Rigid tapping in the lathe, repeatability question

Hi All:
I've got another impossible part to quote, and I need advice on the repeatability of the rigid tapping process.
The thread is a double start ACME (actually the metric equivalent).
Major diameter: 3mm
Pitch: 1.5 mm
Lead 3.0 mm
Minor dia: 1.5 mm
Length: 5mm
Material: free machining brass (thank God!!)

Normally with a double start ACME internal thread I'd single point bore it, but this one is not possible to do that way because the minor diameter is so tiny.
Making an electrode and sinker EDM burning it with conventional orbiting is out too; the electrode cannot be made because if it's small enough to go in the hole, it won't have any center core to hold it together.

So I'm thinking of making a series of "two tooth" taps, ganged together side by side and of progressively larger size.
I can wire cut the teeth dead nuts accurately, so I'll know exactly where they are relative to one another both in X and in Z

My question is: if I just rigid tap with them one after the other, will the C axis on the lathe repeat accurately enough for them to progressively nibble out the thread without mis-indexing?

The rigid tap cycle is attractive because I can follow the spiral both into the part and back out.

What I obviously don't know is if the spindle always picks up the same index pulse when it co-ordinates the C and Z moves in a rigid tap cycle.
Normally you wouldn't need to care because with regular rigid tapping you only go into and back out of the part once with your tap unless you're peck tapping.
I want to go in and out a bunch of times, and always pick out the same helical path again, just with a different cutter each time.

So, those of you who know how the rigid tap cycle is controlled, your wisdom will be greatly appreciated.
The control is a Fanuc Oi Mate TC on a Prodigy gang chucker with C axis and rigid tap enabled.

Cheers

Marcus
Implant Mechanix ? Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining
Clarus Microtech

It'll do the same thing on each cycle... You don't have to worry about the machine, you have to worry about
how you set the second tool. If its rotated just a little, or the length is off a tiny bit, it won't follow...

Maybe rigid tap on the first shot, and then use the rigid tap cycle with a floating holder for the second tap...
If you are making the taps, make the first tooth or so the same profile that was already cut with the first tap, and
it should go in pretty nice..

Hi Bob:
Thanks for responding; I hoped the answer would be as you described.
What I plan to do, to keep all the different cutters in register, is to mount the fluted blanks in a gang block first, and then wire EDM cut them all together so I know exactly where they are relative to one another.
Since I'm only making one tooth for each start of the thread, I'll still have to guide it down the spiral path positively and can't rely on a previous tooth to pull the cutter in.
I understand exactly what you're advocating, but then I have to cut multiple adjacent teeth, and the diameter of the EDM wire gets in my way unless I relieve it at the junction of the flank and the minor diameter of the tap.
It's already so damned skinny there that I'm nervous about making it even skinnier to get the relief.

It's going to be interesting...I wonder what I'm going to quote for the job and if they'll go for it.
Apparently nobody else wants to try it; the customer was told it's un-manufacturable.
Cheers

Marcus
Implant Mechanix ? Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining
Clarus Microtech

Hmmm...figure the price, multiply by 3x, and add $2K. Seems to be a lot of "unmanufacturable" going around. If you get the job, we'll ALL want to know how it turns out.

Marcus, please please let us know how this works out, your plan seems reasonable to me, that is what I would try.
If anyone can do it, you can I'm sure.

Robert

I was wondering what was the need for a double start. Single start with the same pitch would only need to be given 2x turns for the same amount of axial movement.

sinha said:

I was wondering what was the need for a double start. Single start with the same pitch would only need to be given 2x turns for the same amount of axial movement.

Click to expand...

Hello sinha,
One very common reason for a multi start thread is that the depth of the thread is less (its calculated on the Pitch, not the Lead of the thread), and therefore the minor diameter is greater, resulting in a stronger male thread, particularly when the thread is a small diameter, as is the case with the OP's example.

To Marcus,
You will find that the won't have trouble with the repeat-ability. Does the Rigid Tap cycle on your machine involve the "C" axis or just the live Main Spindle? Rigid Tapping using the Main Spindle is an option even on machines that don't actually have a "C" axis and live tools.

