Silver Steel (Drill Rod) Problems

[Roger B]

As part of my fuel injection system I need to make the pump plungers and injector needles to micron tolerances. I am using Silver Steel rod which is ground to a reasonable finish, hardening it and then lapping with a 10mm wide split brass lap and fine diamond paste. The first pieces were successful and worked using petrol (gasoline). I am now having difficulties making a second set.

I am starting with 2mm diameter SS which is usually around 1.995mm. This is hardened and then lapped to 1.98mm. I am happy that the bore of the injector pump is ok. If I use a commercial 1.98mm pin gauge it slides smoothly through the bore and using a light machine oil the plunger will sink in around 10 seconds with a weight of 4.5kg applied. This is around 140 Bar or 2000psi.

The plunger I am making measures slightly under 1.98 (~1.977) but is a tight fit and does not seal so well 
I have read somewhere that SS is centerless ground and under certain circumstances the diameter can measure correctly but it is non round like a UK 20 or 50 pence coin.

I don't have any means of measuring roundness but as a simple trial I tried rolling the plunger and the pin gauge between a V block base and my granite surface plate. The pin gauge just sounds like it is rolling but there seems to be a sort of rhythm/modulation to the plunger (turn the volume up).

Any thoughts? Any ideas of how I can check further? As the pin gauges cost CHF 10 and will make two plungers I may just use them instead of trying to make my own.

<a href="https://www.youtube.com/watch?v=sti1nUFiED4" target="_blank">http://www.youtube.com/watch?v=sti1nUFiED4</a>

<a href="https://www.youtube.com/watch?v=sti1nUFiED4" target="_blank">http://www.youtube.com/watch?v=sti1nUFiED4</a>

[Stuart]

yes you have the three lobed round

it looks like the rotor of a Wankel engine its not round but measures as if it is there is a name for it

search YT for NYCCNC John did a video that shows it in a sphere very exaggerated they looked like rounded triangles but put a plate on thee and it stayed level


if I can find it I will post a link
<a href="https://www.youtube.com/watch?v=wyFCeCAJMec" target="_blank">http://www.youtube.com/watch?v=wyFCeCAJMec</a>

if you what to see some very high precision lapping to sub micron level with very detailed information on how his final lap is 1 micron film followed with 1 micron diamond slurry on paper

have a look at Robin's YT channel  his level to detail is totally mind blowing

https://www.youtube.com/channel/UCn4U3aEr6L2nLe1m_3as6JQ

[b.lindsey]

Seems like the logical move is to just use the pin gages since the test with them is working well!!  You could always try a different piece of drill rod though, it might be "rounder".

Bill

[Stuart]

Bill

that thought crossed my mind as well but maybe the pin gauge would need some machining for other reasons , or is it that diesel does have a small water content and my corrode a steel pin

but a valid point anyway

[b.lindsey]

Stuart, I doubt that corrosion would be an issue and if so it would affect both the pin gage and the drill rod. Diesel should be oily enough to prevent that with reasonable care. AS to further machining though, that could well be an issue. Starting with a larger piece of drill rod though and grinding the OD and then lapping might be another option.

Bill

[Roger B]

Thank you for the thoughts and confirmation of the problem. I remembered reading somewhere about the out of round product from centerless grinding but could not find any confirmation of this.
The pump plunger requires an M2 thread on one end which had initially put me off using the pin gauge. I did manage to anneal just the end and thread it (you can see this in the video).
I have thought about means of checking for roundness. If I had an good 2mm ring gauge (expensive) and checked that the diameter was 2mm and that it would pass through the gauge then it should be round. If the diameter was 2mm and it would not pass the 2mm gauge then it must be out of round.
I will make some more measurements of my SS plunger. As I have lapped a few microns off the top of the lobes I may be able to measure a different diameter at different positions around it.

[sco]

GHT mentions this out of round problem in his Workshop techniques book - although a micrometer wont show it, a dial gauge and vee blocks should.

