Accurate angle on a scraped prism

I'm in the process of making an accurate 55 degree prism, partly for the purposes of trimming up my micro mill dovetails, but mainly for the fun of it.

I've got the two surfaces scraped to the point where they are reasonably flat, but nowhere near bearing quality.

Before I put the finishing touches on it I want to get the angle accurate. I have a Lidl angle measuring tool which reckons it's a 0.1 / 0.2 degrees out, but it's quite hard to take an accurate measurement.

to be fair 0.1 degree out is more than adequate for the micro mill, but I'm treating this as an exercise is precision to see what I can overcome my natural hamfistedness.

According to my back of an excel spreadsheet calculations, if I take a point 15mm in from the sharp bit, at 55 degrees it should be 21.422mm high. at 54.9 degrees it is .08mm (3 thou) lower and at 54.8 degrees .16mm lower.

Of course the edge is not knife sharp (I've softened it, to stop chipping) and I'm not sure how I would measure from it in any case.

I've found references that tell me how to get a 45 prism spot on (it's a variant of the whitworth 3 plate approach), but I'm not sure how to do other angles.

Thanks

Iain

If you can get a bit of it flat enough to reflect, in a darkened room, you might be able to do a variation of your measurement above over longer distance using a laser pointer.

I would think some variation on the sine bar approach would be the way to go...

Actually, BikePete, that makes a lot of sense.

I'd been thinking about how to measure horizontal and vertical accurately which was throwing me.

The sine bar allows me (in theory) to set up the target angle and either measure the distance top and bottom or blue. Also, sine bars appear on our favourite auction site at not ridiculous prices.

Thanks

Iain

As a purely mathematical problem, you could measure the height at two different positions along the prism and use simultaneous equations to determine the horizontal error at the apex. Simply put, if the error were zero, the tangent of the angle would be constant wherever measured, so you could solve for the unknown error which would be the same for both positions. But doubtful that your measurements could show the error to any greater degree of certainty over such a small range.

Iain,

I've just found a reference to an interesting utility: SINE.ZIP on this page: **LINK**

http://www.myvirtualnetwork.com/mklotz/#shop

[quote] A second program removes the need for making the separating link in favor of an approach that uses two butted cylinders of differing diameters. [/quote]

Sorry; I can't test it [I'm on the iPad] ... but the idea is good.

MichaelG.

hi, not done it yet.

the maths isn't the issue for me (I've used maths at work for a long time and am helping no. 1 son with Maths A level).

I would need to measure the X distance between the two points to a high level of accuracy and then the heights. I don't have a registration point for the x distance which makes it hard to get this accurate. I'd need to intersect the prism face (say) at 2mm height and use that as a registration and then have my indicator (say 15mm further in) get the height.

the issue is (mainly?) getting the distance from the registration point to the indicator. apart from the indicator having a ball on the end (bit more maths), it is not designed to be rigid side to side,

Iain

Iain,

The obvious answer is a sine bar & not sure why a spread sheet is involved? This is basic trigonometry, lets not re-invent the wheel.

Tony

Where are you Iain ? two or three sine bars here in my box, I am north of Lichfield Staffs.

How important is a specific angle? Don't you just make the mating part match the first part?

Martin

HI, Tony.

The spreadsheet just saves me typing in to a calculator. nothing clever.

Chris - I'm in Harrogate which is just a bit too far from Staffordshire. Happy to pay postage and a sensible price, though if you have something I can use. ping me an IM...

Martin - I don't really need an exact angle for the reasons you've indicated. However, I think there may be some drift along the bar. But the more important reason is I just want it to be right! I appear to have found a fascination with the scraping side of life and I want to see if I can make something with that level of precision....

Clearly insane...

Iain

Iain,

For the job you describe Martin is correct it doesn't have to be exact, in fact the only thing that matters is that you simply need your dovetail straight edge to have an angle less than the dovetail you are attempting to scrape in. If there is not sufficient "gap" getting an accurate bluing is tricky. It should be sufficiently less so that you can easily offer up the blued straight edge in to the dovetail without bringing the flat bearing in to play.

