Belt-drive design

[cnczoner]

Can anyone help me choose components for a belt-drive design for my 7x10 mini-lathe? I want to reduce by 1:2 for now, and have the option to reduce 1:3 or 1:4 if I need more torque. I'll live with the lower speed if necessary.

What size and types of belts/pulleys are good to use? I looked thru sdp-si.com, and somewhat randomly chose from the sizes available for the shaft size on my stepper. I came up with a MXL-pitch (.080) belt with .375" width, and pulleys with 15 and 30 grooves and 20 and 40 grooves for 1:2 reduction.

Is this wide enough? Strong enough? Should I use plastic or aluminium pulleys? What about the belt -- kevlar, fiberglass, polyester, or ... ???

The Z-axis shaft on the lathe is 9mm I believe -- should I just drill out a 5/16" pulley for this? Or can I find a matching-groove-size pulley with a metric hub somewhere?

Is there a good resource that would help me choose/design this properly? I've already done a decent mechanical design, but I just need to pick the right components.

Thanks,
-Neil.

[RotarySMP]

I used HTD belts on my mini-lathe conversion. Have had no issues with them.

Used Al pulleys. I'm sure nylon pulley would do the job though. Boring out a pulley is a trivial job for a guy with a lathe (like you).

http://www.wrathall.com/Interests/CN...ide_drive2.htm

What motors are you using? With 187oz and the 2:1 reduction my Machine works very well. I had to slow down the rapids on the Z axis, as the bed thickness is not constant, and I have the gib plates pretty tight. If I ever get around to addressing the bed thickness the rapids could be nearly doubled. I am using 4mm lead screw on Z though.

[Chagrin]

(Note: I'm not an expert, but I did stay at a Holiday Inn last night)
http://www.sdp-si.com/D260/PDF/part1.pdf

From that document:

HTD was developed for high torque drive applications, but is not acceptable for most precision indexing or registration applications.

Not that I'm saying that RotarySMP's machine is wrong - I'm sure it works quite well (and much better than mine, which, of course, doesn't exist).

Gates' HTD belts have a semicircular tooth profile which will require special pulleys to match. Most timing belts have a trapezoidal tooth profile.

MXL .375" wide belts have a working limit of 12 lbs of tension per the above document. Take the radius of your pulleys and and strength of your motors and calculate if that'll be within limits. I.e., if your motors are 120 oz-in then they pull 120 ounces (7.5lbs) 1 inch from the center of the shaft.

[avsfan733]

I now know 75 pages worth of really interesting things about belts...awesome site chargin defnitely nicked the pdf

[cnczoner]

RotarySMP: I checked the link for the HTD info, but got to a website in German, and ict not bin ein berlinner.

Chagrin: Not sure how I bypassed SDP-SI's whole tech section, but wow -- that'll answer most of my questions and then some.

Much thanks,
-Neil.

[RotarySMP]

Considering the rigidity of these machines, and the fact that Neil is going to use the existing leadscrews, the backlash of any of the various timing belt systems is likely to be negligable. The biggest driver for me was that I had got a piece of scrap rectangular tube that looked promising, and then searched the Maedler catalog for a pair of Al pulleys wihch would give 2:1 reduction. In XL, you ended up with one Nylon pulley, and I like Al. Hows that for exacting engineering design

I asked a similar question on the CCED Yahoo groups before I made mine, and was led to believe the HTD belts have less backlash. The belts are matched with the pulleys.

Try the www.Mcmastercarr.com catalog.

[cnczoner]

Thanks for the info. All of the HTD options I've found at McMaster are for very large diameter shafts (.5" and above).

Here's what I see as my options at this point...

Using SDP-SI's neat tech info I've have come up with the following parts/design...

Drive gear = polycarbonate, 2-flange, XL, 16 grooves, 0.25" bore, alum insert w/set screw.
Driven gear = polycarbonate, 2-flange, XL, 32 grooves, 0.3125" bore, alum insert w/set screw.
Belt = 45 grooves, urethane, XL, 3/8" wide.

About $16 for all three pieces.

I'll get 1:2 reduction with this, but can can later change the driven gear to 48 grooves to get 1:3 reduction if necessary. The motor will be a 142 oz-in unit, but I also have a 200+ oz-in unit on the way, so the 1:3 may (should) be unnecessary. But I've designed it to all for that just in case .... (since I'm basing a lot of this off unspecific requirements).

I can get a similar config (XL, not HTD) from McMaster, but I'm leaning towards the SDP option since I'd like to try them out and since they have other things I need (like couplers for my direct-drive cross-slide, etc).

Any reason I should not proceed with this?

Also, is it required, expected, recommended, or pointless to use an idler/tensioner? Other then easily being able to adjust the belt tension (which I can do anyway with the slotted holes I made in the brackets), it would let me get more teeth in mesh between the belt and pulleys.

Cheers,
-Neil.

[cnczoner]

BTW, the folks at SDP we're very helpful and found me a configuration using GT belts/pulleys, which they recommended as very low backlash. So I bit. What did it for me was that the smaller pitch made the 20T and 40T pulleys smaller, so will fit easier into my mounts. Will be here early next week hopefully.

Cheers,
-Neil.