# DIY Articulating Arm Hollower



## Thorn495 (Feb 28, 2014)

I thought I'd give it a try making one. Using a handheld Sorby hollower or the easy hollower was pretty fatiguing after awhile trying to control the tool with so much of the shaft off the tool rest.

This articulating arm was cheap to build ($20-30) from stuff at Home Depot. The only trouble I've been having with it is threading a hole with a tap in order to use a screw to hold the cutting bit in place. After breaking 2 taps I gave up and used a small pair of vise grips to demo the tool. I guess the taps I have are pretty cheap. I drilled holes 1/16" smaller than the tap and they still broke off inside.

Any other ideas on how to hold the cutting bits in it? Maybe there's an arm/shaft that comes ready made with a 1/4" hole for bits and some set screws I can find somewhere.


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## FrankC (Aug 24, 2012)

Are you using taping fluid, if it is steel ordinary lard works well, there are charts that give the size of bit to use, 1/16" means nothing without size and TPI of tap. An easy way to check drill size is find one that is a good fit in a nut, or just a bit larger. Turn 1/4 turn, then back up to break chips, then another 1/4 turn, back up.


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## Bill Boehme (Feb 9, 2014)

If you do a web search, you can find charts that give the correct drill size for a tap depending on the material that you are tapping. Generally, number size drills are used for holes that will be tapped. Unless you are tapping a really large hole, 1/16" undersized is way too small. It might also be that the bar that you are tapping has been hardened and, if so, you will need to anneal it before tapping.

Finally, all the hollowing tools that I have do not use a set screw. The cutter is just cemented into the hole with thick CA. That should be all that is necessary because the cutter should be a very snug fit in the hole anyway.

As you have discovered, an overhang of about 9 inches is just about the limit for handheld hollowing. Beyond that, the balance tips in favor of the lathe.


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## Thorn495 (Feb 28, 2014)

I wasn't aware of tapping fluid. Screwing around, I tried clamping a handheld tool in it. The results turned out better and easier than if I did it by hand. Now I'm thinking of making a round wooden clamp to hold the butt of my hollowing tools. The arm jig keeps it all pretty stable.


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## hwebb99 (Nov 27, 2012)

I haven't ever used "tapping fluid". A heavy weight oil workers good for me. You have to frequently back out the tap. Not completely out just about a turn backwards. I think your biggest problem is too small a hole. If you are tapping 1/4-20 use a 13/64 fractional drill bit or a 7 numbered bit. If you were able to drill the rod without difficulty I doubt it is hardened. Since you bought it at a hardware store reinforces my doubt that it it is hardened.


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## Bill Boehme (Feb 9, 2014)

For a small bowl like you have pictured, it would be easier to just use a bowl gouge and then a finishing scraper. The Sorby hollowing tool you show is going to leave a pretty rough surface and it is best used for hollow forms with small openings where the inside surface smoothness isn't very important.


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## Thorn495 (Feb 28, 2014)

Bill Boehme said:


> For a small bowl like you have pictured, it would be easier to just use a bowl gouge and then a finishing scraper


It was just a test piece to see how the tool would cut and behave in the arm jig. It actually cut pretty smooth in the jig

I'll have to keep the tap info in mind the next time I attempt to tap some holes.


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## 44260 (Aug 29, 2013)

The drill bit size is sometimes stamped or marked right on the tap under the tap size


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## hwebb99 (Nov 27, 2012)

dirty-curty said:


> The drill bit size is sometimes stamped or marked right on the tap under the tap size


 That is convenient, but hardly ever the case. Even then it is probably not in a measurement system you have drill bits for.


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## epicfail48 (Mar 27, 2014)

The internet is your friend my, erm, friend...
http://www.tedvisaya.com/images/Drill-Tap.jpg

Print that out, slap it somewhere in the shop and never drill the wrong size tap hole again. According to that chart, a 1/4 tap needs a #7 drill hole, roughly 13/64 in fractional sizes. Going 1/16 smaller than the tap is, as you found out, a great way to break a tap. 

Next time you need a new tap though, id recommend picking up one of the kits with the drill included, like this:
http://www.lowes.com/pd_62223-281-1765538_1z13e72__?productId=3732945&pl=1&Ntt=tap

So, so very handy if you dont have a collection of lettered bits


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## Thorn495 (Feb 28, 2014)

Thanks for the chart. Hmm, the tap I used (and broke) was 5/16-18. The chart seems to say that I need a drill that's .2570" or .29"? A 1/4" bit seems close to that or am I reading it wrong?


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## hwebb99 (Nov 27, 2012)

If I'm not mistaken a 1/4" bit would be .4" too small. The specified hole size is already tight. I wouldn't be surprised if .4" smaller caused your tap to break.


