Step Block Angle for Shop Made Strap Clamps

[SightlessSeer]

I'm in the process of making a set of strap clamps and I've come to the point where I need to cut the serrations in the back side where the clamps engage the step blocks. In commercially made clamps I see that the steps are angled so they lean back and pull the step block into the clamp (see picture below)



I'm wondering how important that angle? It would be easier to make if they were just at 90° because then I could just use an end mill from the top. If they are important does anybody have a suggestion on how to cut it? I'm thinking maybe a dovetail cutter held in the horizontal spindle of my mill, but I would have to go out and buy one. Is there a better way to do it?

Another Idea I had was to use a normal end mill in my vertical spindle but tilt the head to create the lean back on the steps. I think that would work.

I roughly sketched up all three of ideas. Take all dimensions with a heap of salt. I was going to a visual comparison, not the design of the actual part.



Furthest left is closest to a commercial clamp and I think I would do it with a dovetail cutter, middle would be an end mill with no tilt to the head, and right is an endmill with the head tilted.

The Mk. I eyeball says that a 20° lean back angle looks good, but If I were to buy a dovetail cutter it would be a 60° one so I can use it elsewhere.

What do you guys think? I'd appreciate a second opinion.

 

[mmcmdl]

Use a fly cutter . How many you making ?

 

[GrifterGuru]

the steps are angled so they lean back and pull the step block into the clamp

@SightlessSeer You have answered your own question with the section of text I have quoted.

When the clamps are tightened down to the blocks, they pull into the blocks as well as down. This keying together stops the strap clamp from slipping off the block, as well as the block slipping from under the clamp. They are designed as they are so that they do not fail (id-est slip apart) under the forces imparted during machining, which can, at times, be far greater than you would think they would be.

If you were to make your own, I would suggest you look at using a D/T cutter as you suggest so that you maintain the relationship as per commercial Strap and block sets and the integrated safety that design holds.

 

[GrifterGuru]

Use a fly cutter . How many you making ?

Would that not create a radiused step?

 

[mmcmdl]

No , tilt the bock , the tool would have the 20 degree ground on it , just mill across the block . IMO , it's easier just to purchase them as they're not that expensive . Reason I was asking how many , I have 100 or so here and I'll be up in Canada in 10 days .

 

[GrifterGuru]

Ahh, now I see what you are getting at as regards the fly cutter.

 

[SightlessSeer]

Use a fly cutter . How many you making ?

I thought about that, but I'm not sure how precisely I could grind a cutter for my fly cutter. I only have HSS bits and a very crude grinding setup with that combined with the angle built into my fly cutter, I'm not sure that I could get an accurate form tool for it.

For the moment, I'm making 4 clamps and 4 step blocks to go with them. I wouldn't be surprised if I make more later, but this is all that I need for now. I'm planning to make a gib for a lathe that I'm working on fixing up. I have an appropriate piece of cast iron bandsawed out of a scrap wood lathe bed, but I don't have a good way to secure it. The idea is to hold it to the table with 3 or more clamps and play musical clamps as I work the top surface, then flip, rinse, and repeat.

[...]
If you were to make your own, I would suggest you look at using a D/T cutter as you suggest so that you maintain the relationship as per commercial Strap and block sets and the integrated safety that design holds.

Wouldn't the design that I sketched on the right achieve the same thing? My thinking is that the benefit of using a dovetail cutter (or flycutter based form-tool like mmcmdl suggested) on my clamps would be interoperability with commercial clamps and step blocks.

I alsp thought of another reason that they might use a dovetail style cutter after I made the initial post. My though was that that for a given depth of step, a dovetail cut step takes up less depth. This makes the overall slope of the step block less, likely with no real difference in strength. This would also let the manufacturer use (slightly) less steel for each clamp and step block.

You can see what I mean in the picture below. In this sketch, I constrained all of the steps (marked with the red arrows) to be the same width and you can see that the dovetail cut one uses slightly more than half of the depth that the endmill cut one does.

