Enlarging a hole in knurling wheels

I am making a caliper-type knurling tool for use in my lathe, using 3/4 inch diameter knurls from a source in Australia. The hole is 1/4 inch diameter and the intention is to use silver steel rod as the pivot. My problem is, now that the parts are all made, the silver steel rod passes through the caliper but steadfastly refuses to enter the knurl. Of course, the knurls are hardened and although my only 1/4 inch drill goes through, a hand reamer not surprisingly won't touch it. I don't have the facility to heat the knurl so I was wondering if I could buy some 3 mm grinding tools for my Dremel to open the hole up a gnat's. (Precision is not my middle name.) Unless someone can come up with a spiffing wheeze to get me out of the (undersize) hole in which I find myself.

John

This is going to get some fun answers I am sure.

You need a pack of those crappy grinding stones-on-a-stick for your dremel and a diamond dresser. Get the stone down to the size you want and have at it. It is not precision but it will get you out of jail.

The other (better) alternative is to use the correct diameter rod and make some bushings for the holes in the arms.

The best solution is to get the right diameter rod and make new arms and just suck it up.

The hole isn't really undersize, your holes in the arms are oversize!

graham.

Hi John,
I would just use emery cloth wrapped round some 1/8" or 3/16" rod. Put the knurl wheel in the lathe chuck (With some tape or thin aluminium sheet around it to protect it.) Run the lathe as fast as you can and run the rod with the emery cloth in and out until it is up to the size you want. An alternative would be to lap it out wit grinding past on an aluminium lap.

Les.

Hi John - I have sympathy, but those knurls are super hard.

Surely the best way is to grit teeth and modify the caliper, after all you will get pretty annoyed in the future having to open out every different knurl that you want to use.

Cheers - Will

I had a similar issue in the end I turned the pins to suit the size of the Knurling wheel and just ran with the pins being a couple of thou slack in the arms seems to work OK.

Paul

Hi John,

Make a copper lap [search Google] and open up the knurl wheel hole until your SS pin goes through. The pin, caliper & wheel are all 1/4" but with different tolerances.

Tony

I used bronze pins, turned to size, for mine

Neil

As a follow up you don't want your SS pin turning in the caliper so fix the pin with a small grub screw, a flat on the pin would also be beneficial for the screw to tighten onto.

Tony

Make a bracket to mount your dremel tool on the lathe toolpost. Clamp knurl in lathe chuck and rotate slowly in back gear. Feed dremel in and out very slowly, taking tiny depths of cuts until the hole fits your pin.

I made such a bracket out of 12mm aluminium alloy, screw cut a thread in the one end for the nose of the dremel to screw into, at the rear end, just bored a hole in the ally plate to fit the body of the dremel, hacksawed it in half and used a couple of socket head cap screws to clamp it down onto the dremel. Works beautifully. Got my 75 year old three-jaw chuck back to half-thou accuracy again by grinding out the bellmouthed jaws.

You will find that the bore on the knurl wheel will not run true enough to make this a viable solution, have you a 4 jaw chuck?

Tony

Some clever answers here, but, the Knurl wheels are hardened and as such deserve to be kept to size. The answer is to drill out the side plate holes and fit bushes, Bronze is best and work on from there. The knurls should be lubricated in use and the lube will get to the bushings .

Clive

Thanks to everyone for your useful and thought-provoking ideas. I will take them all into consideration and try them one at a time. Unfortunately, the design on which I have based my efforts (by Graham Meek, in EiM), means that the cheeks of the calipers either side of the knurls are only 2.5mm wide, so that rather precludes the use of bushing, but I may have to persue a similar route, if other solutions don't work. I could make some larger calipers but that would mean ordering some more material, involving a wait until September when I next expect to be in the UK, or horrendous postal charges!

The silver steel is spot-on 1/4 inch so the "fault", if such it is, lies in the manufacture of the knurls themselves.

I WILL get around this, somehow; just needed a bit of help. i'll post pictures in my album when it's sorted.

John

Since it hasn't been suggested and it might be food for thought .... what about making the pin in two halves so that you can accommodate both the smaller knurl ID and the caliper larger holes and still assemble it? Probably key the two halves together with a counterbore on one and a shoulder on the other with a though screw to join the two halves of the pin (counterbored if you don't want any projection).

Thinking out loud ..

Edited By OMG on 06/07/2014 16:54:30

> The silver steel is spot-on 1/4 inch so the "fault", if such it is, lies in the manufacture of the knurls themselves.

i wouldn't consider A 0.250" shaft in a 0.250" hole a running fit.

Why not make a stepped bolt with a 1/14" shoulder and turn a small 1/4" diameter register on the nut?

Neil

What seems to be missing is the magnitude of the problem, are the bores under size by 2 tenths or 5 thou?. If 2 tenths then emery or dremel should work. 5 thou needs some proper engineering. I would hold your shaft material in you lath chuck and reduce its diameter for wheel +side cheek length until wheel fits on it, part off with the extra full diam. cheek length on it. Get hold of caliper, put a ball on one of the side holes, smack it with a hammer, this will have drawn some material in and downwards making the hole smaller. Turn caliper over on to this side put in wheel and insert pin thin end first, finish with a hammer.

Frank

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John,

I'm not sure how much this helps, but:

There is an occasional defect with Silver Steel ... it sometimes turns out trochoidal in shape [three-lobed, like the rotor of a Wankel engine]: The "diameter" will measure correctly, but the "radius" varies [or the centre of the diameter moves, if you prefer]. This is a defect of centreless grinding, 'though I would struggle to explain how/why it occurs.

MichaelG.

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Ooops ... probably better to reference the Reuleaux Triangle, not the Trochoid [although they are related]

Edited By Michael Gilligan on 06/07/2014 19:02:52

You've all been very helpful. Thanks. Time now for a little quiet contemplation, methinks. I've formulated a solution in my own mind, now I've got to think it through before execution. " Measure twice, cut once" comes to mind!

John

Edited By John Hinkley on 06/07/2014 20:58:13

If they are that close to size, then ten minutes work with a rat-tail diamond file of appropriate size should do the job.

Martin.

This effect can be seen if you measure a 20p or 50p coin.

Martin.

Done it! The hole turned out to be only half a gnat's undersize. After a slight delay to proceedings to mend the ride-on mower, I found a local DIY shop that sold Dremel accessories and bought some 5.2mm diameter grinding wheels. I put each knurl in the lathe chuck in turn andwith the Dremel going at half speed, lightly ground the bore with the lathe on its slowest speed (170rpm - no back gear). Result - pins slip in nicely with no apparent slop and a beautifully smooth bore, to boot! Now I've just got to put it all together to see if it works!

With luck there should be a photo of the bits in my album and hopefully, later today, the finished article.

Thanks, once again to everybody for your interest and assistance.

John