Making a tap

[David]

I have cut a LH 2S female square thread. It is 8mm dia, 1.5mm pitch, 3mm lead, the crests should be 0.75mm wide and on the screw they should be 0.75mm deep. I'm going to make the female thread only 2/3 depth at 0.5mm. Thus the tapping hole will be 7mm and I doubt I can make a cutting tool to fit in there.

The nut will be brass.

I'm going to use silver steel for the tap.

I have the silver steel tool I used to cut the male thread. It has been hardened/tempered. I think the temper went to blue - there isn't much time to get that right on such a small tool. I'll try using a V thread carbide tip to remove some initial material and see if that helps.

The length/taper for the tap - does anyone have a photo of a home made tap they can post to give me an idea of what is required?

Should I go 2, 3, or 4 flutes? I'll be milling them using a 5C collet block held in the milling vise. This won't provide any relief in the flutes and only what I can file on behind the cutting edge but I'm hoping I can get away with just that.

So I'd love to see any photos or read about how people made their own taps!

[jackrae]

David,
Not an answer to your question but a bit of info that may be useful for other projects
On e-bay there are lead screws and nuts on offer for 3D cad printers such as :-
www.ebay.co.uk/itm/8mm-Lead-Screw-Rod-500mm-4-Start-Z-Axis-Linear-Rail-Bar-Shaft-For-3D-Printer/202594565852?epid=2117242765&hash=item2f2b93bedc:g:g1IAAOSwJVBcYd24:rk:2:pf:1&frcectupt=true
The ones I bought were 8mm diameter, 4-start square thread with a lead of 8mm per rev

[David]

If I can find one on Aus eBay I might go that way. Left-handed would be even better. I'm not hell-bent on making this thing if I can buy it.

[RGR 60130]

David,

For a small boring tool for a one-off job in brass, a 6" nail has been known to work as a tool blank, certainly for making internal 'O' ring grooves. With a bucket full of luck you might get away with using one for your internal screw thread. Always worth a try!

Reg

[David]

I have ordered one of those lead screws, about $7.20 AUD. The lead is a bit aggressive and it's not LH, but worth a go at that price.

I'll still have a go at the tap. I spent 3 or 4 days learning to cut the male thread so it would be a shame to give up now. If the tap proves too hard I won't chase it forever.

I'd still like an idea of how long to make the tap and what sort of taper to put on it.

[jackrae]

If you could drive the lead screw with a couple of gears at a 3:8 ratio you'll get both the desired left-hand action and the 3mm pitch

[ettingtonliam]

I've done quite a few square thread taps, though mostly bigger than yours for machine feedscrew nuts. I use silver steel, and I cut the threads using the same set up as cutting the male thread, if its not too late, before I take the cutting tool out of the lathe, so the tap and male thread are exactly the same. I'd turn the tap down from 10mm, to about 8.1mm, to give extra clearance at the top of the threads and cut about 0.1mm deeper to give clearance at the bottom as well. Square thread nuts like to 'float' a little on the male thread. A good long taper is needed, say 2 - 3 inches, with a short parallel portion 7mm dia, so the tap locates in the nut before starting to cut.or, if you really enjoy cutting these threads, make 2 taps, one 7.5 mm OD and one 8.1mm OD, so you can take 2 goes at cutting the nut.
I presume the nut is brass? Not Phosphor bronze, because it tends to bind up on the tap. 4 flutes if you can. I cheat and don't smooth off the burr, so the taps cuts a little big. Harden, then temper to blue, you want toughness, not wear resistance for a tap you'll probably only use once. I have even used them unhardened, and got away with it.

Richard

[David]

Thanks Richard, that's surprisingly close to the method I used. I also arrived at the 8.1 dia and deeper thread for clearance. Although 'deeper' in my case was about the same as the screw because I knew I'd using an oversize tapping hole. The parallel part on mine is more like 20mm, and the taper is maybe 40 - 50mm.

