Centre finding

[HopperParticipant @hopper]

Posted by Steviegtr on 24/04/2020 23:46:04:

Posted by Vic on 24/04/2020 22:32:03:

Centre drills aren’t for starting holes, you should use a spotting drill. A few of mine.

It's amazing how many regular you tube Guys use a centre drill. Not many use the spot drills. I am lucky as got a few left from Starret cutters when I worked.

Steve.

Centre drills were traditionally used for many years in many machine shops. Spot drills seem to have arisen in popularity since CNC pioneered their widespread use in the interest of faster machining times. I've only just recently converted myself. Now I'm a believer. Especially as it's getting harder to get centre drills of the quality they used to be unless you pay the high price for top name brands.

[SteviegtrParticipant @steviegtr]

From the 2 above posts , do I assume that I use my dro & a solid drill or bar & touch off , then zero the dro. Then move to the other side & do the same & then deduct the dia of the bar.

Steve.

[Bill PhinnParticipant @billphinn90025]

I don't use a drill or bar, but a simple £5 edge finder, Steve.

There is no need to do any subtraction when using the edge finder in this way with the 1/2 button; just edge find on one side, zero, move to the other side, edge find, press the x or y axis button [depending on which axis you're working on] press the 1/2 button, then move the handwheel until you get to zero, which is the mid-point between the two edge-found sides.

[SteviegtrParticipant @steviegtr]

Brilliant, thanks.

Steve.

[JasonBModerator @jasonb]

Here you got Steve. I think on yours you need to press X then half rather than 1/2 then X as in the video. Red light electronic one for me.

You are not actually using a ctr finder but using and edge finder to locate two opposite edges and then letting the DRO work out the mid point.

 

Edited By JasonB on 25/04/2020 07:06:37

Edited By JasonB on 25/04/2020 07:07:08

[not done it yetParticipant @notdoneityet]

Posted by Hopper on 25/04/2020 01:31:36:

Posted by Steviegtr on 24/04/2020 23:46:04:

Posted by Vic on 24/04/2020 22:32:03:

Centre drills aren’t for starting holes, you should use a spotting drill. A few of mine.

It's amazing how many regular you tube Guys use a centre drill. Not many use the spot drills. I am lucky as got a few left from Starret cutters when I worked.

Steve.

Centre drills were traditionally used for many years in many machine shops. Spot drills seem to have arisen in popularity since CNC pioneered their widespread use in the interest of faster machining times. I've only just recently converted myself. Now I'm a believer. Especially as it's getting harder to get centre drills of the quality they used to be unless you pay the high price for top name brands.

If you did not have a dro and were using the crude touch-off with a known drill or bar, it would be better to work from one side only, using the known width of drill or bar and the item to be centred then backlash does not need to enter the fray.

About close enough for 2-4 thou I suppose, depending on the skill and nous of the operator. Errors occur in two measurements, while ‘touching off’ and possibly a bit in backlash – but still likely close enough for many quick and dirty jobs. Like do you know the real size hole the drill cuts, is the bar perfectly round and centred in its holder concentric being just some of the potential errors (which may be summative for the total error).

[Ron LadenParticipant @ronladen17547]

I dont have a DRO display with all the calculations I just have a basic readout from the 3 DRO,s and I have a £5 edge finder with a 6mm tip. I just work from one side having accurately measured the item I am centering, I find the edge move in 3mm and zero the DRO then wind in half the measured amount to find the centre, works for me.

Also like Hopper its only recently that I have moved over to spot drills I was using centre drills previously.

[John RutzenParticipant @johnrutzen76569]

That's interesting , how do you get spotting drills? I've always struggled starting holes with centre drills. Back to the thread I have the same problem with edge finders. I've been trying to work out how to make an optical edge finder.

[Tony Pratt 1Participant @tonypratt1]

Posted by John Rutzen on 25/04/2020 08:14:41:

That's interesting , how do you get spotting drills? I've always struggled starting holes with centre drills. Back to the thread I have the same problem with edge finders. I've been trying to work out how to make an optical edge finder.

