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02-07-2008, 03:34 AM #1
Centre finder for milling operation
If I am milling round stock held along the x-axis in a mill, what is the most accurate measuring device to find the centre of the bar? I have been touching off on the y-axis and moving minus the radius of the tool/finder and minus the radius of the bar but I thought there may be a cool little device to do this quicker and more accurate.
02-07-2008, 04:55 AM #2
AN EDGE FINDER is the most commonly used tool for this. They most commonly come in 3/8" or 1/2" diameter and about 3" long. My favorit is a 3/8" diameter with a .200" tip. You get it spinnning at around 450+ RPM, you touch by hand the .200 tip too get it to run off center. Then you use the offset .200 to touch off on the job workpiece and when it runs concentric, it will quickly pop visibly offset, this is your zero point on the workpiece and then you subtract the .100" (1/2 of .200) and you will be on the zero position on the workpiece. Do this whole process twice and slowly to retain accuracy!. Accuracy is +/-.001" if used properly. Godd-Luck!!...
02-07-2008, 05:51 AM #3
The best way, as far as I'm concerned, bisect with a dial test indicator held in your spindle.
First chuck up a test indicator. Find an approximate center by eye, come down on one side of the round stock, up and down, find the high point, then rock your spindle to find the low point, that is your reading, rotate the spindle and repeat on the other side(you may need a mirror), adjust your perpendicular axis(Y in your case) rinse and repeat until you are happy. I would put this as the most accurate and time consuming way. If you are on a manual machine that has no DRO, this is about the only way, since once you find center, you don't have to move anything.
Edge finders are fine, I do it all the time. My preferred rpm is 1100 on the same 3/8 shank, .200 end. That rpm is where I have found myself to be most accurate. To double check, hit the opposite side, and divide.
OTTOLUCK, why do you try to get it to run off center, when you get anywhere near the part it will start running concentric anyways, its going to break at the same point? I've had some older guys work for me that insisted they "flick" it. Is it possible I run mine faster 1100 rpms, vs your 450+? So that when it is not on the part/stock/jaws, it will run out a bit anyways?
Just one of my pet peeves, like miniature poodles, whats the point?
02-07-2008, 05:55 AM #4
I’m not convinced that an edge finder is the best tool for picking up the edge on ROUND stock or bores. You can never be sure that your at Top Dead Centre of the radius on the round stock.
You asked for the most accurate method, surely that’s a finger type Dial Test indicator, mounted in the spindle. That will give you readings on both planes, X & Y
I generally use a centering holder, but we call them a goose neck. There’s any amount of propriety and home made indicator holders to do this.
A co-axial indicator, like a Blake is also O.K for this.
02-07-2008, 06:59 AM #5
All good recommendations. I like the idea of using a centre finder on the edge.
You can just get a dial indicator and after removing the tool from the spindle fix the magnetic base to the spindle. Get the indicator on one side and just rotate the spindle. Then you just move the table on the x an y axis to get to centre. It is very fast and easy.
02-07-2008, 12:29 PM #6
Stephen: I like your idea of the indicator on the spindle itself. I have not done that before. I have been using the edge finder up to now and that works fine but I always seem to be in a hurry and was looking for some quick and fancy accurate gadget. I will try the indicator on the spindle and sweep from side to side. That should get "top dead centre" pretty accurately as well.
02-07-2008, 12:52 PM #7
For keyway cutting and most other milling operations not requiring tenths accuracy, lightly skim the top of the work with a rotating mill cutter. This will produce a very narrow flat on the part. Locate the center of this with a wiggler and get on with your life.
02-07-2008, 01:40 PM #8
Using the .200 edge finder, edge find one side and zero the machine. Then go to the other side and zero the edge finder again. Read the number off the readout and go to the middle of it.
Edge finding the center of a circle works just fine. Eye ball the center and edge find the near surface. Zero the machine. Then go to the far side on the same axis and edge find again. Read the number and go to the half way point (as above.)
Now you are in the middle of the part on THAT axis; so now do it again on the other axis then you'll be in the center. No adding or subtracting needed.
To help proove you are there, go touch off the points again and they should all read the same.
It probably depends on what you are used to using, as to which device you use.
