Drill grinding jig

[Alan RawlinsParticipant @alanrawlins60482]

I am in the process of making a drill sharpening jig and am trying to get all moving parts to operate smoothly without any noticeable looseness in the moving parts. At my first attempt I bought ready threaded 12mm bar. After tapping a hole in the female parts I found that the bought bar, was very loose in the tapped parts. I then discarded the bar and threaded a piece of mild steel bar with a 12mm die. Now I have the opposite effect, they are too tight when I put them together. I used a tap and die from the same set and I checked that they were the same thread. I drilled the hole to be tapped to a size given on a threading and tapping chart I have. The bar I threaded was 12.3mm in diameter and didn't bother to put it in the lathe to turn it down to exactly 12mm. ( I now wonder if this is the cause of my problem) I used the lathe to get the tap and die started to make sure they were threading true. Any thoughts on what may be causing this tightness of parts and what, if anything, I can do to make them less tight as threading the bar took 30 minutes of quite hard graft for an old chap with arthritis in his shoulders area so I am not too keen to have to do it again.

The design is made up from different designs on the Internet but I hope it will be better and easier to use.

[Alan RawlinsParticipant @alanrawlins60482]

[Bob RodgersonParticipant @bobrodgerson97362]

Alan, if the die is a split die then just slacken the centre grub screw in the die holder then tighten the other two about 1/4 turn each. Nip up the centre screw a little then run the die over the thread again trying the nut after the full width of the die has been cut. Repeat until the thread is a good fit.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Alan Rawlins on 28/07/2015 07:51:23:

… The bar I threaded was 12.3mm in diameter and didn't bother to put it in the lathe to turn it down to exactly 12mm. ( I now wonder if this is the cause of my problem) …

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Alan, I would say that is almost certainly the cause of your problem.

First thing might be to just go over the threaded bar with emery cloth, to remove any roughness on the crests of the threads. Then [assuming that this is a one-off item, and the thread form doesn't need to be exact] lap the two parts together, using an abrasive polish like Solvol Autosol, or an abrasive household cleaner.

Don't forget to thoroughly clean everything when you've done !!

Good Luck

MichaelG.

[Alan RawlinsParticipant @alanrawlins60482]

Thank you for your help, will do as suggested.

[paul 1950Participant @paul1950]

the bar being over size will not be the problem with the final size just makes it harder work, if you are not using a split die that is the problem.

[John HaineParticipant @johnhaine32865]

Alan, I assume that this is for a feedscrew? I have used M6 stainless studding for this perfectly successfully, and conveniently it has a 1 mm pitch. For a drill grinding jig this should be plenty rigid enough. Easy to tap the female thread, and use brass for the nut as it's easy to tap and also a good bearing surface. A little bit of play doesn't matter, it'll wear that way anyway soon enough. If you are really concerned then add a spring to take up the backlash in the right direction.

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