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hoping for repetative identical parts

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savarin

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#1
I need 6 taper pins all identical to so I've come up with the following process and work flow as the easiest way to go.
What I'm after is some comment on whether I'm on the right track or if there is a better way than this.

Chuck a length of 8mm stainless in a collet chuck.
Turn down a 12mm length required for a 6mm thread.
Use the die holder in the tail stock to thread it up to the shoulder.
Part off at the length required for the tapered section.
slide more rod through the chuck and repeat till I have all 6 identical lengths each with a threaded 6mm end.
Now drill and tap the length left in the chuck to 6mm.
screw each cut and threaded length of rod into the end left in the chuck.
Set the compound to the required taper and cut the taper.
Unscrew and screw the next part in and repeat.
My feeling is that as the collet chuck is still holding the rod with a threaded hole in virtually the same orientation as the cut off lengths were threaded then hopefully they will all screw back in and remain in the same alignment.
 

markba633csi

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#2
I don't think you'll get concentric thread to taper alignment even though it seems like it should. I've tried to do tricks like that and the results are often disappointing. Course with a turd of a lathe like mine...
Myself I would cut thread and taper without taking the piece out, then part off. Do that 6 times.
Mark S.
ps you didn't say how severe the taper is- could be a factor
pss nothing wrong with grabbing the threaded end with a collet- no need to make the female piece u mentioned and the accuracy would be reasonable
 
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hanermo2

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#3
Imho .. it depends.
1. Std ie normal threads will not locate the part well in TIR. Wobbly.
2. Std threads will not make the part align with the lathe z axis. The part end will not be concentric with the centerline.
3. I feel You will not be able to make 6 parts, of 8 mm D, with a thread of 6 mm and some length for taper, anywhere near accurately.
The part will be very wobbly, as it is extremely long in length/D, for 6 pieces.

It *could* be done.. again imho..
+using a female taper dead center to support the end, improving concentricity to 0.01 mm or so, for one piece, 1 of 6.
+using high precision threads. These will not screw on to normal parts.
Normal dies will be useless. Need to single point the thread or to make a very accurate die.
+using a travelling steady for each part, along with the very accurate threads.

I would suggest turning between centers, one part at a time.
1.
Chop em all off, overlong.
Make a simple fixture, with 2 points, and one end a sliding fit. A drill rod bit, hardened, in a drilled/bored/reamed hole. 10 min job.
A single hammer punch, at one end, and both ends have a divot for turning between centers.
5 secs per piece.

2.
Now..
Turn them between centers, and the threads will be concentric, the tapers will be concentric, etc.
If you need speed and the threads are less important, you can use a die to either start/help or finish the threads, if your lathe cannot make the thread fast enough in passes/work hours/cycle time.
2.1
Alternatively hold them in collets, and part off the HS end against a fixed temp. stop you clamped to the bed, so the rod does not bend.
2.2
Alternatively/optionally part off the TS end against a fixed steady.

Look at turret lathe videos for inspiration.
Some examples by "feel".
A manual turret lathe might take 5-8 secs, per op, for 4-5 ops to drop off finished parts of great accuracy in perhaps 40 secs total cycle time.
The threading would be 1 op, using a box tool.
These are available for about 100€ nos/used/clones, on ebay.

A modern cnc tool like a high speed production lathe (swiss ie sliding bushing), 2-8 secs per part
high speed cnc lathe - 5x3=15 secs, using 2 tools at once
rotary transfer or multi-spindle, 1 sec per part.
 

savarin

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#4
Maybe some more info regarding the end use.
Total length of the taper will be 15mm
They will be fixed in place with a nut into a slightly oversized hole so they can be moved into the correct position then fixed.
The purpose is to make three pins to engage in holes to ensure alignment of two modules (tubes of my giant binocular) so they always align the same way everytime (some very minute variation allowed) then the tubes can be locked in place with three screws.
 

RJSakowski

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#5
There is still some missing information, What is the OAL? What is the taper angle? What are the tolerances for "identical" parts? a detailed drawing would be helpful in determining the best strategy.

Swiss screw machines work well for this type of part.

At the very least, I wouls single point the threads. At least to get a good concentric start and then finish with a die A lot also depends upon the equipment. How much TIR do you have in your collet chuck? At the chuck haws and asome distance out, say 4"?
 

mksj

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#6
I think your method will work, in particular because you are using a slightly oversized thread mounting hole and then locking the taper pins in place for final alignment. The step shoulder to the thread will set the perpendicular angle, the threaded portion is only used as a retainer not alignment. When cutting the taper, the shoulder will also set the perpendicular angle given that there is a little wiggle room in the threads. As long as the step shoulder is perpendicular, they should be fine. You could cut the taper to a portion of the pin so the final alignment is set by the dowel portion and the receptacle portion is round hole. Probably use something like a 0.001" clearance between the hole and the pin.

You do get some deflection of the parts when holding it only by a 6mm thread, and as others have noted if cutting a full thread with a die there will be some tolerances issues. Wehn possible I use a CNC extended point live center to secure the end furthest from the collet. Normally I cut around 90-95% of threads doing single point and then the finishing thread with an adjustable die sized to the tolerance I need. This can be more difficult for many individuals since metric threads usually require the half-nut needs to remain engaged through this process. I do not see that single point threading is necessary in this case.

If you had the ability to do very precise drilling for the male to female alignment holes, you could recess the shoulder or have a full dimensional base of the taper pin pressed into the piece they mount too, similar to your typical alignment pins.
 

savarin

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#7
I really like the idea of using the straight shank after the taper to finalise the alignment so I think I will go that way thanks.
Now I can cut the taper, chuck up the straight part then cut the thread.
Next job to model this in 3d so I can get the actual dimensions.
Thanks everyone, most helpful.
 
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