Need to make an accurate 8mm hole in brass plate

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whateg0
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Re: Need to make an accurate 8mm hole in brass plate

Post by whateg0 » Wed Mar 13, 2019 4:28 pm

Using calipers to measure the actual holes may work depending on the diameter of the holes and the accuracy needed. The flat on many calipers prevents truly getting in the concave edge of a hole, so your measurement will be big on the small side and small on the big side. A better way when possible is to use a round object that is smaller than the diameter of the hole. Force it to the inside/outside edges of the holes then measure. This way it's a convex surface being measured, which calipers can do.

Dave

whateg0
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Re: Need to make an accurate 8mm hole in brass plate

Post by whateg0 » Wed Mar 13, 2019 4:33 pm

chriscam wrote:
Tue Mar 12, 2019 5:14 am
I have thought for a few moments and need a bit of clarification. I have noted the remarks over my use of a center punch being inaccurate and that I should measure from edges and or screws. I did this and used deviders to measure 4 points as only 3 were needed for triangulation. The problem seems to be in the actual measurement of these distances where if measuring a center of an existing small hole some error is inevitable as is in the human eye when measuring. The difference between these scrible marks was small and as the tolerances are not high I chose the mid point which for this instance will be fine. However how can you accurately transfer measurements from one plate to another to reduce errors/
Regards
Chris
Still depending on the amount of accuracy needed, even this is bound to lead to error. First, the dividers must be set perfectly accurately. next the center punch must be placed exactly in the intersection. Third, the center punch must be ground perfectly conical. When working from an edge, it is better to use an edge finder and the dials on the table to get to the center of the hole. Often, I cheat and use a dowel, or endmill of known diameter, set the dial to half its diameter, then bump the stock up against it before clamping it in the vise. Raise the tool or dowel out of the way and move to 0. The edge of the work should be at the center of the dowel. Make sure that you have taken any backlash out of the screws first. This assuming that you have measured the hole locations from the first part accurately.

So, you state that you need accurately placed holes, but here you say the "tolerances are not high". Can you clarify what you are attempting to achieve? This goes back to the question, how accurate is accurate?

Dave

John Hasler
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Re: Need to make an accurate 8mm hole in brass plate

Post by John Hasler » Wed Mar 13, 2019 7:00 pm

whateg0 wrote:
Wed Mar 13, 2019 4:28 pm
Using calipers to measure the actual holes may work depending on the diameter of the holes and the accuracy needed. The flat on many calipers prevents truly getting in the concave edge of a hole, so your measurement will be big on the small side and small on the big side. A better way when possible is to use a round object that is smaller than the diameter of the hole. Force it to the inside/outside edges of the holes then measure. This way it's a convex surface being measured, which calipers can do.

Dave
All of my calipers have "knife edge" tips with a radius smaller than that of any hole I can get them into.

pete
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Re: Need to make an accurate 8mm hole in brass plate

Post by pete » Wed Mar 13, 2019 10:18 pm

Maybe a bit outside of what you were asking Chris, but doing some research and understanding how it's done is still useful information. Exact (or as close as possible) hole placement is a big issue while jig boring or grinding master gauges used to check that parts being made are within tolerance. The old school method that works just as well in a home shop today "IF" the accuracy required is high enough to justify the time is using what are called tool makers buttons and a dial test indicator. The hole locations are roughly laid out using standard marking out methods and even center punching them. The part then has both edges accurately found in X,Y on the mill so those coordinates are known. Then the rough hole locations are drilled and tapped for the bolts that hold the tool maker buttons to the material. Those buttons are hollow heavy wall tubes hardened and ground and the fastening bolts are well under size for the buttons hollow section. That allows them to be lightly bolted in place and then tapped in whatever direction is required to get each button centered at the desired hole location in X,Y.

The DTI is used to do that centering and is rotated around each button while held in the spindle. That zeros your spindle C/L to where the hole C/L needs to be. If more than one hole is being done then multiple buttons are used, one at each hole position and a micrometer can be used to measure across the buttons until there tapped into the correct position. It's a long slow process and only as good as your measuring equipment and your skills are. But it can approach jig boring accuracy with normal accuracy mills. Each hole is drilled one at a time, usually bored to size or a bit under size and then reamed. Then the next button center is found and the same done. Less accuracy but faster is using coordinates and the machine dials, better would be dial indicators to measure the X,Y movements, best would be a good dro.

At the basic level all accurate machining including the very best CNC machines made today is done by using what's called the Cartesian Coordinate System. That would be X,Y and Z for the first 3 of the coordinates. There's more than that for rotary etc movements but were keeping things simple. Google will give you multiple hits about it. It's not optional to learn and understand how it works. Once I found out and understood how it's used I was able to make a big jump in my accuracy and speed with a whole lot less mistakes. As always it's a wee bit more complex than that and a lot depends on what you have for equipment, but it works very well.

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GlennW
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Re: Need to make an accurate 8mm hole in brass plate

Post by GlennW » Thu Mar 14, 2019 7:46 am

pete,

To clarify a little, when using buttons, you usually need a more accurate measuring system to place them than is available on the machine you will be doing the final hole boring on. If your milling machine handwheels aren't accurate enough to get the accuracy needed, you would place the buttons using the mill, then move the part to to something with greater accuracy such as a surface plate and use a height gauge, gage blocks or Cadillac, and a DTI to accurately adjust the buttons to the correct X/Y locations as an example. You could then use the mill, or a lathe face plate to hold the part and indicate over the accurately placed buttons for final boring/sizing. Same for accurately locating the part on a rotary table for contour work. The buttons allow you to final bore/size/locate holes on a machine that basically has no coordinate system.
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!

pete
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Re: Need to make an accurate 8mm hole in brass plate

Post by pete » Thu Mar 14, 2019 9:10 am

I've no doubt your correct Glenn. The only information I've ever seen for there use was from a book written around 1910 and it showed them being used for boring master gauges on a lathe face plate. It makes a lot more sense to use a surface plate and the most accurate equipment available for setting them.

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GlennW
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Re: Need to make an accurate 8mm hole in brass plate

Post by GlennW » Thu Mar 14, 2019 9:29 am

In the context of this thread, boring the brass plate on a lathe face plate is an option if a boring head is not available.
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!

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