How to align a V accurately with a slot?

[SillyOldDufferModerator @sillyoldduffer]

Today's project suddenly got more difficult than expected and I'm at the stage where I suspect my methods are wrong.

I'm building 'A Fixture for Slotting Screw-Heads' as described in the June 25th 1942 edition of 'The Model Engineer'. It's in mild-steel.

My version of the plan is this:

The finished item (together with the sliding clamp not shown in the drawing) looks like this:

From a different angle:

In use the screw to be slotted is gripped between the Vee and the sliding clamp and the whole fixture is held in a vice. In theory, a hacksaw blade inserted through the guide slot will be perfectly aligned across the centre of screw-head.

In practice I'm having terrible trouble aligning the Vee with the guide slot above. A small misalignment spoils the screw-head. At least I think so!

In making the thing, the guide slot was cut with a band-saw and as far as I can tell is straight.

The Vee was cut as follows,

• I measured the distance from one side of the fixture to the centre of the guide slot
• I used a hermaphrodite calliper to mark the centre of the Vee on the lower part of the fixture
• I mounted the fixture in a milling vice at 45° using a set-square
• I centred a 4mm end-mill on the line with a wiggler
• I lowered the end-mill until it just touched the up-hill side of the 45° slope and then cut a slot 4mm deep into the work.

The fixture looked good until I tried cutting a test slot in the head of a rod. It was slightly 'off'.

I attempted to shift the centre of the Vee with the mill to compensate and although I got close at one point, I can't get it spot on. The Vee is growing like Topsy and is already too large to grip small screws.

My trial and error approach isn't helping and I've walked off the job to type this in after starting to make stupid mistakes.

Is there a better way of aligning a Vee with a distant feature of a part in the first place?

And given where I am now, is there a good way of correcting the alignment of the existing Vee?

I'm humbled by the belief that in 1942 the original would probably have been made by hand with a file. Standards have clearly dropped!

Thanks,

Dave

[SillyOldDufferModerator @sillyoldduffer]

Screw Slot Cutting Fixture

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SillyOldDuffer on 15/09/2016 15:38:13:

… Is there a better way of aligning a Vee with a distant feature of a part in the first place?

.

Yes [or so I think] … Define some arbitrary location as zero/zero/zero, and then produce each feature with reference to that.

The posh way of doing this is/was to use a Hickey Ball [a.k.a. an Ickey Ball] … But you can improvise.

MichaelG.

[HOWARDTParticipant @howardt]

Create the vee first. Place a roller in the eve and measure off that to get centre for the slot.

[SillyOldDufferModerator @sillyoldduffer]

Michael: Thanks for the Ickey Ball pointer, an idea new to me. I found they are also called ''Ball Reference Pins' and that led me to me to this interesting supplier.

Part of what made this difficult was losing the reference point after the 45° tilt. It's good to know there's a way of dealing with that. More thought required though before I try it for real.

Howard: I like the idea of aligning the guide slot to the Vee! I should be able to do that more accurately. I'll have a go tomorrow.

Many thanks,

Dave

PS if anyone needs a jig for cutting ugly off-centre slots in screw-heads, I have one.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SillyOldDuffer on 15/09/2016 20:02:35:

PS if anyone needs a jig for cutting ugly off-centre slots in screw-heads, I have one.

.

I think you're under-selling it, Dave

… If you consider an adjustment cam, it's useful to have an offset slot as a reference.

Give me a modest Royalty if you market it.

MichaelG.

[duncan webster 1Participant @duncanwebster1]

Can't you take a bit off the relevant side of the vee until it is central?

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SillyOldDuffer on 15/09/2016 15:38:13:

… I attempted to shift the centre of the Vee with the mill to compensate and although I got close at one point, I can't get it spot on. The Vee is growing like Topsy and is already too large to grip small screws.

.

Duncan, for info. ^^^

MichaelG.

[Ed DuffnerParticipant @edduffner79357]

I know it would mean some extra work, but the vee could be cut into a seperate adjustable plate and tweaked to perfection, then pinned.

Ed.

[Enough!Participant @enough]

Before cutting the slot final cut the V with a chamfer end-mill? Then (same setting) use the chamfer end mill to strike a line (small-V) to locate the saw guide.

[HopperParticipant @hopper]

How tight of a fit is the bandsaw slot on the hacksaw blade? Is it allowing too much movement of the blade? Is the large hole at the end of the slot allowing too much wandering of the blade? It seems that ideally the slot could be cut with a slitting saw to the exact width of the hacksaw blade itself, not the offset teeth, plus a thou or two working clearance. Then the hole might want to be just big enough to clear the offset teeth of the hacksaw blade, which is slid in from the end and then attached to the hacksaw frame.

[Steve SharmanParticipant @stevesharman33815]

Could you not just drill and tap a pair of cross-holes in the blade-guide part and then use a couple of bolts to push the hacksaw blade slightly one way or the other?