In general terms the nominal angle doesn't matter you are simply scraping the parts to fit. If you want to hit a specific angle on your dovetail way then you may end up removing more metal than you need to. If you do that is there sufficient adjustment in your gib? Be careful......

Which brings me to the fun part. If it were me I would anticipate a lot less fun being had if I was scraping a dovetail to an absolute angle than simply to fit.

What is the nominal angle of the dovetail you are attempting to scrape? I have scraped in dovetail straight edges for the purposes of scraping in a bridgeport which has 50 deg dovetails. My castings had a nominal 45 deg dovetail. This gave sufficient room to offer up the straight edge.

A nice scraping project to go insane over angles that is benefical is e.g. to aim for 90 deg on an angle plate. You can turn something worn out or badly made junk in to a precision tool.

Steve

Edited By SteveI on 30/10/2016 22:13:09 - edit

Edited By SteveI on 30/10/2016 22:20:53

Is this not a job for a surface table, reference square and some different diameter dowels under the height gauge?

Mark

Regarding sine bars, a sine table might be more use but rollers should give some indication of the state of the prismatic face

Edited By Mark C on 30/10/2016 23:02:31

There's a smartphone app which is amazingly accurate

For accurate angles a half decent smartphone can be used as a digital angle measure, the one here is good to 0.1 degrees and free. Android App: Bubble Level by Antoine Vianey (free).

I used it to check the ways on my Drummond, the flat glass screen makes an ideal surface for accurate measuring

.

Steve,

Based on Iain's earlier post ... I think one might reasonably assume 55°

[quote] I'm in the process of making an accurate 55 degree prism, partly for the purposes of trimming up my micro mill dovetails, but mainly for the fun of it. [/quote]

MichaelG.

As Marks suggested, how about something like this - the sizes have been exaggerated for clarity.

Extract from the .TXT file in the link that I posted yesterday:

If I take two cylinders of diameters d1 (smaller) and d2 (larger) and lay them in contact with each other on a horizontal plate, a flat plate placed across them will form an angle, theta, with the horizontal given by:

d1/d2 = (1-sin(theta/2))/(1+sin(theta/2))

... - the cylinders provide their own spacing if you adhere to the formula above.

.

That ^^^ has to come-in useful sometime !!

MichaelG.

MichaelG,>>

I was attempting to clarify the angle of the dovetail way specifically without the use of an assumption. The reason being that using a 55 deg dovetail straight edge to blue up a 55 deg dovetail way is not considered good practice. I know that the OP thought it would be a fun exercise (and fun was part of the motivation) but I am not sure that the OP understands why that will hinder progress with scraping the dovetail bearing. If they do then fine, but then this thread is more about how to get to 55 deg and not about accepted technique to scrape in a dovetail way. Perhaps the OP can clarify their primary concern.>>

It is normal when scraping to hinge the part to track and determine the quality of the bearing and for a dovetail this might be none trivial so instead the scraper would hinge the dovetail straight edge, and to do this requires some gap and a fair bit of feel by the scraper hand. Hinging helps to determine the progress in the quality of the bearing to get the hinge points to be close to the theoretical airy points.>>

Gap is needed because when scraping dovetails the straight edge will slide in to the "corner" of the flat surface and the dovetail. (Now the OP is by seeking matching angles specifically setting out to try and do this.) Accepted practice is to avoid this with a narrower angle on the straight edge, and by actively trying to counter balance that effect of sliding in by holding the straight edge out of the corner to allow a proper assessment of the bearing.>>

For completeness I also commented that the approach might also be inefficient in the use of the scraper hands time and present risk of scraping more bearing surface away than needed which may then have consequences for the gib adjustment and alignment of lead screws.

>>

Steve

Steve,

I defer to your wisdom

... Frankly, I think you lost me somewhere in that explanation

MichaelG.

Michael,

Ok I'll bite....

I was attempting to explain in a constructive way that your assumption was only reasonable if the OP intended to follow a flawed approach to improving his machine ways. I.e. your comment was unhelpful and potentially misleading to the OP and any future reader of the forum as it could be interpreted as an endorsement of bad practice..

Perhaps my explanation is poorly worded and confusing, I wrote it in the hope it would help to explain why the approach is flawed. Thereby making a positive contribution.

Steve