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## Bill Boehme (Feb 9, 2014)

Thorn495 said:


> Thanks for the chart. Hmm, the tap I used (and broke) was 5/16-18. The chart seems to say that I need a drill that's .2570" or .29"? A 1/4" bit seems close to that or am I reading it wrong?


You're reading it wrong. For tapping steel, you want 50% thread engagement which requires a letter size J drill which is 0.2770".

The 0.2570" that you referenced is for 75% thread engagement in soft metals (aluminum or brass) and plastic. And, "close" only counts in horseshoes and hand grenades. Going even smaller than 75% is a guaranteed way to break off a tap ... but I guess that you already know that. 

I don't know what the 0.29" inch that you referenced is referring to.

BTW, I think that your screw is much too large. You can actually get better holding by going to an 8-32 set screw because the finer pitch thread gives you much greater mechanical advantage than the 18 TPI screw.

Even better ... use an interference-fit hole and set with thick CA. When it is time to replace the cutter, heat with a propane torch and the CA will melt and then use Vise Grips to pull the old cutter out.


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## saculnhoj (May 18, 2015)

There's one other thing that is just as important. You didn't say what you were using to turn the tap. If you apply any sideways pressure on a tap you will break it. Use a good tap handle and not a wrench and be very careful to keep the pressure straight down. 
If you use a hole that is too small simply take very short cuts, say 1/4 of a turn or less before backing the cutter up. I've never had a lettered set of drills so I have only used fractional drills to drill the pilot hole for taps over 1/4". Never had a problem so far. The vast majority of taps I've broken are 6/32. don't know why that size because i tap holes all the way down to 4/40 and smaller. 
Kerosene mixed with oil makes a good tapping fluid but I have used straight oil without problems.


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## Thorn495 (Feb 28, 2014)

Alright, I replaced the tap and tried it again with success. Thanks for the help. The arm's been working pretty well. I was thinking of trying to do this with it: 

https://www.youtube.com/watch?v=TyN-RZOPB4M


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## Ron Rutter (Jan 18, 2011)

Using a starter tap, backing up frequently, with the correct size predrill & fluid works great!! ( assuming you have a through hole!)


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## pburri (Jan 17, 2014)

*drill sizes for taps*

All taps have recommended size drills to be used for them. You can probably find a list on the internet. If not, tell me what size tap you're using and I'll tell you what size drill to use.

for example: 1/4-20 tap uses a #7 drill.


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## tom brouillette (Jan 15, 2015)

Your rig looks good. I wish I had the time and equipment to do it. I just bought a Hollow Fast tool system, which has been working very well for me. Based on my limited experience with my system, the only recommendation I would make is to add another arm, making it double jointed. Think about finishing the bottom of a conventional vessel. In order to "sweep" the bottom you must be able to move the tool back and forth without it going in and out. Does that make sense? It also makes it easeir to reverse turn, which I did this weekend for the first time. You seem pretty handy with tools, this would be easy-peasy.


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## hwebb99 (Nov 27, 2012)

tom brouillette said:


> Your rig looks good. I wish I had the time and equipment to do it. I just bought a Hollow Fast tool system, which has been working very well for me. Based on my limited experience with my system, the only recommendation I would make is to add another arm, making it double jointed. Think about finishing the bottom of a conventional vessel. In order to "sweep" the bottom you must be able to move the tool back and forth without it going in and out. Does that make sense? It also makes it easeir to reverse turn, which I did this weekend for the first time. You seem pretty handy with tools, this would be easy-peasy.


 There are no special metal working tools required to make one of these. A simple woodworking drill press and a hacksaw would be sufficient. A jigsaw or sawzall with a metal blade would be faster than the hacksaw.


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## tom brouillette (Jan 15, 2015)

The set I just bought is pretty tight. I'm not sure I could have maintained those tolerances without a machine shop, and I have an industrial maintenance background.


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## hwebb99 (Nov 27, 2012)

tom brouillette said:


> The set I just bought is pretty tight. I'm not sure I could have maintained those tolerances without a machine shop, and I have an industrial maintenance background.


 I'm sure that a quality store bought one has tight tolerances. If it didn't I would be pissed, but I think a homemade one would work good enough.


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## Thorn495 (Feb 28, 2014)

Well, I haven't really messed around with it much since the day I made it. I got a steady rest to help keep pieces from bending/vibrating so much and the handled hollowing tools I have have been behaving much better. I guess I'm getting better at working with them. I found that faster RPM's (as long as the lathe isn't moving) and lighter cuts are all it takes. I was being too aggressive before and the wood was moving all over the place making the tool kick back at me, so I made the arm, lol. I might add a few more joints and mess around with it again someday. I've been busy with Christmas projects lately.


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