Dicing with Delrin - A Repair Job.

[Pete W.]

Hi there, all,

Despite some trepidation about offering my latest efforts alongside the posts of the stalwarts of Mad Modders, I've decided to have a go.  If only because some Modders may be able to tell me where I went wrong and how they would tackle the job.  So, here goes:

A friend managed to damage a rather high quality binocular loup and asked me if I might be able to fix it for him.  I said I'd attempt a repair, strictly on a 'best efforts' basis with no guarantee, but that wouldn't be a quick job - I'd have to finish some other jobs first and also to make some tooling and fixtures along the way.

In this first post, I'll describe the diagnosis, identify the damaged part and summarise my repair plan.  (Photos at the end of the post.)

The loup comprises a pair of magnifying optics which mount on to a bar with an adjustment for inter-ocular distance.  That bar, in turn, mounts on to a head-band.  The optics resemble rifle sights but have a shorter focal distance (about 30 cms) and the mounting bar is shaped to angle their optical axes to converge at that distance.

My friend had managed to drop the loup which had caused it to become 'boss-eyed'.  I gave the loup a thorough inspection and was relieved to find no damage to the optics units themselves but the mount that attaches one optic to the bar had been distorted by the impact.  This component (aka 'the mount') is shown in the photos.  It is plastic, I assume it is a moulding, and it has a stepped hole through which the ocular end of the optic fits, retained by a threaded ring and an M2 grub-screw.  On its upper flat face, there is a tongue that is a sliding fit in a slot in the mounting bar where it is secured with a washer and a plastic-headed M2 thumb-screw.  The tongue on the damaged component had suffered a permanent twist which had caused the loup to become 'boss-eyed'.

I don't have facilities for plastic moulding so the only way I could see to make a replacement mount was to hew it from solid.  My measurements suggested that 8 mm sheet would be the suitable raw material and I opted for Delrin (I'll come back to choice of material later in this thread).  I bought a piece of Delrin sheet on eBay - it was billed as 8 mm but turned out to be nearer 9 mm which had me scratching my head at first!  I sawed off a 1" wide strip using my table saw and divided that into 1½" lengths.  This gave me five blanks (aka 'dominos').

I decided that it would be easier for me to make a matched pair of mounts than to exactly duplicate the undamaged original.  So I resolved to work on all five dominos in parallel in the hope of achieving two usable mounts rather than having to start over if I suffered any problems along the way.  The tongue seemed to be the trickiest feature so I opted to start with that, then progress to the stepped hole and finally to profile the mount to something like the originals.

As Bob says, 'OK, photos':

[Pete W.]

To continue:

I don't have a mill but I do have a vertical slide for the Myford ML7.  (A friendly neighbour did say I could use his mill but my modus operandi is to do a bit and then think a bit and I don't think it's workable to time share a mill!)

After some trial and error, I reached the stage where I had five dominos, each with a usable tongue on one end.  (One domino also has an unusable tongue on the other end - if at first you don't succeed ... )

The procedure was as follows:

Fit a length of 12 mm PGR in the lathe collet, insert the domino into the machine vice with its best edge against the rod and tighten the vice.  Remove the PGR & collet and mount the milling chuck with a four-flute end mill.  With the lead-screw half-nuts closed, take a clean-up facing cut across the end of the domino, noting the lead-screw dial reading.  Move the domino clear of the end mill and advance the carriage by the length of the tongue.  Touch off the rear edge and underside of the domino, zeroing the feed-screw dials in each case.  Wind the vertical slide down by the width of the lower shoulder and feed the cross-slide in by a pre-calculated distance.  Then feed the vertical slide down by the thickness of the tongue plus the cutter diameter, this cuts the 'near' edge of the tongue and positions the end mill ready to cut the upper surface of the tongue.  Next feed the cross-slide out by a pre-calculated distance and then feed the vertical slide up until the underside of the domino is level with the top of the end mill.  This SHOULD give a rectangular tongue of the right width and thickness!

It sounds easy but real lathes have back-lash.  In one or two cases I must have miscounted the turns of the cross-slide dial which put the tongue 100 thou out of position.  Furthermore, my milling arrangement gave lots of swarf and paper edge that obscured visibility of the job (see photos).  I guess a four flute end mill at the maximum open speed of the ML7 is sub-optimum for machining Delrin but it's the best I've got.

Before removing the domino from the machine vice, I replaced the milling chuck and end mill with a drill chuck and drilled and tapped the end of the tongue for the M2 thumb-screw.