What is so fantastic about optical edge finders? Cutwel, Cromwell tools, Drill services, Ebay etc. for spotting drills.

Tony

[colin vercoeParticipant @colinvercoe57719]

spotting drills also pre chamfer the hole ready for tapping etc, they work very well in a milling machine as chamfer milling cutter.

Colin

[SillyOldDufferModerator @sillyoldduffer]

Posted by Tony Pratt 1 on 25/04/2020 09:00:59:

Posted by John Rutzen on 25/04/2020 08:14:41:

… Back to the thread I have the same problem with edge finders. I've been trying to work out how to make an optical edge finder.

What is so fantastic about optical edge finders?

Tony

Quick and easy to use. Not quite as accurate or repeatable as the ordinary type but the bright obvious indicator is an advantage.

Likely to be worth me going optical if my eyesight continues to deteriorate. Up-close focus gets harder with every passing year…

Dave

[VicParticipant @vic]

Lots on the auction site.

Spotting Drill

Spotting Drill

Edited By Vic on 25/04/2020 10:05:02

[Clive FosterParticipant @clivefoster55965]

The secret to reliable, adequately accurate (for the job in question) edge and centre finding is the same as for pretty much every other aspect of model engineering, home shop work and engineering in general. Tooling of sufficient quality to give repeatable results to the required accuracy and practice, practice, practice. Being familiar with what you use invariably beats taking the latest ultra wiz bang super device out every second blue moon and expecting better than brochure claim results.

Heck I bought a Hamier 3D Taster for a bargain £100 about 8 years ago for "that special job" which still sits in its box unused save for bragging rights. Too pussy to use it in case I break it! Those ceramic links are expensive.

Practice is the difficult bit for we home shop types. Its hard enough to scare out time to make the things we want to make without spending ages doing the equivalent of a budding pianists scales. Doesn't help that a goodly proportion of what the less experienced want to do is the equivalent of handing a second year apprentice a job that would make a seasoned toolmaker sit down with the drawings for a good think first. Which is about where any sensible apprentice screams "Help!". Fortunately the internet has made getting a response to "Help!" a lot easier for home shop guy (or gal).

For us a true spinning rod wiping a damp cigarette paper off the side of a job, or better, smooth vice jaw, can, in practiced hands, give results just as fit for purpose as that Hamier on my shelf. Too cheap to meter against over £250 new is no contest. But you have to practice.

In my view the Huffam wiggler gives the best balance between least practice to get good results and accuracy. Especially when used with the DRO 1/2 button. The Huffam pivot is much lower friction than the less expensive collet versions so its very easy to stop feeding when the stem slowly starts to crawl off to one side. Not only is it a nice clearly visible shift but you soon get used to getting it moving across at very much the same speed every time. So its very repeatable. Generally working from one side is more than fine but touching both sides and using the 1/2 button is a little more accurate and saves on maths. Its what I always do with vice held jobs.

Touching off on the insides of the jaws is just so easy. If you don't have a DRO and 1/2 button you can do the same thing with adjustable dials but its more of a fiddle. Set to zero for one side, move to the other, note reading, set dial to half that, move back and find the zero. All movements going the same way to avoid backlash. Always set the zero. Don't try to hold the numbers in your head. You will make a mistake, somewhen, invariably well into a job you seriously don't want to cock up.

Most folk don't take full advantage of adjustable dials, or DRO input abilities, for making work easier and reducing the chances of errors. Setting the dials or DRO so you finish on zero not only makes mistakes due to reading or memory errors less likely but also means what you "set is what you get". Which can be a great help when tying to figure out why you didn't get what you expected. Generally a full turn error is easily seen so should be unlikely. I have, however, proven that "should be unlikely" isn't the same as "will never happen".

Clive

PS If you have wall space set up a whiteboard (or blackboard for traditional type) to write cuts, positions helpful diagrams et al on in nice big readable from the machine script. Paper or notepad scribbles always seem to be too small and the shop gremlins love running off with the paper or pouring oil all over it!

