Making the screw box has been an exercise in frustration. At the end of the last post, I had threaded the hole in the rear block and was trying to make a cutter from a too-small piece of steel. I ended up buying a short length of 1/2" x 1/4" O-1 steel and was able to make the cutter from that.
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| The interior shape is almost complete here |
With the first attempt at a cutter, the interior and exterior facets extended back 1" from the front edge. This time they only go back 1/2". I think this will help in two ways. First, there will be a greater angle at the cutting edge, which will strengthen the edge. Second, the escapement is supposed to cover the entire inside "V" shape and having that shape go back 1" would make the escapement larger than I'd want.
The new cutter was shaped with hacksaw and files. Later it was honed with a triangular stone and a strop. It's a whole different thing trying to sharpen a V-cutter. So far, I'm not satisfied with the overall cutting edge I'm getting.
With this new cutter, I had to deepen the mortise in which it will reside. I made it deep enough so that the bottom of the "V" was in line with the peak of an inside thread in the rear block. In the picture below, I still need to make the mortise a little deeper.
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| Positioning the cutter to get the vertex of the V to line up with a thread |
The workpiece to be threaded has diameter of 2 1/8" and this block started out with 1 3/4" inner diameter. You have to cut away the first thread or two to allow the workpiece to get to the cutter.
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| First thread or two cut away |
But when I insert the piece to be threaded, it doesn't contact the cutting edge!
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| Workpiece inserted, but contacts the cutter halfway along the bevel |
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| Pointer shows where the workpiece hits the cutter |
Per Sir Roy's book, I was aiming to have the leading edge of the cutter roughly lined up with the centerline of the block. It seemed like I needed to move it to the right to get the workpiece to contact the cutting edge. In the pic above, I've already shortened the cutter by 1/8", moving it to the right by that amount. And I'm still contacting the cutter's bevel. It looks like I'll need to move it back more, but then, due to the curvature of the hole, I'll need to move it down a bit to allow it to cut a full thread.
If there is any reader out there who can shed some light on proper positioning here, please leave a comment. I could use some help on this.
Fine Woodworking has an article on making screw thread boxes. It from the early years of the publication. Other than FWW, Roy is the only other source I have ever come across.
ReplyDeleteThanks, Ralph. I've never subscribed to FWW, but I've got Roy's book with this stuff in it. It's just lacking some crucial details.
DeleteIn the Roy Video (https://www.pbs.org/video/woodwrights-shop-screw-box-wooden-threads/ ) he says at about 6'30" that the cutter is canted back.
ReplyDeleteIllustration from the Roubo book:
https://archive.org/details/gri_33125009321973/page/n874/mode/1up?view=theater
The cutter is also canted back.
Arrgh! My replies to your and Paul's comments got lost. Here goes again ...
DeleteIn the book, Roy suggests canting the front edge of the cutter about 10 degrees. In the Woodwright video, he says about 15-20 degrees. I'll look at increasing my angle to see if that helps.
This is the DS Fine Tools cutter. The notch is facing right in the profile view. There's a secondary bevel in the groove that's hard to see - it starts at nothing at the tips and widens to a couple mm at the bottom of the groove. I stopped it with a shaped piece of wood loaded with compound.
ReplyDeletehttps://imgur.com/a/TqSjhnQ
https://imgur.com/a/TqSjhnQ
I have to say...I wish all you guys would get off Blogger. It's a GD mess on mobile devices.
DeleteWas going to say, I think the wings of the cutter need to hit the piece first, lifting away the wood before the bottom of the V can be cleared.
DeleteHello Paul. Yes, that's how I understand it too: the wings of the cutter need to cut before the bottom of the V. Dieter Schmidt's cutter seems to have a MUCH greater angle than what Roy suggests. I'll look into changing the angle. Thanks.
DeleteI thinknwhat'sncausingbthe difference in angle is the fact that the outside of the V isn’t just sloped back the way you have it but is brought back parallel to the sides. I've added a note to the profile view pointing out that the dark area in the photo is ground to a triangular cross section.
DeleteErr,..wait, maybe not. It’s actually the fact that the V profile of the outside is slightly more obtuse than the V of the inside.
DeleteI think I understand what you mean. I wanted my cutter to have a stout angle at the leading edge - no less than 30 degrees. But it's not easy to measure - I was just looking to see if it looked right. I might try making a new cutter with the outside facets ground the way you describe. It might be a while, though - I have to get through a different honey-do project first.
DeleteThe explanation (in old French) is starting here (4 pages):
ReplyDeletehttps://archive.org/details/gri_33125009321973/page/n681/mode/2up
"§. 1. Des Taraux & des Filières en bois à l'usage des Ebénistes"
The iron is canted in such a way that it will first cut the periphery of the dowel; otherwise it will make crumble out of the dowel, especially with soft wood.
Some additional things:
- the iron should be a little ticker than the thread pitch;
- one must ensure the iron will not retreat under pressure;
- the shavings escapement is the same depth as the iron housing (although I don't think it is critical as long as it is at least the same depth);
- when doing a long screw, the box is unscrewed and lubricated with a bloc of soap from time to time;
- if the guiding part is separable, it allows to thread the dowel a little further. Then one can finish the last thread with chisels.
- the starting end of the dowel is slightly tapered to facilitate entry.
Thank you for those additional details, Sylvain. Now I need to get back in the shop and see what I can do.
DeleteYou are on the right track moving the cutter to the right. Don't worry about getting the cutter on the center of the block for your first one.
ReplyDeleteThe major problem is that you have a negative clearance angle. That is compounded by the V profile which widens as you move away from the cutting edge.
Sight at the cutting edge and move the tail of the cutter up and down. You should see the back of the bevel emerge when the clearance goes negative. Another experiment is to carve a V in a flat piece of wood while varying the angle of the cutter. You will notice the heel of the bevel's inability to get in the cut behind the edge.
From the photos, it looks like you are near the right position. Further benefit would come from relieving the bevel and keel of the cutter a bit more. Grinding the point of the V back so the wings lead will help too but you'll need to work the whole bevel to keep your clearance positive.
Look at what carvers do to modify their V gouges from the factory. The keel needs some material removed too.
Steve, it took me a day and several readings of your comment to understand it, but I now think I know what you're saying - at least in the third paragraph. I'm still not sure what you're saying in the fourth paragraph.
DeleteOne thing is for sure, though - I've been trying this on too large a screw. I'm going to work on smaller screws to see if I have more success.
Paul Bouchard said: "I think what's causing the difference in angle is the fact that the outside of the V isn’t just sloped back the way you have it but is brought back parallel to the sides."
ReplyDeleteHave a look at how the cutter is grinded on the Roubo plate (in the lower right corner).
The backing is about 25/26° on the plate.
You could screw your chiseled screw in the threading block first and then put the cutter and see how it comes in contact with the existing screw.
Sylvain, I was just looking more closely at the Roubo drawing last night and noticed the same thing. I think that could give me more clearance when the leading edge of the screw dowel first contacts the cutter.
DeleteAs to your last comment, I was planning to do that, too. Next time I get to work on these screws I'll try these things. Thanks for the comments.