Author Topic: Getting a tacho drive fixed on a classic racing bike  (Read 31055 times)

bogstandard

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Getting a tacho drive fixed on a classic racing bike
« on: September 03, 2009, 07:45:53 PM »
A few days ago, a friend brought around some bits and pieces of a classic racing BSA Bantam. A 1960's+ 175cc two stroker. The road bikes were very quickly found to be easily tuneable, and a new class of racing was born. This bike had been sitting in a ditch for over 2 years, with half the engine under water, and was thought to be a gonner, until he got the engine apart and found it was clean as a whistle inside.

Now to the tacho drama.

The bike had suffered some damage, hence it's life in a ditch. These first few pics show what it was.


The drive piece that actually transferred the engine revs to the right angle tacho drive was rock hard, and as such, the end had snapped off, hence no drive.




A racing bike has no need of a speedo, but it is of paramount importance to know your engine speed for gear changing so that you can stay within the engines power band. This engine as a stock item doesn't have a tacho drive, so what is done, they get hold of a drive off another model of bike, and bang it on. In this case, it was welded (and by the looks of it, not using TIG, this one was done the old way with oxy/acet).
The gear sits in the big hole, with the shaft disappearing into the depths.




And popping out here, on the inside of the timing cover.




Where you see that well manicured machinists finger pointing is the drive off the end of the crank, the sprocket is held on with a 1/2" BSF nut, and onto that nut was a piece that is now missing, that used to engage with the drive gear spindle.
So I need to make a new nut with a drive on it. Unfortunately, I don't have my lathe ready enough yet for single point cutting. I just don't know how accurate imperial threads will come off it until I have tried it out. Another problem, I don't have any 1/2" BSF taps. But I know a man who does.




A quick call to Stew, and very soon I had in my grubby hands just what is needed.




So now to the fix.
I have decided that the hardened drive is just too brittle, so if I put something on the end that is tough and fairly hard, it should do the job.
I am going with a piece of silver steel that will be silver soldered onto the end of the shaft. If I do it right, the shaft will stay glass hard for use as a bearing surface in the bronze bush, and the silver steel won't harden too much so that it gets to a brittle stage.




There is no way I can turn the shaft, it is just too hard. So out came the toolpost grinder and I soon had a spigot ground onto the end of it.




After shortening the spigot on the offhand grinder, this is what it looks like.
The slug on the end will have a fairly close fitting hole drilled into it, then that will sit over the spigot, and both parts will then be soldered together.




Thats as far as I have got at the moment, the next instalment when I get a round tuit.



Bogs




Offline John Hill

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Re: Getting a tacho drive fixed on a classic racing bike
« Reply #1 on: September 03, 2009, 07:54:37 PM »
My James used to leave Beeza Banties in the dust!   :med:


I thought imperial threads would be perfect if cut with the 120/127 change gear?  Maybe there is some other factor I dont know about... :scratch:
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bogstandard

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Re: Getting a tacho drive fixed on a classic racing bike
« Reply #2 on: September 03, 2009, 08:06:50 PM »
John,

I used to run around on a James with a Villiers 2T engine, it would go on forever. But the best Villiers engined bike I ever saw was a Greeves scrambler owned by a friend of mine.
It just so happened that the Bantam was very prolific in the 60's because I think it was classed like a Honda 50, a get you to work and back bike, so that is why it was used, availability.

I don't want to get into thread bashing on this post, but just as a summary, imp threads on a metric machine, even with the conversion gears as we have, as far as I know are only super close, not exact. I will know more in a few days, as I have to make a dummy spindle, and that has to be turned by single point tool, a die will just not be straight enough for the job it has to do.

John

Offline John Hill

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Re: Getting a tacho drive fixed on a classic racing bike
« Reply #3 on: September 03, 2009, 09:09:45 PM »
John, my James was in truth, a very miserable machine though the engine did run well for about 30 miles before I had to clean the whiskers from the plug points.  Nowadays I would know how to select fuel and oil to prevent that but back then I went by the inscription on the fuel cap.  Bantams were popular here too but in this country they would have had to compete in the same price range with various pre-war and war surplus bikes of much bigger capacity of both British and American origin.

Mine had a pressed steel frame which must be been twisted as it would never run straight but eventually someone bought it and I trust they lived to 'enjoy' it.

