Tapping a large diameter hole in wood
Firast let me start by saying I'm a woodworker, and only have access to woodworking machines.
I have been trying to figure out a way to tap a hole for a wooden screw, for bench screws for vises. The inner diameter is 1 1/2" and the outer is 2 1/2".
The hole is 3" deep and the piece of wood its in is 6' long. The wood is hard maple.
And I have to make 40 of these holes for workbenches for my woodworking club. (10 benches, 4 vises each)
For 1 or two holes I could make a tap by inserting a cutter in a wooden screw and slowly scraping it out but I'm looking for an easier way.
Does anyone know of a method or where I can buy a tap this size?
It would seem that a small high speed cutter, ala router bit for lack of better analogy would be the tool of choice to efficiently create the threadform.
However, getting the rotating cutter to traverse a helical path with the proper lead is another, greater challenge. It could be affixed to a metallic screw with the same thread pitch, with hopefully some lateral adjustment for the "depth of cut". Keeping everything aligned and concentric is another issue to be resolved.
This isn't a step-by-step solution, but a concept which might be worth developing for 40 pcs.
Matt, that is what I was thinking. Mount a low profile cross slide to a face plate on lathe, and mount an electric or air router or die grinder to that. Do whatever necessary to keep overhangs short, and rigid. Don't chintz, "even though this is only wood".
Or, hold the tool (router) in a large 4 jaw chuck, set to cut the desired diameter. Just be sure all is foolproof secure.
Then mount a vise or angle plate fixture to the cross slide or compound to hold the length of wood at the desired center height.
Select the gears for desired lead. Fire up the router, and feed it in by slowly turning the lathe over by hand. Be aware of the cord or hose, and have some method of conveniently winding it up and unwinding as the tool is backed out.
The cross slide allows fine tuning for diameter.
Or reverse the process and chuck the wooden piece up in a four jaw chuck and put the router onto the tool post. Then you don't have to deal with the cord rotating.
my simple 3 step process
step 1: find buddy with cnc mill.
step 2: do something nice for him.
step 3: let him helical interpolate the threads on a mill. in a few seconds a part. bam, done.
I'm assuming, from your description, that you're trying to tap the hole -- that you already have the wooden screws. Not quite sure I fully understand Matt and Stephen's ideas, but it would seem a challenge to get a rotating cutter inside the 1.5" clearance you have. Not many high speed right angle heads that will allow you the depth of cut you need to tap the threads using a rotating cutter inside the bore.
Another approach might be to make a 2 part casting of the thread form you need using a wooden screw as the master -- sort of like half nuts on a metal lathe. There are probably reinfforced epoxy casting compounds that would be strong enough and with a bit more work this could be done in aluminum or another alloy. Just a thought . . . --
you can make a tap like this . Turn a wooden screw so it has a taper like a number 1 tap. rout 3 longitudnal grooves similar to the flutes on a tap a good 3/8 deeper than the threads. Now epoxy 3 strips of steel on to the leading edge of the grooves. THe strips are the depth of the routed groove, with a die grinder grind the profile of the threads into the steel strips.
Originally Posted by TTalma
Mount the wooden piece to the end of a screw that you want to duplicate the thread for. The screw is supported by a threaded nut or block, so when you turn the screw it advances at the pitch you want to duplicate. Use a router with either a thread mill (of appropriate form) or a keyseat cutter, for a box thread. AB Tools (www.abtoolsinc.com) makes precision shank extenders that you can use to hold the cutter in a router. The router is fixtured so you can mill a thread in the hole wall as the screw advances back and forth. You can probably do it in one pass.
Quote: "Or reverse the process and chuck the wooden piece up in a four jaw chuck and put the router onto the tool post. Then you don't have to deal with the cord rotating"
That's a pretty good sized lathe: he said the work piece is 6' long & the hole is probably toward one end, thinking of wood vises. But maybe I misunderstood. There is some clear information lacking.
If you can shorten the work piece to clear the overarm on a wide pin router Perry's idea would work slick as can be and very fast. Heavy face place mounted to heavy metal screw (acme allthread & nut from MSC or similar).
Why not simply purchase the correct tap from the
vise manufacturer? Then use that to tap the hole.
Um, this is triple-start square thread form, right?
11-3/4 threads per inch, I think that's the standard?
Check out this site for a wood tap, I have used these for wood screw nut cracker and bench screws for years.
Beall Tool Co. sells apparatus for making wooden threads, male & female. You would be amortizing the cost of the tools over many benches, so it might be feasible
Or, you could use more traditional methods. One of the Woodwright books by Roy Underhill shows how to make and use the traditional tools for making make and female wooden threads. (The titles are "The Woodwright's Shop", "Woodwright's Workbook" etc. try Amazon or B&N on an author search for Roy Underhill.)