Simon.

[Stuart]

I would have thought you would need an electronic dti with a separate head unit that could measure down to millionths its a small size pin and heat from your hand would skew the reading take a look at Robins YT were he sorts out Tom Lipton level just breath on it and you get a reading . And that is not in a temp controlled lab as you know they use lasers as long nil mass levers , much like the old meters that used mirrors and al light beam to show the scale reading

[Florian Eberhard]

Hey Roger

How does your lap look like exactly? Just saw your picutre. Well - maybe you try using a lap that has three segments instead of two. I did use such a design for the few times I did lap rods and it seemed to work quite well although I never checked if the lapped rods were round.

Could it be that the rod was already out of round shape before you started lapping?
Or did the hardening affect the roundness?

And as Simon said, the vee-block and the dial gauge should help to find out what happened to your rods.

[Roger B]

Thank you all again for your thoughts 

I did a little more investigation. First I tried measuring around the problem plunger with a micrometer but this was not conclusive. There appeared to be a variation of a few microns but not showing an angular pattern.

My smallest V block has a 1.7mm groove at the base which is not ideal for a 2mm rod but I tried anyway. Using a 1 micron resolution dial gauge I tried various rods and plungers. The commercial pin gauge was almost perfect, the indicated variation was a couple of tenths of a micron which was probably due to me rotating it. A previously made plunger that worked showed a variation of around 1 micron. The 'problem' plunger and the remaining length of SS that it was made from showed a variation of a couple of hundredths of a mm. This didn't appear to be a 3 lobe problem but more of a corrugation along the length 
Another length of 2mm SS (from Conrad) showed a variation of a couple of microns but keeping a 50cm length steady was difficult so the actual variation may have been less.

Conclusion 1: There is a problem with the length of 2mm SS that I was using. This has now been binned.

Conclusion 2: It is easier and quicker to use the commercial pin gauges as plungers and needles.

[Stuart]

Glad you have found the source of your problem

Bet you will test any new gauge pin and/or any silver steel you use

As we all know when the component gets smaller these small but as you have demonstrated measureable errors in the stock become a problem

Looking forward to a working pump/engine

Stuart

[petertha]

Well - maybe you try using a lap that has three segments instead of two. I did use such a design for the few times I did lap rods and it seemed to work quite well although I never checked if the lapped rods were round.

I've often wondered about this. In this kind of lapping tool, the lap itself is slit longitudinally or at an angle depending on the model. But the holder that imparts force is kind of a pseudo radial design. I say pseudo because its not like tapered collets with alternating slits. The holder still a tension bolt on one side but I would think the radial slits must help.
http://www.americanlap.com/External%20Laps.htm

My drill rod experience was somewhat similar, micro-oval shape when new & barely measurable, at least with my micrometer. My idea was to lap off a few thou for consistent diameter & clean up the finish. When I blued it beforehand & examined early, it evidenced stripes of intermittent lap contact.

[Florian Eberhard]

I would think the radial slits must help.

Yes, that is what I think too but to make such a slit is not too easy (at least a spiraled one) with hobby equipment. You could just cut with an angle to the lap bore though.
The spiraled slit design should be better if the surface is irregular or even has something like a gap or a hole.

I have seen those laps from americanlap.com and I want to to build one with a similar design myself.

Florian

[georgineer]

yes you have the three lobed round

it looks like the rotor of a Wankel engine its not round but measures as if it is there is a name for it

It's a Reuleaux triangle. The coin shape is a Reuleaux heptagon, also called an equilateral curve heptagon.

George

[Mcgyver]

it can be a tricky thing to measure - out of round with constant widths as it can occur with different numbers of nodes.  I've never done it, but recall  the test is to rotate the work in various V blocks that have different angles to ensure its not lobbed.  In other words a 90 degree V block test is not conclusive

I'm surprise this happen if you lapped it; you're using a split lap?