Hex Bar and Sliding dovetails

[bigegg]

I'm making a thing [1] for which I need to make a sliding dovetail
about 3ft long.
The dovetail will slide horizontally across a vertical face, and will need to support a maximum weight of about 2 or 3kgs
Ideally, I would have liked it to be machined from a cast iron, but the budget doesn't run to it.
I don't want to use aluminium extrusion except as a last resort, so I'm thinking of fabricating something from 11mm a/f hex bar.

How accurate is it? (as regards twist),
and what sort of price would I be looking at (for a 6m? length)?
All my regular suppliers are closed, and I can't seem to find an online price (except for the 'bay)

I know I could probably use something like a sliding door hanger, but I *like* sliding dovetails.



[1] Haven't quite decided what it going to be yet -
the concept has numerous applications in home machining.

 

[scott0031]

Hi Bigegg,

Although you give some information its a bit limited. The advice you get will be dependent on the info you give. Most is really dependent on exactly what you want to do, has this thing got to move once in a blue moon or 10k times a day? How fast does it have to travel? Is wear on the slide mechanism an issue?

BDMS (bright drawn mild steel) is pretty good accuracy wise but over longer lengths it can have a tendency to twist by the nature of how it is made, in short its has a surface tension. With the sections and lengths you are talking about i dont think it will be a major issue.

Standard length for brights is 3 mtr/10 ft it wont be expensive at all but you will most likely get clobbered for minimum charge and delivery, unless you can arrange otherwise.

You can look here for a price guide. This will be inflated on a price from a steel supplier.


http://www.maidstone-engineering.com/index.php?route=product/category&path=1_48_96

Acton bright are one company to look at and they do give free delivery, but unsure how that would stand with you if you are not a business.

Its also worth noting there are many slide mechanisms available as standard these days for home build cnc guys, linear rails and probably round rails may be available. Why not consider round rails, they are a bit easier to work with.

There are many different routes you can take with this, its a matter of how far you want to go, the actual application and how much you want to spend.


Kind regards,


Scott

 

[scott0031]

To give an idea this was built before the commercial rails were available. It uses round bar, probably stainless or silver steel (cant remember now) because they are ground finishes. Bearing blocks made to suit with i think plastic bushing. Sorry it was made 20 years plus ago now.

 

[bigegg]

I'm making a variation on a "router boss" or woodrat.

 

[scott0031]

You need to sort the image for it to be any help.


Kind regards,

Scott

 

[bigegg]

<fixed>

so many forums, so many different rules on pictures...

 

[bigegg]

Is it possible to buy steel in standard profiles?
such as ground to a 45deg right triangle?
or equilateral triangle?
or (better still) a dovetail shape?

Would anyone have a rough idea how much it would cost to have a 1m x 200mm length of 10mm plate bevelled to 60deg on the long edges?

 

[astro_al]

Why not just use regular linear bearings? Like the shop bought version of Scott's pic?

Al

 

[bigegg]

Ideally, I'd like the entire slide to be around 25mm thick - I've not really looked into linear bearings, but I can't see how I make them fit in the space available.
Plus, they're expensive, and seem fragile (at least compared to dovetail ways)

 

[Keith 66]

How long do the linear bearings have to be?

 

[bigegg]

The travel of a 300mm plate needs to be about 1m, so at least 1.3m

 

[Keith 66]

Pity, i just picked up a pair of linear rails plus 4 cars out of a skip.
Bars are approx 20mm square but only 400mm long.

 

[bigegg]

Lucky...

I'm looking at about £300 for linear bearings in the size i need.
And even with 25mm bars, I'm worried about deflection due to cutting forces at the centre
(since they will only be supported at the ends)

 

[scott0031]

Is it possible to buy steel in standard profiles?
such as ground to a 45deg right triangle?
or equilateral triangle?
or (better still) a dovetail shape?

Would anyone have a rough idea how much it would cost to have a 1m x 200mm length of 10mm plate bevelled to 60deg on the long edges?

No, not that ive seen in my Engineering life.

Its not as simple as that, it's a bearing surface. Materials move when machined, longer it is the worse it is, hence why things are hardened and ground to keep them in shape and to act a proper bearing surface. Its all possible but it costs.

Ideally, I'd like the entire slide to be around 25mm thick - I've not really looked into linear bearings, but I can't see how I make them fit in the space available.
Plus, they're expensive, and seem fragile (at least compared to dovetail ways)

They are fragile to a degree if unsupported but you have to remember what you re doing here.. its a router, not a machining center.......... and linear rails are used all the time in industrial machines. You are right in the sense the are not known as being as ridged as solid slide ways............. but again its a router. Dont try to crack nut with bull dozer when a toffee hammer is totally adequate.

The only other option you have is to make your own arrangement as in my pics. Round bar and and oilite bushes or something similar housed in a block/ blocks. To make it run nice and smooth it has be be accurate unless you make everything adjustable, which will mean a lot of fiddling to get it right. The design will have a large impact on the end result.

Have a good read here round rails are available.

http://www.worldofcnc.com/


There was also a site called zapp automation which was very good but it seems to have disappeared.



Kind regards,


Scott

 

[bigegg]

Having spent all afternoon researching linear bearings, CNC, and anything related I can find, it would appear that I'm searching for the "holy grail" of DIY machines.

I've come to the conclusion that this project is probably a little *too* ambitious for my budget and equipment at the moment, so I'm going with plan C:

I'm going to reduce the slide travel to 600mm, and make the slideways from plyboard with grooves cut for steel runners (made from 20mm square bar and 25x25x2.5 angle)
I have a large pile (two pallets about 4ft high) of furniture grade (13 ply) 8mm plyboard, which should be stable enough for this job, and the equipment to mill the grooves to within a couple of thou

 

[Shox Dr]

Why not use a supported rail. Marchant dice supply such: 12mm Dia per 100mm £9.36

 

[bigegg]

like this:

 

[Robotstar5]

Guide rails don't have to be round, we use THK linear guides, the lowest profile & bearing will fit in about 25mm gap.

 

[bigegg]

can the "supported rail" be used such that the mounting plate is fixed to a vertical face? (with the rail running horizontally?)
Still looking at £300 tho (2 lengths of 1m each + fittings)

 

[Robotstar5]

The THK system is used flat, vertical and even inverted on the welding manipulators.