Edited By Clive Foster on 25/04/2020 10:33:10

[Anonymous]

For ordinary work, and always for Z, I just touch off the tool with a fag paper trapped between work and tool as an indicator. For more reliable accuracy I use one of these:

To find the middle of work I use the above in conjunction with the 1/2 function on the DRO. On the CNC mill I do the same, but just divide the readout value by 2 with a calcuator. I have broken a couple of probes on the Haimer in 10 years, both due to finger trouble. The golden rule is don't rush the job and stay calm!

For what I call centre finding a hole or spigot I use one of these:

Prior to getting the Centro I used Osborne's maneuver as publicised by Guy Lautard, It works well, but is tedious and gets trickier as the hole gets smaller.

I hate centre drills for anything other than use on the lathe. And even then I don't like 'em. So I've been using spotting drills for many years. Having said that for most of my drilling on the vertical mill I don't need to use a spotting drill. On a flat surface a 4-facet drill will start on it's own with sufficient accuracy for most purposes. I only use the spotting drill where I need greater accuracy when multiple holes need to mate with another part, and/or I haven't got flat surface to start, like round bar or the holes in this casting:

When using spotting drills the big endian versus little endian argument translates to do you use a spotting drill with an included angle bigger or smaller than the drill to follow?

Andrew

[HopperParticipant @hopper]

I dont think the angle of a spotting drill matters. Its closer to drill bit angle than the traditional centre drill which has worked just fine for 100 years or more. As long as there is something there for the tip of the main drill to seat in, all good I reckon. As all my main drill bits are hand sharpened, they may or may not be dead on 118 degrees anyway. (Mostly not I am sure!)

I quite often use a large centre drill as a spotting drill too. Just use the 3mm or so diameter tip to put a small divot in the job enough to start the following main drill, if it is a small drill.

Edited By Hopper on 25/04/2020 13:20:38

[JasonBModerator @jasonb]

If you are using the spotting drill to add the CSK at the same time as spotting you don't get much choice in what angle to use unless you fancy making your own 120deg CSK screws.

[SteviegtrParticipant @steviegtr]

Posted by JasonB on 25/04/2020 07:05:35:

Here you got Steve. I think on yours you need to press X then half rather than 1/2 then X as in the video. Red light electronic one for me.

You are not actually using a ctr finder but using and edge finder to locate two opposite edges and then letting the DRO work out the mid point.

Edited By JasonB on 25/04/2020 07:06:37

Edited By JasonB on 25/04/2020 07:07:08

Great Jason. Will get some practice doing that.

Steve.

[Howard LewisParticipant @howardlewis46836]

Not all wigglers are consistent. A cheap one of, probably, Indian origin has a cylindrical probe which is 0.100" diameter. The ball ended probe is not 0.250", as it is supposed to be, but 6 mm. That caught me out, until I checked and found the mismatch.

Rather than rely on the wiggler having consistent friction every time, I would feel inclined to measure the size of the bar, on the two axes, and calculate the half dimension. Having found the edge with the wiggler, Zero the dial. Advance the work by half the diameter of the probe, and reset the dial to zero. Advance by the dimension that you calculated, and you should b eon the centre line in one plane.

Repeat the above for the other axis, and you should now be over the centre of the work.

This assumes that square bar is truly square and not tapered in either face.

Howard fat fingers again!

Edited By Howard Lewis on 25/04/2020 16:40:41

[not done it yetParticipant @notdoneityet]

This assumes that square bar is truly square and not tapered in either face.

If possible I often centre using the vise jaws rather than the workpiece. If only 3 inches long and out 5 thous over that distance, has the OP checked his vise is not causing this error? It is just surprising that the centre line/drill line is actually altering as the table is moved, rather than this problem being one which would be expected to be a random error if only simply drilling holes was being considered

[Anonymous]

Posted by Hopper on 25/04/2020 13:18:13:

As long as there is something there for the tip of the main drill to seat in, all good I reckon.

