Need to make an accurate 8mm hole in brass plate

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chriscam
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Need to make an accurate 8mm hole in brass plate

Post by chriscam » Mon Mar 11, 2019 11:59 am

Hi I have only just learnt the basics on my Sherline 5400 Mill. I wish to make a 8mm hole in a brass plate. the hole will function as a bushing so i need the hole in the correct position which is center punched. So the question is should i just drill, should I end mill or should I ream. when it comes to reaming I cannot see if the reaming bit needs a pilot hole or the end mills away the metal. Any help gratefully received.
regards
Chris

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

Post by GlennW » Mon Mar 11, 2019 12:58 pm

Your best chance to make an accurate 8mm hole as far as size , roundness, straightness, and location would be to use a boring head after drilling a hole about 7mm as a start.

Reamers do not cut a hole, they only size it and you would usually drill to about .015" under size and then use the reamer to take it to the final size.

Drilling may end up in a tri-lobed hole and possibly tapered. An end mill will most likely end up in an over sized hole, and reaming can end up with a multi-lobed hole and possibly over sized as well as tapered.

A lot depends on how accurate the accurate hole needs to be!
Glenn

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

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

Post by Harold_V » Mon Mar 11, 2019 3:38 pm

What Glenn said. If you hope to create a precise hole, it's done by boring. A reamer has been known to do so, but it offers many opportunities for a less than good hole, and it has no ability to relocate a hole, unlike boring.

Something to consider. A center punch may offer an easy start for a drill when drilling with a drill motor, but it's not a great way to locate a hole when location is critical. In such cases, you're far better served to locate the hole by other means (working from edges, using the machine screws or a DRO), then starting the hole with a spot drill, or a center drill. A spot drill is NOT a requirement, and is not necessary unless you have one. Most home shops have center drills, and they work perfectly well, in spite of the protestations of those who are CNC trained.

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

Post by pete » Mon Mar 11, 2019 3:44 pm

Everything that Glenn and Harold said. You don't say what the part is for, but it's generally the best design practice to make any bushing location as a replaceable pressed in or threaded part even if the parent material can work well as the bushing itself. And as Glenn said accuracy levels vary. Since you mentioned just learning the basics of your mill so far then I hope part of those basics was properly tramming both the head and column of that Sherline mill. The heads alignment keyway is still only approximate and good accuracy would require both the head and columns tram be at least double checked with an indicator to be sure.

And depending on the actual accuracy level required center punching a starting hole location can be a fairly inaccurate method. If I wanted less than .010" location accuracy I wouldn't depend at all on marking out and center punching where I wanted the hole to end up.

And it looks like Harold types faster than I do. :-)

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

Post by DavidF » Mon Mar 11, 2019 8:37 pm

Sometimes its just a matter of tolerance.... Could you get away with just punching through a pre drilled holed with an 8mm endmill??.
Its up to you to find out.....

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

Post by chriscam » Tue Mar 12, 2019 5:02 am

Thanks guys some really useful stuff for me to reflect on.
Chris

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

Post by chriscam » 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

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

Post by GlennW » Tue Mar 12, 2019 6:44 am

chriscam wrote:
Tue Mar 12, 2019 5:14 am
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.
Work from the edge of the hole and know that the center will be half of the diameter from the edge.

If you want to determine the distance between two holes, measure between the inner two edges (with calipers if no other way) and then add the radius of each hole to find the center distance. That will get you to within the accuracy of your calipers...

Same if using gauge pins in the holes to work from. Measure to the outside of the pins and then subtract the radius of each hole to determine the center distance.

Gauge pins are quite useful when working with small holes for both determining hole size and location. Drill blanks or drills can work too.

If you really need to know, you can set the part up on the mill and use a DTI to center over the holes and use the dial readings (or DRO) to determine the location and distance of multiple holes. You can then transfer your readings to CAD and figure out anything you need to know (within the accuracy of your readings) about it as you would be working from the actual centers so there is no diameter involved to add to error when calculating.

If you are duplicating holes in a plate, you can set the original plate up in the vise or on the table using locating stops. Indicate over a hole using a DTI. Then remove the original plate and install the new plate and center drill, drill, ream, bore, or whatever. Then install the original plate again and locate the next hole and do the same until all of the holes are in the new plate. If you have threaded holes, you're back to gauge pins again for determining location which can be used with the DTI...
Glenn

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

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

Post by Harold_V » Tue Mar 12, 2019 3:23 pm

chriscam wrote:
Tue Mar 12, 2019 5:14 am
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.
That's a common problem if you're using visual instruments, but there's a way around it as well. It involves the use of a surface plate, angle plate, height gauge and DTI. A Cadillac PlaChek can be useful, but not a requirement. The piece is set up on an angle plate, then measurements are established by picking up the bottoms of the holes. The diameter must be known, and the distance from the hole edge to the surface plate is determined. The part edge is located on the surface plate, but can also be located on parallels, so long as you know the height of the parallels. Rotating the part 90° and repeating the readings will yield dimensions from two reference edges, which, for manual mill operations, tend to be the left hand side of the part, and the side away from the operator. That way the screws of a machine read in the proper direction. It is not in keeping with CNC operations, however.

This method is in addition to those already mentioned by Glenn. A part can be fully mapped accordingly. This method was the one used where I was trained when performing inspection of machined parts. It removes doubt due to visual error.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

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

Post by earlgo » Wed Mar 13, 2019 8:35 am

Glenn said,"If you want to determine the distance between two holes, measure between the inner two edges (with calipers if no other way) and then add the radius of each hole to find the center distance."

I agree with this, but I often measure between the two inner edges, and then the two outer edges and average the numbers. This allows for the holes to be different sized, and still get an accurate center to center measurement.

If you use gage pins in sheet metal be sure the gages don't tip and throw off the measurement.

JMHO
--earlgo
Before you do anything, you must do something else first. - Washington's principle.

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 8:53 am

earlgo wrote:
Wed Mar 13, 2019 8:35 am
Glenn said,"If you want to determine the distance between two holes, measure between the inner two edges (with calipers if no other way) and then add the radius of each hole to find the center distance."

I agree with this, but I often measure between the two inner edges, and then the two outer edges and average the numbers. This allows for the holes to be different sized, and still get an accurate center to center measurement.

If you use gage pins in sheet metal be sure the gages don't tip and throw off the measurement.

JMHO
--earlgo
That's a good idea. It also means that you don't have to assume that the holes are on size or measure each of them. One less measurement and less measurement error. You can do the same thing when measuring from an edge.

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

Post by Inspector » Wed Mar 13, 2019 2:31 pm

If the holes are the same size then measure the hole with the points of the caliper and zero it. Then measure to the outer edges of two holes and don't compensate. This of course means you are using an electronic caliper ;) and the tolerances are not extremely tight. If they are then use other means.

Pete

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