I will be interested to read of your imperial threading experiences in due course.
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bogstandard

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Re: Getting a tacho drive fixed on a classic racing bike
« Reply #4 on: September 05, 2009, 10:40:22 AM »
I got to the stage last time where the softer end bit needed to be made and fitted. So a piece of silver steel (drill rod), just a little longer and larger diameter than what is required was drilled to give a loose 0.002" (0.05mm) fit on the spigot. This is to allow the silver solder to form and flow thru capilliary action all thru the joint. If you don't leave a gap around this size, your silver solder will most probably only be sitting on the surface and will result in a very weak joint.

The two bits ready for jointing together.




And this is how it will look when finished (well almost).




Tiny rings of solder were placed down into the hole, and around the ground spigot. You have to try to imagine how much solder will be required to fill all the voids in the joint after it is completed. That can only come with experience, and no calculations can be given. The more you do, the better at it you get.
Flux was then spread around the spigot and down into the hole. ALWAYS use a good quality flux, old wives recipes are just no use, and by doing it correctly, you will end up with a perfect joint and very little cleanup required.





A little hearth was made up out of broken firebricks, and because I was trying to protect the previously hardened shaft, that was protected with a bit as well.
The long bolt serves a purpose here. When the part is being warmed up, the flux expands, and like a piston in a bore, will try to push the two bits apart. The bolt holds everything together by pushing the bits together in the initial warming up stage. When the right temp is reached, you can see the solder flowing, then another push with the bolt displaces any excess solder out of the joint.
It was left to cool down naturally for a few seconds, until the solder had solidified, then the part was quenched in water, but with the silver steel bit not being quenched, being held slightly clear of the water.




A quickie rub over with a bit of worn emery shows just how little excess soldering material there is. A file test was done, and it showed exactly what I was after. An original very hard shaft, and a softer drive end to absorb any shocks during running.




The toolpost grinder was brought into play again, and once the original shaft was set up for zero runout, the end attachment was ground down to exactly the original shaft diameter.




Finished with the grinding now. If you look really hard, you can just about see the silver soldered joint. Later, the soft end can be machined to the required drive paddle profile.




It fits perfectly thru the bronze bush and is just slightly overlength. The inside of the timing cover will be machined and cleaned up, to make the area a bit more presentable, and give me a bit more space to work with.




A bit of prodding with a rod and marking it up at certain depths then allowed me to get a rough sketch made of the nut that is required. Get that made, and it will just be a matter of tweaking things to fit.



The next bit of the exercise will be to make the required nut. That will mean I will have to single point cut an imperial thread. A thing I have never done on this machine, so a bit of a learning curve on my part will be required.


Bogs
« Last Edit: September 05, 2009, 02:12:51 PM by bogstandard »

Offline arnoldb

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Re: Getting a tacho drive fixed on a classic racing bike
« Reply #5 on: September 05, 2009, 04:09:00 PM »
Bogs,Great job, and thank you very much for the step-by-step on the silver soldering  :bow:

It is most inspiring to follow your reasoning behind each of the steps involved and then the execution; reminds me of playing a good game of chess.

Regards, Arnold

Offline sbwhart

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Re: Getting a tacho drive fixed on a classic racing bike
« Reply #6 on: September 05, 2009, 04:39:34 PM »
John

If the thread you've got to cut is 1/2 BSF I've got a die, or must it be thread cut.

Cheers

Stew
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bogstandard

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Re: Getting a tacho drive fixed on a classic racing bike
« Reply #7 on: September 05, 2009, 05:14:29 PM »
Stew,

Because the nut has to be perfectly accurate for it to match up to the shaft end I have just done, a die cut thread will just not be accurate enough. It only has to be the smallest amount out of square, and the runout on the end of the nut drive will break the end off the new shaft. As it is, I will be allowing a couple of thou free play in the slot which has yet to be cut.

But I will still borrow it if you don't mind, any slight innacuracy in the thread pitch can be put right using the final threading cut of a die.

John
« Last Edit: September 05, 2009, 10:21:17 PM by bogstandard »

Offline John Stevenson

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Re: Getting a tacho drive fixed on a classic racing bike
« Reply #8 on: September 05, 2009, 05:35:33 PM »
Oh nostalgia, things aren't what they used to be.

I used to make these for Manx Nortons and AJS 7R's years ago, there were 4 main types 1:1 right hand and left hand rotation and the same in 2:1.
There were others but these were the popular ones.

Ironically a few weeks ago whilst sorting out Tardis No 3 I found 10 bare casting in bronze for these.
There is about 3 odd gears in a drawer in one of the toolmaker cabinets but all the jigs and drawings are missing.
Cable end was 1/2 x 26 cycle, can remember that.
If I could have found the drawings I was going to put them on Ebay, one casting with drawings.

