Designing a flat bottom thread cutter

[FGarbrecht]

I want to try to make a thread cutting tool to cut flat bottom threads in wood (pen caps and barrels). It seems like a simple project; as in the attached picture it would have a 1/4" shank for the 1/4" collet chuck, a short length of 1/8" shaft for clearance depth, and then the single (or double) wing cutter section that sticks up 1/8". Question is how to size dimension 'A', the length of the cutting blade?

Here's what I am thinking. If I want to create 10 tpi symmetrical threads (threads and grooves between the threads equal), then the thread pitch is 0.1 inch, and it seems like having the cutter size the same as thread pitch will create grooves and threads the theoretically correct size and spacing, but do I need to build in some slop?

 

[Curly]

Yes some "slop" is needed. Take the pitch of your thread, lets say 10 tip. That would make it .100 divided by 2 because that distance is shared by the inside and outside threads. But wait the thread is on a slope so you need to allow for the pitch angle being cut by a vertical cutter. This kind of thing. /[]/ I would have to experiment to get the right amount of width the cutter needs by cutting both threads and seeing if they fit. Shortening the cutter until I liked it. Depth of thread is a factor too because the deeper you cut the wider the groove gets. I don't have the math skills to calculate it but I'm sure someone here does.

 

[bmachin]

Fred,

You're always going to need some clearance. How much, I can't be of any help other than to say that that the groove will necessarily be wider than the land, so your tool would need to be 0.1+.
Maybe John Albert will chime in here.

I would be concerned about a square tool leaving a pretty rough cut vs a "V" shaped cutter in wood on a cross grain cut. Guess the only way to find out it to try.

Bill

Edit: OOPS! Pete's right about the tool width. Should be 0.05+

 

[Curly]

I think John Albert single points his threads. He uses the metal lathe to move a stationary cutter in the tool post along the rotating part. I'll be happy to sit corrected if I'm wrong though.

 

[magpens]

Somehow, I don't think you should call it "slop" ! However, some "allowance" (for whatever) may be necessary.

The general shape you show is the general shape of a standard boring bar ... you might find a boring bar of a size that you can grind down to do what you want.

You are talking about flat bottom threads, and that might work.

You may also wish to consider the design of Acme threads, which have a trapezoidal cross-section, rather than rectangular. . The reason I suggest this is that Acme threads are used quite a bit for some applications and should be well documented. . Some design issues might come up through the use of approprate search parameters, and you might get some refinement to your idea from such a search.

 

[FGarbrecht]

Somehow, I don't think you should call it "slop" ! However, some "allowance" (for whatever) may be necessary.

The general shape you show is the general shape of a standard boring bar ... you might find a boring bar of a size that you can grind down to do what you want.

You are talking about flat bottom threads, and that might work.

You may also wish to consider the design of Acme threads, which have a trapezoidal cross-section, rather than rectangular. . The reason I suggest this is that Acme threads are used quite a bit for some applications and should be well documented. . Some design issues might come up through the use of approprate search parameters, and you might get some refinement to your idea from such a search.

I took a look at my boring bars and they are all ground at around 88 degrees instead of 90, so if I went that route it would be necessary to square the cutting portion. My smallest boring bar was fairly expensive as I recall so I may try making my own out of some mild steel rod first.

I looked at Acme threadmills but haven't seen any that are tiny enough, and without a mill I don't think I could manage the angles necessary to fashion one up.

 

[Curly]

Mal I just went with what was in the first post. I'm not so sure a trapezoidal thread can be easily cut with a live tooling (cutter revolving and not a stationary one as is usually done on metal lathes).

 

[bmachin]

I think John Albert single points his threads. He uses the metal lathe to move a stationary cutter in the tool post along the rotating part. I'll be happy to sit corrected if I'm wrong though.

You're right. John does single point. However I think that he would have valuable input on cutter width for a given pitch. I believe he uses 8 tpi.

Bill

 

[Curly]

Single point cutting is done along the centre line with the sides of the cutter ground back to allow clearance. With live tooling the cutter is cutting both above and below the centreline at the same time. The two don't necessarily have the same shape to achieve the same clearances. This /[]/ verses this /-/.

 

[bmachin]

Single point cutting is done along the centre line with the sides of the cutter ground back to allow clearance. With live tooling the cutter is cutting both above and below the centreline at the same time. The two don't necessarily have the same shape to achieve the same clearances. This /[]/ verses this /-/.