The process was:

1. Cut the male thread in silver steel just the same as the screw done earlier. I just guessed at the length I'd need. The hardened silver steel tool cut the soft silver steel no problems, but I only took 0.002" per cut with lots of spring cuts. I sounded quite ugly at the ends of the cut where the tap wasn't supported by anything but I got away with it.
2. Cut the taper. I mucked about with this and eventually arrived at 1deg for about 60 - 70% of the tap, and then 2deg to ease the start a bit more.
3. Cut the flutes on the mill using a square 5C collet block for indexing. Cut to just below the thread bottom. I don't know if I should have gone deeper for more clearance but I thought that might weaken the tap. The cutting edges of the tap are on it's centre line. That was a guess at the correct location for them.
4. Try to clean the burrs out of the threads with a needle file. Waste of time.
5. Put the tap back in the lathe, pick up the starts manually and turn the chuck by hand. Runout in the lathe chuck meant what should have been a skim to remove burrs turned into an interrupted cut, that broke the threading tool halfway through the last cleanup pass - thank goodness it wasn't earlier!
6. Back into the 5C collet block to cut the squares for the tap wrench.
7. Harden. I saw the whole tap go through the colours which would have been useful in the tempering, but...
8. Try to temper, but because I couldn't clean the tap very well after hardening I have no idea what the temper is because I couldn't see the colours.. I feel the tap may have got to red again which I obviously didn't want.

I tried running a stone along the flat part of the cutting edges but I don't think it did anything. They don't feel sharp.

The tap wouldn't go into my first tapping hole in the nut. At these sizes my smallest increments are 1/64". I tried the next drill size up the tap went in but wouldn't start no matter how hard I pushed. I may have been turning it the wrong way. Anyway, figuring it wasn't working at this point so I had nothing to lose I went another 1/64" and going the correct way this time the tap started!

It was hard going and I brought it right out every turn or so because there isn't much clearance in the flutes. Soon I needed to use a spanner to get more leverage so you can imagine how worrying that was. But it went through eventually, and I ran it though the other way assuming that would give me some clearance.

The nut went on smoothly, with no backlash in it, either start, in either direction!

The tap isn't pretty, sharp, shaped correctly, and it has no relief anywhere but it did the job. However, here is what I was quoted for tools to do the job:

1 only 16mm Acme Thread Turning Tool Holder, $129.0+gst
5 only 8tpi Acme Inserts, $35.90 per insert +gst, min qty 5 only.
1 only 5/16 2 Start, 8tpi per start LH HSS Tap, approx 80% depth Acme Form, $350.0ea +gst
Freight $16.0 +gst
GST = $53

So $584 AUD. That's about 3 pounds in England, but it's a lot here.

[ettingtonliam]

David

I'm pleased it worked well for you, and yes it looks just like most of the ones I've made, including the fluting. Just fine for one off jobs. Recently in a job lot of milling cutters I've acquired a 'proper' fluting cutter, but although the last tap looks better, I can't say that it cuts any better than the ones fluted with an end mill.

Richard

[David]

Thanks Richard. What is a proper fluting cutter? What shape does it cut?

[ettingtonliam]

The one I have is about 2" dia, 1" bore and about 1/4" thick, for a horizontal mill, marked 'Buck & Hickman Ltd' Toga Improved Type on one side and No.3 - 1/4" to 3/8" taps on the other side. Initially I thought it cut a semi circular groove, but on closer examination, the groove is assymetric, one side (presumably for the cutting edge of the flute) cutting to a 90 degree face, and the other (trailing) side to about 45 degrees. So its important when cutting a tap to make sure the cutter is the right way round!

Its the only one I've ever seen, but they are listed in my copy of Buck and Hickman's catalogue for 1930, numbered 1 to 8. They are not listed in the 1958 catalogue though. There were also cutters of a slightly different profile for fluting reamers.

[jon38r80]

It would appear that 8mm acme threads are used a lot in cnc printing if the ads on Aliexpress are anything to go by only they are 2mm pitch commonly. Brass nuts , anti back lash or or plain are also on there all for a few pounds. probably too late now and the pitch is probably too coarse for your application. Interesting though.
Taps available only seem to start at 3/8 inch though ?!

[David]

I have one of those printer screws coming. I'd ordered it because they were so cheap and didn't know if my efforts would work out. They are 8mm lead, 4 start and right handed. They'd look better than a normal screw thread though.