In this article I will describe how I made myself an acme tap for tapping soft materials.
Step 1: Fabricate Guide Blocks
The first item I made for this project was a pair of guide blocks to hold the threaded rod stock with. I made mine uniform, and square, then drilled holes out of the center of each. I used some phenolic cellulose material I had on hand, I am sure other materials are equally as suitable to use.
Step 2: Add Set Screws to Guide Blocks
I want my guide blocks to remain fixed on my threaded rod stock so I tapped holes for set screws into the sides of them. I took this picture at the end just so I would have an image for this step. That is why one block is already cut in half in it. I'll be getting to that shortly. Tap your blocks now. The set screws you use must be flush below the block faces when tightened onto your rod stock.
Step 3: Machine One Block As the Front Half Block
The front block will need to be partially cut away to allow access for grinding to take place on the threaded rod material. I chose to perform this step on my table saw with a fence and the blade tipped to 45 degrees. I'm sure other methods would be equally as effective. This image was staged after the project was completed just so I had a picture to include for this article. It is probably a good idea to perform this operation with the set screws removed if you are going to use a power tool.
Step 4: Prepare Threaded Rod Stock
Taps have a lead in chamfer on the end of them to help get the tap started. I ground mine by chucking the rod into a cordless drill then running it on a belt sander. When doing this it helps to feed into the motion of the machine. Doing so draws the work into the tool and yields more even results with less skipping of the workpiece. I'll draw a simple graphic to explain.
Step 5: Jig Up for the Job
If a picture is worth 1,000 words I'd better add two for this step. Anyhow the definition of a machine is a device that constrains motion to perform a task. That is exactly what we will be doing here. Two constraints will be accomplished with this setup.
One, a channel will be constructed so as to fit the guide blocks and guide the threaded rod material under the abrasive wheel. I used some scrap wood and a few screws to accomplish this. I suppose I could have routed a channel into a piece of wood as well. Whatever causes the guide blocks to follow a straight line will suffice.
Two, the abrasive head needs to be controlled for feed. I used a hose clamp, a U bolt attached to a scrap of metal with some handy holes in it, and a turnbuckle to affect this. It worked good here. Figure out what you have and make something work good for you too. It doesn't have to be pretty just so long as it is straight and fits the guide blocks snugly but allows you to move the rod under the cut off wheel.
Step 6: Perform the Operation
Now slowly feed the cutting head down so it contacts the work and pass the rod under the head to grind your slot. Pulling the work out and flipping it over yields the desired symmetrical cut grind on the opposite side. This is so simple to do I'm sure if you've come this far you'll have no problems. I'll add an image to show what the final product should look like.
Step 7: Finish Tool
Now having ground the slots into the tap it is time to cut the tool off the end of the threaded rod. I used a bandsaw, you can use a hacksaw, your chop saw or whatever. Taps have square drives on their tops so they can be held in tap handles I wanted this feature as well. Examine the photo to see how I accomplished it using my guide block over as an alignment block to get a square grind.
Step 8: Conclusion
I am pleased with the results I have achieved for my intended use of this tool. I only need it to tap some HDPE blocks for lead drive nuts for a CNC machine I am building. A cheap imported 1/2 X 10 TPI Acme tap costs $47 plus shipping at the time I wrote this. It is more than I am willing to invest for this item. Setting all of this up took me some time but I had fun doing it all. I've seen other people on the net freehanding these with rotary tools like Dremels and their results motivated me to build this rig.http://www.splitreaction.com/homemade-acme-tap/
Mine came out good enough I could probably make them and sell them. I'd have paid $10 for such an item. But as far as I know nothing like that exists in the world.
Having performed this entire operation myself I would like to note now that if I was to do it all over again I'd use a smaller diameter abrasive wheel on my cut off saw. The big one was total overkill and I experienced some flutter using the one I did. Good luck if you decide to try my method of making home made taps. Really I don't think there is any luck involved, if you do the work it should work for you too.
Step 9: Afterword
My tap worked right out of the gate as described but I decided to polish mine a bit more to remove burrs and improve its cutting action. While I was researching my project I came across several resources on the Internet that described how taps work and physical attributes that allow them to do so as well. Things like relief angles and the like. I encourage anyone that plans on attempting this project to familiarize themselves with the geometry of a tap before they begin. This is one page I found:http://mckgyver.pbworks.com/w/page/20654129/AcmeTap
My fluttering cut off wheel had a tendency to undercut my cutting edge in a manner I do not fully understand so it might not have been such a bad thing that it misbehaved as it did on me. I do not know for sure exactly what happened honestly. As they say it all happened so fast ... I further refined my relief hook with a small round file then a piece of sandpaper wrapped around a small diameter metal rod I sanded the groove by hand with.
Some adjusting of the threaded rod stock in the guide blocks while performing the groove grinding may help in achieving a good relief angle for your cutting teeth. Possibly wedging your entire guide channel at an angle may help as well I think.
I only made one tap and I'm happy with how it works now so my efforts in regards to this project are at an end. I feel though that more refinements and improvements to the technique remain to be discovered. Feel free to fiddle around some yourself as I offer this only as inspiration and certainly not the last word in homemade tap making. Post your observations and results in the comments of this article for all to benefit from too.
I hope this guide becomes the best resource on the Internet for cobbling together cheap homemade taps! Now let the threading begin.