John S.
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bogstandard

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Re: Getting a tacho drive fixed on a classic racing bike
« Reply #9 on: September 05, 2009, 06:07:56 PM »
John,

In the old days you could do things like this, wack it on and try it out. Didn't matter where it came from, if it worked, great, if not, try something else.

What you have got must be like rocking horse s**t by now. I would stick them on ebay anyway, someone who has a broken one will soon learn how to machine it, by using the old one as an example.

Mechanical things are now out of the door, it's all electroconical now.

Bogs

Offline Brass_Machine

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Re: Getting a tacho drive fixed on a classic racing bike
« Reply #10 on: September 05, 2009, 09:03:20 PM »
Looking at the first picture... I was  :scratch: trying to figure out how you were go to fix it. Thought you were going to be cutting up a new gear! Nice job!

Eric
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bogstandard

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Re: Getting a tacho drive fixed on a classic racing bike
« Reply #11 on: September 05, 2009, 10:16:38 PM »
Eric,

If you think about things, most items can be repaired, just like the drive shaft. I could even have it just like original if I wanted, hardened all the way along. That would have been achieved with a little change at the end of silver soldering exercise.

There is a saying, 'If it can be fixed, it was never broken in the first place, just awaiting redesign'.

These are the sorts of jobs that used to be done in the backstreet jobbing shops, unfortunately, all gradually disappearing now in favour of the "buy a new one one and bung it in" brigade. The problem is most times on something like this, where do you get a new one from?

It then makes a repair procedure very cost effective.


John

Offline chuck foster

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Re: Getting a tacho drive fixed on a classic racing bike
« Reply #12 on: September 05, 2009, 11:06:28 PM »
good looking work as usual john, thanks for the pictures and the write up  :thumbup: :clap:

chuck :wave:
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Offline John Stevenson

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Re: Getting a tacho drive fixed on a classic racing bike
« Reply #13 on: September 06, 2009, 07:01:11 AM »
Eric,



These are the sorts of jobs that used to be done in the backstreet jobbing shops,

John

HEY I resemble that remark, any Moore like that and I'll have to challenge you to a duel on neutral ground.

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bogstandard

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Re: Getting a tacho drive fixed on a classic racing bike
« Reply #14 on: September 06, 2009, 07:24:42 AM »
If I could get there John, I would take you up on that duel.

I know you are a master jobber, and to me, to be one is like like being a magician that can work miracles as well, and nothing to be ashamed of.

What will happen when we are all dragged out and buried?

Anything older than say 10 years will become instantly recycled.

"Sorry, that is a very old model (5 years old) and you can't get the spares any more" will be the answer everywhere, quickly followed by " but we do have a new model that will only cost you ten squillion doodahs".

Goodbye heritage.

John

Offline John Stevenson

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Re: Getting a tacho drive fixed on a classic racing bike
« Reply #15 on: September 06, 2009, 08:29:24 AM »


What will happen when we are all dragged out and buried?


John

I have already worked that out.
It was a toss up whether to be buried knowing that I owned  a 6' x 6' x 4' plot and the bastards couldn't send me a council tax bill or be cremated.
The cremated option worked best and the hinges pins on the coffin are going to be pieces of EN32. The cremation process will harden these to 56 Rockwell.

My ashes will then be sealed in a glass flask that pivots on these said hinge pins and I will then become an egg timer so I can carry on working.

John S.
John Stevenson

Offline Stefan Pynappels

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Re: Getting a tacho drive fixed on a classic racing bike
« Reply #16 on: September 07, 2009, 07:26:29 AM »
Wow Bogs,

That is pretty amazing, having been born in the replace it or bin it generation, I'm very impressed!

Do you use an ordinary Butane/Propane mix for silver soldering or do you use something like MAPP gas?

bogstandard

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Re: Getting a tacho drive fixed on a classic racing bike
« Reply #17 on: September 10, 2009, 02:23:41 PM »
Before I start, just to answer Spynappels question. I use standard butane/propane mix for all my small torches, and standard propane for the large burner.

Now onto this massive post for such an easy little job. This is definitely a 'takes longer to show than do'.


First off, I need to make a new drive nut for the tacho. So the correct sized drill (7/16") for the 1/2" BSF thread was put into the end of the bar to the depth that was needed.




