Change gear table meaning - Page 1
Forum section: electronics / general

author: Detzi
author_rank: Regular Contributor
timestamp: November 09, 2021, 04:34:04 pm
content: Hey there i have a really small lathe witch came with change gears and this chart: (See attachment) . Say i want to cut 0.5mm threads, does someone know what the right configuration of gears for that would be? What do the blank spots mean, what the "H"'s and what do the vertical "connector"-lines suggest?Thanks in advanceDetzi

author: jpanhalt
author_rank: Super Contributor
timestamp: November 09, 2021, 05:05:40 pm
content: You have to show the picture on the right so we can tell which pulleys are which. The black vertical line may indicate a connecting V-belt.Alternatively, do you have a link to the manual or model number?EDIT: Sorry about the misinformation. My mind was immersed in routing a perfboard, so everything was 90° to normal. Yes, I know threading is done with gears to the spindle and not belts.

author: Benta
author_rank: Super Contributor
timestamp: November 09, 2021, 07:42:58 pm
content: The pulleys on the right are irrelevant. They're just for spindle speed adjustment.The "H" just means a spacer, either one supplied with the lathe, or a small gear wheel that can be used as spacer without being active.This should be depicted on the top of your chart, which we can't see.Open the left side of your lathe to show the gear wheel arrangement (picture), then we can say more.

author: Carel
author_rank: Regular Contributor
timestamp: November 09, 2021, 07:49:34 pm
content: Quote from: Detzi on November 09, 2021, 04:34:04 pmI have put some numbers through the calculator but they dont add up. I do not see a correlation between for example 1 and 2 mm pitch, it should be half or double. So there is more. But the theory is simple, you want to make a pitch of 0.5 mm.If the pitch of the thread spindle of your lathe is for example 2 mm, the chuck has to turn 4 times to turn the thread spindle 1 time.Resulting in a threaded pitch of 0.5 mm.The table seems to indicate that gears with vertical lines drive each other, if the line switches, the middle gears are connected. The H seems to be a non participating gear, a bush, a dont care.The gears should all be marked with their number of teeth.Take a picture of the complete gear train on the left, then it will be clearer.

author: Detzi
author_rank: Regular Contributor
timestamp: November 09, 2021, 08:09:15 pm
content: Thanks so far @all. I made a Picture where all of the drawings is visible as well as two of my Gear train.@Carel yes the gears are numbered and i did manage to find all of the ones that are needed by the chart. I tried to set them up as suggested by the drawing but the blank spot confuses the heck out of me. So far i'd agree that a H stands for a "hole" (?) and also that the vertical lines indicate "touching" gears. What i dont get yet is as i already wrote what "the blank" field is standing for and how i would connect to it as the vertical line indicates. I thought it might be just misprinted in the wrong spot but i really cant tell.

author: Benta
author_rank: Super Contributor
timestamp: November 09, 2021, 08:18:43 pm
content: Right, it makes sense now.Unfortunately (or not, I don't know), you have a lathe with an imperial (inch) leadscrew. That explains the oddball gear ratios.Your lathe has a lead screw with 2.0 mm pitch, just like mine.The top chart shows the gearing when using the lead screw for longtitudal feed. Not the best idea, but common on cheap Chinese lathes.The middle chart shows the gearing for metric screw cutting.The lower chart shows the gearing forapproximateimperial screw cutting.Here's the chart from mine:

author: Carel
author_rank: Regular Contributor
timestamp: November 09, 2021, 08:53:59 pm
content: Your spindle pitch = 2mm.Now put the numbers for 0.5 mm in the calculator:You start with the chuck spindle with 40 tooth, It drives gear A with 52 tooth. There is no gear B.Gear A drives gear D with 80 tooth. Reduction 40:80 = 1/2 . Gear A is used as a bridge, doesn't add to the reduction, but changes the direction not.Gear D is connected to gear C with 40 tooth, it drives gear F with 80 tooth, reduction 40/80 = 1/2. There is no gear E.Total reduction is 1/2 * 1/2 = 1/4. Direct calculation is 40/80 * 40/80 = 0.25.OK, The labeling of the gears is a bit confusing, but it works out, that matters.On the photo you can also see some "H"s, the axles can hold 2 gears, but if you only need 1, you have to fill the space.

author: Benta
author_rank: Super Contributor
timestamp: November 09, 2021, 09:10:09 pm
content: Quote from: Detzi on November 09, 2021, 08:09:15 pmNo, the "H" stands for spacer to keep the gears aligned. Each shaft has space for two gear wheels. It can be two gear wheels, or a gear wheel plus a spacer.The blank spots are for a "joker", meaning you can place a spacer or a gear wheel of any size there, because there's space enough on the the change gear bracket/banjo for the gear wheel to not interact.

author: Detzi
author_rank: Regular Contributor
timestamp: November 10, 2021, 09:16:05 am
content: Thank you both @Benta and @Carel.I think i understand the drawing much better now but have still two questions:- how do you guys know that i have a 2mm pitched lead screw? From the top table?- The vertical "connector" for 0.5mm indicating to connect "wildcard" to "40" teeth is then just a miss print and the connection should be made from 52 to 80?@Carel let me sum up the gear train to make sure i understand correctly:- 40t head stock connects to 52t-this is almost 1:1 but not quite i figure a second 40t gear would be optimal?- 52t to 80t gear almost 1:2- 80t to 40t gear 1:1 since connected via a shaft- 40t to 80t gear 1:2This results in 1*0,5*1*0,5=0,25 paired with a 2mm lead screw ->0.5mm pitch. Is that right?

author: Carel
author_rank: Regular Contributor
timestamp: November 10, 2021, 09:52:18 am
content: You are right. If the table states you need those gears for 0.5 mm pitch and the calculated reduction is 1:4 then 4 chuck revolutions is 1 thread spindle revolution which leads to 0.5 mm pitch.Look at the gear as a mechanical divider (or multiplier), where maintaining the direction of rotation must also hold true. So in this case there is no optimum for gear A. It is just a reverser. Without it you would make a left hand thread.Just play with it, it will become clear.

author: Benta
author_rank: Super Contributor
timestamp: November 10, 2021, 10:15:22 am
content: You have to imagine that at the top of each column in the table there's an additional "40", which is the spindle gear.Now, gear wheels in series don't change anything (except direction), so you only have to look at the "40" in relationship to the gear wheel on the lead screw. For 1 mm pitch it's "80" = 40:80 =1:2.The other gear wheels are just chosen for best distance fit on the bracket. The technical name for those is "idler gear".The interesting part comes when you need transposers, which are two coupled gear wheels on the same shaft. This gives many more combinations as you see from the table.

author: Detzi
author_rank: Regular Contributor
timestamp: November 10, 2021, 10:39:09 am
content: Thanks to both of you, you helped me a lot.

author: Detzi
author_rank: Regular Contributor
timestamp: November 10, 2021, 02:08:48 pm
content: for anyone that wants to print his own gears, this seems pretty useful:https://www.igus-cad.com/default.aspx?treeid=10301

author: Carel
author_rank: Regular Contributor
timestamp: November 10, 2021, 02:34:35 pm
content: Always consult a seasoned professional when you start messing with homemade gears. Load, module, backlash, noise, concentricity, approximation of the perfect tooth, material, books can be and are already filled.And, if possible, the art of prime numbers.

author: Benta
author_rank: Super Contributor
timestamp: November 10, 2021, 04:55:18 pm
content: I'd say plastic is not really the right material for this application. There's more load on the gear wheels than you think.