 

[mmcmdl]

I just looked up what the Te-co sets go for . Prices shot up , maybe tripled ???????? Think I better go dig up the back yard .

 

[SightlessSeer]

I just looked up what the Te-co sets go for . Prices shot up , maybe tripled ???????? Think I better go dig up the back yard .

I ended up taking your advice and using a fly cutter. Initially, I though that you meant to grind the cutter with 20° (or whatever) relief use it as a form cutter to make a dovetail cutter approximation, but after I slept on It I realized that I could tilt the head and grind enough relief on the cutter that with the vertical head tilted I would get a vertical rise and leave whatever angle on the step that I had the milling head tilted at.

In the sketch below the blue dotted line is about how the cutter was relative to the work. I made the step blocks to the dimensions on that drawing.

Spoiler: Step Block Sketch

And here's a photo dump of the clamps from more or less the start to the finish (I don't have pictures for most of making the strap part of the strap clamps, but I'm sure you can fill in the blanks yourself).

Spoiler: Build Pictures

I didn't take any pictures of the squaring up, or machining of the straps. The slots are 1.1" long by 3/8" wide and I cut them with a 3/8" slotting mill (after plunging with a center-cutting 4 flute 3/8" end mill)

I used the bandsaw to seperate the four clamps from the single piece

I ended up cleaning up the bandsaw cut surfaces with the fly-cutter, but that was more of an aesthetic choice than a functional one. One thing that bothers me is that I have the slots about 0.5 mm off of center. It's not enough to make me go in with an end mill to even it out, but it is enough to bother me.

For the step blocks I used a chunk of 2.2"ish steel plate that I got from a local scrap yard. The block was roughly 2.2" x 2.5" x 3.25" before I cleaned it up



I was cheated a little bit with the layout line here. I measured the offset that I needed from either side to get the angle that I wanted from my CAD sketch ... unfortunately, I was dimensioning off of the wrong point. The angle I scribed ended up shallower than I needed it to be, so when I was cutting the steps with the fly cutter I had to hog out about 2.5x the material that I should have by the time I was cutting the bottom step.



I wanted to cut the steps into the step blocks in a single setup, but I was about 1/4" short on Y axis travel on both ends. I couldn't do the cut on the X axis because I can only tilt my vertical head, not nod it.

This was the actual setup I used to cut the steps. The block on the left side is to balance the vice jaw. I only cut on the right.


This part took a really long time

Here I'm trying to show the error in the angle of my band saw cut. You can see that the finished steps on the front block are proud of the back block near the top, but are short of it on the bottom.

Eventually, I had all of the steps cut

The last steps were to cut matching steps (no pun intended) into the straps and then split the two step blocks
And here we have the finished clamps. I was too lazy to clean up the bandsaw cut surfaces on the step blocks. Maybe later I will at some point down the road if I decide it matters enough (I doubt it).

Finally, a contrived setup to test their functionality. I haven't cut anything using them yet, but they hold a T Nut to the table pretty sturdily

If I had to guess, it probably took me about 24 hours total including machining, bandsaw, and deburring. A lot of that time was probably because I used the fly cutter for almost everything (the exception was the slots for the studs in the strap part of the clamps). If I was using an end mill to square up and rough out the parts and then fly cut only when absolutely necessary, I probably could have cut the time down a lot.

I made a point of making the step blocks tall because I wanted to make them as versatile as I could, but I think I might have overdone that. If I were to make them again, I would probably go for 0.100" tall steps instead of 0.150", but keep the total number the same. The granularity is probably more useful than the height.

That was almost certainly not worth the time invested on a purely economic scale, but it was a fun project and is the first one of any substance that I've done so far with the mill.

 

[GrifterGuru]

Very nice work. Just shows how much work is involved and, just perhaps, why the darned things cost so much these days given machine time, materials and "labour", even though, I suspect, they would be manufactured using CNC machines...