A Microscope Repair.

[Pete W.]

Hi there, all,

Here is one of my first projects of 2015.

Several years ago, a friend gave me a Cooke, Troughton & Simms polarising/petrological microscope.  It was in need of a repair to the substage cluster, which should look like this:



At the very top, immediately beneath the stage, the condenser and its iris diaphragm sits a filter holder and beneath that the polariser (the component with the 0 to 360 degree graduations).  Next down is one of the two lugs that support the entire swing-out sub-stage cluster, followed by another filter holder.  The lowest swing-out component is a supplementary condenser lens beneath which is a second support lug.  The threaded portion of the spindle next to the major diameter of the tapered section screws into the hole in the upper mounting lug and the lower end of the plain section is secured in the lower mounting lug with a tiny grub-screw.  The polariser unit is retained by a nut that fits the other threaded section of the spindle.  The assembly is interspersed with several plain and one spring washers.  Oh, and I forgot to say that the upper filter holder, swinging on the plain section of the spindle, is retained by a collar and grub-screw.

However, some previous owner (not he who gave the microscope to me!) had modified (butchered!!!) it.  I presume that the polariser unit had suffered damage to its polarising material and so had been removed, complete with the upper filter holder.  In the process, the spindle on which all the swing-out components pivot was sawn in two.  When the microscope came to me, the empty polariser unit and upper filter holder came separately, complete with half the spindle!

I had put off doing this repair for far too long despite having acquired a complete polariser unit via that well-known on-line auction site but I've decided to make it one of my first workshop projects for 2015.  The first step was to strip out the part of the spindle that was still mounted on the microscope and to have a measuring session.  The two sections of the spindle, arranged to simulate a new spindle, look like this:



To give an idea of the size of this part, the threaded sections are ¼" by 40 tpi ME thread (i.e. Whitworth form) and the overall length of the entire spindle will be roughly 1½ inches.

The polariser unit swings on the tapered section of the spindle.  I could measure the length and maximum diameter of that tapered section easily but could only get an approximate measurement of the minor diameter.  I plan to set over the top-slide of my lathe and machine a dummy spindle with just the tapered section then check the taper in the polariser with engineers' blue.  Then adjust the set-over until I get a good enough fit and machine the tapered section of the replacement spindle at that setting.

The original part was probably drawn with dimensions referenced to some particular point on the spindle - I have no way to retrieve the original manufacturers' dimensions.  So I just have to measure each feature of the spindle sections, compare them with measurements of the mounting lugs and proceed as best I can.  If the first attempt doesn't work, I'll try again - I just need to avoid resetting the top-slide setting until I'm sure I don't need it again!!

I'm going to machine the new spindle from EN1A material. 

Watch this space!!        

[awemawson]

Hi Pete, glad to see you active in the workshop 

Being a lazy sod, I'd just face up those two parts, bore them, and either hold them together with a stud and loctite, or a pin and silver solder. If much material has been removed between them then a suitably shaped pin would make up for it I would think.

[vtsteam]

Looking forward to this...  

[Jonny]

Good one Pete, if a jobs worth doing, do it it right or not at all.

Have both studs to be cock on inline, most likely. If so machine in one from bigger dia bar to avoid flex, I do it all the time.

[BaronJ]

Hi Pete,

One way of getting a very close idea of the diameter of the small end of that taper would be to cut a hole in some thin card and press the small end into it.  I would use my drill press to get enough pressure without doing any damage to the part. Then measure the diameter of the depression in the card.

[Pete W.]

Hi there, all,

Thank you all for your replies and suggestions.

Andrew, the approach you suggested was the same as the advice from a couple of posters on the Yahoo Microscopy list.  While that might be a good approach in a commercial environment or if I were busting to get the microscope working for some urgent observation, I've decided not to go that way, not at first anyway.

Please don't be offended.          

The thing is that I want to know if I can master all the processes required to make a complete replacement spindle - I'll never know that unless I try it.  Another reason is that I've been carting my Myford around with me for forty-four years now and I can't justify that if I only use it as a drilling machine!          

