure.

Off topic, I have been watching your MG restoration. I just sold my 1953 TD last year and miss it already, but it was time to go. I love MG's and have had 2 Midgets, a B, 2 A's and an Austin Healy Sprite (basically a re-badged Midget) Still have some parts kicking around so let me know if you are looking for something and I will "go mining" in the spare parts boxes at dad's house. I've also got a hard top and extra fuel tank for the TD if you are interested.
 
John,

The bass sounds like a fun project. What are you using for a body, solid I assume? You know, there are brass inserts available for use in wood. You just drill the hole a bit oversize and screw them in. Check out the following:

Screw inserts for wood

I am also an MG guy. I won't say ex because I don't think it ever wears off. I restored a 1958 MGA, my first restoration, and replaced the wimpy 1500 engine with an 1800cc B engine for a little more fun. I also owned a 1975 B-GT. They were a lot of fun but I don't miss the adventures of the Lucas electrical systems. :mad:

Tom
 
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