PM Research Engine Number 1

[gary.a.ayres]

Stuart -

Glad to hear that the hole positions are better than you originally thought.

After all, it's impossible to drill a hole in the right place, because the right place is a specific point, and a point is notional, with no dimensions at all. It therefore doesn't exist, and if it doesn't exist it can't be drilled into, right?

On the other hand, the universe contains an infinite number of wrong places to drill into, so drilling into the wrong place is inevitable. 

You may not fully agree with this theory, but I have a 'point', yes? 

So congratulations on having drilled the holes in wrong places that were so close to the non-existent right places that no-one will ever notice and the running of the engine will not be affected at all. 

As for the hole sizes, I am a rank beginner but intuitively I feel that Tony is correct. If it's 2 thou oversize that means a gap of 1 thou all round for the oil film. Would that be more or less what it should be?

As for why the hole is bigger than you expected - could it be because for every reamer size some reamers are slightly over and some slightly under? I recall Jason telling me about this - the different sizes are coded with a letter (which I can't recall at the moment). Could that be the reason?

Anyway, looks like great work to me...

 

[mklotz]

Another reason that reamers may cut slightly oversize is taking too large a "bite" and having swarf accumulate in the grooves while the reamer is cutting.

I like to undersize holes to be reamed by about 0.015" and, even then, flood with cutting oil while reaming.

[cnr6400]

From Gary:

"On the other hand, the universe contains an infinite number of wrong places to drill into, so drilling into the wrong place is inevitable.  "

That explains what happens in my shop a lot of the time then. "Everything in our favour is against us" as an old toolroom foreman I knew used to say. 

[propforward]

Thanks all! Very much appreciate the insight Gary! I think I shall ponder that over a glass of red this evening. Or a glass of single malt. It is saturday, after all.

Marv, I will be sure to use that approach on my next reamed hole, thank you for the specifics. I think you are right - I did attempt to clear more material on this hole, because previous reamed holes seemed to not take out any material, but I very likely went too far the other way.

Some progress today.

On my base casting, from previous poor measurement attempts, the edges of the rails were not centered, leaving one rail about 0.017 closer to center than the other, although I had hit the nominal rail to rail dimension dead on. After mulling this over for months, I decided I could not stand the asymmetry any more, so I set the part up and dusted off the wider rail.



This went very well, and now the rails are even about center of the bore in the end of the casting - they are also matched in width.



This means that the mating part will have to be adjusted, but I'm OK with that.

Speaking of the mating part............



I set measured up the casting and discovered it had a good amount of stock all round, and would accommodate the changes in dimension to match my rails (I did that before adjusting the rails), so set about getting some datums.

I set up as level as possible for a first cut - I got a run iut of very roughly 0.005 along the length of the part.



And took off the minimal amount of material possible.



Then on to parallels to clean the opposite side.



The two faces measured parallel as best as I can figure it, so I then used those surfaces to set up and register to clean the back side.





Then it was time to get on to the nitty gritty. I machined the width of the guides and the thickness in two set ups. I set up like this so that I could mill all around the guide tab on one side.



The result...........



This gave me a solid set of perpendicular surfaces to clamp the part square, and get registration to match the features on the opposite side. Notice how the 1-2-3 blocks are staggered - so that later on I can register off the 1-2-3 block face, and match the upper tab to the lower one.



As I machined around and got close to final thickness, I actually mounted a DTI to the quill, and measured the position of the tab face relative to the 1-2-3 block - and hence the lower tab face, so I was able to keep the tabs co planar within about 0.002" as near as I can measure.

I measured the part after machining - pretty much within 0.001 to 0.002 on all dimensions. So I had to do a quick fit check.........



It's perfect!



After so many bungled attempts on previous parts, I was thrilled and overjoyed at how this part has progressed. Everything is square, parallel and true, and it fits into the base casting just so well - it's a real boost. No wiggle or play, and it slides between the rails really well.

So back to the mill, and set up for the tapped hole.



After drilling and tapping that, I popped it back on the bast, along with my temporary aluminum packing, with a drill bit inserted into the tapped hole just to get a feel for alignment with the bore.



I'm very happy with this part - everything went according to plan, no drama, just enjoyable machining. I think this will clean up a treat and look very nice on the finished engine.

Couple of ops left on it - first I need to drill and ream the cross hole, then mill the slot for the connecting rod. But I don't have a 1/4" reamer on hand, so I'm going to wait a bit before finishing this. Should have one this week some time.



Anyway, thanks for looking in.

[crueby]

I may have missed this on an earlier post, but why put the 1-2-3 blocks inside the vise jaws and around the part, rather than clamping directly on the part?