Regards, Steve

[Tim StevensParticipant @timstevens64731]

How about cutting the slot with a slitting saw, and using the same setting (endways) to nick the surface where the V is going to be? This square-bottomed notch would then act as a location for the V. And if you use a V-ended cutter (ie a posh carbide countersink, or a chamfer end-mill) it could be done all at the same endways setting.

And if the V is now too deep, take a cut off the flat surface each side of it.

Cheers, Tim

Edited By Tim Stevens on 16/09/2016 09:48:38

[SillyOldDufferModerator @sillyoldduffer]

Glad I had a look at the thread before having another go this morning. As always the forum is a fount of good ideas.

MichaelG: Good idea. I've decided to patent the effects of poor workmanship. If everybody comes down to my standard the royalties will soon make me rich beyond the dreams of avarice.

Duncan: Can't you take a bit off the relevant side of the vee until it is central? This is what I can't do with sufficient accuracy. I'm still not sure why. Thinking about it last night, it may be something to do with taking the work off the mill to test it and then not putting it back properly. Perhaps I need a jig to make the jig.

Ed. Putting the Vee into the clamp would also have the advantage that it's a lot less work to make another clamp when I make a mess of it! However, having come unstuck cutting a Vee in the right place, I'm nervous of my ability to keep the parts aligned whilst drilling and pinning the holes. Temporarily gluing the parts in alignment before drilling might work.

Bandersnatch. Using the mill to spot the location of the guide slot without moving the work is a really good idea! If I end up remaking the jig from scratch it's the way to go.

Hopper: How tight of a fit is the bandsaw slot on the hacksaw blade? The bandsaw cut a slot very slightly larger than the hacksaw blade: it's only just wide enough to get the teeth through the slot. The 1942 article doesn't say why the tooth clearance hole is relatively large but I assumed it was to allow for the taller heads of big screws. Perhaps it's actually to allow the fixing pin on the end of the hacksaw blade to pass through the jig before being re-attached to the handle? Anyway I shall experiment with the effect of blade fit before doing anything else. It would be wonderful if the problem turned out to be due to the blade leaning slightly in the slot because I should be able to correct that simply by making sure the blade is centred on the screw-head before committing to to the cut.

Steve: Now that's an interesting idea! Easy enough to try and it might save me a lot of work.

Tim: quick comment as an edit because your post was made while I was typing.  Reducing the depth of the Vee – big thanks!

Thanks all. Recharged and invigorated I'm off to the workshop.

Dave

Edited By SillyOldDuffer on 16/09/2016 10:20:26

[Tim StevensParticipant @timstevens64731]

Silly old duffer says: I've decided to patent the effects of poor workmanship.

Sorry, old chap, but to patent something you need to show that it hasn't been done before. And I am not alone in admitting it.

PS what you might find handy for screw heads are jewellers or clockmakers 'warding files'. Not easy to find, but parallel in thickness from one end to the other. I have even seen files with teeth only on the edge, for exactly this work. The resulting slots have much less of the waviness that the kerf on a hacksaw blade produces.

Cheers, Tim

[Ian S CParticipant @iansc]

I'v also seen it suggested that the kerf of the hacksaw blade should be ground off for cutting screw slots.

I need something like that, some of the slots in my screws are a bit off centre.

Ian S C

[Michael GilliganParticipant @michaelgilligan61133]

Dave,

Another moment [and I think this one is important] …

Given the date of the original article, it seems unlikely that the author would have a milling machine in his home workshop: A shaper, maybe; but he quite probably did everything in the lathe.

[my guess]

• Make a between-centres arbor that can take a slitting saw and a fly-cutter
• Mount the work on the cross-slide [or even clamp it in the tool holder]
• … Proceed carefully and logically.

MichaelG.

Edited By Michael Gilligan on 16/09/2016 11:58:56

[SillyOldDufferModerator @sillyoldduffer]

I'm pleased to report that the jig now works to my satisfaction. Not perfect but 'good enough'.

Inspired by Hopper, I started by checking if it was possible to centralise the cut by twisting the blade in the guide slot. By twisting hard the maximum play at the bottom of the Guide Slot is about 1.5mm, which was just enough to hit screw-head centre at one extreme. No easy solution there unfortunately. For the same 'bends the blade too much' reason I didn't try Steve's adjusting screw idea.

Then I picked up on HowardT and Bandersnatch's idea, but reversed it by trying to locate the correct position of the Vee from the Guide Slot. This isn't easy because the work has to be rotated to cut the Vee and all the references move.

Before cutting a new Vee I followed Tim's suggestion and restored sanity by skimming down the misplaced over-deep Vee.

Then, by putting some shim in the Guide Slot with the work rotated I was able to physically locate Vee centre with a wiggler pin stuck to the end-mill. A magnifying glass helped.