Another reason is that Graham, the now deceased microscopist who gave me the microscope (and lots of other help, advice and assistance) wasn't a 'quick fix' kind of guy and I'd feel shivers down my spine while standing at the lathe if I took an easy way out!          

So I say again, please don't feel offended, guys, I'll go for the complete replacement spindle and hope you don't have reason to say 'we told you so' if I make a pig's ear instead!!
                   

[awemawson]

Pete I quite understand where you are coming from, and after all, you still have the possibility of doing it that way if the taper turns out wrong.

Of course if you take the "Conservationist Approach" you have to preserve as much of the original as possible and make a clear distinction between the old and the new. So in your case you must use a contrasting material that obviously isn't original. Not an approach I agree with, where in old buildings you see new oak scarfed into the old, but not dressed to match, and glass monstrosities attached to Grade 1 listed cottages 

[Pete W.]

Hi there, all,

Andrew, this microscope is venerable but I don't think it's listed!            

So my intention is to make a spindle that is a F³ replacement (you know, 'Form, Fit & Function) for the original one.  As to material, I already stated it's EN1A.  It seems to be free-turning.

I'm not using oak but I have been given four nice pieces of mahogany which I plan to use to make the accessory drawers that fit inside the microscope case to hold the spare objectives and eyepieces.

Today's task has been refinement of my taper test piece.  I did attempt to measure the included angle of the original 'stump' using a 'Bevel Box' and standing the 'stump' on a mini surface plate (actually a 3" x 3" sample of granite from a stone merchant's) but it didn't help me much, the degree scale on my top-slide is too coarse and has about a degree of zero error!

So, having made a first attempt two days ago and persevered this afternoon, I achieved this:



My tin of 'Micrometer' engineer's blue has been kidnapped by the shed gremlins so I used a Sharpie, I was quite pleased with that result having arrived at that fit by trial and error.          

So, just to test the result, I removed that piece of rod from the chuck and turned a taper on the end of another piece - it fitted too.         

My next task is to contrive a tool to turn the run-out groove at the ends of the two lengths of ¼" x 40 tpi ME thread, it needs to be quite narrow.  I do have a grooving tool holder that takes TC tips but I only have 2 mm tips, I need more like ½ mm.  I was wondering if I could grind up a piece of hack-saw blade but all my old hacksaw blades are bi-metal.  Then I saw a slitting saw blade being used for parting on Stuart's thread on here.

So, as I'm in no great hurry, I shall go into cogitate mode for a while.          

[Pete W.]

Hi there, all,

I've been pottering-on with this project.  It's all quite simple stuff compared with most of you other guys' posts but it means that I'm getting some workshop time,
so share my joy!!          

The new spindle will have each end turned down to 0.150" diameter and then a section of ¼" x 40 tpi ME thread each side of the tapered section.  So I have to machine one end, then remove it from the chuck or collet and turn it round.  The initially machined end will be a bit awkward to hold.  So, today's task has been to make a fixture to hold the spindle while I perform that second set of operations.

Starting with a short piece of ⅜" EN1A rod, I faced both ends and centre drilled one.  Then I through-drilled a pilot hole and opened it up with a #24 drill:



Then I opened about half the length of the hole with a #1 drill (tapping size for ¼" x 40 tpi ME thread according to G.H.Thomas):



I then tapped the enlarged section of the hole ¼" x 40 tpi ME thread, starting with the taper tap held in the tail-stock drill chuck and pulling the vee-belt by hand to turn the job.  When I'd got enough thread to locate the tap, I finished the operation with the second and then the plug taps in a hand tap-wrench:



I completed the fixture by turning it round in the collet and cleaning up the drill exit hole with a centre drill.

I've found that my ¼" x 40 tpi ME thread die has a pronounced lead so I don't think it will thread as close to a shoulder as the Cooke, Troughton & Simms original part!  I do have a 40 tpi Whitworth form chaser but it's too deep to fit in the Dickson quick-change tool-holder. 
So I've had to dig out the original ML7 tool-clamp with its elephant-foot screw - just one snag with that, the shed gremlins have kidnapped the spherical washer!!           so finding that is going to be Saturday morning's first job!