[propforward]

It just helps me keep the table centered relative to the quill. Frankly, not a very good reason, and not even necessary. It's a habit I got into when I used a 6" vise, which was too big, and required a block against the rear jaw in able to use it at all. On that thing, anything I clamped against the rear jaw could not be brought far enough forward to get under the center of the quill. I don't need to do it any more, this is a 4" vise and a perfect size for this mill. Much better than the rubbish import vise, too.

File under "seemed like a good idea at the time". 

[crueby]

Gotcha - my mill has similar issues, had to offset the plate the vise sits on to center things better. 

[AOG]

I handled the awkward spacing on my Kurt by putting a few parallels behind the fixed jaw.

Tony

[propforward]

Remember folks, if I appear to be a bungling amateur, it's only because I am.

Continuing on - I realized last night that on my cross slide I probably hadn't assessed the center of the rounded boss properly with respect to the other final features on the casting - and sure enough when I did a measurement this morning, when I put the cross hole in at the correct dimension relative to the rear of the casting, it will appear off center relative to the cast feature. Haven't decided how to address it yet - I can actually shorten the casting and skim a little off the rear end, or attempt to re round the casting. Or leave it. It will be functionally fine. Probably I'll skim the rear of the casting.

Based on these experiences, and some excellent set up info on you tube, I have resolved to ask myself a new set of questions when writing out a plan for machining castings, to try and force myself to understand the casting before cutting anything.

Anyway, today I worked on the rail and spacers. I saw a technique using gauge pins to visually center the mill to casting bosses, as part of achieving a pleasing overall look as well as dimensional accuracy, so I tried that. The first rail didn't work out so well, and I altered the technique a bit for the second rail. Which frankly didn't help in the end, so I need to revisit this approach. Before machining, the underside of the rails were cleaned up on my little belt sander. That worked well at least.

EDIT: The following pics don't show every step. I cleaned the undersides of the rails first and got them flat. Then squared the side of the rail to my vise using a machinists square. I was playing with the gauge pin approach in the pictures, but then secured the part with the various clamps before finishing all the locating. Obviously I didn't attempt to drill with just the middle clamp, the parts would have ended up spinning across the shop, probably.







As you can see, I ended up off center one end.



I still ended up off center even after drilling assessing both bosses, and then splitting differences in X and Y location.



I tried a similar approach for the center hole. The first one didn't work out, but the second was a little closer.




Anyway, after all was said and done, and having made some simple spacers, the cross slide does go together and the slider slides without rattling, it actually fits quite well.



So, a few steps forward, a few back, but on average more steps forward than back. It can be frustrating at times, but I think I'll win out in the end.

[Admiral_dk]

So, a few steps forward, a few back, but on average more steps forward than back. It can be frustrating at times, but I think I'll win out in the end.

I can guarantee you that it has been the same way for all the experts here too .... at some point in their life, projects and otherwise .... Just goes to show that you're in good company           

Per

[propforward]

Thanks Per!

One thing's for sure, I'm not going to improve by not making parts - so I'll keep making parts!

[Jo]

It is looking rather nice and there are a few tricky castings but they all add to the fun, in this rather desirable model engine casting set 

Remember folks, if I appear to be a bungling amateur, it's only because I am.

We all learn by doing. There are few model engines that have been made without their individual identifying features being applied by their makers  .

I know there are some people who won't post until they have finished the entire model to make sure no one knows that they too make mistakes in their builds 

Jo

[propforward]

Thanks Jo.

It's never "nice" to show off things that went wrong of course, but I do it so that if anyone sees some obvious error I am making, they can chime in and help guide me to a better way. I've had many hints and tips from this forum, and they are gratefully received, no matter how small they may seem - some good advice in the above posts, for example.

I'm just glad to be off and running and making progress on this engine again. Already have my next engine castings waiting in the wings.

[Misterg]

Thanks for an interesting and very readable account. I've just read through this in the context of a couple of builds of the same engine I've been following on yoochube -

FWIW, I think you're doing great - at least as well as the "pros". It seems there are a few 'gotchas' hidden in the drawings for this engine!

 

[propforward]

Thanks Misterg! I am genuinely glad you are enjoying the thread. I have to admit, I really like posting what I’ve done at the end of a session. Kind of rounds it off.

I’m probably following the same youtube channels. I think Joe Pie is an inspiration and knows what he is doing. I wrote to him and said as much.

The other builder - Not so much.

There are some gotchas waiting - but to be fair to PMR, all the info is there on the print. You just have to follow the print. I chose to ignore the dimension start point on the drawing for the base casting - don’t ask me why - and as a result the engine doesn’t look quite right.