The original tongue has rounded ends.  I wasn't able to reproduce these so resorted to bevelling the ends of the tongue with a craft knife.  This gave a shape that works but isn't pretty!  Similarly, removing the paper edges, again with a craft knife, wasn't a precise operation.

OK, photos:



 

[Pete W.]

That's all I've got on photos at present, though I have done a bit more work.

I plan to drill three holes in each domino so I've made a gizmo to position the holes in the hope of getting all the drilled dominos the same - it doesn't warrant the title of 'jig' but maybe it's fair to call it a template.  It's made from mild steel sheet, about 2 mm thickness.  After carefully marking the hole positions I used my recently purchased optical centre punch to prepare for drilling.  I also plan to cut a square of 3/8" aluminium alloy plate to mount in the four jaw chuck.  I'll use the template to position holes in that plate.  One hole will mark the position of the stepped hole in the mount - the other two will be tapped and used to attach the domino to the plate for boring.  I'll probably contrive some sort of clamp for the tongue end of the domino.  If that sounds confusing, wait for subsequent posts. 

[krv3000]

hi Pete I see nothing wrong in the way you is attempting the fix 
brill

[Pete W.]

hi Pete I see nothing wrong in the way you is attempting the fix 
brill

Hi there, Bob,

Thank you for your encouragement.

I've progressed a bit further - I'll post a description and a few more photos. 

[Pete W.]

As I said to Bob, I've made a bit more progress even though Mr. Bozo got in on the act!          

The first photo (see below) shows the drilling template and four of the five dominos after drilling.  Those reading this who are experienced in making jigs & fixtures will probably have anticipated the problem I experienced.  You will notice that three corners of the template are rounded while the fourth corner is left sharp - I did that to denote the reference corner and the associated reference long edge.  Aligning the template with each of the dominos in turn seemed to make me all fingers & thumbs and I had to juggle a bit to get the domino & the template properly aligned without the tool-maker's clamp obstructing the hole positions.  I should have designed and made a more complex jig with stops to press the dominos and the template against to ensure accurate alignment.  It didn't seem worth the effort for only five-off dominos.  The hole marking the eventual position of the bored hole for the optic is a nominal 0.100" while the other two are a snug 4 BA clearance.

After I'd drilled all the holes, I recognised Mr. Bozo's contribution - the hole pattern is 100 thou too far from the edge with the tongue.  Oh, well, at least they are all the same and I think the mounting to the head-band will enable the optics to still be set to eye level.

The next operation was to make a boring fixture.  I cut a square of 5/16" hard aluminium alloy plate and milled the edges to clean up after the hacksaw.  I made a slight error with the last side as you might be able to see in the second photo - the diagonal scribed line doesn't quite end at the corner!  Then I used the drilling template to position a set of holes astride the diagonal with the centre hole over the centre of the fixture.  I opened two of the holes and tapped them 4 BA.  I was concerned that the tongue end of the domino would need some support in case the domino tried to 'walk up' the drill or boring tool so I contrived the clamp shown in the photos - it's cut off the end of a computer hard drive slide, 'waste not, want not'!  The 'back end' of the clamp rests on the head of another 4 BA screw while the clamp is held onto the domino with another 4 BA screw.

The next operation is to mount the boring fixture in the four jaw chuck and set the centre hole to run true, then to mount each domino in turn and bore the stepped hole to take the optic.  I've stamped each domino so I know which face to bore.  Before I can do that, I have to make a 'wobble bar',  á la Doubleboost.  Once that's all done, I have to profile the parts to (approximately) match the shape of the original mount, I think that's going to be a bit of a challenge!   

Through all these stages, I've found my optical centre punch to be very helpful.

Right, photos:

[tom osselton]

 I found this the other day have you thought of casting it with plastic resin?
I'm going to have to try this! 

[SwarfnStuff]

Interesting Link Tom, duly bookmarked in the "try this perhaps" folder. Could be useful in getting impressions of fiddly bits. Or just to mess with? 
John B

[tom osselton]

Thats what I thought I even bought some corn starch the other day and still have a half tube of silicone sitting in the kitchen.

[SwarfnStuff]

Ha ha, Last tube of silicone I bought had set solid in the tube when I finally needed it.  Thought I would have to bodge something up, fortunately a mate had something that did the job.
John B

[tom osselton]

I hear you I haven't used silicone from the tube since probably 1981 so after cutting the tip and  ratcheting the plunger wondering when its coming out I remembered to puncture the seal

[SwarfnStuff]

No such luck with mine, Tube had been used and my usual bung some plastic supermarket bag over the main nozzle then screw a new dispensing tip over. Usually works but obviously tooooo long between uses.
Kinda makes the advert for small tube sizes make sense if only a little job. More expensive per squirt but not when the remainder sets solid.
Ah well,we live and learn,"Hopefully".
John B

[Pete W.]