I held the work in the collet today, for better concentricity and to minimise overhang, but I found that it's sometimes difficult to get the tool near the work-piece - the three-jaw gets the work-piece within reach of the tooling.   

[philf]

Pete,

A bit late to suggest something different perhaps but couldn't you make the spindle with the two plain ends and a threaded centre portion. The taper section could be tapped and then be 'loctited' on to the threaded portion. The threads would then be complete right up to the taper.

Phil.

[Pete W.]

Hi there, Phil,

Thank you for your suggestion.  I did actually consider that approach but I concluded that turning the tapered section to length with clean ends would be tricky.  Difficult to hold the part.  Also, any slackness in the thread might put the polariser unit out of square.

It is a problem that I shall have to solve sometime, though.  I'm thinking my next project after this one ought to be a mandrel handle and that needs a tapered plug with a threaded through hole to act as an expander.  The thread probably won't be ¼" x 40 tpi ME though, more likely ¼" BSF. 

[vtsteam]

I've found that my ¼" x 40 tpi ME thread die has a pronounced lead so I don't think it will thread as close to a shoulder as the Cooke, Troughton & Simms original part!  I do have a 40 tpi Whitworth form chaser but it's too deep to fit in the Dickson quick-change tool-holder.

Pate, I usually just turn the die around backwards after threading the normal way, to thread to a shoulder.

Looks like you're making great (and fun) progress!

[philf]

Hi there, Phil,

Thank you for your suggestion.  I did actually consider that approach but I concluded that turning the tapered section to length with clean ends would be tricky.  Difficult to hold the part.  Also, any slackness in the thread might put the polariser unit out of square.

Pete,

Not if you machined the taper after it had been loctited on.

Phil.

[Pete W.]

Hi there, all,

Put in some more workshop time today.

I'd pondered on the best sequence of operations and realised that, in the later stages of machining, the work-piece was going to be held by a narrow spigot one end while the other spigot, the thread and the tapered section were machined.  To give the job some support, I fitted a piece of brass in the tailstock drill chuck and drilled it from the headstock, drill #24, a running clearance for the diameter of the two spigots.  Here's a photo, sorry the camera auto-focus decided it preferred the background!



I had originally planned to hold the raw material in a ⅜" collet to minimise overhang but, for reasons cited in my earlier post, that didn't allow the tools to reach the job!  So I used my 'Sunday Best' three-jaw instead.  Further thought revealed that I needed to win some clearance between the top-slide handle and the tail-stock body (remember, the tail-stock is set over in preparation for machining the tapered section; I didn't want to disturb that setting.)  So, I extended the tools further out of the QCTP holders than I usually have them.

I started by machining the long spigot, first with a right-hand knife:



Then cleaning the corner with a 'Mini-Thin' tipped grooving tool:



Next, I machined the bar to the diameter of the major diameter of the tapered section and, using the 'Mini-Thin' tipped grooving tool, marked the extent of the threaded portion:



I then reduced the diameter of that section to ¼" and threaded it ¼" x 40 tpi ME using a tail-stock die-holder.  I had to refine the undercut at the end of the thread by repeated applications of the grooving tool.  Before proceeding, I checked that the shape so-far achieved fitted nicely in the sub-stage bracket of the microscope.  Sorry, forgot to take photos of that operation.  I then trimmed the long spigot back to the correct length.

It seemed advisable to remove as much metal as possible with the job still supported on the end of the raw material bar.  So I put in two further grooves to denote the boundaries of the tapered section and the other threaded section.  I next withdrew enough of the bar from the chuck and supported the long spigot using my tail-stock steady while I plunged the sections of the bar whose diameters needed to be reduced.  I reduced what was to become the second threaded section to ¼".



Again, I had to apply the grooving tool to refine the depths of the grooves as this work progressed.  When the short spigot section of the job was finished to the required diameter, I removed the job from the chuck, sawed the work-piece off the raw bar and reversed it in the chuck, holding on the long spigot.  I was relieved to find that it revolved true.