This is a bad pic because I couldn't get the camera to focus on such a small part. What it is, is an undercutting tool, and this was fed into the bottom of the hole and a small area was relieved. This is normally done if you are single pointing an internal thread, to allow an area for the tip to run into. But in this case, Stew had loaned me the correct sized tap, and I wacked a thread in with that.




With the thread cut, I machined a small 'washer' face and give the thread a good lead in chamfer.




In this shot, you can see the undercut I mentioned.




And the 'nut' fitted perfectly onto the thread on the engine.




For the nut and drive to be made perfectly concentric, I required a true running thread, the same as the one on the engine, to be made.
In this instance, I mounted up some mild steel, and got to work. This mandrel will not be removed from the chuck until all machining operations are completed on the nut.




It is well over a year since I have done any single point threading, and this is the first time this lathe has done any at all. Plus, because I am using a metric lathe to cut an imperial thread, I will not be able to disengage the leadscrew, but reverse the lathe back to the start point each time. So this is a bit of a learning curve for me as well.
I am going to be doing it 'by the book' for my first attempt.
First off, turn the blank down to the required OD of the thread, in this case 1/2".




Then a small spigot was turned on the end that is the same diameter as the root of the thread, 0.420" (the thread depth is 0.040" for 1/2" BSF), then the spigot was shortened slightly so that it would not interfere with the nut being screwed onto the thread.




A recess for the tool to run into was made to the same depth as the root diameter.
That is the metal prepared for cutting the thread, now for the machine.




Unlike American (unified) or metric threads, BSF uses 55 degrees as a thread form instead of 60 degrees. I need half of that figure, so 27.5 degrees.
I put my topslide (compound) in line with the crosslide, and then swung it back by 27.5 degs. You remove this figure from 90 degs, NOT add to 0. So 62.5 on a normal 90 deg scale. If you had a 60 deg angled thread, then you would set your topslide to 60 degs (90 - 30= 60).
On my machine, I have put extra index marks around the crosslide top to allow me to do this indexing a lot easier. If you have a 90-0-90 or 0-360 scale, then you will be able to do this on your normal index mark.




With the topslide now set at the correct angle, then the toolpost was slackened off and the tool tip set using a threading gauge (slightly different to an American 'fishtail' gauge, which doesn't have the BS angles on it). Once set, the toolpost was locked up solid, and the angle rechecked.
As you can most probably gather, I am using the offset angle method to do this threading, not the direct feed in method.
I am playing about with my lathe here, and if I can do it this way, which is that hardest way, then I will have no trouble using other methods.




First off, I touched the tool onto the thread top OD using the crosslide dial.




The crosslide dial is then zeroed. Thru the whole threading operation, this dial will be either wound back to retract the tool, or after retracting, it will be wound forwards to this zero setting. All cuts will be put on using the topslide.




With the crosslide at zero, the tool was fed forwards using the topslide until the tool touched on the spigot for the thread root diameter. The topslide dial was then zeroed.
So if I now only use the topslide for feed, when the crosslide dial is at zero and the topslide dial is also at zero, the thread should be at the correct depth.




The machine change gears and dials were set for what the chart told me to do.




To show better what I am doing, I blued up the bit of steel.
With the crosslide at zero, I brought the topslide in until it just touched on.




At that setting, I engaged the half nuts and took a cut. The half nuts will now stay permanently engaged until the full thread is completed.
This 'swipe' was checked with my thread gauge to make sure things were OK.
It was now a case of retract crosslide, reverse the machine to wind things back to the start, forwards on the crosslide to zero, put a bit more cut on with the topslide and take another cut, then repeat from the beginning of this sentence.
When you start to get close to the zero on the topslide, you start checking with the nut for fit.
What I do, when very close, I do no more topslide cuts, just minute cuts with the crosslide. This cleans up the threads. When very, very close, I put no more cut on at all. Just let the tool go down the thread a couple of times at that setting. You will find that this removes minute amounts from the thread and really cleans them up.




Thread cut.




Nut fits just fine.

I did in fact have a die for this thread, kindly loaned to me by Stew, and I was going to use it to finish this thread off. But then I thought, lets see if this lathe can cut an almost true thread, and it looks like it does. Confidence in cutting an accurate imperial thread on this metric lathe has just gone up by 1000%.




The little root depth spigot was turned off and the thread was given a nice chamfer on the end. I now have a perfectly true running thread to cut the nut on.




Oh! sRF^&**(

I cut a bit too close to the nut end, and the end fell off. That is just me being too confident and in a rush to get the job finished.