There must be a good traditional way to do this, as decent-looking wooden threads are found on many pre-industrial-age items.
Here is a set of very large taps being offered on ebay.
Use the largest of the bunch to tap the wood... then take the tap to a machine shop and ask them to make a hole in a suitable chunk of aluminum and tap it using your tap to create a die for creating the wooden "screws" to match. That way you would have the ULTRA-tap and die set for future benches or repairs...
Perhaps a local machine shop may have a large tap like this they might let you use on wood since wood couldn't hurt it...
If there is a VO-Tech that teaches machine shop... maybe you could get an instructor to use your problem as a class project and have a large "tap" and a "die" of the appropriate size created out of Cold Rolled Steel that you supply... it wouldn't even have to be hardened to work on wood.
BTW, I am not sure I understand your measurements... 1.5" inner diameter and 2.5" outer diameter? does that mean that your thread depth is an INCH? please explain.
Here is another ebay tap that might work for you:
Look here. 1 1/2 inch thread box and through and bottom taps, not in stock but due in August, 37.99.
Now that will be 1 1/2 inch OD thread. I don't know if that is good enough for you, but with hard maple, and is this a "leg vise", it should be all you need.
Metalworking taps work really well in wood. 1 1/2" x 6tpi taps are available from J&L and ENCO.
ACME taps are available from MSC. The ACME threads are 1 1/2" x 4 tpi.
If you are really handy, you could get some drill rod and forge and harden your own tap but it looks like 1 1/2 drill rod runs about $75.00
WOW, Thanks for all the great advice.
To clarify, the holes would be drilled on one end of the 6' board, the center would be 8" from one end, and the board is 5" tall, 3" thick.
I have to agree that finding someone with a cnc mill would be our best bet. I never thought of that as an option. I have sent an email to all the members (400+) asking them to see if they know anyone with a mill willing to help. If no members have access to a mill, I will try bidding it out.
These benches are for our woodworking club we "rent" the space from another non profit the "hand up group" that teaches special needs people trades. Our rent is these benches and the maintanice of thier equiptment. A good deal for both groups.
So far all of the materials and labor has been volenteered. Our budget for the entire project is $600, $250 of it is a home depot gift card (thanks HD). And that isn't alot since all we have is the lumber.
Back to the benches. The reason we decided to use wooden screws instead of metal is simple, it's easier on the tools. The special needs people using these benches won't be very gentle with the equiptment, and since we are maintaining it, we're trying to avoid tool damage.
The reason we picked a large diameter screw is from members experience and what we have seen in the real world. The 2 1/2 screws seem to hold up fine to lots of abuse. One member has an old bench and the screw is missing large chunks from the teeth, and it doesn't affect the performance. The 1 1/2" screws like those offered by the beal company work well, but lack the support strength we will need (this is based on members experiance).
We're mostly following a design from a bench used daily at a local living history muesem. The date on the bench is 1844 and we figure if it's held up that long it will work for us.
And last but not least this bench was designed by commity and we all know how that goes. I voulentered to head this group, and no good deed goes unpunished.
Thank you for all the ideas, and please keep them comming.
Also could someone explain how to read the tap info such as the ones referenced by the ebay links. Our screws would be 2 threads per inch and the thread would be 1/2" deep. how would that look on a chart or something like that.
(please ignore my bad spelling)
We make wooden bench screws all the time for the workbenches in our historic area. We have a German tap and die set that was here when I came in 1970.It is about 2 1/2" dia.,and 3 threads per inch..I saw a large tap and metal die set on ebay a while back,but it wasn't cheap. A tip: soak your wood screws in mineral oil real well before cutting them,and they will tear less. The die still has to be very sharp,and properly adjusted. Since the proper thread angle for wood threads (V) shape is 90*,It is easy to cut beautiful threads with a router fed in at 45*. The hole is your problem,I know. For what I need,I just make a tap,if I don't have one. I make the kind that has a hole bored through the end of the first thread.The end of the tap is turned to the I.D. of the thread. The tap is bored out hollow.The chip goes down the hole,and into the hollow tap. Large taps like yours need a 2 stage cutter. One does the bottom half of the thread,and the top half is done by the next hole following along behind. There are ways to thread holes with a wooden tap with a steel tooth.It gradually scrapes its thread out. You need to study a folk culture book to see how to make one. they are slow to use,though,when you are making so many parts.
Is this the Rochester group? I know at least one of the members there/former president owns a factory full of screw machines & VMC's (Rol-Tru). Besides theri primary business, he is an avid woodworker, wood tool developer & all around good guy.