Regards,

Bill

Hi Bill:
Thank you for responding; I appreciate it as always.
With regard to your question, the machine has C axis positioning and interpolation down to 0.001 degree for use with live tooling.
What I do not know is whether the C axis encoder is used during a rigid tapping cycle and, of course, whether the rigid tapping cycle will always grab the same encoder position or whether it just grabs the closest one.
After all, during normal use for rigid tapping it doesn't matter where the thread starts, only that Z and C be co-ordinated during the cycle.
I'm presuming it does; I see no reason for it not to, and both BobW and you confirm that it does, so the method should theoretically work.
I will certainly let everyone know how it works out if the customer goes for it.

I'm still pondering pricing: I get my ass handed to me too often on jobs like these but I have such an irresistible drive to solve the puzzle that too often I don't care enough to make a good business decision, so I tend to underprice the work in order to bask in the glory.

No wonder I'm so damned poor!

Cheers

Marcus
Implant Mechanix ? Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining
Clarus Microtech

implmex said:

I'm still pondering pricing: I get my ass handed to me too often on jobs like these but I have such an irresistible drive to solve the puzzle that too often I don't care enough to make a good business decision, so I tend to underprice the work in order to bask in the glory.

No wonder I'm so damned poor!

Cheers

Marcus

Click to expand...

Marcus, Yes, you can absolutely do what you are considering. I've relied on the fact that turning centers typically use the index pulse of the encoder to start certain operations. I've also done similar to what you desire with custom tools. Other than recommending to forge forward trust your gut, and feel free to call if you want, you might check with Royal (and others ) to see about a special tap, similar to what you're planning, but in one tap... I've done that too. ( in fact, I think you and I discussed it, back then ) A bit pricey, but worth the omission of a significant amount of fiddle farting with things... As regards pricing, I've ridden the same boat. Triple your price, up front. It should make the losses just a little more palatable. Good luck! Let us know how it goes.

Hi Bill,
Let me understand it more clearly. I am missing some point.
I am thinking in terms of single point cutting.
When we want to produce a single start thread, we use F = pitch.
If we want to make a double start thread, we use F = 2 x Pitch, and make one more thread after shifting the start position by pitch distance. We thus get two equi-spaced helices. The depth of thread remains same whether it is single or double start. So should be the strength. Confused I am!

sinha said:

Hi Bill,
Let me understand it more clearly. I am missing some point.
I am thinking in terms of single point cutting.
When we want to produce a single start thread, we use F = pitch.
If we want to make a double start thread, we use F = 2 x Pitch, and make one more thread after shifting the start position by pitch distance. We thus get two equi-spaced helices. The depth of thread remains same whether it is single or double start. So should be the strength. Confused I am!

Click to expand...

Hi sinha,
You're correct when comparing two Threads of the same pitch; I didn't pick up specifically your use of pitch. But if the criteria of a 3.0mm Lead and Major Diameter in Marcus's Thread could not be changed, then there would be nothing left of the Minor Diameter by the time a Single Start, 3.0mm Lead Thread was cut to the correct depth for the 3.0mm pitch. A 3.0mm Lead, Two Start Thread will have the same strength as a 1.5mm Lead, Single Start Thread. In the length of Marcus's Tread of 5mm, the extra 1.6666 turns required seems insignificant expressed in that term, but its still double the number of turns. It all comes down to the end use specifications.

Regards,

Bill

I am still confused. Please bear with me.
I may be wrong but this is what I believe:
Even in Marcus' case, the depth of thread would be calculated on the basis of 1.5 mm pitch even if it is double start. So the question of insufficient material does not arise just because we make double start instead of single start.
Only the angle of helix changes, to make room for another helix, symmetrically placed in the gap.
The only difference would be the number of turns required for tightening or loosening. A double start thread can be tightened in half the turns that are needed for tightening a single start thread. So, it is quicker. If that is the consideration, then double start is better. Otherwise, it is not different from single start threads in other aspects.
I have seen double start thread in a car jack. I could not think of any explanation other than the fact that it would lift the car in half the time. Of course, more effort would be needed in lifting.

