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Altering a tool shape

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spike7638

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#1
I'd like to take a plug-cutter (a tool for making wooden plugs that then are inserted into holes to cover up fasteners) and slightly alter it. Here's a picture of a plug-cutter set, so that you get the idea:

upload_2017-8-17_12-8-3.png
The resulting plugs --- created by plunging the cutter into the face of a board, and then bandsawing the board to make the plugs call out --- are more or less cylindrical. The fancier plug-cutters actually make plugs that are slightly chamfered on the very top (if you plunge deep enough).

I need some plugs that are "half-inch oversized", i.e., the plug has a diameter of about 0.53 inches. In fact, I'd really like to make a plug that tapers from 0.55 down to about 0.51. (Some plug cutters already have taper, but none are quite this size, alas).

I thought that perhaps I could start with a plug-cutter like the one above, and on a lathe, remove some of the material inside using the compound -- is that the right term? --- to produce the taper, and something like a boring bar to hold a bit that'd do the cutting.

But I expect that the cutter is made of tool steel and very hard, so maybe this idea is just stupid. Or maybe it needs to be ground (which is probably beyond my skills). Or maybe I should start from scratch and make something that does the job well enough to produce 8 or 10 plugs, which is all I need.

Can anyone give me advice on the feasibility of the "modify an existing cutter" approach, and how I could go about it?
 

francist

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#2
The plug cutters that I have like that are very hard -- too hard to modify on the lathe as is I think. However, they are not High Speed Steel which suggests that they could be softened enough to to be cut with your boring bar. If you don't need many you may get away with not even having to harden the cutter again after you've modified it.

Grinding is the other option that comes to my mind, but finding the appropriate sized and shaped point could be a pain. On the other hand, might be less effort in the long run than going the heat treatment route. An inexpensive grinding point could be dressed into a slight taper easily enough.

-frank
 

JimDawson

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#3
Depending on what equipment you have, making a tool post grinder would be my first choice.

Here is how I did mine. Air die grinder from Harbor Freight.

 

RJSakowski

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#7
Turning or grinding an interrupted cut can cause a rounding over of the cutting edge. This will prevent the plug cutter from cutting properly. Whenever I have tried a modification like this, I take a light cut and have the cutting edge be the trailing rather than leading edge. Grinding would be preferable to turning or boring.
 

spike7638

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#8
Thanks, everyone, for the ideas. I'm now wondering whether a die-grinder might do the job, with the original cutter chucked in my drill-press (which I can do at home, rather than setting up something messy like this in the shop at work). I'll have to experiment.

The "use the lathe to reduce a larger plug" idea is a nice one, and I could cut some 5/8 plugs and try this...but as Frank points out, the grain runs perpendicular to the lathe axis, and this leads to three problems: (1) irregular cuts producing elliptical cross-sections, (2) a tendency to "grab" at two points in the cycle, and (3) a tendency to split along the grain. (Indeed, in teak, which is the wood I'm using, this along-the-grain splitting even happens sometimes with a top-notch plug-cutter.

Then again, starting from a 5/8 plug and doing something like sanding ... perhaps a dremel hooked up to counter-rotate and sand as the drill press is turning the plug .... could conceivably work. I guess it's time to go back and noodle a bit more.
 

RJSakowski

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#9
I had to turn some mushroom shaped plugs a couple of years ago. The grain would chip out or if I tried to remove the plug (they were meant to be removable), the lip would break off. I saturated the blanks with super glue and let them cure overnight. This gave me a much stronger plug. They were roughed out on the lathe and the cap machined using the CNC mill with a ball end mill, followed by sanding in the lathe.
Newel Post Plug.JPG
 

brino

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#10
My first choice would be what @Groundhog suggests and simply turn the part you need rather than a custom cutter to make a part.

My second choice would be to forget trying to modify a hardened, commercial plug cutter and make your own.
Since you need less than a dozen, even unhardened steel should work.
Maybe start with a piece of pipe with an ID that meets the minimum diameter, and cut/grind the taper to suit.
Then using a grinder/dremel cut the gaps for the cutter teeth.

