M/Cycle Forklegs as slides

[BillTodd]

Continued from: http://www.cnczone.com/forums/showthread.php?t=1985

Hi Graham,

I though I'd better move the discussion here before we upset the moderators

Hi Bill, thanks for reply. I like the look of your engraver 3 axis machine. Is the stainless threaded M8 rod standard stock material or specially ordered? I can see a plastic nut in one of the pictures. Were the nuts bought ready made? Have you measured the accuracy/repeatability of X and Y? One of the lesser reasons why I didn't press on with the Taig CNC conversion was because it has limited movement about 9.5" x 5.5". On the other hand it is said to be accurate to better than 1 thou.

Yes, the M8 threaded rod is off the shelf from CPC (although I had to order a few lengths before I got one straight enough), the nuts are simply made from 3/4" diameter PTFE extrusion. The attached pic shows the Y axis nut with M12 nylotron(?) mounting nut. I added brass nut thinking I'd need to reduce the backlash, but the PTFE stretches around the tap as it cuts the thread, so it forms a snug fitting nut without it.

The M8 x 1.25 thread and stepper motors don't make it the fastest thing in the world (~15mm/s) but over such a small bed it's not really a problem.

>Have you measured the accuracy/repeatability of X and Y?
No, not yet, but it's fairly good (hey, the projects in its tenth year , no need to rush )

I am really interested more in woodwork at the moment so the accuracy is plenty good enough but I would like more movement. One of my aims is to be able to cut out pockets and inlays to insert a contrasting material into a wood panel, like marquetry but perhaps using metal, mother of pearl or similar as the inlay. I saw some good examples on this site for, I recall, guitar necks. Just had a measure up in the garage, my two bikes have: 1) 38mm dia x 750mm long approx and 2) 42mm dia x 610 long approx stanchion tubes. The total tube length is a best guess as I can't measure the part inside the alloy leg casting. The first mentioned came from a Kawasaki KLR650 or similar trail bike and would give as you say 18" and perhaps 24" of travel as machine axes.

The steel tubein the slider will be the length of the slider minus an inch or so at the bottom (where the damper fixes), minus the fork movement.

If the alloy casting runs direct on the stanchion then I guess they would ideally need a gaiter and regular oiling to reduce wear although they have a lipped seal at the top of the slider the arrangement of which I might be able to replicate at the cut end. I remember some old British bikes had a bronze bush in the top of the leg and one on the bottom of the stanchion. I think those forks had about 4" travel and they often leaked!

I wouldn't have thought wear would be a huge problem at the loads you'll be running them at; A simple oil-soaked felt wiper at each end would suffice surely?

I can't start on anything else just yet but will enquire about supply of of old forks from the breakers. Avoiding bent tubes may be a bit hit and miss but for woodwork if say 1mm out of plane was the maximum acceptable in the total length I should be able to see that by rolling the two tubes against each other. However, breakers may not be too keen to let me strip their forks down for inspection so perhaps just choosing ones with a nice smooth action without any sticky points or play would be good enough. Once bought I have a friend with a machine shop who could measure them up.

Have a good look around the bottom triple-clamp, if they've been thumped it'll usually show there.

That has reminded me of someone advertising they could straighten and replate fork tubes, however, that is probably an expensive service deviating from my very cheap objective!

I wouldn't worry about a little pitting.

Regards,

Bill

[grahamweekes]

Hi Bill,

thanks for more info. I hope you can follow my reply below, the quote system doesn't work the way I expected.

"Yes, the M8 threaded rod is off the shelf from CPC (although I had to order a few "lengths before I got one straight enough), the nuts are simply made from 3/4" diameter PTFE extrusion."
I hadn't heard of CPC before but they look pretty useful and cheaper than RS! If the test piece mentioned below goes ok I might try buying some threaded rod from Screwfix, they have S/Steel and plated MS in 0.3 m and 1.0 m lengths. Smallest pack of 1m lengths is 5 pieces but I would have no postage if I pick it up from a trade counter.

"The attached pic shows the Y axis nut with M12 nylotron(?) mounting nut. I added brass nut thinking I'd need to reduce the backlash, but the PTFE stretches around the tap as it cuts the thread, so it forms a snug fitting nut without it."
I was worried about straightness mainly. I have some odd bits of threaded rod, probably rather knocked about but ok to experiment with, and metric taps so will try making a test piece lead screw and nut.

"The M8 x 1.25 thread and stepper motors don't make it the fastest thing in the world (~15mm/s) but over such a small bed it's not really a problem."
Speed doesn't worry me either at my proposed size of machine.
">Have you measured the accuracy/repeatability of X and Y?
No, not yet, but it's fairly good (hey, the projects in its tenth year , no need to rush )"
Ten years makes me feel slightly less guilty at my lack of progress. The accuracy is not too critical for my proposed use.

"The steel tube in the slider will be the length of the slider minus an inch or so at the bottom (where the damper fixes), minus the fork movement."
That is about how I made my estimate. I should remember the stanchion lengths as I have had these forks apart.

"I wouldn't have thought wear would be a huge problem at the loads you'll be running them at; A simple oil-soaked felt wiper at each end would suffice surely?"
I think I agree, fitting a second seal would be quite a challenging job. However, I like the idea of keeping swarf out of the moving parts so would keep the idea of the gaiters. I saw a professional CNC router table which had rubber gaiters to cover the horizontal axes ways.

"Have a good look around the bottom triple-clamp, if they've been thumped it'll usually show there. I wouldn't worry about a little pitting."
I was thinking of straightness rather than pitting but my comment applies to that too. I would do better to make sure the forks were straight before buying.

I couldn't find any standard for fork stanchion straightness in my references but one of my workshop manuals refers to "stanchion maximum warpage" of 0.1 mm, less than 4 thou. Shame I don't know what warpage is. It does specify clearance between stanchion and lower leg as 0.050 mm to 0.114 mm, 2 to 4.5 thou which should do me for woodwork I guess the sensible thing to do would be to make up one axis and measure the accuracy and repeatability of that completed axis arising from the combination of tolerances and any backlash.

Regards

Graham

[BillTodd]

I hadn't heard of CPC before

They're part of the Farnell group. They sell a huge range of stuff from microprocessors to tow ropes.

Smallest pack of 1m lengths is 5 pieces but I would have no postage if I pick it up from a trade counter.

I had to buy a few from CPC before I got a straight one . It's best to buy them in a pack if they're going by post; there's less chance of them getting bent in transit.

I should remember the stanchion

Stanchion! That's the word I was struggling to remember

Shame I don't know what warpage is

I can only guess it's the deviation over the whole length ????