Hi there, Tom and John B,

Thank you for your posts.  That pink goo looks interesting but I don't think it's suitable for my project for two reasons.

First of all, the mould to form my part would be prohibitively complex as my part has features on faces at right angles to each other (see the photo in my first post).  Remember I said that I'm working on five dominos in case of accidents along the way - I actually only need a pair of the finished items.

Second, I doubt whether it would be stiff enough to support the M2 tapped hole against the weight of the (very expensive) optic.

I elected to use Delrin because I found some of the right thickness on eBay.  Since then, I've discovered that the same seller also sells HDPE in the same thickness and it's cheaper!  I don't know whether that would have been a better choice but at this stage I'll persevere with what I've got and see how it turns out.

I hope to post some more photos and description of the next stage of the project in a few days' time. 

[Pete W.]

Well, I managed to get some shop-time, Hurrah, Hurrah!!          

So I've made some progress with this little project.

I showed a photo of the boring fixture in my last illustrated post.  Before I could use it, I had to make a bar to mount a dial indicator in my tool-post and a wobble-bar to use to centre the fixture in the four-jaw.  Both wobble-bars and indicating in the four-jaw chuck are well covered in YouTube videos etc. so I haven't bothered to show those.  So the first couple of photos (see end of post) show the fixture in the four-jaw.  Once it was centred, I machined a recess about 1/16" deep in the face to give a relief for the boring tool(s) when they penetrated the rear face of the Delrin 'dominos'.

The next photo shows a domino mounted in the fixture ready for boring.  As I'd made the mounting holes fixing screws a really snug fit on the 4 BA. screws, I didn't indicate each domino but relied in the fixture and the dominos all having been drilled using the same template (see earlier posts).

Then there's a photo of a domino after boring through and opening-up part of the bore to match the bore of the part I'm replacing.  The domino has a small pilot-hole in the required place, I opened that up with a 5.5 mm end mill followed by a 12 mm slot drill (they both happened to be at hand) then continued to the required diameters and depths using a small carbide-tipped boring bar.  I was pleased to find that these tools did a good job, cutting easily and producing a fine swarf.  There was a slight 'paper-edge' at the junction of the through bore and the counter-bore which my de-burring tool wouldn't reach nut my trusty Swiss Army pocket-knife coped with it OK.

The next photo shows a bored domino alongside the part I'm replacing and the final photo shows a trial assembly of the domino, the optic and the bearer bar.

The next stage of the project is to bore the remaining four dominos and then to contrive a set-up to profile them to match the original mount.

Right, photos:

[Spurry]

Looking good so far.    A lot of extra work there, with not having a mill.
Pete

[Pete W.]

Hi there, Pete,

Thank you for your post.  Yes, it would have been nice to have a mill - I think I did mention earlier in the thread that a neighbour had offered to let me use his mill.  However, since I couldn't complete the project in one session, I declined the offer - I don't think it's reasonable to 'time-share' a mill!

These parts are quite small, the through bore is approximately 5/8" diameter and the remaining wall is quite thin so holding the part would be challenging, even in a mill.  When I get to the profiling stage of the job I'm intending to try a method recently shown by Dale Derry in one of his YouTube videos - it doesn't require a rotary table.

After I posted my previous post I remembered that I'd left out one operation.  The Delrin sheet from which the dominos are cut is about 65 thou too thick so I turned the domino around on the boring fixture and relieved the back surface around the bore by that amount.  In anticipation of doing that, I'd taken particular care to position the two fixing holes symmetrically about the centre-line of the domino.  There was a misalignment error of maybe 15 thou but I don't think it'll show once the part is profiled.   

[awemawson]

Coming along nicely Pete 

What are you plans for the 'mill less profiling' ? I'm imagining it mounted on a spigot turned to fit the bore, and fixed on the cross slide using the lathe as a mill 

Do you have a small rotary table to mount it on, on the cross slide?

[Pete W.]

Hi there, Andrew,

Thank you for your post.