But as usual, I had a setup spare in my back pocket, and it was duly finished off to obtain exactly what I wanted.




The engine couldn't be checked out until this nut was made.
Unfortunately, when it was fitted, and the cover was put on, I stuck a transfer punch down the hole that the other bit feeds into and gave it a tap, turned the engine thru 180 degs, and then another tap.
As you can see, the pop marks should have been in the middle of the nut. This shows that the welded on bit isn't in line with the centre of the crank.



So it looks like that was the cause of the original failure of the spade on the end of the gear shaft.
Now that I know what the problem is, the cure is a fairly simple, but a largish set up on the mill, and a little more turning.

A small 'just' job has turned into a major fix.

But that is what friends are for.


Bogs

Offline sbwhart

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Re: Getting a tacho drive fixed on a classic racing bike
« Reply #18 on: September 10, 2009, 03:25:59 PM »
Blimey John how are you going to get things lined up bush and re drill the case  :scratch:

Lovely work..

Stew
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Offline John Hill

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Re: Getting a tacho drive fixed on a classic racing bike
« Reply #19 on: September 10, 2009, 05:10:39 PM »
Great write up John!   :clap:

BTW, :offtopic:I dont think you need have any fears about cutting imperial threads provided you have the 120/127 change gear, you will recall the table I calculated some months ago and the perfect ratios that were identified.  The lathe will cut imperial threads just as accurate as metric which I presume depends upon the accuracy of the metric feed screw.
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bogstandard

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Re: Getting a tacho drive fixed on a classic racing bike
« Reply #20 on: September 10, 2009, 06:02:42 PM »
Stew,

It is actually a fairly easy exercise.
Clamp the one half of the crankcase to the mill table. Centre up on the main bearing. Put the outer cover back on, and bore away to fit a new threaded bush. Spot on centre.

John,

I didn't want to do any single point cutting until I was properly prepared for it, and to really check everything out, and catalogue my findings. Unfortunately this job came along and I was forced to do it.

I had already given your charts a good dose of looking at, but as you know, sometimes there is a great deal of difference between theory and practice. This time, it looks like your theory is spot on, and I will have a lot less trepidation when it comes for me to do some more imperial threading. Nice one.

But one thing I must get done is the retracting toolpost. What I was doing today was a real PITA.

In fact this lathe has really surprised me. I have used all sorts of change geared lathes, and have spent hours setting up gear trains to get just what I wanted, but this one was so easy, with the built in selectable box and a couple of gear swaps, it was all done in about 5 minutes. The only problem is that now I have to get back in there and put it all back to how it was. I had set it up for really nice fine feeds.

Bogs

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Re: Getting a tacho drive fixed on a classic racing bike
« Reply #21 on: September 10, 2009, 06:11:22 PM »
In fact this lathe has really surprised me. I have used all sorts of change geared lathes, and have spent hours setting up gear trains to get just what I wanted, but this one was so easy, with the built in selectable box and a couple of gear swaps, it was all done in about 5 minutes. The only problem is that now I have to get back in there and put it all back to how it was. I had set it up for really nice fine feeds.

Bogs

John, your comments give me great confidence that this, my first and doubtless only lathe, was a good choice.

I think my favourite handle on the gear head is the one marked 'I' 'II',  instant change between fine and really fine! :med:
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bogstandard

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Re: Getting a tacho drive fixed on a classic racing bike
« Reply #22 on: September 10, 2009, 06:20:00 PM »
John,

My favourite as well, but I wish it had synchromesh on the screw fwds/rev lever, it is a real pain stopping the machine to feed the other way.


John

Offline Bernd

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Re: Getting a tacho drive fixed on a classic racing bike
« Reply #23 on: September 10, 2009, 06:28:00 PM »
John,

Very nice write up on threading.  :thumbup:

You explained it exactley the way I learned it, except for the imperial thread cutting on a metric lathe. People should now be pointed in this direction when they ask how to single point threads.

Bernd
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Offline John Hill

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Re: Getting a tacho drive fixed on a classic racing bike
« Reply #24 on: September 10, 2009, 06:29:22 PM »
John,

My favourite as well, but I wish it had synchromesh on the screw fwds/rev lever, it is a real pain stopping the machine to feed the other way.


John

Fortunately the fwd/rev lever has a neutral position so maybe a cunning Madmodder could fit a seperate electric motor to drive the feed box?  Cutting feeds only as that would not be practical for thread cutting.
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