Stepper Motors -- a back story to my scraper-sharpener project.
While working on my "low speed motor" side of the project, I encountered a problem. When I turned the power supply on, often as not the stepper wouldn't spin -- it would just buzz loudly. This happened for several different steppers, including a new one. So what was going on?
I did figure out that I could get the stepper to turn by giving the diamond disk a good spin with my hand, but wasn't sure why until I read an article written by some college students who designed and built their own stepper motor controller. The controller included speed and acceleration control. They mentioned that a stepper has limitations with regard to acceleration. If requested to accelerate too quickly, the stepper can't follow so it will lose steps.
I think my simple oscillator circuit is the culprit. It immediately starts oscillating, so the stepper is requested to basically instantaneously accelerate. That's not going to happen so it exhibits the ultimate in lost steps -- it loses them ALL and just turns into a noisemaker.
To solve this I am working on a modification to my 555 oscillator that will produce a ramped output frequency. My first attempt using a capacitor on the control pin didn't pan out so back to the drawing board.
I suppose I could replace my oscillator with a microcontroller but that just seems like overkill. But then I could claim I have the beginnings of a CNC sharpening setup <g>.
Monday, February 15, 2016
Sunday, February 14, 2016
Making the tools to make the tools
Recently I've started learning how to scrape metal. The purpose: improve the fit of machine tool surfaces, like milling machine dovetails. Practically speaking, I want to improve the inexpensive tabletop lathe and milling machines I currently own. They are a good illustration of "you get what you pay for". Although I do have to say that I've made useful stuff on them, doing no improvements other than adjusting them. But as I've become a little more proficient on them, some of the shortcomings have started to annoy me. So began an unexpectedly long (still in progress) sequence of getting tools to improve other tools, only to discover that they're not so great, either.
So I found my self in a sort of bootstrap mode. The starting point was a granite surface plate and what turned out to be a very non-flat surface gauge. So, scraping had to start there before I could even begin to work on my mill. But in fact, I had to go back even further. First, about scraping.
The scraping process involves marking the surface of the item you want to improve. A very flat granite surface plate is loaded with blue pigment, and the work is rubbed over the surface plate. The high spots on the work are marked. They are removed using a sharp scraper. Repeat. A gross oversimplification, but that's basically what is done. At the end, you should have a very flat surface. But not necessarily one that is parallel or perpendicular to anything else you care about. To achieve those other things, you need a good dial indicator and a surface gauge to hold it. I got a decent dial indicator, but the cheap surface gauge I bought quickly revealed itself to be insufficient. It rocked when placed on the surface plate, to the tune of about .006" -- impossibly bad for the kind of standards that scraping can achieve. So my first task became clear: fix the surface gauge. Except it really wasn't the first step, as it turned out.
Through a number of false starts I arrived at the _real_ first step I needed to take: build a setup to sharpen tungsten carbide to make proper scrapers. The photos below show what I came up with. And it works.
Photo above shows the diamond lap (charged w14,000 grit diamond), tool support and tool holder. The scraper is held in place with a set screw that is in one of the steel blocks attached to the holder.
I got the diamond disks from a lapidary supply company. I got 150, 320, 600, 1200 and 3000 grit disks, and 14,000 grit diamond powder. I used the back of one of the coarse disks and charged it with the 14,0000 grit. A disposable aluminum loaf pan is used to hold water -- the disk rotates thru the water for lubrication. The 14,000 grit disk was used dry.
Closeup of holder and 3/4" carbide scraper. The left end of the holder is designed to accept brass inserts with different radii. The support has a fence on the end closest to the diamond wheel (shown at the top of the photo). The brass insert is pushed against the fence, so the end of the scraper describes an arc. This is how I ground a known radius on the scraper.
A photo showing my "low speed motor". It's a stepper motor with a driver. The step pulses are generated with a 555 timer-oscillator.
This is a slow-speed setup so there is little chance of heat buildup. The idea was to have a setup that can also be used to touch up HSS tool bits, and maybe the odd knife or two.
This setup worked very well. The scraper now removes metal pretty effortlessly. I'll sharpen my smaller carbide bits with smaller radii so I can use them for the fine work.
Recently I've started learning how to scrape metal. The purpose: improve the fit of machine tool surfaces, like milling machine dovetails. Practically speaking, I want to improve the inexpensive tabletop lathe and milling machines I currently own. They are a good illustration of "you get what you pay for". Although I do have to say that I've made useful stuff on them, doing no improvements other than adjusting them. But as I've become a little more proficient on them, some of the shortcomings have started to annoy me. So began an unexpectedly long (still in progress) sequence of getting tools to improve other tools, only to discover that they're not so great, either.
So I found my self in a sort of bootstrap mode. The starting point was a granite surface plate and what turned out to be a very non-flat surface gauge. So, scraping had to start there before I could even begin to work on my mill. But in fact, I had to go back even further. First, about scraping.
The scraping process involves marking the surface of the item you want to improve. A very flat granite surface plate is loaded with blue pigment, and the work is rubbed over the surface plate. The high spots on the work are marked. They are removed using a sharp scraper. Repeat. A gross oversimplification, but that's basically what is done. At the end, you should have a very flat surface. But not necessarily one that is parallel or perpendicular to anything else you care about. To achieve those other things, you need a good dial indicator and a surface gauge to hold it. I got a decent dial indicator, but the cheap surface gauge I bought quickly revealed itself to be insufficient. It rocked when placed on the surface plate, to the tune of about .006" -- impossibly bad for the kind of standards that scraping can achieve. So my first task became clear: fix the surface gauge. Except it really wasn't the first step, as it turned out.
Through a number of false starts I arrived at the _real_ first step I needed to take: build a setup to sharpen tungsten carbide to make proper scrapers. The photos below show what I came up with. And it works.
I got the diamond disks from a lapidary supply company. I got 150, 320, 600, 1200 and 3000 grit disks, and 14,000 grit diamond powder. I used the back of one of the coarse disks and charged it with the 14,0000 grit. A disposable aluminum loaf pan is used to hold water -- the disk rotates thru the water for lubrication. The 14,000 grit disk was used dry.
This is a slow-speed setup so there is little chance of heat buildup. The idea was to have a setup that can also be used to touch up HSS tool bits, and maybe the odd knife or two.
This setup worked very well. The scraper now removes metal pretty effortlessly. I'll sharpen my smaller carbide bits with smaller radii so I can use them for the fine work.
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