making large diameter taps

I'm in the market for a specialty tap size, for threading bicycle bottom brackets, like this expensive tool:

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Is there any technical reason this thing should cost $500, or is it just a case of a small market?

Since I have a lathe, mill, and rotary table, it got me to wondering whether I could make one of my own and after machining, heat treat it for hardness. Do I have a prayer of getting a working tool by going that route, or does this type of thing require grinding with specialized equipment (that I don't have)?

They have a good picture of the cutting surfaces in these pictures:

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more pictures are on this page:
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This looks different than the smaller taps I'm familiar with. I've never tried making a tap, but I get the idea from this cutting surface profile that each cutting tooth advances just a bit, sort of like moving in the compound feed on a lathe when cutting threads. In that case I guess it would be tapered, even beyond the initial starting taper. This would be one of the more complicated parts I have tried making. I have never cut tapered threads for example.

Thanks,

Dave

Reply to
david.margrave
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Likely the small market.

I'd make one if I needed such a tap. I'm assuming you can cut the thread pitch on your lathe.

Are you building a bicycle from scratch or are you trying to repair a damaged bottom bracket?

As far as the tapered part, I've found only bottom taps in the bin and have hand ground the taper on them to get by. If I can do it anyone likely can that tries.

This is big enough to machine easily relative to length to diameter. No chance of breaking it, and touching it up with a dremel tool if it dulls is childs play.

Charge on, if you can make a decent frame, I bet you can handle this bit of machining.

Wes

Reply to
Wes

Don't unlock half nuts cutting metric assumming it is inch by nature.

Good luck, I hope you can get the old frame rolling again.

Not sure if this is something of interest but wiki has an entry.

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Wes

Reply to
Wes

Yes, I've got one of the cheap 7x12 mini lathes with a metric and inch leadscrew. I get by with it.

I want to make a frame from scratch, but I also have an old oddball frame I picked up for next to nothing on cragslist - A french frame with 'swiss' threads in the bottom bracket (M35x1.0 I believe, RH on the left and LH on the right). I'm considering boring that out and putting in a sleeve and cutting standard ISO threads. Or maybe I'll just cut that frame into pieces and use it for scrap and brazing practice.

Reply to
david.margrave

You might try pricing a set of custom made taps. Check with a local machine shop supply or perhaps on the web.

Here is one place that will make any tap you want:

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|1060&forward=1 (You may have to cut an paste that link manually if it does not work just by clicking.)

As to making your own you can. I think I would cut it slightly over size, heat treat and then have a thread grinding shop finish it to size.

If you are only doing a few parts then you might get by with turning your own out of a piece of tool steel and hardening it with a torch. If you try this way you might want to make some test threads before you work on the bike.

Another ides is to take the frame to a bicycle mechanic that has the tool and just farm out the rethreading job.

Reply to
Roger Shoaf

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notice that the tap is also hollow - it looks like you install the bearing assembly and then let the assembly guide the tap - this is not how "normal" taps work, though I see no reason why you can't make it

Reply to
Bill Noble

I see what's different about this thing. Look at the starter threads, and then the remaining threads on the tap - they are spaced twice as far as the first few starter threads. Is that to keep it getting jammed up with chips or is there another reason? The taps I'm used to have the full thread profile.

Reply to
david.margrave

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I assume you are rethreading the bottom bracket. If so, this means that tap life will be longer. Anyway, to make this tap you can use tool steel and thread it on your lathe. The tap shown looks different in one case because it is partly a "skip tooth" tap. A skip tooth tap has alternate teeth removed to provide for chip clearance and to lower the torque required to turn the tap. Some taper pipe taps are made this way. They work especially well in aluminum. For your use I don't think you need to bother. Just turn and thread like you would a screw. Turn or file a taper on the front of your custom made tap, you don't need to taper the actual thread. So even though the front of the tap is tapered on the O.D. the pitch diameter of the thread will be constant. For the left hand thread start the tool at the end of the thread closest to the headstock and feed away from the headstock. Be sure to make the tap larger in diameter than the part that threads into the hole you want to tap. After threading you can put your custom tap in the mill and cut the grooves like the example. After cutting the grooves very carefully deburr the tap and then heat treat. After heat treat use a stone to clean up the faces of the "teeth" that were the result of the grooves you milled. As far as making the tap oversize goes you need to measure the pitch diameter of the piece that threads into the bike frame. The best way for you to do this is the "three wire method". Use two wires laid in the threads on one side of the tap or part and the other wire directly opposite the other two. Measure over the wires. This link:

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the three wire method. It probably doesn't matter too much in your case to get the best wire size exactly or even to calculate the actual pitch diameter. You just need to make sure your tap is a few thousandths bigger that the part. So if the measurement over the wires is, for example, 1.2500, make your tap 1.2530. If you want to, you can look up the clearance specs for the particular pitch you are cutting and use the three wire method to give you the numbers to plug into the formulas in the above link to figure out what the actual pitch diameter is that you are cutting. ERS

Reply to
etpm

I design and cut odd threads all day and think I understand them. However the one time I tried just what you suggest, it just didn't work. I suspect you need precision grinding to make a decent tap, not just cutting.

Consider that some calculations are required that I've yet to find documented. Machinery's Handbook may document finished thread dimensions, but tap dimensions differ from the referent thread.

Reply to
Richard J Kinch

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Note the diagram on this page titled Rake Angle. It's done with specialized grinding machines.

Reply to
Mal Boswick

Actually I was referring to the _Relief Angle_ shown in that diagram.

Reply to
Mal Boswick

I can't grind a circular relief without making some sort of rotating offset center fixture but a short flat relief angled 5 degrees has worked. The coarse relief behind it doesn't contact the work and can be ground by hand. The relief supports the cutting edge and keeps it from digging in too deeply. I've seen 1/32" to 1/16" width suggested in old tool grinding books.

This was on a Delta Toolmaker set up as a normal surface grinder with home made centers to hold the rusty 3/4-10 tap. I used that tap again last week and it still cuts steel as easily as a new one.

Jim Wilkins

Reply to
Jim Wilkins

Thanks for the informative reply. I have a little experience with the three wire method, and even have a set of these measuring wires of unusual diameters which make the math easier:

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to know that I should go a little oversized, I might not have thought of that.

Reply to
david.margrave

Do you have a commercially tapped frame to fit it to? That's the easy way as long as you thread it between centers so you can put it back in the lathe in exact position. The handle looks like it would make a good arbor.

Reply to
Jim Wilkins

I should note that even though you can make this tap it will not cut as easily as a properly ground tap. There will be drag on the teeth. That said, if you are just chasing threads in an already threaded hole, it will work fine. ERS

Reply to
etpm

I hope to make a tool that can cut new threads, but I also have an idea on the old Swiss-threaded bike frame I've got. What would happen if I ran a 'standard' 1.370"x24tpi tap through the existing M35x1.0 threaded bottom bracket? Those dimensions are very close and I expect it would go several turns before the existing thread and the inch tap start interfering and material starts getting removed. I'm just wondering whether a finished tapped hole of several threads worth of engagement would be enough for this particular application.

Reply to
david.margrave

I've seen mention from some of our posters or maybe on some model engineering forums that they sharpen the cutting edges of their taps to refresh them. Sounds like they are using a dremel tool iirc.

If they would speak up on technique, it would be very helpful in your making of a tap for your application.

Wes

Reply to
Wes

Removing half the threads up in the body of the tap reduces the drag of the tap in a deep hole--especially noticeable in tapping S.Steel.--Jerry

Reply to
Jerry Wass

ring forums that

ike they are using

ing of a tap for

The taps I've resharpened that way already had a proper circular relief on the outside, behind the cutting edge.

Reply to
Jim Wilkins

forums that

they are using

of a tap for

I think I'm getting what you are saying. I'll mic a tap tomorrow and see if diameter falls off past cutting edge. That is what you are saying?

Wes

Reply to
Wes

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