But surely if the tip of the drill needs to seat in the spotted location the angle of the spotting drill needs to be larger than the following drill. Which is the opposite of saying it doesn't matter, and also contrary to JasonB.

Andrew

[Former MemberParticipant @formermember32069]

[This posting has been removed]

[HopperParticipant @hopper]

Posted by Andrew Johnston on 26/04/2020 11:41:50:

Posted by Hopper on 25/04/2020 13:18:13:

As long as there is something there for the tip of the main drill to seat in, all good I reckon.

But surely if the tip of the drill needs to seat in the spotted location the angle of the spotting drill needs to be larger than the following drill. Which is the opposite of saying it doesn't matter, and also contrary to JasonB.

Andrew

I dont think so. Angle of a centre drill is 60 degrees, quite a bit less than the angle of a standard 118-deg drill and works just fine. But even if you used a spotting drill that was a few degrees greater angle than 118 the following drill still will follow the existing conical divot in the job. We are drilling a hole here, not splitting the atom.

[JasonBModerator @jasonb]

Hopper do most people go right in to the 60deg angle on ctr drills when just using to locate the position of a hole? I would say most just yes the pilot dia and that is not 60deg.

Personally I tend to use a 90deg 1/8" spotting drill most of the time and just make a small dimple and have not found traditional or split point drills wanting to snatch or go off ctr, maybe more of an issue with the bigger drills Andrew uses. Really no different to using a ctr punch which is 90deg and has served engineers for years.

Also like him a lot of the time I will go straight in with a stub length split point and not spot unless I want to be sure of position.

[Clive FosterParticipant @clivefoster55965]

Howard

As you say, when using the common inexpensive wiggler set having a collet style holder it is unwise to assume that the friction of the pivot ball is consistently low so there will be unquantifiable errors when working from one side. Such errors will, for practical purposes, largely cancel out when working from both sides and setting to the middle of the two readings.

Effective or not, relying on auto compensation of unknown errors is an unsatisfactory expedient.

The Huffam style wiggler has the pivot ball running on a narrow seat with a spring loaded nylon pressure pad holding it in place. Friction is low and inherently repeatable. The G.H.Thomas version has a functionally similar design, albeit laid out differently. If run at a suitable speed, Huffam suggest between 200 and 500 rpm, it is very easy to pick up the knack of feeding until the stem walks slowly off to the side. I reckon to hit about 5° per second walk rate. Any errors are far too small for me to quantify.

I reckon three tries for a neophyte to go from never having used one before to thou or so accuracy with a Huffam. A few more to get really good.

If you have an economy set it would seem well worth the effort to make a couple of holders in Huffam or GHT style for the ball and cylinder probes. Huffam use a hollow tube body with the seat at the bottom and a screw on top cap to apply spring pressure to the pad. The spring holds the pad and cap together so its easy to swop probes. GHT puts the spring and pressure pad in solid body with the ball seat in a screw on bottom cap. GHT calls for a 1/2" long spring of around 20 lb /inch, 4 kg /cm rate compressed by about 3/32", 2.5 mm to give 2 lb force on the ball. Huffam uses a 1", 25 mm, long spring compressed by 1/4", 6mm which, I think puts a little less force on the ball. GHT says spring forces between 1 lb and 3 lb work fine, I'd reckon the Huffam is close to 1 lb.

You pays your money and takes your choice.

Huffam innards and extension rod. Never used the extension myself but I suspect it might be handy in some horizontal mill style jobs. There is a cylinder ended probe for rods too:-

Clive

[Ron LadenParticipant @ronladen17547]

Excuse my inexperience here but it was only a few weeks back that I picked up on a thread which was telling someone they shouldnt be using a centre drill for starting a drilling operation but should be using a spotting drill and centre drills are for putting in the seat for a centre not for starting/positioning drills.

Since then I have got myself some spotting drills but can someone explain why centre drills are not correct for starting a drilled hole, its what I did all the time and I bet there are thousands out there that still do. If they can cut an accurate seat for a centre then dont they also centre a drill well enough..?

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