Critique my patterns please

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chucketn

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I'm building patterns for 3 tools I need/want. The tools are a follower/traveling rest and steady rest for my 7x lathe, and the body and banjo for the L.H. Sparey Dividing Head.
Please look at my patterns and offer any advice/criticism as appropriate. No draft has been applied yet to any of them. Do I need it? Do they need to be split?

Pic's here: http://s571.photobucket.com/user/chucketn/library/patterns

Thanks,

Chuck
 
Hi Chuck,
It's really hard to tell from the pictures but one thing I can say is all patterns need draft on one surface or another. When the mold is rammed up the sand is tight against the pattern and without draft it can be very hard if not impossible to pull the pattern without distorting the mold. Some patterns need to be split and some don't, it all depends on what the shape is. Here again it's a matter of getting the pattern out of the mold. Look at the shape and see which direction it would be easiest to draw the mold, (no pockets or undercuts).
gbritnell
 
George, I'll repaint the patterns with grey paint tomorrow, may make them easier to see. Where would you put the draft on a circular pattern, or a loaf of bread shape?

Chuck
 
where to put the draft depends on how it will be placed in the sand. The top and bottom surfaces can be flat, but the surfaces that need to slide against the sand need to have some draft so they will pull without pulling the sand out also. short sections like rings an inch thick can be hammered in all directions and pull okay. Thicker sections need draft.
 
If I may be so bold, try the attached sketch for draft-angles. Split it as shown. Fill behind the boss, to blend with the main "lump". Fill the two bolt-holes.
Smooth-out ALL those bandsaw marks and paint it as smooth as you can.
It's going to be a fairly hefty casting for a beginner. If possible, lighten it by coring the main chunk, or at least the spindle hole.
I'll let someone else help with making a core.
Geoff

pattern.jpg
 
If I may be so bold, try the attached sketch for draft-angles. Split it as shown. Fill behind the boss, to blend with the main "lump". Fill the two bolt-holes.
Smooth-out ALL those bandsaw marks and paint it as smooth as you can.
It's going to be a fairly hefty casting for a beginner. If possible, lighten it by coring the main chunk, or at least the spindle hole.
I'll let someone else help with making a core.
Geoff

Geoff, that's what I'm looking for. Thank you! I planned on filling the bolt holes. Some of the members here and on other forums were asking how these tools would be mounted, so I drilled the holes to mount them as I would use them. It also allowed proof of concept, which led to some changes in the designs.
Thanks again.
Chuck
 
Hi Chuck, In the first sketch, I've "indicated" a core to remove a lot of the Body's mass but it would be a lot of work. The blue indicates a core for the spindle-hole, which would be well-worth doing regardless - less metal to cast and a LOT less to machine out afterwards! A piece of plastic water-pipe makes a simple core-box.
The second sketch re-arranges your design to remove much of the body-mass without needing to make a core-box. Hmm, very similar to many "old-style" tailstock designs...
Geoff

pattern_&_core.jpg


pattern_mod.jpg
 
Hmm, just checking for certainty, not meaning to patronize, but if you are not sure about draft and split lines, perhaps someone should ask if you have made shrinkage allowances.

Not really, but I've had it brought to my attention. I should allow 1%, min., right?

Hi Chuck, In the first sketch, I've "indicated" a core to remove a lot of the Body's mass but it would be a lot of work. The blue indicates a core for the spindle-hole, which would be well-worth doing regardless - less metal to cast and a LOT less to machine out afterwards! A piece of plastic water-pipe makes a simple core-box.
The second sketch re-arranges your design to remove much of the body-mass without needing to make a core-box. Hmm, very similar to many "old-style" tailstock designs...
Geoff
Does the core dia stay as made or will it shrink, too? For example, if I'm using 1" shafting, should I make the core in 1" id pipe or 3/4"?

Chuck
 
"Does the core dia stay as made or will it shrink, too? For example, if I'm using 1" shafting, should I make the core in 1" id pipe or 3/4"?" For 1" shafting, certainly no more than 3/4" OD, but this will make a very slender sand-core. You'll probably fail a few times before Eureka comes along.
The cored hole will not even be truly "round", so it will definitely need to be machined. One advantage of the casting having a cored hole is you can line bore the final hole dead-on centre.
Flatten the base (by whatever means you have, e.g. mill, bastard file etc), drill the mounting/clamping holes, bolt the casting to the saddle/cross-slide. Poke your boring-bar through, between chuck and tailstock-centre, and engage a slow feed. Many small cutter-adjustments later - voila! One pretty good bore.

I use a 1/2" sq. drive extension for my boring bar, cross-drilled for a small cutter, and a small ball-race in the socket out at the tailstock-end (I don't have a live-centre.)
Geoff
 

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