I am still wondering if double start is really needed in this case. Saving a few seconds in tightening/loosening is not a great thing. Possibly, the reason is that a double start thread requires more effort to loosen it (compared to a single start thread) which might be desirable in implants. Or is there some other reason?
The design must be questioned because manufacturing of this part is so difficult.

sinha said:

I am still confused. Please bear with me.
I may be wrong but this is what I believe:
Even in Marcus' case, the depth of thread would be calculated on the basis of 1.5 mm pitch even if it is double start. So the question of insufficient material does not arise just because we make double start instead of single start.
Only the angle of helix changes, to make room for another helix, symmetrically placed in the gap.
The only difference would be the number of turns required for tightening or loosening. A double start thread can be tightened in half the turns that are needed for tightening a single start thread. So, it is quicker. If that is the consideration, then double start is better. Otherwise, it is not different from single start threads in other aspects.
I have seen double start thread in a car jack. I could not think of any explanation other than the fact that it would lift the car in half the time. Of course, more effort would be needed in lifting.

I am still wondering if double start is really needed in this case. Saving a few seconds in tightening/loosening is not a great thing. Possibly, the reason is that a double start thread requires more effort to loosen it (compared to a single start thread) which might be desirable in implants. Or is there some other reason?
The design must be questioned because manufacturing of this part is so difficult.

Click to expand...

Hello sinha,
We are kind of in agreeance; the confusion is due to the use of the terms Pitch and Lead. In my previous Post I made reference to "if the criteria of a 3.0mm Lead and Major Diameter in Marcus's Thread could not be changed", in that case, the Depth Of Thread of a Single Start Thread having a Lead of 3.0mm is calculated on 3.0mm, as the Pitch and Lead of a Single Start Thread are equal.

If the type of jack you're referring to is a multi-stage where one threaded shaft winds out of the threaded bore of a second threaded shaft, a multi-start thread is used to result in a particular Lead, but with a shallower thread depth, so that the component having both an internal and external thread retains strength compared to a similar diameter part having a Single Start thread of the same Lead cut both internally and externally.

Regards,

Bill

Ok.
Let us see how Marcus machines this part.
The good part is that he loves challenge.

sinha said:

Ok.
Let us see how Marcus machines this part.
The good part is that he loves challenge.

Click to expand...

The good part is that I don't have to do it.

Bobw wrote: "The good part is that I don't have to do it."

C'mon Bobw:
Where's your sense of heady adventure??

Think of the bragging rights.
Think of the penury!
Think of the chest thumping Alpha Male posturing you could adopt at the pub!!

Oh right, I forgot, no one but a fellow machinist knows or cares what you just pulled off.

My Lovely Wife has a perfect saying for this: "That's nice, dear".
She does like pretty blue chips though...and the cheques; she likes the cheques.

This week I'll find out more about the job and whether there's actually any kind of budget for it.

Thanks again everyone; I'll keep you all posted on what becomes of it.

Cheers

Marcus
Implant Mechanix ? Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining
Clarus Microtech

Hi All:
Remember this thread?
Well I got the job, and here's the first try off the lathe.
IT WORKED!!! YIPPEEE!!
Grinding and aligning the taps turned out to be a bit of a bitch; I couldn't wirecut them because the helix angle was way too steep, so I ground two form relieved single point taps.
The tap photo is not great, but you can make out how it works.
It's got two points, one for each start, so I guess it's actually a two point tap, but it single points each thread gullet and I use the rigid tap cycle to push it through at the right lead.
My thanks again to all who helped with advice...it's much appreciated as always.
Cheers

Marcus
Implant Mechanix ? Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining
www.clarusmicrotech.com

I'm impressed... Can you retire now?

Nah BobW;
But I did stick out my tits!!
Cheers

Marcus
Implant Mechanix ? Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining
Clarus Microtech

So, as someone with a biomed background, I'm very curious what the part is for? Can you indulge inquiring minds? Nice work!