Polycarbonate bench dogs for MFT/3

[Dan Pfeiffer]

A while ago I made up some bench dogs from 1" polycarbonate rod.  I see this is a common topic so I thought I would toss up a few photos.  I made them on my milling machine using a 3 jaw chuck that fits into the spindle like a collet.  I put a cutter in the vice and it all works like a lathe though probably not as precise.  If I would get my butt in gear and get my lathe running that would be the right tool for this.  The polycarbonate machines nicely. I also made one from delrin.  The fit is just snug and they seem to work well for me when aligning the fence. 

[attachthumb=#] [attachthumb=#] [attachthumb=#]

 

[vkumar]

Dan looks good.  We had Qwas dogs, now Pfeiffer dogs ;D

Vijay

 

[Alan m]

nice home made dogs.
could i ask you what your avitar pic is please

 

[erock]

Dan nice job!
where did you get the 1" poly from? thanks
eric

 

[Dan Pfeiffer]

nice home made dogs. could i ask you what your avitar pic is please

The avatar is my super cool Tamaya Vega sextant.
[attachthumb=#]

Dan nice job! where did you get the 1" poly from? thanks

I got the 1" polycarbonate at Mcmaster Carr (mcmaster.com).  About $8/ft.  Mcmaster # 8571K16.  They have a lot of other materials too.  I got the delrin there and they have aluminum, stainless and just about anything else and they will have it on your door step tomorrow if you order by 3PM or so. 

 

[tbellemare]

Nice product, Dan.

Have you tried these at various temperatures? We go from ~15 degrees F (on very rare occasion) to ~115 degrees F (also on very rare occasion). It isn't unusual here to go for a week to a month above 85.

The Coefficient of Thermal Expansion of extruded acrylic is very significant.

Tom

 

[Dan Pfeiffer]

Nice product, Dan.
Have you tried these at various temperatures? We go from ~15 degrees F (on very rare occasion) to ~115 degrees F (also on very rare occasion). It isn't unusual here to go for a week to a month above 85.
The Coefficient of Thermal Expansion of extruded acrylic is very significant.
Tom

These are not intended to be products.  Just something I made.  I have not tested them in any manner except to see that they fit the holes in my MFT/3 and give good alignment.  -15 to 115 F is pretty extreme work conditions.  My shop may go from 40 to 85 in winter vs summer and that's where I use them.  Have you tested the MDF top at these temperature extremes?  Rather than using a dog material with low thermal expansion maybe you need one that matches the MDF.

 

[Alan m]

if they are from the same piece of plastic should they not expand equaly. the diamiter orf the dogs doesnt matter as long as they are the same.

 

[mouppe]

if they are from the same piece of plastic should they not expand equaly. the diamiter orf the dogs doesnt matter as long as they are the same.

I think what Dan meant is not comparing dog to dog, but dog material to MDF material.

 

[tbellemare]

What I meant was if they fit snuggly at 40 degrees, will they be stuck at 85?

Though I don't think MDF moves much at all, if it moved, the hole would shrink with temperature rise. The dog would grow and I'm pretty sure it would grow perceptibly with a 40 degree rise. I'll get some data and calculate it...

Tom

 

[Dan Pfeiffer]

What I meant was if they fit snuggly at 40 degrees, will they be stuck at 85?
Though I don't think MDF moves much at all, if it moved, the hole would shrink with temperature rise. The dog would grow and I'm pretty sure it would grow perceptibly with a 40 degree rise. I'll get some data and calculate it...
Tom

I don't want to start some crazy debate here but this is a basic property of materials and a common misconception.  Heat makes things expand.  If the thing has holes the holes expand too.  The top of the work table, whether it's MDF, steel, plastic, marble, will expand with increasing temp.  So will any holes in the material.  They get bigger.  So if you want a truly constant fit of the dogs you want them to have the same thermal expansion properties as the table surface.  This assumes even heating.  If you have the dogs in a plastic box on your dashboard and take them to the relatively cool table you have a different issue. 

 

[mhoy]

If they fit and you're happy, then well, you're happy.  [big grin] 

Here is a link to thermodynamics for everyone.
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/thexp2.html

Mark

 

[tbellemare]

'Sorry, Dan, backward logic about the hole on my part. I know well that if you heat a nut, it can release...

Tom

 

[worknfool]

So that B/A sextant...used to square your MFT rail? 

Dealt w/ McMaster a lot in past years. Not usually the best priced, but they do have everything, and they do get it to you quick like a bunny. Laird plastics is a nation wide concern I believe, and they are priced quite well usually. The one closest to me gives us off fall of all shapes and sizes when we think to ask while picking up other material. Also used to get a lot of usable "trash" from a local sign shop. They were happy to hold stuff for us as long as we took everything and got rid of it for them. Chunks of 1/2" acrylic for router bases that the local Woodcraft was asking $20+ for, free for the asking.