Proceeding carefully, I threaded the appropriate section ¼" x 40 tpi ME using a tail-stock die-holder.  As before, I had to refine the undercut at the end of the thread by repeated applications of the grooving tool.  (The QC tool-post really shows its value when repeatedly swapping tools like this.)  I also used the grooving tool to clean up the shoulder where the short spigot meets the threaded section - if you examine the photo carefully, you'll see that I overdid this a bit!  I then carefully trimmed the short spigot to the correct length.

Because of the ravages of anno domini, I was a bit tired by this time and I should have quit while I was winning!  'Winning'?  Instead, I decided to go for broke and machine the tapered section.  There was just sufficient room at the start of the cut for the right-hand knife tool with the end of the short spigot engaged in the tail-stock steady.

Now, I do know, from folk-lore, from all the books I've read, from all the forums I've browsed and even from some practical experience, that you do have to be very careful turning tapers.  The sharper the taper, the more care is required and this taper is only about 8° included angle.

BUT, I was tired and when it was almost done, I decided to take 'just one more cut'!!!!          



So, this is what I achieved through my day's labours:



Well, that last cut took off about twice the meat that it should have done so the taper protrudes a few thou through the polariser unit rather than being flush.  I might be able to shim it, time will tell.  Also, some of the threads that seemed fine while work was in progress, now seem to have tightened up, maybe some fine swarf in the threads.  So, I plan to sleep on it and do some fitting and fettling and see how things turn out.

Maybe this one will be declared a trial run and I'll machine another - it should be simpler second time around!                   

[Jonny]

Anything accurate you shouldn't keep removing from chuck, it will never line up. Runout at tip may be as good as ok but where held something else and in doing so the taper can be eccentric to the threads plus runout.
That's why I said do it in one from bigger dia to avoid the flex, threads, taper and both ends of shaft all in line. Good bit is can mark all lengths out in situ but retain strength at chuck end doing that last. Doing two things similar now.

Try the mini thin for doing the  start and finish shaft dia on power feed.
Taper best left oversize maybe 2 thou, black up and sand to suit fitment trying often.

Shame you cant try the threaded microscope portion on what your threading. Drill and tap may come out sloppy or tight, see what the old part fits like.

[Pete W.]

Hi there, Jonny,

Thank you for your post.   

I've now had a look at the spindle through a pocket magnifier and tried the fit of some of the threads.  Despite my run-out groove being quite a generous width, the starting lead of the die still results in the very end thread preventing full engagement.  I think one way round that, as I wrote earlier, is to screw-cut the threads using my chaser.  (I have three ¼" x 40 tpi ME dies and they all have a significant lead on both sides so I don't think reversing the die in the holder is going to work.)          

Also, Jonny, I'll machine the next one as you suggested, without removing it from the chuck until the very end.  That means that I shall have to screw-cut the threaded section nearest the chuck so I might as well screw-cut them both.  I don't want to alter the set-over of the top-slide so I shall have to machine the part with the short spigot towards the tail-stock.  Also, in order to be able to try the polariser unit on the taper as I machine it, the taper has to 'point' towards the tail-stock.

I'll machine the short spigot first and then reduce the diameter of the two threaded sections (and what will later become the long spigot) to ¼" and screw-cut them before machining the taper.  Only then will I plunge cut to reduce the diameter for the long spigot (to 0.150") so as to maintain the support for the part for those outer operations.  I'll use my tail-stock steady for all those operations that permit it.  I'll need to be careful to note my cross-slide dial readings while screw-cutting because at that stage only one threaded section will be accessible for checking the fit.

So, what to do next?          

I think the next thing will be to turn another dummy taper or two to check that the set-over of the top-slide hasn't shifted.  Then I think I'll have a practice at screw-cutting with the chaser. 
I bought 4 feet of ⅜" mild steel rod so I think I have enough material!                  

