Thread: Spring loaded centre finder

Feel like I haven't posted any photos for a while probably because I haven't been able to stand up for too long because of the dickey knee.

Anyway things have improved a bit and over the last few days managed to make a 12mm drawbar for the mill and a centre punch centre finder.

I recently purchased an Acute Sharp kit from Eccentric.
It comes as a bunch of fasteners, and precut materials spot drilled which have to be drilled out to size prior to assembly. Watching the accompanying assembly vids and saw Gary's neat little centre finder that is inserted in a mill or drill press enabling accurate position of a workpiece under the chuck. Once the position is established the workpiece is clamped and the finder replaced with the appropriate drill bit.

The parts of the finder are shown in the photo below.
The two main parts are the pointy bit on the right hand side, which I will call the point , and the point holder on the left.

Cfinder.jpg

The spring goes over the shank of the point which then slides into the holder and is held in place by the grub screw that rides in the slot.
To provide further positional stability the 6mm diameter shank of the point slides into a 6 mm diameter hole inside the 12mm shank of the holder.

In the first iteration there was no slot in the point, just a reduce diameter section (R) that together with the grub screw prevented the point from falling out or going back too far.

However, due to various run outs introduced into all the parts all along the way etc when installed in the DP the point wobbled visibly depending on the rotational position of the point so I cut a slot in the point to restrict the rotation and then assembled whole thing and locked the point to the holder with the grub screw.

Then I mounted the the holder shank in the lathe and tuned up the point so that the point so its now concentric with the holder shank.
The run out is thus just run outs from the lathe and the DP and much better than with no slot.

If I was making it again I would not have a reduce diameter section (R).

Works a treat.
Cfinder1.jpg

Hi Bob,

Nice ! I do agree with your comment about reducing that section though. I started to make something similar and never got around to finishing it. Your post has given me incentive to go and look for the bits I made, that is if I can remember where they ended up

Nice work Bob, need to make a couple of them myself.

BJ it's probably quicker to make new ones than look for them.
I dropped a 40mm diameter bearing the other day and spent a couple of hours looking for it, gave up and went on with something else. Dropped that and found the bearing but not the part I recently dropped.
Kryn

The seemingly trivial project and all the run out measurements I did a couple of weeks back has given me a greater appreciation of all these issues. I was tempted to make another one using the 4 jaw to minimise the runout but it these are going to be mainly use in the DP it's probably not necessary to go that far.

Hi Bob,

If you think about it making the body and the shank is only going to have the runout of the worst case chuck jaws. What did you end up getting ? 0.06 mm, just over half a thou.

OK use soft jaws or make a collet !

I'll see what I can do over the next week.

Kryn your right, I'll never remember or find those bits.

Hi Bob, Kryn, Guys,

Bob: I don't want to hijack your thread, so if you don't mind I'll start a new one.

Kryn: I found the bits I made, just where I put them, in that safe place. You know I'm convinced that when you are looking for something, you go mentally blind. The bits are there in the draw, that safe place, but your brain stops you from being able to see them.

Originally Posted by BaronJ

Hi Bob, Kryn, Guys,
Bob: I don't want to hijack your thread, so if you don't mind I'll start a new one.

Feel free to post here if you like

Hi Bob, Guys,

This is a spring center that I started making some time ago, then I got distracted and forgot about it until Bob posted about the one he made. So I've taken some photographs to show the bits I've made. I still have not fully completed it and still have to find the taps that I bought to finish it.

21-08-2018-002.JPG 21-08-2018-003.JPG
This is the body of the device, looking from both ends. The material I think is silver steel, 1/2" in diameter. I intend to harden it at some point. The body was drilled right through and reamed 1/4" before drilling the other end 8 mm diameter for a depth of 1 1/4" inches about 32 mm. This diameter was chosen because the screw had a head diameter of 7.9 mm and 8 mm is the tapping size for an M9 X 1 thread. I squared the bottom of the hole with an 8 mm slot drill.

21-08-2018-004.JPG 21-08-2018-005.JPG
These are pictures of both ends of the probe. One end has been machined to a 90 degree point. The other end has been drilled and threaded M4, to suit the screw that will go in here. The material is 6 mm chromed steel rod ! It was salvaged from a printer that I scrapped several years ago. I actually kept all the motors and guide rods for stock. They have turned out very useful. It is an excellent fit in the 1/4" reamed hole.

