djaxon
starting to spend too much on rocks
Member since February 2015
Posts: 159
|
Post by djaxon on Oct 9, 2015 18:08:53 GMT -5
I have read the term undercut on many "rock" sites and think I would know it when I see it but I would like to have one of you experts tell me what it is. Am about to build a polisher and am getting ready to study procedures for polishing stones. Undercut seems to come up a lot but no explanations are given on the pages I'm reading. TIA
|
|
|
Post by orrum on Oct 9, 2015 18:23:17 GMT -5
A marshmallow has peanuts embedded in it. U start grinding on it and the marshmallow between the peanuts grinds away faster between the peanuts leaving the nuts sticking slightly above the marshmallow.
Welcome!
|
|
djaxon
starting to spend too much on rocks
Member since February 2015
Posts: 159
|
Post by djaxon on Oct 10, 2015 10:55:47 GMT -5
Thanks much! This explains the word and makes me hungry at the same time. I appreciate your help. dj
|
|
|
Post by captbob on Oct 10, 2015 14:51:32 GMT -5
Hematite softer than the red jasper = "undercutting"
|
|
micellular
has rocks in the head
Rock fever is curable with more rocks.
Member since September 2015
Posts: 640
|
Post by micellular on Oct 10, 2015 18:40:30 GMT -5
Pretty sure if you started grinding marshmallow you'd end up with sticky wheels and a big mess. But tasty
|
|
|
Post by Pat on Oct 10, 2015 19:29:40 GMT -5
A marshmallow has peanuts embedded in it. U start grinding on it and the marshmallow between the peanuts grinds away faster between the peanuts leaving the nuts sticking slightly above the marshmallow. Welcome! Excellent explanation and the picture by captbob. The forum ought to have a DEFINITIONS SECTION, OR A DEFINITIONS THREAD FOR EACH SECTION.
|
|
|
Post by Peruano on Oct 11, 2015 8:25:28 GMT -5
If you have a striped rock - like the example above, . . . and you really need to polish it despite its tendency to undercut. 1. Try to concentrate with the rock layering oriented with the direction of travel of the wheel. You will have to deviate from this angle to remove all scratches, but if you concentrate on this angle and a few degrees in each direction from it you will undercut less. 2. Be particularly careful on aggressive wheels. The lighter wheels will not produce the problems as badly.
Undercutting can vary between a wheel (cab machine) and a tumbler so try them both if you face an important challenge. Otherwise, I know no secrets. Tom
|
|
|
Post by rockjunquie on Nov 1, 2015 17:09:59 GMT -5
captbob Great visual!! Perfect representation. Too bad about the rocks. They are beautiful.
|
|
|
Post by 1dave on Nov 9, 2015 9:55:27 GMT -5
You buy a stone because it has the most amazing colors and patterns. With Great Expectations you begin shaping the dream stone of all dream stones . . . Oh NO! Deep canyons and craters appear on that supposed to be smooth surface. It's over. Undercut.
|
|
|
Post by 1dave on Nov 9, 2015 11:28:00 GMT -5
In spite of undercutting, WE still do work like this: dino + rhodochrosite ring- Photo from Peggy Houchin's post in Gembone Connoisseurs - The Beauty of Gembone.
|
|
liz
noticing nice landscape pebbles
Member since September 2014
Posts: 83
|
Post by liz on Nov 9, 2015 18:24:59 GMT -5
That's a gorgeous ring!
|
|
|
Post by Starguy on Dec 3, 2015 20:43:11 GMT -5
djaxon Undercutting is one of the true challenges of being a Lapidary artist. You need to figure out a way to prevent it. Hopefully one of these days I'll figure it out. The important thing is to clean out the previous grit before going on to the finer grit. The number one rule is don't transfer grit from one stage to the next. Sometimes undercutting can look really nice. orrum had the perfect description and captbob shows a great example. micellular is right. Don't grind marshmallow.
|
|