Noob questions on milling scales

[Isley]

So, I have my blade profiles modeled (just x,y axis right now) and naturally plan to model the handle scales on top of the blade profile models. The order of operations would be; Water jet the blades, heat treat the blades, mill the handles. In my standard all in house process, if I want to use 3/32 pins I drill 3/32 holes in an arrangement that makes sure everybody lines up and I wind up with a nice tight marriage between the blade and handle such that the epoxy is just there as a sealant. I'm a little concerned that any dimensional changes resulting from heat treat might slightly corrupt the alignment of my pin holes through the milled handles. Thus, oversize my blade holes by a few thousanths? This will be my first semi-production run so any advice and experience in this process is appreciated. I can build the models and have some familiarity with the challenges of the milling process, but beyond that I don't know what I don't know, so feel free to add to my knowledge.

 

[DevinT]

Size change is different for every steel and the heat treatment can greatly affect the growth. Austenitizing temperature, cryo, tempering temperature, soak time all affect the amount of growth.

Hoss

 

[argel55]

Hard to put a 3/32 pin in a 3/32 hole. Pin material is not exact.

 

[Isley]

Size change is different for every steel and the heat treatment can greatly affect the growth. Austenitizing temperature, cryo, tempering temperature, soak time all affect the amount of growth.

Hoss

Right I get that, so how is this dealt with by larger manufacturers and other individuals who mill, because they definitely do. I could turn 3/32" blade holes into 1/8" and I'm confident that would be a safe range but I'd still be curious as to what measures I can employ (outside of a PhD in materials engineering) to get the best possible mate between handle and blade including holes and profile accuracy.

 

[DevinT]

Right I get that, so how is this dealt with by larger manufacturers and other individuals who mill, because they definitely do. I could turn 3/32" blade holes into 1/8" and I'm confident that would be a safe range but I'd still be curious as to what measures I can employ (outside of a PhD in materials engineering) to get the best possible mate between handle and blade including holes and profile accuracy.

Need more information, is there a guard or bolster? Full tang? What steel? How many holes?

You can measure size change and adjust accordingly. You’re over thinking this.

Hoss

 

[Isley]

Need more information, is there a guard or bolster? Full tang? What steel? How many holes?

You can measure size change and adjust accordingly. You’re over thinking this.

Hoss

Me? Over think things? Never. Full tang, 14C28N, many of my profiles are set up so that I can either use 5 or 7 3/32 perimeter pins or 3 Corby type rivets - each with an additional 1/4" thong hole. I could also use counter bored hex bolts but haven't considered them until I began brainstorming milling as an option. I also wondered about the accuracy of the water jet blade profile vs the same handle profile when milled. I plan on doing some hand finishing for sure but I guess I'm just trying to pin down the level of precision I can expect.

 

[Isley]

Hard to put a 3/32 pin in a 3/32 hole. Pin material is not exact.

I'm aware. Plus micarta tends to shrink up after the drill exits. So I usually knock off a few thou by sanding it with the drill in reverse which has the effect of essentially creating a thread on the pin stock, which I then use to advantage and drill it into the handle holes with the drill running clockwise. Kind of screws the pin stock through the holes.

 

[HSC ///]

I get that you are trying to dial everything in and plan well... but you are likely going to do a prototype run and have to make adjustments anyway

as you know, you can see and feel a .001 gap or mismatch

I think you will learn everything you need to know in the first five or 10 blades that you set up and Mill and have heat treated.

almost every maker including production makers make their knives different and produce differently. Everyone handles the same problem a different way.

 

[TILLER]

You are going to need to do some hand/grinder work to mate up the scales to the tang. I don’t think you can know ahead of time what that will be, precisely. Could be a few thousandths here, .01 there, just depends.

I machine my blades (more accurate than water jet) and my handles and I still follow up with some amount of finish grinding.

One thing I’ve seen to avoid having to mate up scales and tangs with finish work is to recess the scales so that the tang is proud of the scales. Of course then you’d have to dial in a chamfer on the tang.

 

[C_Claycomb]

If you are gluing, and the scales are machined finished size, cleaning any squeeze out off the handle, and not having it leave a darker mark, can be a PITA. Trying to have a lot of small pin holes all close fitting is asking for trouble. I think you want two pins for position and the rest for shear/peel resistance. The latter does not require the tang hole to be such a close fit to the pin. I use a combination of metric and imperial drills to dial in pin fit, metric drills here can be had in 0.1mm diameter increments. Maybe you can get good quality number drills where you are? They are not so widely available here, certainly not good ones. If you are milling the scales I am guessing you will be generating some of the holes with a mill, rather than drilling them all? I would probably generate the two alignment holes, then use those to position a template to drill the rest. Not as accurate as generating all of them, but 3/32 might be small to go generate through 3/8 Micarta seven times. Could you mill a drilling template?

If you can do double sided machining you can have hidden pins, which can make finishing handles easier. The other thing that double sided milling opens up is hidden tangs that retain the side view shape of full tangs, but with significantly less mass, and no bare steel to be cold in the hand, and only one glue join between identical materials, rather than two lines between steel and Micarta.

Best of luck!

Chris

 

[Nathan the Machinist]

My tang holes in the steel are .250-.251, typically .2503-.2507 measured by Deltronic pin gauges.
Same with the scales, but it's not meaningful to measure micarta to 10ths. But typically a light friction fit to a ground dowel pin.

My pins are turned .2490-.2505, which means there is occasionally a light press fit.

Some push in by hand, some need a little tap but none need a press.

I've made many thousands of these without a problem. The blades grow enough in heat treat they won't go back onto fixtures pined by those holes. I think the holes probably move around a few thou. But the scales still fit fine because the micarta has enough give to it to accommodate a tiny amount of movement.

I do it this way because I don't want play or mismatch. So everything is tight, and hole location is tightly controlled. This seems like a pain in the ass, but the additional time spent making accurate holes simplifies everything else in the process so it is worth it to me. We never oversize holes or undersize pins to ease assembly and after a little bit of a learning curve doing it this way we've never had any problems with it. We don't use epoxy except sometimes to bond scales material to a liner.

 

[Isley]

My tang holes in the steel are .250-.251, typically .2503-.2507 measured by Deltronic pin gauges.
Same with the scales, but it's not meaningful to measure micarta to 10ths. But typically a light friction fit to a ground dowel pin.

My pins are turned .2490-.2505, which means there is occasionally a light press fit.

Some push in by hand, some need a little tap but none need a press.

I've made many thousands of these without a problem. The blades grow enough in heat treat they won't go back onto fixtures pined by those holes. I think the holes probably move around a few thou. But the scales still fit fine because the micarta has enough give to it to accommodate a tiny amount of movement.

I do it this way because I don't want play or mismatch. So everything is tight, and hole location is tightly controlled. This seems like a pain in the ass, but the additional time spent making accurate holes simplifies everything else in the process so it is worth it to me. We never oversize holes or undersize pins to ease assembly and after a little bit of a learning curve doing it this way we've never had any problems with it. We don't use epoxy except sometimes to bond scales material to a liner.

Thank you. Since my blade profiles will be water jetted, I'm being told by another source that I should consider over sizing my profiles by .025 for non taper control or .005 with taper control. This oversize could be cleaned up by a mill or in my case by hand. Does that sound about right? Additionally, would you reccomend a 6/32 stand off as a standard way to connect the scales? Seems I see that alot.

 

[C_Claycomb]

For what it is worth, I think 10-32UNF would be better than 6-32UNC, if the screws are the primary fixing. I intend to use something like that if and when I get around to producing my CNC machined handles.