With a 4mm end-mill the point of the Vee should be correctly centred when the end-mill is 4mm deep. I went in 3.5mm and then, without moving the work, I tested the Vee's alignment to the Guide Slot with a Centre Drill (i.e a rod with a pointed end!)

Alignment was off by about a millimetre, so I took the cut down to 3.9mm and tried again. The Centre Drill test showed that this was closer. but I still needed to go deeper. In the end I deepened the cut four times by 0.1mm before the Centre Drill looked right. (I couldn't get a decent photo of this test being done in the mill but next picture shows what I did.)

The moment of truth:

Bingo:

So, root-cause analysis time.

I made the jig from Black Mild Steel bar stock in the belief that the body was 'square enough'. It's actually slightly out, which put the Vee and the Guide Slot off reference. The error happens to be accumulative and was further compounded by the way I marked the job out. Not squaring the metal off properly was a mistake!

My first attempt at cutting the Vee had me removing the work from the mill-vice every time I checked the alignment. I now think this was a bad mistake – an improperly squared job tilted at an angle, being repeatedly removed and replaced to check the effect of shaving metal from one side only of the Vee. It's not unlikely that metal was accidentally removed from the wrong side of the Vee as well due to reset errors.

I also failed to think about the best way of locating the Vee relative to the Guide Slot before starting work. If I was doing it again I'd do it the other way round

Very many thanks for all your excellent help and suggestions. I may well incorporate some of them in a more carefully made Mark 2 version.

Cheers,

Dave

[SillyOldDufferModerator @sillyoldduffer]

Posted by Michael Gilligan on 16/09/2016 11:58:01:

Dave,

Another moment [and I think this one is important] …

Given the date of the original article, it seems unlikely that the author would have a milling machine in his home workshop: A shaper, maybe; but he quite probably did everything in the lathe.

[my guess]

• Make a between-centres arbor that can take a slitting saw and a fly-cutter
• Mount the work on the cross-slide [or even clamp it in the tool holder]
• … Proceed carefully and logically.

MichaelG.

Edited By Michael Gilligan on 16/09/2016 11:58:56

You got me thinking more about how this would have been done in 1942. I was fooled into approaching this as an easy job when it's actually got hidden depths.

The original short article (by W.M Halliday) spends most of it's time describing what the tool is and how it is used. There is no advice on how to make it. I suppose there were fewer self-trained amateurs with workshops back then who needed the "words and music".

If I've understood your suggestion correctly I don't think it can be done in a lathe with a slotting saw and fly-cutter. The reason is that the over-hanging Guide Slot block doesn't allow enough space to get a fly-cutter in to cut the Vee slot. (At least the way I imagine doing it!)

The geometry also makes it difficult to file the Vee by hand because the stroke is limited to about 10mm.

If the Vee was done in a lathe, I think it must have been cut like a key-way bycranking a tool back and forwards to make the groove with the chuck locked. If so it must have been hard work.

I can't help feeling that I'm missing something obvious and that an experienced craftsman would have made easy meat of this one.

Cheers,

Dave

[Michael GilliganParticipant @michaelgilligan61133]

Dave,

Good to see I got you thinking about it.

If Neil has no objection: Would you post a scan of the article, so we can all have a go at working-out how Mr. Halliday might have done it.

Thanks

MichaelG.

[Ian S CParticipant @iansc]

I think the V could well be cut by planing it in the lathe as you would to cut a key way after you put a cut with the hacksaw down the center of the V.

Ian S C

[SillyOldDufferModerator @sillyoldduffer]

Posted by Michael Gilligan on 17/09/2016 11:27:50:

Dave,

Good to see I got you thinking about it.

If Neil has no objection: Would you post a scan of the article, so we can all have a go at working-out how Mr. Halliday might have done it.

Thanks

MichaelG.

On the basis that it's easier to apologise than to get permission…

Cheers,

Dave

Edited By SillyOldDuffer on 17/09/2016 18:50:36

Edited By SillyOldDuffer on 17/09/2016 18:50:59

[Michael GilliganParticipant @michaelgilligan61133]

Well done, Dave

I will have a ponder about how I think he would have made it.

MichaelG.

[Neil WyattModerator @neilwyatt]

I have no problem with short articles from old MEs or MEWs being posted here, just not huge ones or plans that are usually sold by us or others or links to pirate copies of publications. (Small excerpts from plans to illustrated a point are OK).

Main thing to remember is we are a publisher and we want other people to respect our copyright, so we respect theirs!

Neil

[SillyOldDufferModerator @sillyoldduffer]

Thanks Neil. I didn't think you would object to a short item from 1942.

I live not far from a large second-hand book dealer who sometimes sells old copies of Model Engineer for 10p each. I've picked up a few examples from every decade apart from the 1920's and wish I'd bought more when I had the chance.

What's fascinating is just how little the core content has dated in comparison with other popular magazines. Excellent value if you see them, and the adverts are a window into a different world. This forum has it's grumpy side, but there were plenty of characters back then too.

Dave

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