Pattern shape/casting strength

I'm planning on recasting my compound slide from my 12" Seneca Falls lathe in cast iron. A close look at the toolpost holder and just a glance at the dovetails tells the story. Busted and beyond repair. It's part of my effort to get this guy up and running.
http://forums.thehomefoundry.org/index.php?threads/new-lathe-project.368/
There is at least one other part that needs to be recast but that will be pretty straightforward and will be covered in the main thread.
The cross slide will be cast in a general form and will then be machined to fit. I have the machinery and tooling to do it and will have experienced guidance to help me with it.
The question is about the shape of the pattern around the toolpost holder where it transitions into the main shape of the casting. I have made a 2 part pattern that consists of 2 rectangular blocks a bit over dimension with draft and dowels. I have no desire to make the body curved for aesthetic purposes but I'm wondering if the curves that were cast into the original part were to enhance its strength and rigidity. The part has obviously failed but not at the area in question.
I'm guessing that it really wont make much difference but I thought I'd throw it out there.







Pete

 

I'm guessing it's just the way they made them in the day.
The break occurred away from that area and was probably from overhang and a long reach with the tool holder.
What does that part weigh?

 

It's a bit over 6lbs. That's undoubtedly an over hang break. After the first break and repair it probably never fit right again so likely continued to take a beating. Judging from other signs of breakage and repairs elsewhere on the machine it's clear that this lathe spent many years quietly weeping.

 

I think the top slide is awfully thin in the web between the dovetails where the tool post attaches (if I have the pics figured out correctly). If you can aesthetically stand deviating from the original design, I would be inclined to thicken that web to between 150 and 200% of the original design. That would, of course, elevate the point at which a tool post would attach. But that might not matter if using wedge type quick change toolpost instead of a lantern. If this is a user lathe, a QCTP is soooo much more useful than a lantern. I would only consider a lantern for a display lathe.

Denis

 

unless you are more Knowledgeable then the original Engineer/ designer, do not change anything . alot of design elements that are thought of as ornamental had a purpose , changes in material and techniques made the older designs obsolete in many cases, but if you dont know, dont experiment

V/r HT1

 

Back when that lathe was designed and cast WCTP’s did not exist so allowance for the height of a lantern had to be maintained. If you mock-up/borrow an AXA toolpost and check to see the cutter height can be made to be on the centerline you will not regret the added meat of increasing the web thickness. But as originally designed it brok and I would think it will easily break again if so cast. Absolute worst case of too thick web is a half hour extra milling time to remove metal from a too thick web. But it will be harder to add metal after casting. ;-)

Denis

 

I'm listening carefully to all of this. Others I have spoken to have given me the same advice as HT1. Respecting that advice is why I'm casting it instead of just milling it out of steel. Damage to the gears or spindle bearings would mean the end of the road for the machine, but boy that is awful thin isn't it?

 

But some folks here are design engineers, and it is completely in the realm of what we can do to redesign the part and make it better.

.

 

Yes

 

There may indeed be design engineers here, but I’m not one of them.

I am surmising that the argument for not bringing that part up to what I would (based only on experience and intuition but no formal engineering training) think would be intended-use-appropriate strength is that by leaving weak it could be a sort of break-away safety link that might protect the gear train of the lathe? Is that the advice you’ve gotten?

Denis

 

Agree with DavidF. Another factor against its scanty design is that flimsiness (and overhang which can’t as easily be corrected) strongly promote chatter of the tool. That will not be a fun lathe as designed to do much parting off! And taking a modest cut simply turning stock will be challenging due to chatter. Making it stouter will diminish flexion and the increased mass will counter oscillation/chatter.

Obviously your call. Just making the argument for stouter.

Denis

 

I wanted to let this Dog Lay, but I wont be able to Sleep. I apologize in advance

in castings, as opposed to forgings , strength does not increase linearly with thickness , this is very important in structural shapes , this is caused due to Dendrite formation, often generalized as directional solidification, the first two Chapters of the Navy Foundry Manual go over this briefly, in short as a web gets thicker, the grain structure gets courser and courser, until it eventually forms micro-porosity in the center.. and later a shrink defect
Bob Puhakka or Dr Campbell mentioned this in the Balancing issue on the impeller in one of the videos... same idea
. in general the metal closest to the sand has a superior fine grain structure. at the very least by making an over-sized member and machining off the exterior you are removing the structurally superior metal and leaving the inferior. everyone agrees with me this is a bad idea... Right???

Yes yes I know in many alloys this can be mitigated with heat treatment , but not removed

there is a massive amount of thought that goes into the design of some parts we take for granted, go take a look at the underside of a large bandsaw or drill press table . spend 15 minutes reading chapter 2 of the Navy Foundry manual, it give just enough information to tell you leave design to the experts, and if you are an expert you should know you cannot redesign a part effectively with a few pictures and no measurements or design specifications .


V/r HT1

P.S. if i offended anyone, that was not my intention I apologize

 

I have to agree with HT1 on this. (man that hurts) I'm not much of a design but I am an experienced Mechanical Engineer. My take on it is that most old lathes and other machine tools were well designed and have passed the test of time. This lathe has obviously been abused, significantly. A minor increase in thickness is appropriate, in part because you probably won't deliver the same quality casting as the original. But you won't abuse this baby and it will last a very long time.

 

It should never be offensive to discuss a question if done in a calm and deliberate manner without personal attack or undue ardor. No need to let a dog lay or be distressed if there is additional debate. Put out the pros and cons, even if consensus is not reached, there is benefit in mulling the idea over.

There should be little debate that thickening the web will strengthen the part and if the OP does a few minutes measuring and drawing it should be possible to see that the part in question could be considerably thicker given the fact it was originally designed for a quite tall, compared to a wedge QCTP, lantern tool post.

If for some very unforeseen reason he needs to revert to a thinner web(maybe makes an error measuring?) then if he machines off the base, in view of better grain structure near the surface, the the base is in compression and the Upper surface is relatively undisturbed. How much real difference would that make ? Or better, just adjust the pattern and cast it thinner.

The engineering on this part might or might not have been great. How do we know? What harm is likely to come from thickening the part? What was the original melt recipe? If a new part is cast will the alloy be better or worse? Was the original part heat treated and in what manner, flame hardened, shot peened?

I would take a different path ( not necessarily better) and realize I can’t truly replicate the original part as there are too many unknowns and therefore I’d do what makes sense.

Whether everyone or no one agrees doesn’t matter. The OP asked for advice. That’s my advice. I am interested in seeing other advice and find it not annoying, but interesting and worth consideration.

Denis

 

Denis: My experience is that old machinery was very well engineered by people who knew what they were doing. We make more mistakes now by computer jockeys using design programs which may or may not be suitable for the specific task. Boeing is unfortunately mired in a disaster of that kind right now. I would not want to be trying to cope with their issues. I would wager the compound was broken by the machinist stepping away to adjust another machine. I've seen good machinists running up to three machines at once. Just like juggling. If I've got a long cut I may step away to grab a bottle of water but if you go to get a tool, and it's not there, and you start looking, and then you hear it crash to a stop.

 

It is a bit tricky to get a good view of the part, so I am not even sure how to approach the design of it.
I tend to think it was a crash into the chuck that did it in, and it probably would have never broken otherwise (as others have already mentioned) .


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