You're partially right.  I plan to trim off as much of the excess material as I dare using a junior hacksaw.  Then the blank (it'll be the wrong shape to be called a 'domino' by that stage) gets mounted, as you say, on a spigot, secured by an inverted cup washer or clamp-plate and cap-head screw.  I think I showed the spigot in an earlier post.  The spigot will be mounted on a horizontal plate gripped in the vice on the vertical slide.

The first stage of alignment is to position the carriage so the cutter is the right distance from the centre of the bore of the blank.  The carriage gets locked in this position for the remainder of the process.  I'm thinking of using an end-mill in my Clarkson AutoLok chuck in the lathe mandrel.  Alternatively I might use a small side & face cutter if it will reach the work-piece.  I can't use a between centres mandrel because the vertical slide gets in the way.

Then the blank is aligned so the straight flank (still concealed within the blank!!) is in-line with the cutter.  Traverse the cross-slide to make the cut, hoping that the clamp-plate is tight enough to stop the cutter from sucking the blank round on the spigot and into the teeth!  Re-align and repeat for the other flank.

Then, machine the circular part of the profile as an infinite number of infinitely small flats ('infinite' means as many as my patience will allow!!).  I'll set the cross-slide to set the centre of the bore of the blank on to the lathe axis and lock it, then release the clamp-plate and turn the blank in azimuth a little bit between cuts and re-clamp.  Delrin seems to cut better than it files. 

Have I confused you?  If not, say so and I'll try again!                     

[Pete W.]

Hi there, again, Andrew,

Here's a link to the YouTube video that gave me the inspiration for my proposed profiling method:

 

[seadog]

Not a bad way of doing it, for metal. I had some hinge parts to make for a neighbour's gazebo, quite intricate, but still, made in Delrin. There was a radius called for, so I set up a cutter in my lathe and arranged a pivot, at the correct radius from the chosen end mill, and then just rotated the part by hand against the cutter. Obviously you're back milling so it's not snatched into the cutter. In you case you've a larger radius to cope with but, with a little imagination I think it may be the easiest way to go.

[Pete W.]

Hi there, Seadog,

Thank you for your post.

It looks from your pictures as though you had more room to take hold of your work-pieces than I have.  You'll see that in the pictures in my next post. 

[Pete W.]

Well, I've made a bit more progress.  I turned a couple of thick washers (aka clamp-plates) from aluminium alloy bar.  Then I cut another piece of the 3/8" aluminium alloy plate and milled the edges clean & square - all routine stuff so no photos of that.  Then I bored a hole to take the lower part of the spigot.  I had to get a bit creative because the hole is quite close to the edge of the plate and I needed to avoid the boring tool hitting the chuck jaws.  I stuck some small pieces of 1/16" Perspex to the jaws with double-sided tape to provide some clearance behind the plate.  I cleaned the spigot and the plate and applied some Loctite before clamping the plate and the spigot together with one of the washers and a 1/4" BSF cap-head screw.  Despite my cleaning the parts with IPA and leaving them clamped-up overnight, the Loctite didn't cure!  Still, I was given that bottle of Loctite because its shelf-life had expired and that was about 20 years ago!          

I've included a photo showing the facing of the rear of one of the dominos to remove the surplus thickness of the material where the optic will eventually seat.

The remaining photos (below) show the profiling process in progress.  The captions on the photos are fairly self-explanatory so I won't write much here.  I've achieved a good enough profile by taking a cut, then stopping the cutter with the counter-shaft clutch and winding the cross-slide to move the job away from the cutter.  Then I slacken the securing cap-head screw, turning the blank through a small angle, taking the next cut and so on, 'rinse & repeat'.  I tried smoothing the profile of the first piece with a fine file but I think I prefer the micro-faceted appearance of the second one.

Once I've profiled all five blanks, I'll need to remount each one in turn on the profiling fixture and drill & tap two M2 holes in each one to take an M2 grub-screw. 

Right, photos:

[awemawson]

Excellent work Pete - coming along nicely ... Why do they have the letter 'A' impressed into them 

[seadog]

Nicely finished Pete.

[Joules]

Just out of interest, Delrin has a melting point of 175 ℃ it would be feasible to make an alloy mould use a slug of delrin in a tube and a bottle jack (close fitting piston).  Heat the mould and tube to 175 ℃, digital thermometer to keep an eye on temperature, then squeeze the delrin slug into the mould.   Bit like the guys making Delrin nuts for lead screws, might not be worth the effort for a one off, but maybe worth gaining the knowledge especially if you are likely to have repeat work or multiple components.
I just offer this thought as I am in the process of making a mould to shoot some delrin components for a client     Not using a bottle jack I might add