Good Luck and be safe!
Please let us know how you make out.

-brino
 

T Bredehoft

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#11
Might I suggest your plugs finish at 5.08, not 5.3. Point 3 inches is quite a bit to drive into a .500 hole. You probably would have trouble starting a .510 teak plug in a .500 teak hole. You'd have to hammer it in and the wood would probably split in the wrong place.
 

spike7638

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#12
I appreciate your concern, Tom, but it's misplaced here.

The problem is that the holes into which these plugs go have had multiple plugs installed and removed over the last 47 (!) years, so they're actually considerably rounded out and enlarged. I measured the diameters with calipers, and the largest was a bit over .52, so 5.08 wouldn't really do the job; fortunately, they're mostly reasonably round, despite the enlargement. I have to assume that prior pluggings did something like using epoxy to set the plugs, and then they were drawn out, they took some of the surrounding material with them.

In general, the trick with tapered bungs in teak is to tap them in until they're JUST snug, carefully lining up the grain so that when the wood expands/contracts, nothing bad happens, and then chisel off the top, and finally either plane or sand them to match the underlying wood's profile. If you dampen them with just a little varnish on the way in, they seal up quite nicely. I actually know my wood and my goals pretty well --- it's the machining that I needed advice on.
 

spike7638

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#13
My first choice would be what @Groundhog suggests and simply turn the part you need rather than a custom cutter to make a part.

My second choice would be to forget trying to modify a hardened, commercial plug cutter and make your own.
Since you need less than a dozen, even unhardened steel should work.
Maybe start with a piece of pipe with an ID that meets the minimum diameter, and cut/grind the taper to suit.
Then using a grinder/dremel cut the gaps for the cutter teeth.

Good Luck and be safe!
Please let us know how you make out.

-brino
We're thinking along the same lines, Brino -- the first thing I did was look around for a pipe with the right ID...but no joy (at least not in my basement!). Of course, starting with a pipe, I still have to devise a way to chuck it into the drill-press. I've got a long drive to do tonight, so I'll muse a little more...
 

Ulma Doctor

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#14
turn a pipe of the desired dimension from a single piece of stock.
reverse the stock in a 4 jaw chuck and indicate true.
turn the drive end of the newly formed blank to a dimension acceptable for your drillpress
relieve and create the cutting edges on the bench or wherever you can
if you make it from O1 you can harden it
 

WoodBee

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#15
I agree with Ulma Doctor. I have made several in the past from quick and dirty to all out out of silver steel and hardened for larger batches of plugs. If you turn a slight taper on the inside of the tool it promotes a close fit, and you can compensate for slight size differences by choosing which part of the plug you use (if made "extra long")
The wall thickness of the tool does not need to be thin. In my experience this only leads to quick deformation and dulling of the teeth.
I have also made comparable tools to cut broken screws out of wood and GRP ( inside hole to just clear the screw). To minimize hole size I used a thin wall thickness but these needed touching up of the teeth a lot.( Luckily I usually don't need to a lot of these in the same job....)
Peter
 

Ken from ontario

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#17
If altering an existing plug cutter dosen't work Iwould try the next size up in metric , something like a 14mm plug cutter, it is 0.55".
 

spike7638

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#18
I finally made my plug-cutter. I drilled out a piece of rod, turned down the OD a bit, then bored it at about a 2.5 degree angle, put a bevel on the cutting end, made a shaft on the other end, and used a collet-block to hold it while cutting four slightly-angled grooves to make cutters. I screwed up the first one by not offsetting it far enough, but the other three came out OK, and I used it to cut a teak plug that came out just as I had hoped, although it turns out I should have used a 1 or 1.5 degree taper --- what I've got is a bit too extreme. But the whole job was surprisingly easier than I'd anticipated, which was great.

Thanks to everyone for the suggestions, etc. It didn't turn out pretty, but it's pretty enough, and I can make a second one a lot faster and better. Here's a picture of the result.


IMG_20180426_222336.jpg
 
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