Thursday, March 4, 2021

How I Do Full Tang Bolster in Stainless Steel

A picture tutorial on shaping and pinning two-part bolsters on a full tang blade. 

The bolster shape that I chose for the KN48-5 was a shape in my bolster patterns

Starting with some flattened 3/8" 416 stainless steel bar stock I rough cut the basic shape on the band saw. With some layout dye and a scribe I mark the final shape on the steel. Then bring them close to the line on the belt grinder. 

I use the tang bolster pin holes in the tang to drill and check the pin alignment. I cut  two pieces of  plain old 1/8" steel rod as temporary pins to hold the two bolster piece together while they are shaped. 

Here I used a 2" diameter wheel to shape the backs of the bolster pieces. Keep an eye on squareness and symmetry. 

Using the 45° table, I cut the bevels in the bolster. I have to careful to preserve symmetry, so I alternate left side and right side. 
Yes, I am wearing band-aids as I feel they give better grip when wet and reduce the pain on the fingers from heat build up. This little block of steel will get hot. Have a bucket of water handy. 

Once the bevels are worked to 120 grit, I will put them on some flat sand paper and lightly work each side to remove any lateral scratches. I will sand to 220 and 320 in this manner. We want no visible large  scratches.

After 400 or 600 grit I go the buffer with green compound. Only polish the fronts. The rest of the faces will get finished when the scales are on.

I cut the pins to be 1/4" longer than the the tang and bolster pieces all added up. In this case, I make them stick out 1/8" on each side. For these pins I will use the real 416 stainless pin stock. It's important to use the same material as the bolster pieces as this helps hide the pins. 

Pins are given a quick rough up with 220 grit sandpaper and bolster pieces get soaked washed in acetone. 

I make sure the insides of the holes are cleaned as well. I cannot stress enough how important clean pins and holes are. Clean the ricasso area of the blade as well. No grease and no grit allowed. 

I dry the parts and assemble the bolster and give the pins a little peening with a hammer to keep them from slipping in the holes. I carefully place four #6 flat washers and head over to the hydraulic press. 

The press squashes the pins a little first then it hits the washers. These act to press the bolster together and hard against the tang. I crank it a full 20 tons or whatever the max a "20 ton" hydraulic press will do. 

The result is the pins are expanded and the bolster pieces are super tight to the tang. If everything is flat you'll have nice lines around the pieces with no gaps. 

A quick zip on the belt grinder and the pins dissolve into the bolster. 

From this step, I will fit the scales and liners on. After glue-up I will shape the bolsters round on the top and bottom to fit the fingers.



Tim said...

Do you taper the holes at all? I've been trying to get my bolster pinning right when making slip joints and using a 7 degree taper reamer, peening with a hammer with mixed results. I've very curious now about how this would go in a press.

D. Comeau said...

Hi Tim.

No I don't taper the holes. The expansion of the pin pushes outwards and completely fills the hole. The value of tapering the holes is to apply some force to bring the bolster pieces together. With this hydraulic press method the compressive force is applied before the pins start to expand. That's the idea behind the little washers.

One might use a 1/8" straight reamer to clean the holes in the bolster pieces. Spin it through with some acetone and wipe the ream with a clean shop towel; repeat. Most important is sanding and cleaning everything. A small amount of left over cutting oil in the holes or mill scale on the pins will reveal an unwanted black ring. Shine up the pins, clean with acetone. Clean the holes and you should be good to go.

Also, dissimilar metals can produce rings. Example using 304 pins on 416 bolster pieces, or brass pins and bolster pieces made from two different kinds of brass. I once made a 416 bolster with 304 pins and the pins would randomly appear as circles and disappear when wiped a finger. Kind of like ghost pins, neat but not what I wanted.

Hope this helps,