Ok I am a little ashamed to admit I dont know a good way to inspect for hole location. The last shop I worked in spoiled me. They had a brown and sharp tesa hite gauge. They call it a digital hite gauge but its really more like a CMM. A tesa hite is on the wish list for my home shop but it will be a while before I will have my own. So what is the next best way to inspect for hole location?
I believe I have seen (although I cant one right off hand) unimikes that use gauge pins for the anvil. Thats my only real idea for an accurate hole location measurement. The problem with that is they would never work on a recessed hole.
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- From: Lawrence London <lfljvenaura@gmail.com>
- To: machinist@lists.ibiblio.org
- Subject: [machinist] How to check hole locatin?
- Date: Sun, 27 Jan 2013 21:17:38 -0500
How to check hole locatin?
http://www.practicalmachinist.com/vb/general/how-check-hole-locatin-259443/
Cast Iron
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How to check hole locatin?
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01-27-2013, 12:30 PM #2
Titanium
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You need to be a lot more specific about what type and size of workpiece you want to check. We also need to know how many accurate digits you require in your data. Tell us about the equipment you have in inventory. There is probably a way to get it done with existing equipment with lots of time and originality.
Pre-CMM days the inspection would be done on a surface plate with a height gage. If better accuracy is required than the height gage provided you work to gage blocks or a Cadillac gage (height master). Some features might be checked with the scribing foot but I always preferred a test indicator. Measure hole radius and add that to the location of the side of the hole you can pick up. Tapped holes can be checked either using tap shanks or minor diameter pins.
After you get the first direction finished you rotate the part exactly 90 degrees and do it again. Sometimes you can fasten the part to an angle plate for an easy 90 degree rotation.
Sometimes the part can be inspected using a machine as a CMM. The machine needs to be qualified or calibrated before using this method, just the same as your other inspection equipment. -
01-27-2013, 12:40 PM #3
Diamond
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I'm agreeing 100% with gbent. To even begin to give you an answer we'll have to know much more about what it is you wish to measure other than a "hole location".
Originally Posted by bellinoracing
1.Typical hole diameters?
2. Hole to hole, hole to edge or both?
3. How accurately do you need to measure?
4. Based on what tolerances?
5. How much would you be prepared to spend?
Shall we assume you have a surface plate?
Start with that and I'm sure you'll get intelligent and useful responses.
Gordon -
01-27-2013, 12:47 PM #4
Diamond
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Quick and dirty would beSometimes the part can be inspected using a machine as a CMM.
...clamp part in vise or to the table of a bridgeport or equivalent...
...indicate hole #1 center with test indicator..
...zero DRO on your manual mill....
....indicate hole #2 center with test indicator...
....note X and Y difference and use trig to establish the actual centerline distance....
I think that's probably the best I could do in my own shop. I believe using that method I could measure it within .001" of what a whizbang (meaning proper) CMM could do. Also thinking about it in reverse, that's probably the most accurate method I have of holding a dimension between two hole centers as well. The DRO takes out issues with backlash or wear in the leadscrews. Obviously its accuracy could be compromised over longer distances unless some kind of a survey was done to understand its non linearities. -
01-27-2013, 01:53 PM #5
Cast Iron
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In a home-shop environment, where relatively low accuracy is required, I could see Matt's method being reasonable. One major issue to be aware of however, is just because a DRO reads some particular number doesn't mean it is actually the distance features are apart from each other. The problem is due to stack effects of the knee, saddle and table. This can account for an alarming amount of error. At work, we had a relatively new manual knee mill with a 2-axis Newall DRO. When using a coaxial indicator on a part near the edge of the table, I discovered that even with the gibs tight, the table swung a full +/-.002/.003" from the feature without the DRO picking up the movement. This is because the stack of the table parts weren't moving very much relative to each other, but out at the edge of the table, this small movement translated into a fairly large amount of error. I was pretty disgusted with the machine after that. I checked my own older, but U.S. made machine with 3 axis DRO, and it had similar issues, primarily with the Z-axis when the lock lever was engaged.
Something to keep in mind to check for before using a mill as a CMM. -
01-27-2013, 02:45 PM #6
Diamond
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'Bottom end' of the accuracy list, but something many of us keep forgetting to make or buy, year-in-year out, is a set of add-on hole-centre-finder cone-point legs for our ubiquitous calipers - range immaterial. I say 'make' 'coz those sold are usually lost on anything but rather small holes. Meanwhile - turn a set of pins to fit precisely if none are on hand that do, then work off the external surface with the usual math. (A major reason we DO forget to grab those cone-points..) Originally Posted by bellinoracing
Next-up is the surface plate and its partners, grand or humble, with the tedious contortions already mentioned.
Beyond that, sometimes one just has to make or adapt gages and fixtures specific to the task. Which CAN be the fastest route of all. And can be vetted by a third-party.
For your 'recessed' situation, that might be a drilled plate on the end of a rod, pins in the plate, ONE pin fixed, the other mounted in an off-center disc such that it can be rotated to align precisely. The end of the holding handle, for example.
Get it to fit, pull it back out to measure-over with common outside micrometer or the like, and do the math on the pin diameters.
Accuracy is all in how well you implement and handle it. If it repeats both in the recess and on test items out in plain view, it works as well as store-bought. ELSE NOT.
