The Home Machinist!

I guess that I should trust my mill's micrometer dials when spacing holes over a very short distance as Harold says.

Just remember to move in the same direction used for taking out any backlash in both the table and the mill's cross slide.

After locating and drilling all of the holes in the heart shaped link blanks, they ended up being a perfect light push fit on all of the cross beam pins. As I mentioned before, my 1/8" dia. HSS drill seems to drill a few tenth's undersize probably because I have used this drill so often the spiral lands on the drill are worn down that much. Rectify this by finish reaming the holes or get a new drill.

The internal heart shape in the links were machined next. The upper part radius between the 1/8" drilled holes is 5/32" and the larger side radii are 7/16" centred on the top 1/8" drilled holes. If you have a nifty way of machining this, you can tell me as the side radii end as a point/line at the bottom where they join together.

These radii are not critical but if you want them and the finished parts to look correct then...

I have done this before so here is how I do it.

I draft the internal heart shape on some tracing paper and then with paper glue spray, glue the tracing paper to the metal blank (using a wallpaper seam roller) in the correct position. It is best if there are extended centre lines both on the tracing paper and on the metal blanks which can be exactly aligned.

Then chain drill a series of very small holes (say using a # 55 drill) to just inside the paper heart shaped layout line, cut through any residual bridges with a fine jewellers coping saw and finish the internal heart shaped outline by die filing.

A lot of slow laborious effort to be sure but if you have a better way, I would like to hear from you.

Carrdo wrote: ↑Tue Jul 09, 2019 2:01 pm my 1/8" dia. HSS drill seems to drill a few tenth's undersize probably because I have used this drill so often the spiral lands on the drill are worn down that much.

Something to ponder.
Twist drills are not straight. They are made with a (slight) taper towards the shank, which minimizes contact of the periphery (which is circular ground), so friction is minimized. That results in a drill that will generate a slightly smaller hole as the length of the drill is reduced by repeated sharpening.

Twist drills are made with the flutes tapered the opposite direction (they get shallower as you approach the shank). That results in an ever thickening of the web.

Twist drills are notorious for wandering, and for producing holes that are not truly round. That means that a hole that may measure a given diameter won't allow a pin of the same measured size to fit. That condition is exacerbated when drilling deep holes, and minimized with shallow holes.

Twist drills are known to yield three sided holes when drilling thin material.

A drilled hole can be somewhat improved by reaming. It can be improved even greater if the hole is drilled undersized, allowing the reamer to establish the finished size. That will result in a hole that is straight, although it may not be round, and that's typical of reamed holes. The hole will often have one more side than the reamer has cutting edges. Not perfect, but a considerable improvement over a drilled hole.

H

Machining the internal heart shape on the heart shape links.

This time I tried something different - to actually generate the curves rather than approximating them as previously described by chain drilling and then by die filing.

To do this one has to use the rotary table again. This is probably the most demanding operation I have ever done with manual milling as:

You can't really see what is going on due to the very limited space between the milling cutter, the workpiece and all of it being mostly obscured by the fixture clamping and the part supports.

When milling the side heart shaped radii one has to achieve tangency on the two arc ends all of which demands close visual access and setting of the cutting tool, magnification and exceptionally good lighting all of which are very difficult to achieve with the limited sighting.

We are cutting partial (and largely obscured) arcs so one has to rely totally on the rotary table graduations (say from 39 to 83 degrees and then back again - repeated many times) so extreme concentration is necessary - a momentary lapse here and your eye is going to see the resulting error as you will have an offset at the ends of the arc instead of true tangency.

To determine the rotary table start and stop positions, one needs to make several trial cuts and as one approaches tangency, stop, withdraw the cutter and closely examen the cut under magnification, then, as necessary, proceed a little further until visual tangency is achieved and the final rotary table positions are recorded.

All of the above can be achieved but it will cost you in terms of time, nerves and concentration. Probably this is an ideal application for CNC but for only four parts not worth the programming involved.

And I am not going to be doing this ever again.

The heart shaped links internal side slots finished machined.

This was a real machining nightmare. As careful as I was, I messed up two link blanks getting there.

There is a lot more to the layout and the setup related to the machining than what I am showing. I did make extensive additional hand written notes and sketches as to what is involved.

Just as important, the lower link drilled holes all line up as you will see why this is necessary later.

Further work on the heart shaped links, this time on the external profile.

I always ink and scribe the external profile first as it involves 4 intersecting arcs. Just to be sure that the link external profile is correct as there is not enough information on the drawing as is without making additional measurements.

More rotary table work with half rounding on the bottom end of the link and quarter rounding on the top. Setup after setup.
Gross shaping first with band sawing followed by belt sanding to just outside the scribed outline.

The links are starting to look good but I am not there yet.

Completing the external profile on the links.

What to do when you have lost all of the previous holding surfaces through the profile machining operations?

Create another holding fixture!

I still (deliberately) have a vertical centre line scribed on each link and the holding fixture was also made with a scribed centre line so all one needs to do is align both centre lines.

I will use the holding fixture and the small fly cutter seen in the photo to finish the scallop section (both sides) on all of the links.

Nice work. This all looks very familiar to what I did about a year ago. With the three holes accurately drilled, the fixture only needs to be a couple of close fitting pins and a clamp. Do one slot, then flip over and do the other.

Looks good.
And sure reminds me of the convenience of laser cutting facilities.
RN

Finishing the link's external profile.

In the end I couldn't use the small fly cutter shown previously as the radius of the scallop cut was so small (1/2"), the corners of the fly cutter body fouled the workpiece.

Switch tactics.

The perfect cutting tool for this operation turned out to be a 1" dia. by 3/16" thick Woodruff cutter. Run the cutter very slowly (50 rpm) and progressively, in small increments, and plunge cut to the layout line also very slowly.

Setup matched pairs of links in the holding fixture and carefully align the cutter on the link body. I also used an end stop to position the link holding fixture in exactly the same location for each scalloping operation.

Scallop the nearside and then the far side on a pair of links. I used a mirror to see what was going on on the far side as seen in one of the photos.

Finally, finish the external link profile by touchup drum sanding.

I am very happy with the end result.

Moving on to the lead truck centre trunnion (as Josslin names it) or the bolster as I call it.

This was roughed out from a large solid block of HR steel which was available. Nothing unusual here.

Bandsaw out the rough trunnion piece from the block, "rectangularize" it (to the best of your abilities) to within 1/16" of the finished outside dimensions, band saw and then chain drill out the end of the major slot in the block and then finish the slot to its final width and depth via heavy end milling.

Just a lot of heavy roughing (and time) to this point.

Layout, milling, then more layout and more milling on the bolster block.

Take advantage of every tool, fixture and machine feature you have available.

In the first photo I am using both the vise stop and mill's table stop to precisely position the part and to limit the length of cut.

Further machining on the bolster to bring it to its final size and to install the swing link pins and collars on the bolster.