21-08-2018-006.JPG 21-08-2018-007.JPG
I have no idea where this screw came from. Only that it is M4 X 10 mm, with a suitable head on it.

21-08-2018-008.JPG 21-08-2018-010.jpg
This shows the screw partially screwed into the probe and how it fits through the body.

21-08-2018-009.jpg
The spring is 1" long and fits comfortably in the 8 mm hole on top of the spring. Again I have no idea where the spring came from. But it is perfect for this application.

21-08-2018-001.JPG
And not least a family picture of all the bits except the cap that screws into the end to stop the spring popping out.

I will post further when I make the cap that goes in the end.

Hi Guys,

I found some pictures that should have been in the previous post.

21-08-2018-011.JPG21-08-2018-012.jpg
The spring is about four turns above the end when the probe is fully seated.


21-08-2018-013.jpg
This picture showing the nylon collet that I made to ensure that the hole drilled through the body was as concentric as I could get it prior to reaming to size. The red marker is so that if I needed I could put it back into the chuck in exactly the same place.

I made one virtually identical to John's a few years ago. I mount it in the tailstock and PD chucks to provide pressure to the back of a T-handle tap wrench when tapping threads.

Decided to blue the centre finder using the repeated nitric and hydrochloric acid vapour exposure method I have used before.
Cfinder2.jpg

I tried to speed up the process in that I have been using a 20L plastic drum as the exposure chamber but instead decided to try a 1L plastic chamber. Even though the acid evaporation surface area to evaporation chamber volume ratio was maintained a small amount of acid vapour condensed onto the parts so when I went to check what was happening about an hour after first inserting the parts there were a few small bubbles on the surface (all that should be showing is light rusting) indicating the acid was etching the surfaces. The parts started out as reasonably well polished and if all goes right this process should maintain the level of smoothness of finish of the original. The situation was probably not helped by the fact that in the larger chamber the parts are suspended ~300mm above the acid containers but in the 1L chamber the parts are almost alongside the containers.

The light etch the parts received during their first expsoure is what then gave the surfaces a mat like finish. The etching must have been very light because the point is still a very neat fit into the holder.

Ideally I should have re-polished the surfaces and started again but decided to treat it as a learning exercise and after the first exposure I reverted to the 20L tank and the above shows the result of 7 total exposures.

Nice deep blue/black Bob. Can you provide any more details of this method?

Originally Posted by jack620

Nice deep blue/black Bob. Can you provide any more details of this method?

Rather than reinvent the wheel a decent summary of the process I use is described as Fume Bluing on Wikipedia
https://en.wikipedia.org/wiki/Bluing_(steel)

It's by far the slowest of the methods but produces an excellent finish and when lightly oiled is one of the most rust resistant bluing methods around.
There are much faster methods but I treat it as one of my Shed Zen processes and do it in between other jobs.

I did put up a bunch of pictures in 2012 when I did my first lot of bluing but as these have been lost I will post them again.
Details of the exposure times here. //metalworkforums.com/f65/t1558...ng#post1522423

Below is the chamber/tank - a 20L plastic drum - the smaller containers in the bottom contain about 10 mm deep layer of concentrated HCl and HNO3 and the contained lids are removed during the exposures and the chamber lid is sealed. The PVC pipe is jammed into the inside of the chamber and used to suspend the pieces being blued.
IMG_3064.jpg

IMG_3062.jpg

The top handle shows the amount of bluing typically obtained after a couple of exposures compared to an unexposed piece.
Th first couple of exposures produce a light proceeding to dark grey colour as seen on the handle.
Bluing1.jpg

Then the next 4-5 exposures produces a grey-brown and then it starts to go brown-black and then blue-black.

This is the original tool post and small rotary table bracket made from just a ordinary piece of 6mm thick angle.
toolpostjpg2.jpg

After polishing and bluing (12 exposures)
toolpostjpg3f.jpg

Here is a link to a bluing project I did in 2015 that shows the process in some detail
//metalworkforums.com/f65/t1971...ghlight=bluing

Thanks Bob. It’s certainly a labour of love. I’m probably too impatient to see that process through. Gives great results though.

Originally Posted by jack620

Thanks Bob. It’s certainly a labour of love. I’m probably too impatient to see that process through. Gives great results though.

Yeah I agree the fume method doesn't suit many people but hey, I'm retired and I like the slowness of some processes.