Bill -
01-27-2013, 03:06 PM #7
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I'm not sure I quite understand the source of this problem. On my mill the sensor for the z-axis is mounted to the knee, and for x and y the sensors are mounted to the saddle. Considering just the x-axis (the same would be true for y, although the errors would be even less) slop in the ways could allow the table to tilt down at the left when it was all the way to the left, and up at the left when all the way to the right. Since the x-sensor is roughly in the middle of the machine, if the table tilted in this way it would rotate the DRO's x scale very slightly with respect to the x-sensor, and thus the DRO wouldn't detect any movement. However, if the hole in question were on a part at the far end of the table (say, 20 in. from the spindle), and the table dropped by even as much as 0.010l", that would shift the x position of that hole by less than 0.0002". Originally Posted by pbungum
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01-27-2013, 03:11 PM #8
Titanium
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When I have to "make it just like the sample", because there is no print, (of course there's no print!) here's a suggestion. Clamp to the mill. If it's round, find center. If straight sided somewhere, zero it in. Find the least bashed up, rusted, smashed corner for a datum. Using gage pins in a collet or good chuck, bring the quill down 'til it goes into the stinkin' hole without bind. You can get a pretty good feel for that. Record those measurements and make a print. Oh, and hope for the best.
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01-27-2013, 03:18 PM #9
Diamond
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ACK. Mil-spec work, that is. Originally Posted by Ray Behner
Just MIL-T-FSGDI instead of MIL-T-FP41C
Bill -
01-27-2013, 04:07 PM #10
Titanium
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An old trick I've used to find the centre of bores with a standard height gauge, use a small ball bearing say 0.250 ". Place the ball bearing on the bottom of the bore, touch on lightly with your height gauge. Take a reading, subtract the ball bearing size, add on half the bore size and that's your bore centre height. Regards Tyrone.
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01-27-2013, 04:18 PM #11
Diamond
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Really helps to put them ball bearings into a bit of copper tubing, 'floating' to drape over micrometers, calipers, and height-gauges. Otherwise, better buy 'em by the gross. Originally Posted by Tyrone Shoelaces
Not much else in a shop as wants to run-off, run FAR, then hide as a bearing ball does...
Even predicting the location of a ball-bearing mousetrap is a fool's errand.
Bill -
01-27-2013, 04:31 PM #12
Diamond
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So the way I'm understanding is that the table gibs, more or less the "freeness" that allows the slides to move, can result in racking/twisting the table, where the DRO doesn't measure "bending" of the scale, just the linear displacement. Originally Posted by pbungum
I would say that seems perfectly reasonable.
Is the solution then to lock up the table gibs wherever you are making a measurement? As if you were concerned with the table shifting when drilling/boring a hole, you'd likely do just that.
The cones and sphere/balls seem excellent too...but don't those depend on the sharp edge of the hole where it meets a presumably flat surface being very reliable about its circumference. Another way to look at that is if there were to be a chamfer it's not necessarily concentric with the hole sides. I've put many hundreds of countersink/chamfers on holes with a battery drill, obviously those were to keep people from getting cut and not as a toleranced feature...but more to the point...how can you rely on the chamfer to seat the ball exactly if it was made by me?
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01-27-2013, 05:17 PM #13
Diamond
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The result of getting them actually in the right place - in eight different shops nation wide - circa 1928:
http://img.photobucket.com/albums/v3...ecesofIron.jpg -
01-27-2013, 06:15 PM #14
Plastic
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Even predicting the location of a ball-bearing mousetrap is a fool's errand.
Thermite,
The ball-bearing mouse traps, i.e. tom cats, round this neighbourhood can be relied on to be in my garden at least half a dozen times every night. -
01-27-2013, 06:39 PM #15
Titanium
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A ball bearing mousetrap is about as useless as a politician, but with higher morals. Originally Posted by prawn
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01-27-2013, 06:56 PM #16
Diamond
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Been (mostly) based in the Metro DC area since the sixties. Only way I've seen 'Politician' and 'morals' used in the same sentence is if the next word after 'morals' was 'charge'. Originally Posted by gbent
That lot are FOREVER 'checking hole location' ... and getting it wrong...
Bill -
01-27-2013, 08:00 PM #17
Cast Iron
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Matt - that's it exactly. If you think of the stack effect in relation to a knee that is not very well fitted to it's corresponding column, the amount of tension on the gib/way can have a substantial effect on the X and Y positions. Even Z itself can have quite a bit of movement from only a little bit of fitment trouble with the knee/column. If the saddle is at maximum distance from the column, and there is only a small amount of tilting of the knee in relation to the column when the knee locks are engaged, that small angular change can move the saddle up a fair amount. Now, add that to slight fit problems on the saddle to the knee, and the table to the saddle, and all of a sudden there is a lot of error in the system. The DRO scale doesn't really measure the absolute position of any table axis, but rather it's relative position in regards to the member it is attached to, and does not take into account angular changes very well. We assume that machine ways are usually fit perfectly, and that different regions of the table, saddle, knee, and column are all equal in terms of that fit. Unfortunately, this is rarely the case. For most things we do it doesn't matter much. If, however, you are looking to precisely measure hole -location, I'm guessing now it does matter. If the machine being used in this way is a tight machine, it might be just fine, even with these problems, depending on the accuracy he's looking for. It's just something to consider as an issue. Also, machines with less "stack", such as european style milling machines might, if condition is good, be slightly better as they have fewer stacked machine components typically.
Reply With Quote- [machinist] How to check hole locatin?, Lawrence London, 01/27/2013
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