For the record, Jonny, I only removed the job from the chuck once, to saw off the part-machined part and reverse it in the chuck.  I did, at one earlier stage, draw it further out of the chuck but I took care not to turn it wrt the chuck jaws.  Also, I did as stated try the threaded portion in the microscope sub-stage bracket and it did fit then - but it doesn't now!              

[vtsteam]

Pete, did you miss my earlier suggestion to cut the threads once the proper way with your die, then re-cut the same threads with the die reversed so you can cut full depth threads all the way to a shoulder? I did that many times in the construction of my lathe.

[Pete W.]

Hi there, Steve,

Thank you for your post - yes, I did see your suggestion.  I responded to it in my first main paragraph, the sentence in brackets. 

[vtsteam]

Oh, okay, I finally found it (after reading three times through the thread!) I guess I don't understand what you meant by lead in, because all my dies cut to the very edge of the die, and the start end is tapered, while the opposite end is full depth. so if you reverse them they cut full depth to an edge. But I'm guessing that what you have is a die where the cutting edges don't extend to the end of the die -- and maybe there is some kind of coutersunk hole there. Anyway, I trust you know what you have won't work, so apologies for the repetition!

[Pete W.]

Hi there, all,

Well, after a lot of distractions from workshop time, I managed to get to grips with a second attempt.

I'd previously had difficulty getting a good thread right up to the shoulder (see earlier posts).  Thinking this was because of the lead (aka taper) in the mouth of the die, I'd bought a 40 tpi chaser.  So, before starting on a spindle for real, I'd had a few practices with the chaser on some 1/4" free-cutting mild steel rod.



The camera didn't do that thread justice!  Some of my attempts weren't bad.

I started on the spindle by turning the stock material (3/8" free-cutting mild steel) down to the maximum diameter of the spindle, namely 0.350".  That very ordinary process looks like this:



The end of the material shows an earlier 'dummy run' at turning the taper.  That gets removed in subsequent operations.

Then I used the top-slide and the mini-thin grooving tool to define the lengths of each section of the spindle.  I dialled the required dimensions after correcting for the angular offset of the top-slide (1/Cos 4°), pretty small but I did it anyway.  I turned each groove to the appropriate depth.  The tresult looked like this:



(There's a very thin 'extra' section right at the outer end of the job - this is to allow for the centre drilled hole in the end of the stock, to be removed later.)

Then I turned each of the outer sections to the outside diameter of the thread, 1/4" (intending to further reduce the spigot diameter as a later operation), resulting in this:



Then, with the lathe gear-box set to 40 tpi, I presented the chaser to the 1/4" section.  I had the chaser mounted in a rear tool-post, NOT inverted, with the feed set to move the carriage away from the chuck.  Because there wasn't enough room to clear the chaser, I removed the tail-stock and centre.

This, and cutting the grooves at the chuck end of the job, caused my downfall!  The job rode up over the chaser and snapped, resulting in this:



                 

Oh, well, as my dear old mother used to say, 'If at first (and second) you don't succeed, try, try, try again!'.          

I should have left those grooves at the tail-stock end of the job until later.  It would also have helped if I'd had a piece of 0.218" hardened steel rod with a true 60° point ground on one end and its other end set in a 2MT shank - Set in the tail-stock, it would have supported the job against light cuts.     

[Pete W.]

Hi there, all, again,

After the catastrophe of the second attempt, I retired to lick my wounds (figuratively speaking) and to ponder a 'better way'.

This afternoon, I was ready for the third attempt!

I chucked and centred a fresh piece of material and reduced most of its length to a diameter of 0.350".  I then machined a groove sufficiently far in from the outer end of the bar for the threaded section plus the spigot.  I used the mini-thin grooving tool and cut to a root diameter a few thou less than the core diameter of a 1/4" x 40 tpi ME thread.  I then reduced a section of its outer end to 0.250" diameter, temporarily removed the tail-stock centre and used a tailstock die-holder to thread that outer section, resulting in this:



I used a different die from the one I used for my first attempt.  What I replied to Steve back then about 'all my dies' was plainly wrong - this one cut a good thread and I was able to run a nut right up to the shoulder, like this:



I restored the tail-stock centre support and turned the tapered section, like this:



When it was close to size, I tried the polariser unit on the taper and took VERY careful and light cuts.  I stopped when the top surface of the polariser unit lug was as nearly flush with the top of the taper as I could see by eye.