02-07-2008, 03:08 PM #9
One thing about those rotating edge finders is that in my experience you need to keep a watchful eye on just when it begins to go out of concentricity. IOW, a very slight break is when it's most accurately indicated. If you move the y-axis for example until there's something even around 1/16" of an inch of break it's probably too much. More like 1/64" of break is what you're looking for to give an idea. Takes some good eyes. lol
Btw probably not a bad idea to mention this even though obvious but because most of us need to be real close to see where the break is starting to happen do watch your hair, etc. Maybe a magnifying glass mounted somehow is what I need these days. ;')
02-07-2008, 03:27 PM #10
If you use the edge finder on opposite sides of the stock, and if your y-axis dimension has any play, and if you are reading off the physical dials on the handle, make sure that you take that play into account.
Another technique is to put a point into the chuck and balance a flat ground piece of steel (about the size of a 6 inch ruler, but it has to be flat) on top of the workpiece bar. Lower the chuck so that it just touches the flat, and manipulate your table so that the flat piece is held between the point in the chuck and the workpiece bar is level.
02-07-2008, 05:42 PM #11
Answer to bobw's posting
BOBW, I totally agree with you, but a center finder is a little faster, though it definitely is not as accurate as using an indicator, and with the indicator I end up using a mirrow to read the backside of the indicator. Though I have used my B&S indicator hundreds of times when I feel I need to. I also touch off with a cutter with a cigeratte paper, or just touch of on junk & stupid stuff jobs! As far as RPM, yes 1000+RPM is best, but I am only using a old BP in my home shop, and at times I don't want to switch over to high speed if I will not be using high speed, I hate adjusting belts and flipping lever's unless I have to. Also, I tried to get the point across that you could use high speed in low range if you want to, but I added the (+) sign as a referance. It is as low as you should go and this fellow machinist may have a small mill with little speed selection, he did not say so I only tried to cover basic situations!. I do appreciate your comment, but please try to understand why I wrote it the way I did!. Take Care Guy!!.... Jim L.
02-07-2008, 05:57 PM #12
02-07-2008, 06:59 PM #13
I flick the edge finder off center so that I can quickly crank over to the work. When I see the edge finder going concentric, I know it is time to slow down travel to find the edge. Just seems quicker to me.
02-07-2008, 07:23 PM #14
I have found a good quality edge finder has a much more positive "kick"...crappy ones have a mushier kick and it is hard to decide when they have really kicked :-). A starrett one gives a nice sharp definitive "kick"
02-07-2008, 09:39 PM #15
on the other spun up to 1000rpm, or the next speed available up, gives a very sharp "kick".
We have three of the ones with a black tip and a small shinny flat that were getting so it
was even hard to offset the tip with a finger. I finally washed the movable junction out with
some kerosene and paper and they started to work again but still not as good as the
02-08-2008, 12:43 AM #16
I had to find the center of a hole last week and I put a peg in the hole and mounted the indicator on the drill chuck. I then get the indicator as close to center as I can get it and just spin it as it touches the edge of the peg until the neddle doesnt move. Pretty easy and accurate.
02-08-2008, 02:00 AM #17
Ottoluck, sorry for any offense, that is the problem with the written word and the time delays.
I've done the same, I'm in low gear, I'm staying in low gear, its been a while, but I get it.
Stu, I can live with that explanation, it makes sense.
I had quite a few old timey guys in the shop that I ran. Most from manual machines only or button pushing CNCs, I made them set them up. Some had a really hard time with the edgefinders and .0001 pulse. It wouldn't "snap" hard enough for them. If it was in .001 pulse, they had a much easier time. On .0001 pulse, since they were not sure, they would pull off a couple thou, and flick the edge finder, then go back in. They where .002 off the part, its not going to wiggle anymore! thats where the pet peeve comes in, a two minute job turned into a twenty minute job, drove me nuts.
On edit, Bosley Jr, I've done that also, I don't think it is particuarly accurate, but it is quick, if you don't need to be dead nuts on, perfect solution. Sucks on a CNC, you definitely need a quill handle in your hand. Another taught to me by an old timer.
Last edited by Bobw; 02-08-2008 at 02:08 AM. Reason: just cuz
02-08-2008, 02:56 AM #18
Test indicator is the best to pickup center location, corner anything you want, better than any Co-axis, edge finder.
02-08-2008, 03:23 AM #19
02-08-2008, 06:01 PM #20
I'm not fond of double end edge finders, and here is why.
Sometimes I have used edge finders in drill chucks...if the chuck is worn so that the jaws have a taper, they will often only grip the end of the edge finder your not using, this leaves the thing rattling around in the chuck.
I have never seen purpose in using an edge finder in .0001 steps.... I just use it in .001...odds are the error will only .0005 or less anyway.