Duh!!

Finally see what you're saying. Start wide and narrow down 'til you've gone too far then make another one.

Got that Fred?

You're resident Dunce,

Bill

 

[FGarbrecht]

I think one needs an engineering degree to figure this out, so I'll rely on my usual iterative process -> try, fail, try, fail, try, fail............maybe succeed. I want to thread wooden pens so I need to use live tooling, but single point of the lathe would work for my ebonite pens. @jalbert must use a tool he made to get those nice chunky threads. Looks like something that a modified boring bar might handle.

 

[Curly]

The single pointing of a reasonably hard wood will work with the single point tool. You'll just have to take a bunch of passes a little at a time. You won't be able to do it in one or two.

 

[FGarbrecht]

The single pointing of a reasonably hard wood will work with the single point tool. You'll just have to take a bunch of passes a little at a time. You won't be able to do it in one or two.

I'll have to try this, now that I figured out how to do single point threading. I can experiment on the destroyed cherry burl pen that I started with.

Are there guidelines for initial dimensioning of the barrel tenon and cap hole for single point and/or live cutter threading? Can I assume the same as I would do for standard tap and die threading (tenon same size as tap, hole = tap/die size minus pitch)?

 

[magpens]

You said:
"I want to thread wooden pens so I need to use live tooling, but single point of the lathe would work for my ebonite pens. "

@FGarbrecht

I seem to be missing your point about why you "need to use live tooling" with wood, Fred. . Please, if you don't mind, could you elaborate a little.

My understanding of "live tooling" is that the tool rotates (and advances) to cut the thread and the workpiece (wood) is held stationary.
This is what you have when turning a tap by hand with a wrench and you hold your workpiece in a vise.

I am not sure what the alternative term to "live tooling" is, but an example would be rotating the workpiece clamped in headstock while holding the tap so that it does not rotate but it is allowed to advance as the threading progresses ( I do this on my metal lathe with my tailstock free to slide and supporting a Jacobs chuck which holds the tap. . I rotate the headstock manually. ).

 

[FGarbrecht]

@FGarbrecht

I seem to be missing your point about why you "need to use live tooling" with wood, Fred. . Please, if you don't mind, could you elaborate a little.

My understanding of "live tooling" is that the tool rotates (and advances) to cut the thread and the workpiece (wood) is held stationary.
This is what you have when turning a tap by hand with a wrench and you hold your workpiece in a vise.

I am not sure what the alternative term to "live tooling" is, but an example would be rotating the workpiece in the headstock jaws while holding the tap so that it does not rotate but it is allowed to advance as the threading progresses ( I do this on my metal lathe with my tailstock free to slide and supporting a Jacobs chuck which holds the tap. . I rotate the headstock manually. ).

Everything I've seen suggests that standard threading using taps and dies doesn't work well on wood (I'm making wooden pens). I've only tried once or twice and it wasn't pretty. What I'm referring to as 'live cutter' threading means a cutter that rotates at high speed 10-20k rpm and is advanced into the workpiece. I'm doing this on the metal lathe using the change gear setup to advance the cutting tool into the pen part which is secured to the headstock and rotated manually using a crank. @Curly just suggested that standard single point cutting on the metal lathe may work on really hard woods, so I may give that a try too. I'm sure that a flat bottomed tool would just make a mess of things if used the conventional way on the metal lathe; it may also be a mess when rotated at high speed too, but I guess I'll find out

 

[magpens]

@FGarbrecht
So your high speed cutter would be in a Dremel tool (eg.) mounted on the cross slide of the metal lathe (advancing via the change gear mech.) ?

 

[FGarbrecht]

@FGarbrecht
So your high speed cutter would be in a Dremel tool (eg.) mounted on the cross slide of the metal lathe (advancing via the change gear mech.) ?

Exactly! There's a picture and description of the setup in this thread

 

[More4dan]

Just remember to grind some side relieve on the cutter side facing the headstock to allow the cutter to miss the thread that is "ramping" toward the cutter.

From a side view:

I think I would also add a slight taper. This would allow the cap to tighten when closing using the wedge effect. I would also make sure the slope is 9 degrees or more.

Below 9 degrees is where "sticksion" happens. Remember when the Tupperware cups from our childhood would stick together when stacked and became almost impossible to separate? This happens at about 8 degrees or less with tapered contact.


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