Despite reading some negative comments about modifying the hexagonal heads of set screws and bolts, I set about producing my tee-slot bolts from some "Carriage Bolts" (aka "Coach Bolts") bought at my local 'pound' shop.
The finished result seems to be fine for my purposes. The tee-slot bolts described here have a much bigger flat support area, than the ones I bought from a supplier (this is one of the criticisms made of modified hexagonal bolts). They were also a whole lot cheaper than 'official' tee-slot bolts. These Carriage/Coach bolts seemed to be an almost perfect size for modification to fit my tee-slots.
The beauty of using these Carriage/Coach bolts is the square shank near the circular head which makes it an almost trivial task to measure, align and cut them. The finished bolt is strong and has an 8mm thread diameter. The square shank fits beautifully in the slots and does not protrude above the surface.
I used my lathe with a vertical milling slide and an end mill.
I'm a real novice on a lathe and this is not a precision job (the method could be precise if necessary). Most things were 'eyeballed', with just a couple of careful measurements. I'm hoping that comments from others will highlight any obvious gaffs I have made!
Incidentally, I started using my grinder to produce the bolts. This was taking ages, was very hard to get nice parallel faces and nothing was perpendicular to anything! I rapidly gave up with this approach, there had to be a better way!
Step 1: Choice of Bolts and Equipment
M8 is the largest standard metric thread which will fit in the slot. I was happy with part-threaded bolts because they were for a specific task. For general use, I would have used bolts which had thread right along their length. These bolts come in a wide range of lengths and shank styles.
One supplier says the following about them:
"Predominately used on roof joists, fencing and timber to timber applications these Cup square bolts also known as carriage bolts or coach bolts come supplied with nuts and are in a Bright Zinc Plated (silver like) finish.
Key Features include:-
Square underneath the head for a greater lock into wood
Domed round head for a neater finish"
My bolts had a head diameter of 20mm.
I made some rough measurements of the slots which showed that the biggest part of the slots were about 14.2mm wide. The 20mm diameter of the bolt-heads had to be reduced to this width so that they could fit into the slot.
In addition, the head was a bit over 5mm thick at its fattest part and had to be reduced to about 4mm thick.
The set-up I used can be seen in the photographs. I used my vertical milling slide with a couple of clamping bars (held by M6 bolts screwed into threaded tee slot nuts).
I used a 4-flute 10mm end-mill (the only end-mill I have!) with plenty of cutting fluid daubed on with a brush.
I made use of a converted tyre-depth-gauge which made it easier to measure how far I had cut; but this was not vital.
Step 2: Setting Up for Cutting
1) reducing the head thickness to about 4mm
2) reducing the width of the head to about 14mm
The bolt being worked on was first clamped flush with the edge of the vertical slide. The cutting was done on the side of the end-mill.
For the first bolt I worked on, I just eyeballed the clamping bar to be parallel with the vertical slide (this worked fine). Subsequently, I put a second bolt up high, just as a spacer to make sure the clamping bar was perfectly parallel - this just removed any judgement about the alignment of the bar.
Once it had been thicknessed, I slid the bolt away from the vertical slide by a couple of mm. This enabled the end of the end-mill to do its work of cutting down the width of the bolt, without cutting into the slide itself, or the clamping bar.
Step 3: Reducing the Thickness of the Bolt Head
- Clamp the bolt flush up against the vertical-slide.
- Check that the bolt looks parallel to the vertical-slide tee-slots (ie is at right-angles to the end-mill).
- Bring the apron towards the chuck, until the bolt is in the middle of the end-mill (horizontal movement)
- Adjust the vertical-slide until the bolt is in the middle of the end-mill (vertical movement)
- Clamp the apron to the bed using the clamping screw.
- Switch on, apply cutting fluid.
- Bring the cross slide in, until the bolt just touches the end-mill - zero the cross-slide index wheel (or note its position).
- Do the cutting (raise the vertical-slide, bring the cross-slide in by a suitable cutting depth, move the vertical-slide down, etc, etc)
- Stop when the bolt is the correct thickness (for me it was after 60 divisions on the cross slide - ie cutting 1.2mm).
I was not particularly precise - I used my digital vernier to check the thickness, and stopped once it got below 4mm. After doing a couple of bolts, I knew how far to move the cross slide when using its index wheel. I don't believe there is much quality control in the production of the bolts, so precision would need some more careful measurements of each bolt!
I made my cuts on both the upward and downward movement of the vertical slide - not best practice, due to backlash in the slide.
Next, reduce the width of the bolt head....
Step 4: Reduce the Width of the Bolt Head
- Slacken off the clamping bar and pull the bolt away from the vertical-slide by a couple of mm.
- Adjust the apron and cross-slide so that the end of the end-mill is going to cut the bolt and not cut the vertical-slide.
- You can't clamp the apron, because you need to move it.
- I engaged the half-nuts onto the lead-screw and used the lead-screw hand-wheel to move the apron.
- Switch on. Move the apron until the end-mill is just touching the bolt.
- Zero the digital depth gauge.
- Do the cutting (raise and lower the vertical slide, feeding the cutting depth between each pass)
- Stop when a cut of 2.9mm has been made.
Here is where the square shank of the bolt, comes into its own!
- Slacken the clamping bar.
- Rotate the bolt by 180 degrees (ie onto the appropriate side of the square shank)
- Repeat the cutting process.
The above sizes will need adjustment for differently manufactured bolts and also if you don't have an obvious way of measuring the cutting depth. (A digital tyre depth gauge is VERY cheap and easy to adapt - Google it).
Step 5: The Finished Tee-slot Bolt
The last two photographs show the bolts being used for my next project.If it works, it will be the next Instructable!