Then I used the mini-thin grooving tool again to define the 'back end' of the tapered section, like this:



The next operation was to turn down the section of the job between the base of the taper and the chuck to, again, 0.250", with this result:



I then further reduced a section of the job immediately in front of the chuck to a diameter of 0.150".  This was looking ahead to having to cut off the job and reverse it in the lathe to allow access for the 1/4" x 40 tpi die to thread the section behind the base of the taper.  I'll then support that short section of spigot on a drilled brass bar-end in the tail-stock chuck.  But that comes later!  Here's what it looks like:



The final operation in today's session was another application of the mini-thin grooving tool to define the length of the threaded section above the taper, like so:



Looking ahead, the next operation will be to turn off the surplus thread at the current tail-stock end and reduce the diameter to 0.150" to form the spigot at that end.  Then to cut off the job at the narrow neck at the chuck end, reverse the job in the lathe and use the tail-stock die-holder to thread the section next to the wide end of the taper, then mini-groove to define the desired length of that thread, then reduce the surplus thread at that end to the spigot diameter, i.e. 0.150".  All of that promises to present some centring challenges!!

Watch this space!!                   

[vtsteam]

That looks pretty!

Fingers crossed.......!

[Pete W.]

Third attempt continued:

At the end of my last post I outlined the next operations and today's workshop session more or less followed that sequence.  First of all I removed the surplus thread and finished the spigot to final diameter on the end nearest the tail-stock (i.e. next to the narrow end of the taper).  I left a bit of thread because the tail-stock centre limited how close to the end I could go.  Here's a photo:



Next, I removed the job from the lathe and removed excess material from each end, using a junior hacksaw.  I then trimmed the spigot to length and reversed the job in the lathe using the fixture shown in my first post in this thread.  I supported the short spigot in a piece of drilled brass bar, held in the tail-stock chuck, like this:



I then realised that I was getting ahead of myself,and swapped the tail-stock chuck for the tail-stock die-holder and cut the thread, like this:



You can see that, as with the other end, this die cut a clean thread right up to the shoulder.  I ran a couple of nuts onto the thread to verify.

Next, I used the mini-thin grooving tool to define the required length of the thread and removed the excess thread, finishing at the desired spigot diameter, 0.150".  For careful light cuts, the mini-thin grooving tool also works well as a right-hand knife tool.  I then defined the required length of the spigot.  Here's a photo:



I then removed the job from the lathe and cut off the excess length from the spigot, using the junior hacksaw.  The torque applied during the latest turning operations had tightened the job in the thread of the fixture and I had quite a tussle to remove it!  The only part I dared to hold in the bench vise (with fibre jaws) was the spigot but it wouldn't grip.  Eventually, I gripped the spigot in the tail-stock chuck and the fixture in another chuck and was able to separate job from fixture.  I returned the job to the lathe and used a Swiss file to break the sharp edges and then used the junior hacksaw to start a screwdriver slot in the end of the longer spigot, finishing the slot with a Swiss file.  That produced this:



I think (and hope!) that this concludes the lathework.  The next step is a trial assembly of the spindle, its mounting bracket and all the optical components that swing on the spindle.  I can already see that I've cut the lower threaded section and the lower spigot a bit shorter than ideal but hopefully they'll be good enough.   

[Pete W.]

                   

I decided that one isn't good enough!

I went out to the workshop this afternoon to clean down the lathe and, sort of 'on autopilot', found myself machining another!          

[awemawson]

Well shelf spares are always handy ! I'm terrible in that if I acquire a useful 'thing' particularly if it contains electronics, I grab any spares that crop up and box and store them. Almost guarantees that they'll never be needed.

Example #1 : I have a complete set of electronics, electrics and servos for my Diesinker EDM rescued from one being scrapped.

Example #2 : pretty much a re-run of the above for my Wire EDM